diff --git a/docs/concepts/bootstrapper-architecture.rst b/docs/concepts/bootstrapper-architecture.rst
index 970d36af..5663739d 100644
--- a/docs/concepts/bootstrapper-architecture.rst
+++ b/docs/concepts/bootstrapper-architecture.rst
@@ -82,6 +82,12 @@ Each phase wraps a ``WorkItem`` dataclass that accumulates per-package
 state (resolved version, build environment, build result, etc.) as it
 moves through the pipeline.
 
+.. versionchanged:: 0.103.0
+   Later build dependency installs retain explicit requirements from earlier
+   phases within the same build environment. A backend or sdist dependency that
+   conflicts with an earlier build-system bound fails resolution instead of
+   silently replacing the earlier dependency with an incompatible version.
+
 Bootstrapper and Phase Interaction
 ----------------------------------
 
diff --git a/src/fromager/build_environment.py b/src/fromager/build_environment.py
index 1be2fe5b..0b3a8e73 100644
--- a/src/fromager/build_environment.py
+++ b/src/fromager/build_environment.py
@@ -44,9 +44,12 @@ def __init__(
         req_type: RequirementType,
         req: Requirement | str | None,
         all_reqs: typing.Iterable[Requirement],
+        earlier_reqs: typing.Iterable[Requirement] = (),
     ):
-        self.all_reqs = all_reqs
-        resolutions = []
+        earlier_reqs = tuple(earlier_reqs)
+        all_reqs = tuple(all_reqs)
+        self.all_reqs = earlier_reqs + all_reqs
+        resolutions = [f"{r} (earlier bound)" for r in earlier_reqs]
         for r in all_reqs:
             try:
                 _, version = resolver.resolve(
@@ -93,6 +96,7 @@ def __init__(
         self._ctx = ctx
         self._req = req
         self._sdist_root_dir = sdist_root_dir
+        self._installed_requirements: tuple[Requirement, ...] = ()
         # abiflags adds 't' for free-threaded (nogil) builds
         self.path = sdist_root_dir.parent.absolute().joinpath(
             f"build-{platform.python_version()}{sys.abiflags}"
@@ -213,9 +217,12 @@ def _createenv(self) -> None:
         logger.info("created build environment in %s", self.path)
 
     def install(self, reqs: typing.Iterable[Requirement]) -> None:
+        reqs = tuple(reqs)
         if not reqs:
             return
 
+        all_reqs = self._installed_requirements + reqs
+
         # UV does not compile byte code by default
         cmd = [
             "uv",
@@ -228,13 +235,14 @@ def install(self, reqs: typing.Iterable[Requirement]) -> None:
         ]
         cmd.extend(self._ctx.pip_constraint_args)
         cmd.extend(self._ctx.pip_wheel_server_args)
-        cmd.extend(str(req) for req in reqs)
+        cmd.extend(str(req) for req in all_reqs)
 
         self.run(
             cmd,
             cwd=str(self.path.parent),
             network_isolation=False,
         )
+        self._installed_requirements = all_reqs
         logger.info(
             "installed dependencies %s into build environment in %s",
             reqs,
@@ -347,6 +355,7 @@ def _safe_install(
     deps: typing.Iterable[Requirement],
     dep_req_type: RequirementType,
 ) -> None:
+    deps = tuple(deps)
     if not deps:
         return
     logger.debug("installing %s %s", dep_req_type, deps)
@@ -364,6 +373,7 @@ def _safe_install(
             dep_req_type,
             req_info,
             deps,
+            earlier_reqs=build_env._installed_requirements,
         ) from err
 
     logger.info("installed %s requirements %s", dep_req_type, deps)
diff --git a/tests/test_build_environment.py b/tests/test_build_environment.py
index 28c504af..21a6f5d5 100644
--- a/tests/test_build_environment.py
+++ b/tests/test_build_environment.py
@@ -1,4 +1,6 @@
 import json
+import pathlib
+import subprocess
 import textwrap
 from unittest.mock import Mock, patch
 
@@ -11,6 +13,68 @@
 from fromager.requirements_file import RequirementType
 
 
+def test_install_keeps_earlier_explicit_bounds_across_phases(
+    tmp_path: pathlib.Path,
+) -> None:
+    """A later transitive upgrade must still satisfy the first phase's cap."""
+    ctx = Mock(pip_constraint_args=[], pip_wheel_server_args=[])
+    with patch.object(build_environment.BuildEnvironment, "_createenv"):
+        env = build_environment.BuildEnvironment(
+            ctx=ctx, req=Requirement("pyarrow==15.0.2"), sdist_root_dir=tmp_path
+        )
+    with patch.object(env, "run") as run:
+        env.install(iter([Requirement("setuptools<82")]))
+        env.install(
+            iter([Requirement("setuptools_scm<8"), Requirement("cython>=0.29")])
+        )
+
+    second_command = run.call_args_list[1].args[0]
+    assert second_command[-3:] == [
+        "setuptools<82",
+        "setuptools_scm<8",
+        "cython>=0.29",
+    ]
+
+
+def test_failed_later_install_reports_earlier_bounds(tmp_path: pathlib.Path) -> None:
+    """A conflict report includes the bound from the first phase."""
+    ctx = Mock(pip_constraint_args=[], pip_wheel_server_args=[])
+    with patch.object(build_environment.BuildEnvironment, "_createenv"):
+        env = build_environment.BuildEnvironment(
+            ctx=ctx, req=Requirement("pyarrow"), sdist_root_dir=tmp_path
+        )
+    failure = subprocess.CalledProcessError(
+        1, "uv pip install", output="Because you require setuptools>=82"
+    )
+    with (
+        patch.object(env, "run", side_effect=[None, failure]),
+        patch("fromager.resolver.resolve", return_value=("", Version("81"))) as resolve,
+    ):
+        build_environment._safe_install(
+            ctx,
+            Requirement("pyarrow"),
+            env,
+            [Requirement("setuptools<82")],
+            RequirementType.BUILD_SYSTEM,
+        )
+        with pytest.raises(build_environment.MissingDependency) as exc:
+            build_environment._safe_install(
+                ctx,
+                Requirement("pyarrow"),
+                env,
+                [Requirement("setuptools>=82")],
+                RequirementType.BUILD_BACKEND,
+            )
+
+    assert resolve.call_count == 1
+    assert [str(req) for req in exc.value.all_reqs] == [
+        "setuptools<82",
+        "setuptools>=82",
+    ]
+    assert "setuptools<82 (earlier bound)" in str(exc.value)
+    assert [str(req) for req in env._installed_requirements] == ["setuptools<82"]
+
+
 @patch("fromager.resolver.resolve")
 def test_missing_dependency_format(
     resolve_dist: Mock,