diff --git a/.claude/skills/convert-gha/SKILL.md b/.claude/skills/convert-gha/SKILL.md
index e50796c2..cb1e9eb1 100644
--- a/.claude/skills/convert-gha/SKILL.md
+++ b/.claude/skills/convert-gha/SKILL.md
@@ -54,7 +54,7 @@ Convert existing GitHub Actions workflows (`.github/workflows/*.yml` / `*.yaml`)
    | `actions/cache` | **Not needed — caching is implicit in Harmont** | Harmont automatically caches build artifacts, dependency installs, and toolchain outputs between runs. Remove all cache steps. |
    | `actions/setup-*` (setup-node, setup-python, etc.) | Harmont toolchains (`hm.js`, `hm.python`, etc.) | Toolchains handle installation. Specify version via toolchain config. |
    | `actions/checkout` | **Not needed — source is always available** | Harmont automatically provides the source code to every step. |
-   | `runs-on: ubuntu-latest` | `default_image: "ubuntu:24.04"` | Harmont runs steps in Docker containers |
+   | `runs-on: ubuntu-latest` | (default base is `ubuntu:24.04`; set a per-step `image="..."` to override) | Harmont runs steps in Docker containers |
    | `services:` (e.g., postgres) | Service containers in step config | Check docs for service container syntax |
    | `matrix:` | Multiple pipelines or parameterized steps | No direct matrix — may need separate pipeline definitions or `.fork()` |
    | `env:` / `secrets.*` | `env: {}` on pipeline or step | Secrets must be passed as environment variables |
diff --git a/.claude/skills/write-pipeline/SKILL.md b/.claude/skills/write-pipeline/SKILL.md
index ac349116..fdc62ffa 100644
--- a/.claude/skills/write-pipeline/SKILL.md
+++ b/.claude/skills/write-pipeline/SKILL.md
@@ -58,7 +58,7 @@ Write, modify, or extend Harmont CI pipelines defined in `.hm/pipeline.py` (Pyth
    - Prefer toolchains over raw `sh()` calls when a toolchain exists for the language.
    - Use `.fork()` for steps that can run in parallel.
    - Set triggers (`push`, `pull_request`) appropriate to the project.
-   - Use `default_image: "ubuntu:24.04"` unless the project needs something specific.
+   - Steps run on `ubuntu:24.04` by default; set a per-step `image="..."` only when a specific step needs a different base.
    - Set `env: {"CI": "true"}` on the pipeline.
 
 5. **Validate the pipeline renders correctly:**
diff --git a/README.md b/README.md
index bcc98501..ebc4862e 100644
--- a/README.md
+++ b/README.md
@@ -114,7 +114,6 @@ def project() -> PythonToolchain:
 
 @hm.pipeline(
     "ci",
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci(project: hm.Target[PythonToolchain]) -> tuple[hm.Step, ...]:
@@ -148,7 +147,6 @@ const pipelines: PipelineDefinition[] = [
         project.fmt(),
         project.typecheck(),
       ],
-      { defaultImage: "ubuntu:24.04" },
     ),
   },
 ];
@@ -192,7 +190,7 @@ maps it over for you:
 - `actions/setup-*` → replaced by a typed toolchain
 - `actions/cache` → not needed (Harmont caches Docker layers automatically)
 - `jobs.*.needs` → the DAG `hm` derives from your code
-- `runs-on` → `default_image`
+- `runs-on` → per-step `image=` (the default base is `ubuntu:24.04`)
 
 The result is a pipeline you can run **locally** before it ever hits CI.
 
diff --git a/crates/hm-dsl-engine/harmont-py/harmont/__init__.py b/crates/hm-dsl-engine/harmont-py/harmont/__init__.py
index 9cfc586c..9448277f 100644
--- a/crates/hm-dsl-engine/harmont-py/harmont/__init__.py
+++ b/crates/hm-dsl-engine/harmont-py/harmont/__init__.py
@@ -8,7 +8,7 @@
     Step.fork(label=None)    -> Step
     wait(*, continue_on_failure=False) -> Step
 
-    pipeline(leaves, *, env=None, default_image=None) -> dict (v0 IR)
+    pipeline(leaves, *, env=None) -> dict (v0 IR)
     pipeline_to_json(p, **kw) -> str
 
     @pipeline(slug, ..., triggers=[...], allow_manual=True)  -> decorator
@@ -68,12 +68,12 @@ def pipeline(*args: Any, **kwargs: Any) -> Any:
 
     Factory form — first positional is a list/tuple of ``Step``s:
 
-        pipeline([step1, step2, ...], env=None, default_image=None) -> dict
+        pipeline([step1, step2, ...], env=None) -> dict
 
     Decorator form — no positionals or a string slug:
 
         @pipeline(slug=None, *, name=None, triggers=(), allow_manual=True,
-                  env=None, default_image=None)
+                  env=None)
         def my_pipeline() -> Step: ...
 
     The discriminant is the type of the first positional argument:
diff --git a/crates/hm-dsl-engine/harmont-py/harmont/_decorator.py b/crates/hm-dsl-engine/harmont-py/harmont/_decorator.py
index 67c841b0..e9d77872 100644
--- a/crates/hm-dsl-engine/harmont-py/harmont/_decorator.py
+++ b/crates/hm-dsl-engine/harmont-py/harmont/_decorator.py
@@ -34,7 +34,6 @@ def pipeline(
     triggers: tuple[Trigger, ...] | list[Trigger] = (),
     allow_manual: bool = True,
     env: dict[str, str] | None = None,
-    default_image: str | None = None,
     timeout: str | int | None = None,
 ) -> Callable[[Callable[..., Any]], Callable[[], Any]]:
     """Register a function as a CI pipeline (decorator form).
@@ -56,8 +55,6 @@ def pipeline(
         allow_manual: When ``True``, the pipeline can be triggered manually
             via the UI or API in addition to its configured triggers.
         env: Pipeline-level environment variables applied to every step.
-        default_image: Local-mode Docker base image applied to root steps
-            that lack an explicit ``image`` or ``builds_in`` parent.
         timeout: Whole-build wall-clock budget ("30m", "1h", or int
             seconds). The build is killed and fails as timed out once it
             elapses.
@@ -86,7 +83,6 @@ def wrapper() -> Any:
                 triggers=tuple(triggers),
                 allow_manual=allow_manual,
                 env=env,
-                default_image=default_image,
                 fn=wrapper,
                 timeout=timeout,
             )
diff --git a/crates/hm-dsl-engine/harmont-py/harmont/_envelope.py b/crates/hm-dsl-engine/harmont-py/harmont/_envelope.py
index 972ae3dd..936cb379 100644
--- a/crates/hm-dsl-engine/harmont-py/harmont/_envelope.py
+++ b/crates/hm-dsl-engine/harmont-py/harmont/_envelope.py
@@ -40,7 +40,7 @@ def _render_one(
     except TypeError as e:
         msg = f"pipeline {reg.slug!r}: invalid return value\n  → {e}"
         raise TypeError(msg) from e
-    ir = _assemble(leaves, env=reg.env, default_image=reg.default_image, timeout=reg.timeout)
+    ir = _assemble(leaves, env=reg.env, timeout=reg.timeout)
     resolve_pipeline_keys(
         ir.get("graph", {}),
         pipeline_org=pipeline_org,
diff --git a/crates/hm-dsl-engine/harmont-py/harmont/_pipeline.py b/crates/hm-dsl-engine/harmont-py/harmont/_pipeline.py
index cf601d86..38b25ea1 100644
--- a/crates/hm-dsl-engine/harmont-py/harmont/_pipeline.py
+++ b/crates/hm-dsl-engine/harmont-py/harmont/_pipeline.py
@@ -27,19 +27,24 @@
 if TYPE_CHECKING:
     from ._step import Step
 
+# Across-the-board default image for imageless root steps. The SDK's
+# toolchains assume an apt-capable base (apt-get), so ubuntu:24.04 is the
+# universal default; child steps boot from their parent's snapshot and
+# stay imageless.
+DEFAULT_IMAGE = "ubuntu:24.04"
+
 
 def pipeline(
     leaves: list[Step] | tuple[Step, ...],
     *,
     env: dict[str, str] | None = None,
-    default_image: str | None = None,
     timeout: str | int | None = None,
 ) -> dict[str, Any]:
     """Top-level factory. Returns a JSON-shaped dict (version "0").
 
-    ``default_image`` is the local-mode fallback Docker image: it
-    applies to every command step that lacks both a ``builds_in``
-    parent edge and a per-step ``image`` override.
+    Every imageless root command step (one with no ``builds_in`` parent
+    and no per-step ``image``) is stamped with ``DEFAULT_IMAGE``
+    (``ubuntu:24.04``). Set a per-step ``image=`` on a step to override.
 
     ``timeout`` is a whole-build wall-clock budget (``"30m"``, ``"1h"``,
     or an int number of seconds). When it elapses the build is killed and
@@ -52,15 +57,9 @@ def pipeline(
         )
         raise ValueError(msg)
     out: dict[str, Any] = {"version": "0"}
-    if default_image is not None:
-        out["default_image"] = default_image
     if timeout is not None:
         out["timeout_seconds"] = parse_duration(timeout)
-    out["graph"] = _lower_to_graph(
-        list(leaves),
-        env=env,
-        default_image=default_image,
-    )
+    out["graph"] = _lower_to_graph(list(leaves), env=env)
     return out
 
 
@@ -68,7 +67,6 @@ def _lower_to_graph(
     leaves: list[Step],
     *,
     env: dict[str, str] | None = None,
-    default_image: str | None = None,
 ) -> dict[str, Any]:
     """Walk back via `parent`, topo-sort, emit petgraph-serde graph dict.
 
@@ -86,7 +84,7 @@ def _lower_to_graph(
     for i, s in enumerate(command_steps):
         idx_by_id[id(s)] = i
 
-    # Track which node indices have a builds_in parent (for default_image).
+    # Track which node indices have a builds_in parent (child steps stay image-less).
     has_builds_in_parent: set[int] = set()
 
     nodes: list[dict[str, Any]] = []
@@ -156,11 +154,13 @@ def _lower_to_graph(
 
         pre_wait_indices.append(node_idx)
 
-    # Apply default_image to root nodes (those without a builds_in parent).
-    if default_image is not None:
-        for i, node in enumerate(nodes):
-            if i not in has_builds_in_parent and "image" not in node["step"]:
-                node["step"]["image"] = default_image
+    # Stamp the default image on every root command step that lacks an
+    # explicit one. Root steps boot from an image tag (not a parent
+    # snapshot); child steps inherit the parent's committed snapshot and
+    # must stay image-less.
+    for i, node in enumerate(nodes):
+        if i not in has_builds_in_parent and "image" not in node["step"]:
+            node["step"]["image"] = DEFAULT_IMAGE
 
     return {
         "nodes": nodes,
diff --git a/crates/hm-dsl-engine/harmont-py/harmont/_registry.py b/crates/hm-dsl-engine/harmont-py/harmont/_registry.py
index 43b6ad4c..0eb5c7b8 100644
--- a/crates/hm-dsl-engine/harmont-py/harmont/_registry.py
+++ b/crates/hm-dsl-engine/harmont-py/harmont/_registry.py
@@ -22,7 +22,6 @@ class PipelineRegistration:
     triggers: tuple[Trigger, ...]
     allow_manual: bool
     env: dict[str, str] | None
-    default_image: str | None
     fn: Callable[[], object]
     timeout: str | int | None = None
 
diff --git a/crates/hm-dsl-engine/harmont-py/harmont/_step.py b/crates/hm-dsl-engine/harmont-py/harmont/_step.py
index b3e1bea2..2fb8e090 100644
--- a/crates/hm-dsl-engine/harmont-py/harmont/_step.py
+++ b/crates/hm-dsl-engine/harmont-py/harmont/_step.py
@@ -37,8 +37,9 @@ class Step:
     timeout_seconds: int | None = None
     image: str | None = None
     """Local-mode Docker base image override for this step. Ignored when
-    the step has a ``builds_in`` parent (the parent's snapshot wins);
-    falls back to the pipeline's ``default_image`` when unset."""
+    the step has a ``builds_in`` parent (the parent's snapshot wins).
+    When unset, root steps fall back to ``ubuntu:24.04``; child steps
+    inherit the parent's snapshot."""
 
     runner: str | None = None
     """Step-executor plugin runner name. ``None`` = default (Docker)."""
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_cmake.py b/crates/hm-dsl-engine/harmont-py/tests/test_cmake.py
index a17e1cc5..6615498c 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_cmake.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_cmake.py
@@ -19,7 +19,7 @@ def _cmds(p: dict) -> list[str]:
 class TestCMakeToolchain:
     def test_default_toolchain_installs_cmake_ninja_ccache(self):
         tc = hm.cmake()
-        p = hm.pipeline([tc.installed], default_image="ubuntu:24.04")
+        p = hm.pipeline([tc.installed])
         cmds = _cmds(p)
         apt_cmd = next(c for c in cmds if "apt-get install" in c)
         assert "cmake" in apt_cmd
@@ -28,14 +28,14 @@ def test_default_toolchain_installs_cmake_ninja_ccache(self):
 
     def test_clang_18_compiler_installs_clang_18(self):
         tc = hm.cmake(compiler="clang-18")
-        p = hm.pipeline([tc.installed], default_image="ubuntu:24.04")
+        p = hm.pipeline([tc.installed])
         cmds = _cmds(p)
         apt_cmd = next(c for c in cmds if "apt-get install" in c)
         assert "clang-18" in apt_cmd
 
     def test_gcc_14_compiler_installs_gcc_14_and_gpp_14(self):
         tc = hm.cmake(compiler="gcc-14")
-        p = hm.pipeline([tc.installed], default_image="ubuntu:24.04")
+        p = hm.pipeline([tc.installed])
         cmds = _cmds(p)
         apt_cmd = next(c for c in cmds if "apt-get install" in c)
         assert "gcc-14" in apt_cmd
@@ -48,7 +48,7 @@ def test_invalid_compiler_raises_valueerror(self):
     def test_ccache_false_omits_ccache_from_apt_and_flags(self):
         tc = hm.cmake(ccache=False)
         proj = tc.project(path=".")
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         cmds = _cmds(p)
         apt_cmd = next(c for c in cmds if "apt-get install" in c)
         assert "ccache" not in apt_cmd
@@ -60,7 +60,7 @@ def test_toolchain_shared_across_projects_single_apt_install(self):
         tc = hm.cmake()
         proj1 = tc.project(path="svc1")
         proj2 = tc.project(path="svc2")
-        p = hm.pipeline([proj1.built, proj2.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj1.built, proj2.built])
         cmds = _cmds(p)
         apt_installs = [c for c in cmds if "apt-get install" in c]
         assert len(apt_installs) == 1
@@ -74,14 +74,14 @@ def test_toolchain_shared_across_projects_single_apt_install(self):
 class TestCMakeProject:
     def test_build_produces_configure_and_build_commands(self):
         proj = hm.cmake(path="svc")
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         cmds = _cmds(p)
         assert any("cmake -S . -B build" in c for c in cmds)
         assert any("cmake --build" in c for c in cmds)
 
     def test_warmup_uses_relative_build_dir_after_cd(self):
         proj = hm.cmake(path="infra/agent")
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         cmds = _cmds(p)
         warmup = next(c for c in cmds if "cmake -S . -B build" in c)
         assert "cd infra/agent" in warmup
@@ -90,49 +90,49 @@ def test_warmup_uses_relative_build_dir_after_cd(self):
 
     def test_uses_ninja_generator_by_default(self):
         proj = hm.cmake(path="svc")
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         cmds = _cmds(p)
         configure_cmd = next(c for c in cmds if "cmake -S" in c)
         assert "-G Ninja" in configure_cmd
 
     def test_no_build_type_by_default(self):
         proj = hm.cmake(path="svc")
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         cmds = _cmds(p)
         configure_cmd = next(c for c in cmds if "cmake -S" in c)
         assert "CMAKE_BUILD_TYPE" not in configure_cmd
 
     def test_defines_cmake_build_type(self):
         proj = hm.cmake(path="svc", defines={"CMAKE_BUILD_TYPE": "Debug"})
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         cmds = _cmds(p)
         configure_cmd = next(c for c in cmds if "cmake -S" in c)
         assert "CMAKE_BUILD_TYPE=Debug" in configure_cmd
 
     def test_defines_produces_d_flags(self):
         proj = hm.cmake(path="svc", defines={"BUILD_TESTING": "ON"})
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         cmds = _cmds(p)
         configure_cmd = next(c for c in cmds if "cmake -S" in c)
         assert "-DBUILD_TESTING=ON" in configure_cmd
 
     def test_defines_build_shared_libs(self):
         proj = hm.cmake(path="svc", defines={"BUILD_SHARED_LIBS": "ON"})
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         cmds = _cmds(p)
         configure_cmd = next(c for c in cmds if "cmake -S" in c)
         assert "-DBUILD_SHARED_LIBS=ON" in configure_cmd
 
     def test_defines_cmake_cxx_standard(self):
         proj = hm.cmake(path="svc", defines={"CMAKE_CXX_STANDARD": "20"})
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         cmds = _cmds(p)
         configure_cmd = next(c for c in cmds if "cmake -S" in c)
         assert "-DCMAKE_CXX_STANDARD=20" in configure_cmd
 
     def test_preset_produces_preset_flag_and_no_build_type(self):
         proj = hm.cmake(path="svc", preset="ci-linux")
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         cmds = _cmds(p)
         configure_cmd = next(c for c in cmds if "--preset" in c)
         assert "--preset ci-linux" in configure_cmd
@@ -140,7 +140,7 @@ def test_preset_produces_preset_flag_and_no_build_type(self):
 
     def test_ccache_true_adds_compiler_launcher_flags(self):
         proj = hm.cmake(path="svc", ccache=True)
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         cmds = _cmds(p)
         configure_cmd = next(c for c in cmds if "cmake -S" in c)
         assert "-DCMAKE_C_COMPILER_LAUNCHER=ccache" in configure_cmd
@@ -148,7 +148,7 @@ def test_ccache_true_adds_compiler_launcher_flags(self):
 
     def test_test_produces_ctest_with_output_on_failure_and_parallel(self):
         proj = hm.cmake(path="svc")
-        p = hm.pipeline([proj.test()], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.test()])
         cmds = _cmds(p)
         test_cmd = next(c for c in cmds if "ctest" in c)
         assert "--output-on-failure" in test_cmd
@@ -156,28 +156,28 @@ def test_test_produces_ctest_with_output_on_failure_and_parallel(self):
 
     def test_test_includes_incremental_build(self):
         proj = hm.cmake(path="svc")
-        p = hm.pipeline([proj.test()], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.test()])
         cmds = _cmds(p)
         test_cmd = next(c for c in cmds if "ctest" in c)
         assert "cmake --build" in test_cmd
 
     def test_test_uses_absolute_path_for_standalone_step(self):
         proj = hm.cmake(path="infra/agent")
-        p = hm.pipeline([proj.test()], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.test()])
         cmds = _cmds(p)
         test_cmd = next(c for c in cmds if "ctest" in c)
         assert "cmake --build infra/agent/build" in test_cmd
 
     def test_install_with_prefix(self):
         proj = hm.cmake(path="svc")
-        p = hm.pipeline([proj.install(prefix="/usr/local")], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.install(prefix="/usr/local")])
         cmds = _cmds(p)
         install_cmd = next(c for c in cmds if "cmake --install" in c)
         assert "--prefix /usr/local" in install_cmd
 
     def test_fmt_runs_clang_format_dry_run(self):
         proj = hm.cmake(path="svc")
-        p = hm.pipeline([proj.fmt()], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.fmt()])
         cmds = _cmds(p)
         fmt_cmd = next(c for c in cmds if "xargs clang-format" in c)
         assert "--dry-run --Werror" in fmt_cmd
@@ -190,7 +190,7 @@ def test_fmt_parent_is_toolchain_installed(self):
 
     def test_lint_runs_run_clang_tidy(self):
         proj = hm.cmake(path="svc")
-        p = hm.pipeline([proj.lint()], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.lint()])
         cmds = _cmds(p)
         assert any("run-clang-tidy" in c for c in cmds)
 
@@ -201,7 +201,7 @@ def test_lint_parent_is_built(self):
 
     def test_package_runs_cpack(self):
         proj = hm.cmake(path="svc")
-        p = hm.pipeline([proj.package()], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.package()])
         cmds = _cmds(p)
         assert any("cpack" in c for c in cmds)
 
@@ -214,7 +214,7 @@ def test_package_runs_cpack(self):
 class TestCMakeVcpkg:
     def test_deps_vcpkg_produces_bootstrap_command(self):
         proj = hm.cmake(path="svc", deps="vcpkg")
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         cmds = _cmds(p)
         assert any("bootstrap-vcpkg" in c for c in cmds)
 
@@ -224,7 +224,7 @@ def test_invalid_deps_raises_valueerror(self):
 
     def test_vcpkg_step_has_on_change_cache_policy(self):
         proj = hm.cmake(path="svc", deps="vcpkg")
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         nodes = p["graph"]["nodes"]
         vcpkg_node = next(n for n in nodes if "bootstrap-vcpkg" in n["step"]["cmd"])
         assert vcpkg_node["step"]["cache"]["policy"] == "on_change"
@@ -237,17 +237,17 @@ def test_vcpkg_step_has_on_change_cache_policy(self):
 
 class TestCMakeBareForm:
     def test_bare_build_produces_cmake_build(self):
-        p = hm.pipeline([hm.cmake.build()], default_image="ubuntu:24.04")
+        p = hm.pipeline([hm.cmake.build()])
         cmds = _cmds(p)
         assert any("cmake --build" in c for c in cmds)
 
     def test_bare_test_produces_ctest(self):
-        p = hm.pipeline([hm.cmake.test()], default_image="ubuntu:24.04")
+        p = hm.pipeline([hm.cmake.test()])
         cmds = _cmds(p)
         assert any("ctest" in c for c in cmds)
 
     def test_bare_fmt_produces_clang_format(self):
-        p = hm.pipeline([hm.cmake.fmt()], default_image="ubuntu:24.04")
+        p = hm.pipeline([hm.cmake.fmt()])
         cmds = _cmds(p)
         assert any("clang-format" in c for c in cmds)
 
@@ -284,6 +284,6 @@ class TestCMakeWithBase:
     def test_providing_base_skips_apt_install(self):
         base = hm.scratch().sh("custom base", label="base")
         proj = hm.cmake(path="svc", base=base)
-        p = hm.pipeline([proj.built], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.built])
         cmds = _cmds(p)
         assert not any("apt-get install" in c for c in cmds)
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_decorator.py b/crates/hm-dsl-engine/harmont-py/tests/test_decorator.py
index 7d5aa6c0..abb6f35b 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_decorator.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_decorator.py
@@ -25,7 +25,6 @@ def whatever() -> hm.Step:
     assert reg.triggers == ()
     assert reg.allow_manual is True
     assert reg.env is None
-    assert reg.default_image is None
 
 
 def test_default_slug_from_function_name():
@@ -44,14 +43,13 @@ def ci() -> hm.Step:
     assert REGISTRATIONS[0].name == "Continuous Integration"
 
 
-def test_forwards_env_and_default_image():
-    @hm.pipeline("ci", env={"FOO": "bar"}, default_image="alpine:3.20")
+def test_forwards_env():
+    @hm.pipeline("ci", env={"FOO": "bar"})
     def ci() -> hm.Step:
         return hm.scratch().sh("echo")
 
     reg = REGISTRATIONS[0]
     assert reg.env == {"FOO": "bar"}
-    assert reg.default_image == "alpine:3.20"
 
 
 def test_allow_manual_false():
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_e2e_fixtures.py b/crates/hm-dsl-engine/harmont-py/tests/test_e2e_fixtures.py
index 417046ec..2d106491 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_e2e_fixtures.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_e2e_fixtures.py
@@ -65,7 +65,6 @@ def _build_monorepo_ci() -> dict:
             web_project.run("lint"),
         ],
         env={"CI": "true"},
-        default_image="ubuntu:24.04",
     )
 
 
@@ -75,7 +74,6 @@ def _build_rust_release() -> dict:
     return hm.pipeline(
         [project.build(), project.test(), project.clippy(), project.fmt(), project.doc()],
         env={"CI": "true"},
-        default_image="ubuntu:24.04",
     )
 
 
@@ -103,7 +101,6 @@ def _build_zig_node_polyglot() -> dict:
             web.run("lint"),
         ],
         env={"CI": "true"},
-        default_image="ubuntu:24.04",
     )
 
 
@@ -120,7 +117,6 @@ def _build_kitchen_sink() -> dict:
             py_web.lint(),
         ],
         env={"CI": "true"},
-        default_image="ubuntu:24.04",
     )
 
 
@@ -136,7 +132,6 @@ def _build_cmake_advanced() -> dict:
     return hm.pipeline(
         [project.test(), project.lint(), project.fmt()],
         env={"CI": "true"},
-        default_image="ubuntu:24.04",
     )
 
 
@@ -154,8 +149,13 @@ def test_e2e_fixture(name: str) -> None:
     ir = SCENARIOS[name]()
 
     assert ir["version"] == "0"
-    assert ir["default_image"] == "ubuntu:24.04"
     assert len(ir["graph"]["nodes"]) > 0
+    # Root steps (no builds_in parent) should carry the ubuntu:24.04 default image.
+    nodes = ir["graph"]["nodes"]
+    edges = ir["graph"]["edges"]
+    child_idxs = {e[1] for e in edges if e[2] == "builds_in"}
+    roots = [n for i, n in enumerate(nodes) if i not in child_idxs]
+    assert all(n["step"].get("image") == "ubuntu:24.04" for n in roots)
     assert ir["graph"]["edge_property"] == "directed"
 
     for node in ir["graph"]["nodes"]:
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_elixir.py b/crates/hm-dsl-engine/harmont-py/tests/test_elixir.py
index 5589999e..ea042abd 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_elixir.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_elixir.py
@@ -21,7 +21,7 @@ def _step_by_substring(p: dict, needle: str) -> dict:
 
 def test_elixir_object_form_full_chain():
     ex = hm.elixir(path="apps/api")
-    p = hm.pipeline([ex.compile()], default_image="ubuntu:24.04")
+    p = hm.pipeline([ex.compile()])
     cmds = _cmds(p)
     assert any("apt-get install" in c for c in cmds)
     assert any("erlang" in c.lower() for c in cmds)
@@ -33,7 +33,6 @@ def test_elixir_actions_share_install_step():
     ex = hm.elixir(path=".")
     p = hm.pipeline(
         [ex.compile(), ex.test(), ex.format(), ex.credo()],
-        default_image="ubuntu:24.04",
     )
     cmds = _cmds(p)
     assert len([c for c in cmds if "mix deps.get" in c]) == 1
@@ -115,7 +114,7 @@ def test_elixir_dialyzer_chains_through_plt():
 def test_elixir_with_base_skips_apt():
     base = hm.scratch().sh("custom base", label="base")
     ex = hm.elixir(path=".", base=base)
-    p = hm.pipeline([ex.compile()], default_image="ubuntu:24.04")
+    p = hm.pipeline([ex.compile()])
     cmds = _cmds(p)
     assert not any("apt-get update && apt-get install -y" in c for c in cmds)
     assert any("custom base" in c for c in cmds)
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_envelope.py b/crates/hm-dsl-engine/harmont-py/tests/test_envelope.py
index 430f93be..48fe783b 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_envelope.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_envelope.py
@@ -111,14 +111,13 @@ def ci() -> hm.Pipeline:
     assert cmds == ["a", "b"]
 
 
-def test_pipeline_forwards_env_and_default_image_to_assemble():
-    @hm.pipeline("ci", env={"CI": "true"}, default_image="alpine:3.20")
+def test_pipeline_forwards_env_to_assemble():
+    @hm.pipeline("ci", env={"CI": "true"})
     def ci() -> hm.Step:
         return hm.scratch().sh("echo")
 
     out = json.loads(hm.dump_registry_json())
     definition = out["pipelines"][0]["definition"]
-    assert definition["default_image"] == "alpine:3.20"
     # Pipeline-level env is merged into node env dicts.
     for node in _graph_nodes(definition):
         assert node["env"].get("CI") == "true"
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_examples_render.py b/crates/hm-dsl-engine/harmont-py/tests/test_examples_render.py
index 3b52a8a8..5442a9ae 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_examples_render.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_examples_render.py
@@ -64,7 +64,12 @@ def test_example_renders_to_v0_ir(
     assert definition.get("graph", {}).get("nodes"), (
         f"{example_dir.name}: ci pipeline has no nodes"
     )
-    assert definition.get("default_image"), (
-        f"{example_dir.name}: ci pipeline missing default_image — local "
-        "executor falls back to alpine and apt-get-based examples die"
+    nodes = definition.get("graph", {}).get("nodes", [])
+    edges = definition.get("graph", {}).get("edges", [])
+    child_idxs = {e[1] for e in edges if e[2] == "builds_in"}
+    roots = [n for i, n in enumerate(nodes) if i not in child_idxs]
+    assert roots, f"{example_dir.name}: ci pipeline has no root steps"
+    assert all("image" in n["step"] for n in roots), (
+        f"{example_dir.name}: a root step is missing an image — the lowering "
+        f"should stamp the ubuntu:24.04 default on every imageless root"
     )
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_go.py b/crates/hm-dsl-engine/harmont-py/tests/test_go.py
index 4b0b6fe4..5b9cdf14 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_go.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_go.py
@@ -21,7 +21,7 @@ def _step_by_substring(p: dict, needle: str) -> dict:
 
 def test_go_object_form_full_chain():
     go = hm.go(path="svc")
-    p = hm.pipeline([go.build()], default_image="ubuntu:24.04")
+    p = hm.pipeline([go.build()])
     cmds = _cmds(p)
     assert any("apt-get install" in c for c in cmds)
     assert any("go.dev/dl/" in c for c in cmds)
@@ -30,7 +30,7 @@ def test_go_object_form_full_chain():
 
 def test_go_actions_share_install_step():
     go = hm.go(path="svc")
-    p = hm.pipeline([go.build(), go.test(), go.vet(), go.fmt()], default_image="ubuntu:24.04")
+    p = hm.pipeline([go.build(), go.test(), go.vet(), go.fmt()])
     cmds = _cmds(p)
     assert len([c for c in cmds if "go.dev/dl/" in c]) == 1
     assert any("go build ./..." in c for c in cmds)
@@ -78,7 +78,7 @@ def test_go_action_labels_auto_generated():
 def test_go_with_base_skips_apt():
     base = hm.scratch().sh("custom base", label="base")
     go = hm.go(path="svc", base=base)
-    p = hm.pipeline([go.build()], default_image="ubuntu:24.04")
+    p = hm.pipeline([go.build()])
     cmds = _cmds(p)
     assert not any("apt-get install" in c for c in cmds)
     assert any("custom base" in c for c in cmds)
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_js.py b/crates/hm-dsl-engine/harmont-py/tests/test_js.py
index d41aee79..e1126d9a 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_js.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_js.py
@@ -273,7 +273,7 @@ def test_default_label(runtime: str, expected: str) -> None:
 )
 def test_pipeline_ir(opts: dict) -> None:
     p = js.project(**opts)
-    ir = hm.pipeline([p.run("test"), p.run("lint")], default_image="ubuntu:24.04")
+    ir = hm.pipeline([p.run("test"), p.run("lint")])
     assert ir["version"] == "0"
     assert len(ir["graph"]["nodes"]) >= 4
 
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_json_emit.py b/crates/hm-dsl-engine/harmont-py/tests/test_json_emit.py
index 59b27705..08167f99 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_json_emit.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_json_emit.py
@@ -110,9 +110,10 @@ def test_pipeline_env_merged_into_node_env():
     assert _nodes(out)[0]["env"]["DEBIAN_FRONTEND"] == "noninteractive"
 
 
-def test_default_image_emitted_when_set():
-    out = _emit(pipeline([scratch().sh("a", label="a")], default_image="alpine:3"))
-    assert out["default_image"] == "alpine:3"
+def test_imageless_root_emitted_with_ubuntu_default():
+    out = _emit(pipeline([scratch().sh("a", label="a")]))
+    assert _nodes(out)[0]["step"]["image"] == "ubuntu:24.04"
+    assert "default_image" not in out
 
 
 def test_cache_ttl_resolves_key():
@@ -158,7 +159,8 @@ def test_cache_on_change_paths_round_trip(tmp_path):
 def test_no_optional_fields_when_not_set():
     out = _emit(pipeline([scratch().sh("x", label="x")]))
     s = _nodes(out)[0]["step"]
-    assert "image" not in s
+    # Root imageless steps now receive ubuntu:24.04 automatically.
+    assert s.get("image") == "ubuntu:24.04"
     assert "timeout_seconds" not in s
     assert "cache" not in s
 
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_pipeline.py b/crates/hm-dsl-engine/harmont-py/tests/test_pipeline.py
index 9cbeb7c9..d4259b8d 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_pipeline.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_pipeline.py
@@ -42,12 +42,23 @@ def test_pipeline_rejects_legacy_variadic_step_form():
     assert "hm.pipeline([a, b])" in str(exc.value)
 
 
-def test_pipeline_default_image_lowers_to_dict():
-    p = pipeline(
-        [scratch().sh("echo", label="a", image="ubuntu:24.04")],
-        default_image="alpine:3.20",
-    )
-    assert p["default_image"] == "alpine:3.20"
-    step = p["graph"]["nodes"][0]["step"]
-    assert step["image"] == "ubuntu:24.04"
-    assert step["label"] == "a"
+def test_imageless_root_gets_ubuntu_default():
+    p = pipeline([scratch().sh("echo hi", label="a")])
+    nodes = p["graph"]["nodes"]
+    assert nodes[0]["step"]["image"] == "ubuntu:24.04"
+    # No top-level default_image key is emitted anymore.
+    assert "default_image" not in p
+
+
+def test_explicit_root_image_is_preserved():
+    p = pipeline([scratch().sh("echo hi", label="a", image="alpine:3.20")])
+    assert p["graph"]["nodes"][0]["step"]["image"] == "alpine:3.20"
+
+
+def test_child_step_stays_imageless():
+    root = scratch().sh("echo p", label="p")
+    child = root.sh("echo c", label="c")
+    p = pipeline([child])
+    nodes = {n["step"]["key"]: n["step"] for n in p["graph"]["nodes"]}
+    # parent (root) gets the default; child boots from parent snapshot.
+    assert "image" not in nodes["c"]
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_py_uv.py b/crates/hm-dsl-engine/harmont-py/tests/test_py_uv.py
index c6f4d300..919c99fd 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_py_uv.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_py_uv.py
@@ -26,7 +26,7 @@ def _step_by_substring(p: dict, needle: str) -> dict:
 class TestUvObjectForm:
     def test_full_chain(self):
         proj = hm.py.uv(path="svc")
-        p = hm.pipeline([proj.test()], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.test()])
         cmds = _cmds(p)
         assert any("apt-get install" in c for c in cmds)
         assert any("astral.sh/uv/install.sh" in c for c in cmds)
@@ -37,7 +37,6 @@ def test_shared_install(self):
         proj = hm.py.uv(path="svc")
         p = hm.pipeline(
             [proj.test(), proj.lint(), proj.fmt(), proj.typecheck()],
-            default_image="ubuntu:24.04",
         )
         cmds = _cmds(p)
         assert len([c for c in cmds if "astral.sh/uv/install.sh" in c]) == 1
@@ -142,7 +141,7 @@ def test_image_emitted_on_apt_step(self):
     def test_base_skips_apt(self):
         base = hm.scratch().sh("custom base", label="base")
         proj = hm.py.uv(path="svc", base=base)
-        p = hm.pipeline([proj.test()], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.test()])
         cmds = _cmds(p)
         assert not any("apt-get install" in c for c in cmds)
         assert any("custom base" in c for c in cmds)
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_python.py b/crates/hm-dsl-engine/harmont-py/tests/test_python.py
index 55cf98f0..cace93a0 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_python.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_python.py
@@ -22,7 +22,7 @@ def _step_by_substring(p: dict, needle: str) -> dict:
 
 def test_python_object_form_full_chain():
     py = hm.python(path="svc")
-    p = hm.pipeline([py.test()], default_image="ubuntu:24.04")
+    p = hm.pipeline([py.test()])
     cmds = _cmds(p)
     assert any("apt-get install" in c for c in cmds)
     assert any("astral.sh/uv/install.sh" in c for c in cmds)
@@ -32,7 +32,7 @@ def test_python_object_form_full_chain():
 
 def test_python_actions_share_install_step():
     py = hm.python(path="svc")
-    p = hm.pipeline([py.test(), py.lint(), py.fmt(), py.typecheck()], default_image="ubuntu:24.04")
+    p = hm.pipeline([py.test(), py.lint(), py.fmt(), py.typecheck()])
     cmds = _cmds(p)
     assert len([c for c in cmds if "astral.sh/uv/install.sh" in c]) == 1
     assert len([c for c in cmds if "apt-get install" in c]) == 1
@@ -120,7 +120,7 @@ def test_python_image_emitted_on_apt_step():
 def test_python_with_base_skips_apt():
     base = hm.scratch().sh("custom base", label="base")
     py = hm.python(path="svc", base=base)
-    p = hm.pipeline([py.test()], default_image="ubuntu:24.04")
+    p = hm.pipeline([py.test()])
     cmds = _cmds(p)
     assert not any("apt-get install" in c for c in cmds)
     assert any("custom base" in c for c in cmds)
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_registry.py b/crates/hm-dsl-engine/harmont-py/tests/test_registry.py
index 54f45a44..fe084e9b 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_registry.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_registry.py
@@ -28,7 +28,6 @@ def test_register_appends():
         triggers=(),
         allow_manual=True,
         env=None,
-        default_image=None,
         fn=lambda: None,
     )
     register(reg)
@@ -44,7 +43,6 @@ def test_register_duplicate_slug_raises():
             triggers=(),
             allow_manual=True,
             env=None,
-            default_image=None,
             fn=fn,
         )
     )
@@ -56,7 +54,6 @@ def test_register_duplicate_slug_raises():
                 triggers=(),
                 allow_manual=True,
                 env=None,
-                default_image=None,
                 fn=fn,
             )
         )
@@ -73,7 +70,6 @@ def test_clear_resets():
             triggers=(),
             allow_manual=True,
             env=None,
-            default_image=None,
             fn=fn,
         )
     )
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_rust.py b/crates/hm-dsl-engine/harmont-py/tests/test_rust.py
index fe65c060..6f44b556 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_rust.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_rust.py
@@ -26,7 +26,7 @@ def _step_by_substring(p: dict, needle: str) -> dict:
 class TestRustToolchain:
     def test_full_chain(self):
         tc = hm.rust.toolchain(path="cli")
-        p = hm.pipeline([tc.build()], default_image="ubuntu:24.04")
+        p = hm.pipeline([tc.build()])
         cmds = _cmds(p)
         assert any("apt-get install" in c for c in cmds)
         assert any("sh.rustup.rs" in c for c in cmds)
@@ -36,7 +36,6 @@ def test_actions_share_install_step(self):
         tc = hm.rust.toolchain(path="cli")
         p = hm.pipeline(
             [tc.build(), tc.test(), tc.clippy(), tc.fmt(), tc.doc()],
-            default_image="ubuntu:24.04",
         )
         cmds = _cmds(p)
         assert len([c for c in cmds if "sh.rustup.rs" in c]) == 1
@@ -118,7 +117,7 @@ def test_image_emitted_on_apt_step(self):
     def test_with_base_skips_apt(self):
         base = hm.scratch().sh("custom base", label="base")
         tc = hm.rust.toolchain(path="cli", base=base)
-        p = hm.pipeline([tc.build()], default_image="ubuntu:24.04")
+        p = hm.pipeline([tc.build()])
         cmds = _cmds(p)
         assert not any("apt-get install" in c for c in cmds)
         assert any("custom base" in c for c in cmds)
@@ -151,7 +150,7 @@ def test_warmup_in_pipeline(self):
             ". $HOME/.cargo/env && cd cli && cargo test --workspace --locked",
             label=":rust: test",
         )
-        p = hm.pipeline([t, tc.fmt()], default_image="ubuntu:24.04")
+        p = hm.pipeline([t, tc.fmt()])
         cmds = _cmds(p)
         assert any("cargo build --workspace --tests --locked" in c for c in cmds)
         assert any("cargo test --workspace --locked" in c for c in cmds)
@@ -232,7 +231,7 @@ def test_toolchain_escape_hatch(self):
     def test_with_base_skips_apt(self):
         base = hm.scratch().sh("custom base", label="base")
         proj = hm.rust.project(path="cli", base=base)
-        p = hm.pipeline([proj.test(), proj.clippy(), proj.fmt()], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.test(), proj.clippy(), proj.fmt()])
         cmds = _cmds(p)
         assert not any("apt-get install" in c for c in cmds)
         assert any("custom base" in c for c in cmds)
@@ -246,7 +245,7 @@ def test_labels(self):
 
     def test_pipeline_ir(self):
         proj = hm.rust.project(path="cli")
-        p = hm.pipeline([proj.test(), proj.clippy(), proj.fmt()], default_image="ubuntu:24.04")
+        p = hm.pipeline([proj.test(), proj.clippy(), proj.fmt()])
         cmds = _cmds(p)
         assert any("cargo build --workspace --tests --locked" in c for c in cmds)
         assert any("cargo test --workspace --locked" in c for c in cmds)
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_toolchain_compose.py b/crates/hm-dsl-engine/harmont-py/tests/test_toolchain_compose.py
index d64a7e36..7e5d6167 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_toolchain_compose.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_toolchain_compose.py
@@ -37,7 +37,7 @@ def test_deterministic_emission():
 
     def build() -> dict:
         rust = hm.rust.toolchain(path="cli")
-        return hm.pipeline([rust.build(), rust.test()], default_image="ubuntu:24.04")
+        return hm.pipeline([rust.build(), rust.test()])
 
     assert build() == build()
 
@@ -49,7 +49,6 @@ def test_mixed_pipeline_compiles():
     go = hm.go(path="services/api")
     p = hm.pipeline(
         [rust.test(), rust.clippy(), node.install(), go.build(), go.test()],
-        default_image="ubuntu:24.04",
     )
     assert p["version"] == "0"
     assert len(p["graph"]["nodes"]) > 0
@@ -80,7 +79,6 @@ def test_apt_base_shared_across_toolchains():
     py = hm.py.uv(path="dsls/harmont-py", base=base)
     p = hm.pipeline(
         [rust.build(), py.test()],
-        default_image="ubuntu:24.04",
     )
     cmds = _cmds(p)
     assert len([c for c in cmds if "apt-get install" in c]) == 1
@@ -96,7 +94,7 @@ def test_apt_base_default_label():
 def test_apt_base_custom_image():
     base = hm.apt_base(packages=("curl",), image="debian:bookworm")
     rust = hm.rust.toolchain(path=".", base=base)
-    p = hm.pipeline([rust.build()], default_image="ubuntu:24.04")
+    p = hm.pipeline([rust.build()])
     apt_step = _step_by_substring(p, "apt-get install")
     assert apt_step.get("image") == "debian:bookworm"
 
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_zig.py b/crates/hm-dsl-engine/harmont-py/tests/test_zig.py
index 4bb55296..015c347f 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_zig.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_zig.py
@@ -20,7 +20,7 @@ def _step_by_substring(p: dict, needle: str) -> dict:
 
 def test_zig_object_form_full_chain():
     z = hm.zig(path="svc")
-    p = hm.pipeline([z.build()], default_image="ubuntu:24.04")
+    p = hm.pipeline([z.build()])
     cmds = _cmds(p)
     assert any("ziglang.org" in c for c in cmds)
     assert any("cd svc && zig build" in c for c in cmds)
@@ -28,7 +28,7 @@ def test_zig_object_form_full_chain():
 
 def test_zig_actions_share_install():
     z = hm.zig(path="svc")
-    p = hm.pipeline([z.build(), z.test(), z.fmt()], default_image="ubuntu:24.04")
+    p = hm.pipeline([z.build(), z.test(), z.fmt()])
     cmds = _cmds(p)
     assert len([c for c in cmds if "ziglang.org" in c]) == 1
     assert any("zig build test" in c for c in cmds)
@@ -80,5 +80,5 @@ def test_zig_new_version_uses_new_url_format():
 def test_zig_with_base_skips_apt():
     base = hm.scratch().sh("custom base", label="base")
     z = hm.zig(path="svc", base=base)
-    p = hm.pipeline([z.build()], default_image="ubuntu:24.04")
+    p = hm.pipeline([z.build()])
     assert not any("apt-get install" in c for c in _cmds(p))
diff --git a/crates/hm-dsl-engine/harmont-py/tests/test_zig_toolchain.py b/crates/hm-dsl-engine/harmont-py/tests/test_zig_toolchain.py
index 7f4e744b..7ea94f36 100644
--- a/crates/hm-dsl-engine/harmont-py/tests/test_zig_toolchain.py
+++ b/crates/hm-dsl-engine/harmont-py/tests/test_zig_toolchain.py
@@ -58,7 +58,7 @@ def lib_a(zig: hm.Target[ZigToolchain]) -> ZigProject:
     def lib_b(zig: hm.Target[ZigToolchain]) -> ZigProject:
         return zig.project(path="lib-b")
 
-    @hm.pipeline("ci", default_image="ubuntu:24.04")
+    @hm.pipeline("ci")
     def ci(
         lib_a: hm.Target[ZigProject],
         lib_b: hm.Target[ZigProject],
diff --git a/crates/hm-dsl-engine/harmont-ts/src/pipeline.ts b/crates/hm-dsl-engine/harmont-ts/src/pipeline.ts
index a531fb92..bf745598 100644
--- a/crates/hm-dsl-engine/harmont-ts/src/pipeline.ts
+++ b/crates/hm-dsl-engine/harmont-ts/src/pipeline.ts
@@ -3,15 +3,18 @@ import { parseDuration } from "./duration.js";
 import { resolveKeys } from "./keys.js";
 import type { Step } from "./step.js";
 
+// Across-the-board default image for imageless root steps. The SDK's
+// toolchains assume an apt-capable base (apt-get), so ubuntu:24.04 is the
+// universal default; child steps boot from their parent's snapshot.
+const DEFAULT_IMAGE = "ubuntu:24.04";
+
 export interface PipelineOptions {
   readonly env?: Readonly<Record<string, string>>;
-  readonly defaultImage?: string;
   readonly timeout?: string | number;
 }
 
 export interface PipelineIR {
   version: string;
-  default_image?: string;
   timeout_seconds?: number;
   graph: {
     nodes: GraphNode[];
@@ -41,9 +44,6 @@ export function pipeline(
   }
 
   const ir: PipelineIR = { version: "0", graph: lowerToGraph(leaves, opts) };
-  if (opts?.defaultImage != null) {
-    ir.default_image = opts.defaultImage;
-  }
   if (opts?.timeout != null) {
     ir.timeout_seconds = parseDuration(opts.timeout);
   }
@@ -116,11 +116,9 @@ function lowerToGraph(
     preWaitIndices.push(nodeIdx);
   }
 
-  if (opts?.defaultImage != null) {
-    for (let i = 0; i < nodes.length; i++) {
-      if (!hasBuildsInParent.has(i) && !("image" in nodes[i].step)) {
-        nodes[i].step.image = opts.defaultImage;
-      }
+  for (let i = 0; i < nodes.length; i++) {
+    if (!hasBuildsInParent.has(i) && !("image" in nodes[i].step)) {
+      nodes[i].step.image = DEFAULT_IMAGE;
     }
   }
 
diff --git a/crates/hm-dsl-engine/harmont-ts/tests/e2e-fixtures.test.ts b/crates/hm-dsl-engine/harmont-ts/tests/e2e-fixtures.test.ts
index 546366ea..c5d872c6 100644
--- a/crates/hm-dsl-engine/harmont-ts/tests/e2e-fixtures.test.ts
+++ b/crates/hm-dsl-engine/harmont-ts/tests/e2e-fixtures.test.ts
@@ -70,11 +70,16 @@ describe("E2E pipeline fixtures", () => {
         webProject.run("test"),
         webProject.run("lint"),
       ],
-      { env: { CI: "true" }, defaultImage: "ubuntu:24.04" },
+      { env: { CI: "true" } },
     );
 
     expect(ir.version).toBe("0");
-    expect(ir.default_image).toBe("ubuntu:24.04");
+    const monorepoChildIdxs = new Set(
+      ir.graph.edges.filter((e: any) => e[2] === "builds_in").map((e: any) => e[1]),
+    );
+    const monorepoRoots = ir.graph.nodes.filter((_: any, i: number) => !monorepoChildIdxs.has(i));
+    expect(monorepoRoots.length).toBeGreaterThan(0);
+    expect(monorepoRoots.every((n: any) => "image" in n.step)).toBe(true);
     expect(ir.graph.nodes.length).toBeGreaterThan(0);
     assertFixture("monorepo-ci", ir);
   });
@@ -84,7 +89,7 @@ describe("E2E pipeline fixtures", () => {
 
     const ir = pipeline(
       [project.build(), project.test(), project.clippy(), project.fmt(), project.doc()],
-      { env: { CI: "true" }, defaultImage: "ubuntu:24.04" },
+      { env: { CI: "true" } },
     );
 
     expect(ir.version).toBe("0");
@@ -112,7 +117,7 @@ describe("E2E pipeline fixtures", () => {
         web.run("test"),
         web.run("lint"),
       ],
-      { env: { CI: "true" }, defaultImage: "ubuntu:24.04" },
+      { env: { CI: "true" } },
     );
 
     expect(ir.version).toBe("0");
@@ -125,7 +130,7 @@ describe("E2E pipeline fixtures", () => {
 
     const ir = pipeline(
       [cProject.build(), cProject.test(), cProject.fmt(), pyWeb.test(), pyWeb.lint()],
-      { env: { CI: "true" }, defaultImage: "ubuntu:24.04" },
+      { env: { CI: "true" } },
     );
 
     expect(ir.version).toBe("0");
@@ -141,7 +146,7 @@ describe("E2E pipeline fixtures", () => {
 
     const ir = pipeline(
       [project.test(), project.lint(), project.fmt()],
-      { env: { CI: "true" }, defaultImage: "ubuntu:24.04" },
+      { env: { CI: "true" } },
     );
 
     expect(ir.version).toBe("0");
diff --git a/crates/hm-dsl-engine/harmont-ts/tests/examples.test.ts b/crates/hm-dsl-engine/harmont-ts/tests/examples.test.ts
index 71288277..dec87182 100644
--- a/crates/hm-dsl-engine/harmont-ts/tests/examples.test.ts
+++ b/crates/hm-dsl-engine/harmont-ts/tests/examples.test.ts
@@ -44,7 +44,15 @@ describe.skipIf(examples.length === 0)("examples render to v0 IR", () => {
       expect(ci.pipeline.version).toBe("0");
       expect(ci.pipeline.graph.nodes.length).toBeGreaterThan(0);
       expect(ci.pipeline.graph.edge_property).toBe("directed");
-      expect(ci.pipeline.default_image).toBeTruthy();
+
+      // Every root step (no builds_in parent) must have an image stamped on it
+      const nodes = ci.pipeline.graph.nodes;
+      const childIdxs = new Set(
+        ci.pipeline.graph.edges.filter((e: any) => e[2] === "builds_in").map((e: any) => e[1]),
+      );
+      const roots = nodes.filter((_: any, i: number) => !childIdxs.has(i));
+      expect(roots.length).toBeGreaterThan(0);
+      expect(roots.every((n: any) => "image" in n.step)).toBe(true);
 
       // Verify all nodes have required fields
       for (const node of ci.pipeline.graph.nodes) {
diff --git a/crates/hm-dsl-engine/harmont-ts/tests/integration.test.ts b/crates/hm-dsl-engine/harmont-ts/tests/integration.test.ts
index 8cec87b6..6c022bbd 100644
--- a/crates/hm-dsl-engine/harmont-ts/tests/integration.test.ts
+++ b/crates/hm-dsl-engine/harmont-ts/tests/integration.test.ts
@@ -25,7 +25,7 @@ beforeEach(() => {
 });
 
 describe("full pipeline build", () => {
-  it("creates install -> build -> test chain with cache, env, defaultImage", () => {
+  it("creates install -> build -> test chain with cache, env", () => {
     const install = scratch()
       .sh("npm ci", { label: "install", cache: forever() });
     const build = install
@@ -35,7 +35,6 @@ describe("full pipeline build", () => {
 
     const ir = pipeline([test], {
       env: { CI: "true" },
-      defaultImage: "node:22-alpine",
     });
 
     // version
@@ -58,9 +57,13 @@ describe("full pipeline build", () => {
     expect(buildNode.env).toEqual({ ...base, CI: "true", NODE_ENV: "production" });
     expect(testNode.env).toEqual({ ...base, CI: "true" });
 
-    // default_image applies to root node only (install), not children
-    expect(ir.default_image).toBe("node:22-alpine");
-    expect(installNode.step.image).toBe("node:22-alpine");
+    // ubuntu:24.04 is automatically stamped on root steps (no builds_in parent)
+    const childIdxs = new Set(
+      ir.graph.edges.filter((e) => e[2] === "builds_in").map((e) => e[1]),
+    );
+    const roots = ir.graph.nodes.filter((_n, i) => !childIdxs.has(i));
+    expect(roots).toHaveLength(1);
+    expect(roots[0].step.image).toBe("ubuntu:24.04");
     expect("image" in buildNode.step).toBe(false);
     expect("image" in testNode.step).toBe(false);
 
@@ -171,11 +174,15 @@ describe("JSON snake_case output", () => {
       label: "build",
       cache: onChange("src/", "lib/"),
     }));
-    const ir = pipeline([s], { defaultImage: "ubuntu:24.04" });
+    const ir = pipeline([s]);
     const json = JSON.stringify(ir);
 
+    // Root imageless step gets ubuntu:24.04 stamped on it
+    expect(json).toContain('"image":"ubuntu:24.04"');
+    // Must NOT contain the removed top-level default_image field
+    expect(json).not.toContain('"default_image"');
+
     // Must contain snake_case keys
-    expect(json).toContain('"default_image"');
     expect(json).toContain('"timeout_seconds"');
     expect(json).toContain('"edge_property"');
     expect(json).toContain('"node_holes"');
diff --git a/crates/hm-dsl-engine/harmont-ts/tests/pipeline.test.ts b/crates/hm-dsl-engine/harmont-ts/tests/pipeline.test.ts
index 10764699..81a604ff 100644
--- a/crates/hm-dsl-engine/harmont-ts/tests/pipeline.test.ts
+++ b/crates/hm-dsl-engine/harmont-ts/tests/pipeline.test.ts
@@ -7,12 +7,6 @@ function stepKeys(ir: any): string[] {
   return ir.graph.nodes.map((n: any) => n.step.key);
 }
 
-function buildsInEdges(ir: any): [number, number][] {
-  return ir.graph.edges
-    .filter((e: any) => e[2] === "builds_in")
-    .map((e: any) => [e[0], e[1]]);
-}
-
 function dependsOnEdges(ir: any): [number, number][] {
   return ir.graph.edges
     .filter((e: any) => e[2] === "depends_on")
@@ -48,13 +42,6 @@ describe("pipeline", () => {
     expect(() => pipeline([])).toThrow("at least one leaf");
   });
 
-  it("sets default_image on IR when provided", () => {
-    const p = pipeline([sh("echo", { label: "a", image: "ubuntu:24.04" })], {
-      defaultImage: "alpine:3.20",
-    });
-    expect(p.default_image).toBe("alpine:3.20");
-    expect(p.graph.nodes[0].step.image).toBe("ubuntu:24.04");
-  });
 });
 
 describe("lowering: single chain", () => {
@@ -185,25 +172,32 @@ describe("lowering: dedup", () => {
   });
 });
 
-describe("lowering: default_image", () => {
-  it("applies default_image to root nodes without explicit image", () => {
-    const s = scratch().sh("echo");
-    const ir = pipeline([s], { defaultImage: "ubuntu:24.04" });
+describe("lowering: default image", () => {
+  it("stamps ubuntu:24.04 on an imageless root", () => {
+    const s = scratch().sh("echo hi", { label: "a" });
+    const ir = pipeline([s]);
     expect(ir.graph.nodes[0].step.image).toBe("ubuntu:24.04");
   });
 
-  it("does not override explicit image", () => {
-    const s = scratch().sh("echo", { image: "alpine:3.20" });
-    const ir = pipeline([s], { defaultImage: "ubuntu:24.04" });
+  it("preserves an explicit root image", () => {
+    const s = scratch({ image: "alpine:3.20" }).sh("echo hi", { label: "a" });
+    const ir = pipeline([s]);
     expect(ir.graph.nodes[0].step.image).toBe("alpine:3.20");
   });
 
-  it("does not apply to child nodes with builds_in parent", () => {
-    const parent = scratch().sh("a", { label: "a" });
-    const child = parent.sh("b", { label: "b" });
-    const ir = pipeline([child], { defaultImage: "ubuntu:24.04" });
-    expect(ir.graph.nodes[0].step.image).toBe("ubuntu:24.04");
-    expect("image" in ir.graph.nodes[1].step).toBe(false);
+  it("leaves child steps imageless", () => {
+    const root = scratch().sh("echo p", { label: "p" });
+    const child = root.sh("echo c", { label: "c" });
+    const ir = pipeline([child]);
+    const byKey = Object.fromEntries(
+      ir.graph.nodes.map((n) => [n.step.key as string, n.step]),
+    );
+    expect("image" in byKey["c"]).toBe(false);
+  });
+
+  it("emits no top-level default_image key", () => {
+    const ir = pipeline([scratch().sh("echo hi", { label: "a" })]);
+    expect("default_image" in ir).toBe(false);
   });
 });
 
diff --git a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/cmake.test.ts b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/cmake.test.ts
index efdace7c..1143fde1 100644
--- a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/cmake.test.ts
+++ b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/cmake.test.ts
@@ -117,9 +117,7 @@ describe("cmake with vcpkg", () => {
 describe("cmake in pipeline", () => {
   it("produces valid IR", () => {
     const proj = cmake({ path: "." });
-    const ir = pipeline([proj.build(), proj.test()], {
-      defaultImage: "ubuntu:24.04",
-    });
+    const ir = pipeline([proj.build(), proj.test()]);
     expect(ir.graph.nodes.length).toBeGreaterThanOrEqual(3);
   });
 });
diff --git a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/elixir.test.ts b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/elixir.test.ts
index 4e3423be..b5f9c7a0 100644
--- a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/elixir.test.ts
+++ b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/elixir.test.ts
@@ -150,9 +150,7 @@ describe("elixir install chain", () => {
 describe("elixir in pipeline", () => {
   it("produces valid IR", () => {
     const ex = elixir();
-    const ir = pipeline([ex.compile(), ex.test(), ex.format()], {
-      defaultImage: "ubuntu:24.04",
-    });
+    const ir = pipeline([ex.compile(), ex.test(), ex.format()]);
     expect(ir.graph.nodes.length).toBeGreaterThanOrEqual(5);
     expect(ir.version).toBe("0");
   });
diff --git a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/go.test.ts b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/go.test.ts
index 83442b63..7b0e1f09 100644
--- a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/go.test.ts
+++ b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/go.test.ts
@@ -98,7 +98,7 @@ describe("go install chain", () => {
 describe("go in pipeline", () => {
   it("produces valid IR", () => {
     const g = go();
-    const ir = pipeline([g.build(), g.test()], { defaultImage: "ubuntu:24.04" });
+    const ir = pipeline([g.build(), g.test()]);
     expect(ir.graph.nodes.length).toBeGreaterThanOrEqual(3);
     expect(ir.version).toBe("0");
   });
diff --git a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/js.test.ts b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/js.test.ts
index a5f23f52..d80772b5 100644
--- a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/js.test.ts
+++ b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/js.test.ts
@@ -372,45 +372,35 @@ describe("js.project actions", () => {
 describe("js.project pipeline IR", () => {
   it("produces valid IR for node+npm", () => {
     const p = js.project();
-    const ir = pipeline([p.run("test"), p.run("lint")], {
-      defaultImage: "ubuntu:24.04",
-    });
+    const ir = pipeline([p.run("test"), p.run("lint")]);
     expect(ir.version).toBe("0");
     expect(ir.graph.nodes.length).toBeGreaterThanOrEqual(4);
   });
 
   it("produces valid IR for node+pnpm", () => {
     const p = js.project({ pm: "pnpm" });
-    const ir = pipeline([p.run("test"), p.run("lint")], {
-      defaultImage: "ubuntu:24.04",
-    });
+    const ir = pipeline([p.run("test"), p.run("lint")]);
     expect(ir.version).toBe("0");
     expect(ir.graph.nodes.length).toBeGreaterThanOrEqual(5);
   });
 
   it("produces valid IR for node+yarn-berry", () => {
     const p = js.project({ pm: "yarn-berry" });
-    const ir = pipeline([p.run("test"), p.run("lint")], {
-      defaultImage: "ubuntu:24.04",
-    });
+    const ir = pipeline([p.run("test"), p.run("lint")]);
     expect(ir.version).toBe("0");
     expect(ir.graph.nodes.length).toBeGreaterThanOrEqual(5);
   });
 
   it("produces valid IR for bun+bun", () => {
     const p = js.project({ runtime: "bun" });
-    const ir = pipeline([p.run("test"), p.run("lint")], {
-      defaultImage: "ubuntu:24.04",
-    });
+    const ir = pipeline([p.run("test"), p.run("lint")]);
     expect(ir.version).toBe("0");
     expect(ir.graph.nodes.length).toBeGreaterThanOrEqual(4);
   });
 
   it("produces valid IR for deno", () => {
     const p = js.project({ runtime: "deno" });
-    const ir = pipeline([p.run("test"), p.run("lint")], {
-      defaultImage: "ubuntu:24.04",
-    });
+    const ir = pipeline([p.run("test"), p.run("lint")]);
     expect(ir.version).toBe("0");
     expect(ir.graph.nodes.length).toBeGreaterThanOrEqual(4);
   });
diff --git a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/py/uv.test.ts b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/py/uv.test.ts
index c25463b4..fa6afb0d 100644
--- a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/py/uv.test.ts
+++ b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/py/uv.test.ts
@@ -131,7 +131,7 @@ describe("py.uv install chain", () => {
 describe("py.uv in pipeline", () => {
   it("produces valid IR", () => {
     const p = py.uv();
-    const ir = pipeline([p.test(), p.lint()], { defaultImage: "ubuntu:24.04" });
+    const ir = pipeline([p.test(), p.lint()]);
     expect(ir.graph.nodes.length).toBeGreaterThanOrEqual(4);
     expect(ir.version).toBe("0");
   });
diff --git a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/python.test.ts b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/python.test.ts
index 9b6db68d..a1914ea4 100644
--- a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/python.test.ts
+++ b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/python.test.ts
@@ -98,7 +98,7 @@ describe("python install chain", () => {
 describe("python in pipeline", () => {
   it("produces valid IR", () => {
     const p = python();
-    const ir = pipeline([p.test(), p.lint()], { defaultImage: "ubuntu:24.04" });
+    const ir = pipeline([p.test(), p.lint()]);
     expect(ir.graph.nodes.length).toBeGreaterThanOrEqual(4);
     expect(ir.version).toBe("0");
   });
diff --git a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/rust.test.ts b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/rust.test.ts
index f12a3ef9..028b9c37 100644
--- a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/rust.test.ts
+++ b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/rust.test.ts
@@ -142,9 +142,7 @@ describe("rust.toolchain", () => {
 
   it("produces valid pipeline IR", () => {
     const r = rust.toolchain();
-    const ir = pipeline([r.build(), r.test(), r.clippy(), r.fmt()], {
-      defaultImage: "ubuntu:24.04",
-    });
+    const ir = pipeline([r.build(), r.test(), r.clippy(), r.fmt()]);
     expect(ir.graph.nodes.length).toBeGreaterThanOrEqual(4);
     expect(ir.version).toBe("0");
   });
@@ -237,9 +235,7 @@ describe("rust.project", () => {
   it("with base skips apt", () => {
     const base = sh("custom base");
     const proj = rust.project({ path: "cli", base });
-    const ir = pipeline([proj.test(), proj.clippy(), proj.fmt()], {
-      defaultImage: "ubuntu:24.04",
-    });
+    const ir = pipeline([proj.test(), proj.clippy(), proj.fmt()]);
     const c = cmds(ir);
     expect(
       c.filter((cmd: string) => cmd.includes("apt-get install")),
@@ -249,9 +245,7 @@ describe("rust.project", () => {
 
   it("produces valid pipeline IR", () => {
     const proj = rust.project({ path: "cli" });
-    const ir = pipeline([proj.test(), proj.clippy(), proj.fmt()], {
-      defaultImage: "ubuntu:24.04",
-    });
+    const ir = pipeline([proj.test(), proj.clippy(), proj.fmt()]);
     expect(ir.version).toBe("0");
     expect(ir.graph.nodes.length).toBeGreaterThanOrEqual(4);
   });
diff --git a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/shared.test.ts b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/shared.test.ts
index 4bacbf62..187e4d0c 100644
--- a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/shared.test.ts
+++ b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/shared.test.ts
@@ -50,7 +50,7 @@ describe("aptBase", () => {
     });
     const r = rust.toolchain({ base });
     const p = uv({ path: "dsls/harmont-py", base });
-    const ir = pipeline([r.build(), p.test()], { defaultImage: "ubuntu:24.04" });
+    const ir = pipeline([r.build(), p.test()]);
     const cmds = ir.graph.nodes.map(
       (n: { step: { cmd: string } }) => n.step.cmd,
     );
diff --git a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/zig.test.ts b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/zig.test.ts
index f1494255..1347f12f 100644
--- a/crates/hm-dsl-engine/harmont-ts/tests/toolchains/zig.test.ts
+++ b/crates/hm-dsl-engine/harmont-ts/tests/toolchains/zig.test.ts
@@ -88,9 +88,7 @@ describe("zig multi-project pipeline", () => {
     const tc = zig();
     const a = tc.project("lib-a");
     const b = tc.project("lib-b");
-    const ir = pipeline([a.build(), a.test(), b.build(), b.test()], {
-      defaultImage: "ubuntu:24.04",
-    });
+    const ir = pipeline([a.build(), a.test(), b.build(), b.test()]);
     expect(ir.graph.nodes.length).toBeGreaterThanOrEqual(5);
     expect(ir.version).toBe("0");
   });
diff --git a/crates/hm-exec/src/local/runner/vm.rs b/crates/hm-exec/src/local/runner/vm.rs
index 9c2d2b70..325634fc 100644
--- a/crates/hm-exec/src/local/runner/vm.rs
+++ b/crates/hm-exec/src/local/runner/vm.rs
@@ -78,12 +78,15 @@ async fn run_step_vm(vm: &HmVm, ctx: &StepContext, input: ExecutorInput) -> Resu
     let source = if let Some(ref snap) = input.parent_snapshot {
         ImageSource::Snapshot(SnapshotId::new(snap.0.clone()))
     } else {
+        // Imageless root step: default to an apt-capable base. The SDK's
+        // toolchains all assume `apt-get`, so `ubuntu:24.04` is the
+        // across-the-board default (alpine has no apt-get).
         ImageSource::Image(
             input
                 .step
                 .image
                 .clone()
-                .unwrap_or_else(|| "alpine:latest".to_string()),
+                .unwrap_or_else(|| "ubuntu:24.04".to_string()),
         )
     };
 
diff --git a/crates/hm-pipeline-ir/tests/e2e_fixtures.rs b/crates/hm-pipeline-ir/tests/e2e_fixtures.rs
index e8c93112..7a6fd951 100644
--- a/crates/hm-pipeline-ir/tests/e2e_fixtures.rs
+++ b/crates/hm-pipeline-ir/tests/e2e_fixtures.rs
@@ -53,7 +53,7 @@ fn edge_kinds(g: &PipelineGraph) -> (usize, usize) {
 #[test]
 fn python_monorepo_ci() {
     let g = load_fixture("python", "monorepo-ci");
-    assert_eq!(g.default_image(), Some("ubuntu:24.04"));
+    assert_eq!(g.default_image(), None);
     assert!(g.node_count() >= 15, "nodes: {}", g.node_count());
     let labels = step_labels(&g);
     assert!(labels.iter().any(|l| l.contains("go")));
@@ -72,16 +72,28 @@ fn python_monorepo_ci() {
 #[test]
 fn python_rust_release() {
     let g = load_fixture("python", "rust-release");
-    assert_eq!(g.default_image(), Some("ubuntu:24.04"));
+    assert_eq!(g.default_image(), None);
     assert!(g.node_count() >= 5, "nodes: {}", g.node_count());
     let labels = step_labels(&g);
     assert!(labels.iter().any(|l| l.contains("rust")));
+    // The DSL now injects image on each imageless root step directly.
+    let apt_base = g
+        .dag()
+        .graph()
+        .node_references()
+        .find(|(_, t)| t.step.key == "apt-base")
+        .map(|(_, t)| t.step.image.as_deref());
+    assert_eq!(
+        apt_base,
+        Some(Some("ubuntu:24.04")),
+        "root step apt-base must carry explicit image"
+    );
 }
 
 #[test]
 fn python_zig_node_polyglot() {
     let g = load_fixture("python", "zig-node-polyglot");
-    assert_eq!(g.default_image(), Some("ubuntu:24.04"));
+    assert_eq!(g.default_image(), None);
     assert!(g.node_count() >= 10, "nodes: {}", g.node_count());
     let labels = step_labels(&g);
     assert!(labels.iter().any(|l| l.contains("zig")));
@@ -95,7 +107,7 @@ fn python_zig_node_polyglot() {
 #[test]
 fn python_kitchen_sink() {
     let g = load_fixture("python", "kitchen-sink");
-    assert_eq!(g.default_image(), Some("ubuntu:24.04"));
+    assert_eq!(g.default_image(), None);
     assert!(g.node_count() >= 10, "nodes: {}", g.node_count());
     let labels = step_labels(&g);
     assert!(labels.iter().any(|l| l.contains("python")));
@@ -116,28 +128,28 @@ fn python_kitchen_sink() {
 #[test]
 fn ts_monorepo_ci() {
     let g = load_fixture("ts", "monorepo-ci");
-    assert_eq!(g.default_image(), Some("ubuntu:24.04"));
+    assert_eq!(g.default_image(), None);
     assert!(g.node_count() >= 15);
 }
 
 #[test]
 fn ts_rust_release() {
     let g = load_fixture("ts", "rust-release");
-    assert_eq!(g.default_image(), Some("ubuntu:24.04"));
+    assert_eq!(g.default_image(), None);
     assert!(g.node_count() >= 5);
 }
 
 #[test]
 fn ts_zig_node_polyglot() {
     let g = load_fixture("ts", "zig-node-polyglot");
-    assert_eq!(g.default_image(), Some("ubuntu:24.04"));
+    assert_eq!(g.default_image(), None);
     assert!(g.node_count() >= 10);
 }
 
 #[test]
 fn ts_kitchen_sink() {
     let g = load_fixture("ts", "kitchen-sink");
-    assert_eq!(g.default_image(), Some("ubuntu:24.04"));
+    assert_eq!(g.default_image(), None);
     assert!(g.node_count() >= 10);
 }
 
diff --git a/crates/hm/README.md b/crates/hm/README.md
index a66993a2..1f0821f8 100644
--- a/crates/hm/README.md
+++ b/crates/hm/README.md
@@ -79,7 +79,7 @@ def ci() -> hm.Step:
         "echo independent work",
         label="other",
     )
-    return hm.pipeline(fetch, work, default_image="ubuntu:24.04")
+    return hm.pipeline(fetch, work)
 ```
 
 For larger pipelines, compose with `@hm.target` and typed fixture params:
diff --git a/crates/hm/src/commands/init_templates/cmake.py b/crates/hm/src/commands/init_templates/cmake.py
index b37f0668..a0c4efdc 100644
--- a/crates/hm/src/commands/init_templates/cmake.py
+++ b/crates/hm/src/commands/init_templates/cmake.py
@@ -7,7 +7,6 @@
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci() -> tuple[hm.Step, ...]:
diff --git a/crates/hm/src/commands/init_templates/elixir.py b/crates/hm/src/commands/init_templates/elixir.py
index 46eefb28..085ea0d7 100644
--- a/crates/hm/src/commands/init_templates/elixir.py
+++ b/crates/hm/src/commands/init_templates/elixir.py
@@ -7,7 +7,6 @@
 @hm.pipeline(
     "ci",
     env={"CI": "true", "MIX_ENV": "test"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci() -> tuple[hm.Step, ...]:
diff --git a/crates/hm/src/commands/init_templates/js.ts b/crates/hm/src/commands/init_templates/js.ts
index aeca70eb..5cfc833e 100644
--- a/crates/hm/src/commands/init_templates/js.ts
+++ b/crates/hm/src/commands/init_templates/js.ts
@@ -9,7 +9,6 @@ const pipelines: PipelineDefinition[] = [
     triggers: [push({ branch: "main" })],
     pipeline: pipeline([project.run("build"), project.run("test"), project.run("lint")], {
       env: { CI: "true" },
-      defaultImage: "ubuntu:24.04",
     }),
   },
 ];
diff --git a/crates/hm/src/commands/init_templates/nextjs.ts b/crates/hm/src/commands/init_templates/nextjs.ts
index 7fdd4544..2e4cbccb 100644
--- a/crates/hm/src/commands/init_templates/nextjs.ts
+++ b/crates/hm/src/commands/init_templates/nextjs.ts
@@ -9,7 +9,6 @@ const pipelines: PipelineDefinition[] = [
     triggers: [push({ branch: "main" })],
     pipeline: pipeline([project.run("build"), project.run("lint")], {
       env: { CI: "true" },
-      defaultImage: "ubuntu:24.04",
     }),
   },
 ];
diff --git a/crates/hm/src/commands/init_templates/python.py b/crates/hm/src/commands/init_templates/python.py
index 0d34c3b1..6b122046 100644
--- a/crates/hm/src/commands/init_templates/python.py
+++ b/crates/hm/src/commands/init_templates/python.py
@@ -13,7 +13,6 @@ def project() -> PythonToolchain:
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci(project: hm.Target[PythonToolchain]) -> tuple[hm.Step, ...]:
diff --git a/crates/hm/src/commands/init_templates/rust.py b/crates/hm/src/commands/init_templates/rust.py
index 2a41052c..bcb64f56 100644
--- a/crates/hm/src/commands/init_templates/rust.py
+++ b/crates/hm/src/commands/init_templates/rust.py
@@ -13,7 +13,6 @@ def project() -> RustToolchain:
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci(project: hm.Target[RustToolchain]) -> tuple[hm.Step, ...]:
diff --git a/crates/hm/src/commands/init_templates/skill_convert_gha.md b/crates/hm/src/commands/init_templates/skill_convert_gha.md
index e50796c2..cb1e9eb1 100644
--- a/crates/hm/src/commands/init_templates/skill_convert_gha.md
+++ b/crates/hm/src/commands/init_templates/skill_convert_gha.md
@@ -54,7 +54,7 @@ Convert existing GitHub Actions workflows (`.github/workflows/*.yml` / `*.yaml`)
    | `actions/cache` | **Not needed — caching is implicit in Harmont** | Harmont automatically caches build artifacts, dependency installs, and toolchain outputs between runs. Remove all cache steps. |
    | `actions/setup-*` (setup-node, setup-python, etc.) | Harmont toolchains (`hm.js`, `hm.python`, etc.) | Toolchains handle installation. Specify version via toolchain config. |
    | `actions/checkout` | **Not needed — source is always available** | Harmont automatically provides the source code to every step. |
-   | `runs-on: ubuntu-latest` | `default_image: "ubuntu:24.04"` | Harmont runs steps in Docker containers |
+   | `runs-on: ubuntu-latest` | (default base is `ubuntu:24.04`; set a per-step `image="..."` to override) | Harmont runs steps in Docker containers |
    | `services:` (e.g., postgres) | Service containers in step config | Check docs for service container syntax |
    | `matrix:` | Multiple pipelines or parameterized steps | No direct matrix — may need separate pipeline definitions or `.fork()` |
    | `env:` / `secrets.*` | `env: {}` on pipeline or step | Secrets must be passed as environment variables |
diff --git a/crates/hm/src/commands/init_templates/skill_write_pipeline.md b/crates/hm/src/commands/init_templates/skill_write_pipeline.md
index ac349116..fdc62ffa 100644
--- a/crates/hm/src/commands/init_templates/skill_write_pipeline.md
+++ b/crates/hm/src/commands/init_templates/skill_write_pipeline.md
@@ -58,7 +58,7 @@ Write, modify, or extend Harmont CI pipelines defined in `.hm/pipeline.py` (Pyth
    - Prefer toolchains over raw `sh()` calls when a toolchain exists for the language.
    - Use `.fork()` for steps that can run in parallel.
    - Set triggers (`push`, `pull_request`) appropriate to the project.
-   - Use `default_image: "ubuntu:24.04"` unless the project needs something specific.
+   - Steps run on `ubuntu:24.04` by default; set a per-step `image="..."` only when a specific step needs a different base.
    - Set `env: {"CI": "true"}` on the pipeline.
 
 5. **Validate the pipeline renders correctly:**
diff --git a/crates/hm/src/commands/init_templates/zig.ts b/crates/hm/src/commands/init_templates/zig.ts
index aeb456cc..a755a0b8 100644
--- a/crates/hm/src/commands/init_templates/zig.ts
+++ b/crates/hm/src/commands/init_templates/zig.ts
@@ -9,7 +9,6 @@ const pipelines: PipelineDefinition[] = [
     triggers: [push({ branch: "main" })],
     pipeline: pipeline([project.build(), project.test(), project.fmt()], {
       env: { CI: "true" },
-      defaultImage: "ubuntu:24.04",
     }),
   },
 ];
diff --git a/crates/hm/tests/chain_failure_render.rs b/crates/hm/tests/chain_failure_render.rs
index ca08661e..4df4fe9b 100644
--- a/crates/hm/tests/chain_failure_render.rs
+++ b/crates/hm/tests/chain_failure_render.rs
@@ -10,7 +10,7 @@ const FAILING_PIPELINE_PY: &str = r#"
 import harmont as hm
 
 
-@hm.pipeline("failing", default_image="alpine:3.20")
+@hm.pipeline("failing")
 def failing() -> hm.Step:
     return hm.sh("exit 7", label="oops", image="alpine:3.20")
 "#;
diff --git a/crates/hm/tests/cmd_run_local_autoselect.rs b/crates/hm/tests/cmd_run_local_autoselect.rs
index 740d71bf..71ff0b61 100644
--- a/crates/hm/tests/cmd_run_local_autoselect.rs
+++ b/crates/hm/tests/cmd_run_local_autoselect.rs
@@ -10,7 +10,7 @@ const PIPELINE_PY: &str = r#"
 import harmont as hm
 
 
-@hm.pipeline("only-one", default_image="alpine:3.20")
+@hm.pipeline("only-one")
 def only_one() -> hm.Step:
     return hm.sh("echo autoselected", label="hi", image="alpine:3.20")
 "#;
@@ -40,13 +40,13 @@ fn many_pipelines_still_requires_arg() {
         r#"
 import harmont as hm
 
-@hm.pipeline("a", default_image="alpine:3.20")
+@hm.pipeline("a")
 def a() -> hm.Step:
-    return hm.sh("echo a")
+    return hm.sh("echo a", image="alpine:3.20")
 
-@hm.pipeline("b", default_image="alpine:3.20")
+@hm.pipeline("b")
 def b() -> hm.Step:
-    return hm.sh("echo b")
+    return hm.sh("echo b", image="alpine:3.20")
 "#,
     )
     .unwrap();
diff --git a/crates/hm/tests/cmd_run_local_format.rs b/crates/hm/tests/cmd_run_local_format.rs
index dbb52ba4..78ea72bc 100644
--- a/crates/hm/tests/cmd_run_local_format.rs
+++ b/crates/hm/tests/cmd_run_local_format.rs
@@ -10,7 +10,7 @@ const PIPELINE_PY: &str = r#"
 import harmont as hm
 
 
-@hm.pipeline("formatted", default_image="alpine:3.20")
+@hm.pipeline("formatted")
 def formatted() -> hm.Step:
     return hm.sh("echo formatted-hello", label="hi", image="alpine:3.20")
 "#;
diff --git a/crates/hm/tests/cmd_run_local_orchestrated.rs b/crates/hm/tests/cmd_run_local_orchestrated.rs
index 7180d9d9..78869aaa 100644
--- a/crates/hm/tests/cmd_run_local_orchestrated.rs
+++ b/crates/hm/tests/cmd_run_local_orchestrated.rs
@@ -21,7 +21,7 @@ const PIPELINE_PY: &str = r#"
 import harmont as hm
 
 
-@hm.pipeline("orchestrated", default_image="alpine:3.20")
+@hm.pipeline("orchestrated")
 def orchestrated() -> hm.Step:
     return hm.sh("echo orchestrated hello", label="hi", image="alpine:3.20")
 "#;
@@ -50,7 +50,7 @@ const CHAIN_PIPELINE_PY: &str = r#"
 import harmont as hm
 
 
-@hm.pipeline("chain", default_image="alpine:3.20")
+@hm.pipeline("chain")
 def chain() -> hm.Step:
     a = hm.sh("echo step-a > /tmp/a && cat /tmp/a", label="a", image="alpine:3.20")
     return a.sh("cat /tmp/a && echo step-b", label="b")
diff --git a/crates/hm/tests/fixtures/pipelines/cow_chain.py b/crates/hm/tests/fixtures/pipelines/cow_chain.py
index 70f39528..7ac9650c 100644
--- a/crates/hm/tests/fixtures/pipelines/cow_chain.py
+++ b/crates/hm/tests/fixtures/pipelines/cow_chain.py
@@ -2,9 +2,9 @@
 import harmont as hm
 
 
-@hm.pipeline("cow-chain", default_image="alpine:latest")
+@hm.pipeline("cow-chain")
 def cow_chain():
-    a = hm.sh("echo from-a > /workspace/a.txt", label="a")
+    a = hm.sh("echo from-a > /workspace/a.txt", label="a", image="alpine:latest")
     b = a.sh("cat /workspace/a.txt && echo from-b > /workspace/b.txt", label="b")
     c = b.sh("cat /workspace/a.txt && cat /workspace/b.txt && echo c-saw-both", label="c")
     return [c]
diff --git a/crates/hm/tests/keep_going.rs b/crates/hm/tests/keep_going.rs
index 7b9b572f..0027d055 100644
--- a/crates/hm/tests/keep_going.rs
+++ b/crates/hm/tests/keep_going.rs
@@ -9,7 +9,7 @@ const FORK_PIPELINE_PY: &str = r#"
 import harmont as hm
 
 
-@hm.pipeline("two", default_image="alpine:3.20")
+@hm.pipeline("two")
 def two():
     root = hm.scratch().fork()
     a = root.sh("exit 1", label="fail-step", image="alpine:3.20")
@@ -30,11 +30,11 @@ const CHAIN_PIPELINE_PY: &str = r#"
 import harmont as hm
 
 
-@hm.pipeline("chain", default_image="alpine:3.20")
+@hm.pipeline("chain")
 def chain():
     a = hm.sh("exit 1", label="step-a", image="alpine:3.20")
-    b = a.sh("echo b", label="step-b", image="alpine:3.20")
-    c = b.sh("echo c", label="step-c", image="alpine:3.20")
+    b = a.sh("echo b", label="step-b")
+    c = b.sh("echo c", label="step-c")
     return c
 "#;
 
diff --git a/crates/hm/tests/local_fork_cache.rs b/crates/hm/tests/local_fork_cache.rs
index 411a770d..982eb0fd 100644
--- a/crates/hm/tests/local_fork_cache.rs
+++ b/crates/hm/tests/local_fork_cache.rs
@@ -25,16 +25,17 @@ fn write_pipeline(dir: &Path, marker_contents: &str) {
 import harmont as hm
 
 def build():
-    base = hm.scratch().run(
+    base = hm.scratch().sh(
         "echo base-ran",
         label="base",
         cache=hm.forever(),
+        image="alpine:3.20",
     )
-    child = base.fork(label="child").run(
+    child = base.fork(label="child").sh(
         "cat /workspace/marker.txt",
         label="child",
     )
-    return hm.pipeline(child, default_image="alpine:3.20")
+    return hm.pipeline([child])
 "#,
     )
     .expect("pipeline.py");
diff --git a/crates/hm/tests/local_parallelism.rs b/crates/hm/tests/local_parallelism.rs
index f18175a2..739d900d 100644
--- a/crates/hm/tests/local_parallelism.rs
+++ b/crates/hm/tests/local_parallelism.rs
@@ -23,9 +23,9 @@ fn write_pipeline(dir: &Path) {
 import harmont as hm
 
 def build():
-    a = hm.scratch().run("sleep 3", label="sleep-a")
-    b = hm.scratch().run("sleep 3", label="sleep-b")
-    return hm.pipeline(a, b, default_image="alpine:3.20")
+    a = hm.scratch().sh("sleep 3", label="sleep-a", image="alpine:3.20")
+    b = hm.scratch().sh("sleep 3", label="sleep-b", image="alpine:3.20")
+    return hm.pipeline([a, b])
 "#,
     )
     .expect("pipeline.py");
diff --git a/examples/README.md b/examples/README.md
index c638b8e4..f67c912c 100644
--- a/examples/README.md
+++ b/examples/README.md
@@ -20,7 +20,7 @@ Minimal idiomatic starter projects, each wired up to a Harmont CI pipeline. Ever
 1. Install the Harmont CLI (`cli/` in this repo, or `cargo install harmont-cli` once published).
 2. `cd examples/<lang>` and run `hm run ci`. The CLI uses the project's `.hm/pipeline.py` and executes each step in a local Docker container (the default backend), sharing caches across runs. Use `--backend cloud` to submit the run to Harmont Cloud instead.
 
-Every pipeline uses `default_image="ubuntu:24.04"` and the apt-base / language-install steps are cached forever — only the action leaves (`test`, `lint`, etc.) re-run after a code change.
+Every pipeline runs on the default `ubuntu:24.04` base (set a per-step `image=` to override) and the apt-base / language-install steps are cached forever — only the action leaves (`test`, `lint`, etc.) re-run after a code change.
 
 ## What to copy
 
diff --git a/examples/bun/.hm/pipeline.py b/examples/bun/.hm/pipeline.py
index bec40093..4d95e853 100644
--- a/examples/bun/.hm/pipeline.py
+++ b/examples/bun/.hm/pipeline.py
@@ -7,7 +7,6 @@
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci() -> tuple[hm.Step, ...]:
diff --git a/examples/c/.hm/pipeline.py b/examples/c/.hm/pipeline.py
index f8ea110c..26a06ae5 100644
--- a/examples/c/.hm/pipeline.py
+++ b/examples/c/.hm/pipeline.py
@@ -7,7 +7,6 @@
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci() -> tuple[hm.Step, ...]:
diff --git a/examples/cmake-advanced/.hm/pipeline.py b/examples/cmake-advanced/.hm/pipeline.py
index c74440d3..218292e5 100644
--- a/examples/cmake-advanced/.hm/pipeline.py
+++ b/examples/cmake-advanced/.hm/pipeline.py
@@ -7,7 +7,6 @@
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main"), hm.pr()],
 )
 def ci() -> tuple[hm.Step, ...]:
diff --git a/examples/cpp/.hm/pipeline.py b/examples/cpp/.hm/pipeline.py
index 61cf6cf4..777f1e86 100644
--- a/examples/cpp/.hm/pipeline.py
+++ b/examples/cpp/.hm/pipeline.py
@@ -7,7 +7,6 @@
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci() -> tuple[hm.Step, ...]:
diff --git a/examples/elixir-phoenix/.hm/pipeline.py b/examples/elixir-phoenix/.hm/pipeline.py
index b6bd0206..cc089d42 100644
--- a/examples/elixir-phoenix/.hm/pipeline.py
+++ b/examples/elixir-phoenix/.hm/pipeline.py
@@ -12,7 +12,6 @@ def project():
 @hm.pipeline(
     "ci",
     env={"CI": "true", "MIX_ENV": "test"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main"), hm.pr()],
 )
 def ci(project: hm.Target) -> tuple[hm.Step, ...]:
@@ -31,7 +30,6 @@ def ci(project: hm.Target) -> tuple[hm.Step, ...]:
 @hm.pipeline(
     "deploy",
     env={"MIX_ENV": "prod"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def deploy(project: hm.Target) -> tuple[hm.Step, ...]:
diff --git a/examples/elixir/.hm/pipeline.py b/examples/elixir/.hm/pipeline.py
index 11ba30dc..d81d1671 100644
--- a/examples/elixir/.hm/pipeline.py
+++ b/examples/elixir/.hm/pipeline.py
@@ -7,7 +7,6 @@
 @hm.pipeline(
     "ci",
     env={"CI": "true", "MIX_ENV": "test"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci() -> tuple[hm.Step, ...]:
diff --git a/examples/go/.hm/pipeline.py b/examples/go/.hm/pipeline.py
index 7c145a83..97eba899 100644
--- a/examples/go/.hm/pipeline.py
+++ b/examples/go/.hm/pipeline.py
@@ -13,7 +13,6 @@ def project() -> GoToolchain:
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci(project: hm.Target[GoToolchain]) -> tuple[hm.Step, ...]:
diff --git a/examples/nextjs/.hm/pipeline.py b/examples/nextjs/.hm/pipeline.py
index 010cafba..c3bf8d4d 100644
--- a/examples/nextjs/.hm/pipeline.py
+++ b/examples/nextjs/.hm/pipeline.py
@@ -7,7 +7,6 @@
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci() -> tuple[hm.Step, ...]:
diff --git a/examples/python-uv/.hm/pipeline.py b/examples/python-uv/.hm/pipeline.py
index 3f3c0dcc..78baed7a 100644
--- a/examples/python-uv/.hm/pipeline.py
+++ b/examples/python-uv/.hm/pipeline.py
@@ -13,7 +13,6 @@ def project() -> PythonToolchain:
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci(project: hm.Target[PythonToolchain]) -> tuple[hm.Step, ...]:
diff --git a/examples/react/.hm/pipeline.py b/examples/react/.hm/pipeline.py
index 0e4a8814..79e78cb1 100644
--- a/examples/react/.hm/pipeline.py
+++ b/examples/react/.hm/pipeline.py
@@ -7,7 +7,6 @@
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci() -> tuple[hm.Step, ...]:
diff --git a/examples/rust/.hm/pipeline.py b/examples/rust/.hm/pipeline.py
index e27a1490..b5d01135 100644
--- a/examples/rust/.hm/pipeline.py
+++ b/examples/rust/.hm/pipeline.py
@@ -13,7 +13,6 @@ def project() -> RustToolchain:
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci(project: hm.Target[RustToolchain]) -> tuple[hm.Step, ...]:
diff --git a/examples/typescript/.hm/pipeline.py b/examples/typescript/.hm/pipeline.py
index 9da8404d..0bea8c87 100644
--- a/examples/typescript/.hm/pipeline.py
+++ b/examples/typescript/.hm/pipeline.py
@@ -7,7 +7,6 @@
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci() -> tuple[hm.Step, ...]:
diff --git a/examples/zig-js/.hm/pipeline.py b/examples/zig-js/.hm/pipeline.py
index 0d6fe5ca..f45f0850 100644
--- a/examples/zig-js/.hm/pipeline.py
+++ b/examples/zig-js/.hm/pipeline.py
@@ -53,7 +53,6 @@ def web_project(apt_base: hm.Target[hm.Step]) -> JsProject:
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci(
diff --git a/examples/zig/.hm/pipeline.py b/examples/zig/.hm/pipeline.py
index e38289bf..f70ddb2c 100644
--- a/examples/zig/.hm/pipeline.py
+++ b/examples/zig/.hm/pipeline.py
@@ -7,7 +7,6 @@
 @hm.pipeline(
     "ci",
     env={"CI": "true"},
-    default_image="ubuntu:24.04",
     triggers=[hm.push(branch="main")],
 )
 def ci() -> tuple[hm.Step, ...]:
diff --git a/tests/e2e/fixtures/python/cmake-advanced.json b/tests/e2e/fixtures/python/cmake-advanced.json
index 5a923649..b1ccf9b3 100644
--- a/tests/e2e/fixtures/python/cmake-advanced.json
+++ b/tests/e2e/fixtures/python/cmake-advanced.json
@@ -1,5 +1,4 @@
 {
-  "default_image": "ubuntu:24.04",
   "graph": {
     "edge_property": "directed",
     "edges": [
diff --git a/tests/e2e/fixtures/python/kitchen-sink.json b/tests/e2e/fixtures/python/kitchen-sink.json
index 028331bc..e1fcb4ea 100644
--- a/tests/e2e/fixtures/python/kitchen-sink.json
+++ b/tests/e2e/fixtures/python/kitchen-sink.json
@@ -1,5 +1,4 @@
 {
-  "default_image": "ubuntu:24.04",
   "graph": {
     "edge_property": "directed",
     "edges": [
diff --git a/tests/e2e/fixtures/python/monorepo-ci.json b/tests/e2e/fixtures/python/monorepo-ci.json
index e84a4f3b..ec0ef385 100644
--- a/tests/e2e/fixtures/python/monorepo-ci.json
+++ b/tests/e2e/fixtures/python/monorepo-ci.json
@@ -1,5 +1,4 @@
 {
-  "default_image": "ubuntu:24.04",
   "graph": {
     "edge_property": "directed",
     "edges": [
diff --git a/tests/e2e/fixtures/python/rust-release.json b/tests/e2e/fixtures/python/rust-release.json
index cd40ff9d..9d2af6e4 100644
--- a/tests/e2e/fixtures/python/rust-release.json
+++ b/tests/e2e/fixtures/python/rust-release.json
@@ -1,5 +1,4 @@
 {
-  "default_image": "ubuntu:24.04",
   "graph": {
     "edge_property": "directed",
     "edges": [
diff --git a/tests/e2e/fixtures/python/zig-node-polyglot.json b/tests/e2e/fixtures/python/zig-node-polyglot.json
index 76406024..0ddbebff 100644
--- a/tests/e2e/fixtures/python/zig-node-polyglot.json
+++ b/tests/e2e/fixtures/python/zig-node-polyglot.json
@@ -1,5 +1,4 @@
 {
-  "default_image": "ubuntu:24.04",
   "graph": {
     "edge_property": "directed",
     "edges": [
diff --git a/tests/e2e/fixtures/ts/cmake-advanced.json b/tests/e2e/fixtures/ts/cmake-advanced.json
index 5a923649..b1ccf9b3 100644
--- a/tests/e2e/fixtures/ts/cmake-advanced.json
+++ b/tests/e2e/fixtures/ts/cmake-advanced.json
@@ -1,5 +1,4 @@
 {
-  "default_image": "ubuntu:24.04",
   "graph": {
     "edge_property": "directed",
     "edges": [
diff --git a/tests/e2e/fixtures/ts/kitchen-sink.json b/tests/e2e/fixtures/ts/kitchen-sink.json
index 028331bc..e1fcb4ea 100644
--- a/tests/e2e/fixtures/ts/kitchen-sink.json
+++ b/tests/e2e/fixtures/ts/kitchen-sink.json
@@ -1,5 +1,4 @@
 {
-  "default_image": "ubuntu:24.04",
   "graph": {
     "edge_property": "directed",
     "edges": [
diff --git a/tests/e2e/fixtures/ts/monorepo-ci.json b/tests/e2e/fixtures/ts/monorepo-ci.json
index e84a4f3b..ec0ef385 100644
--- a/tests/e2e/fixtures/ts/monorepo-ci.json
+++ b/tests/e2e/fixtures/ts/monorepo-ci.json
@@ -1,5 +1,4 @@
 {
-  "default_image": "ubuntu:24.04",
   "graph": {
     "edge_property": "directed",
     "edges": [
diff --git a/tests/e2e/fixtures/ts/rust-release.json b/tests/e2e/fixtures/ts/rust-release.json
index cd40ff9d..9d2af6e4 100644
--- a/tests/e2e/fixtures/ts/rust-release.json
+++ b/tests/e2e/fixtures/ts/rust-release.json
@@ -1,5 +1,4 @@
 {
-  "default_image": "ubuntu:24.04",
   "graph": {
     "edge_property": "directed",
     "edges": [
diff --git a/tests/e2e/fixtures/ts/zig-node-polyglot.json b/tests/e2e/fixtures/ts/zig-node-polyglot.json
index 76406024..0ddbebff 100644
--- a/tests/e2e/fixtures/ts/zig-node-polyglot.json
+++ b/tests/e2e/fixtures/ts/zig-node-polyglot.json
@@ -1,5 +1,4 @@
 {
-  "default_image": "ubuntu:24.04",
   "graph": {
     "edge_property": "directed",
     "edges": [