diff --git a/graphify/file_slice.py b/graphify/file_slice.py
index 30dc49cfbd..670c36231b 100644
--- a/graphify/file_slice.py
+++ b/graphify/file_slice.py
@@ -26,7 +26,19 @@
 # `_file_to_text` is a straight ``read_text`` (so a char range matches the bytes
 # the model is shown). Deliberately excludes code (.py, .ts, ...) and binary
 # docs (.pdf) — those are never sliced.
-_SPLITTABLE_TEXT_SUFFIXES = frozenset({".md", ".mdx", ".markdown", ".txt", ".rst"})
+#
+# This set has to keep pace with ``detect.DOC_EXTENSIONS``: anything classified
+# as a document reaches the semantic pass, and anything the pass sees that is
+# NOT listed here is silently cut at ``_FILE_CHAR_CAP`` by ``_read_files``. The
+# two lists drifted as DOC_EXTENSIONS grew — .qmd, .skill, .html, .yaml and .yml
+# were documents that never got sliced, so a 38k-character one reached the model
+# as its first 20k with no warning and no partial marker (#2900).
+# ``tests/test_oversized_document_slicing.py`` pins the relationship so a future
+# addition to DOC_EXTENSIONS fails loudly instead of quietly losing content.
+_SPLITTABLE_TEXT_SUFFIXES = frozenset({
+    ".md", ".mdx", ".markdown", ".txt", ".rst",
+    ".qmd", ".skill", ".html", ".yaml", ".yml",
+})
 
 # Boundary preferences, strongest first. A Markdown heading (``\n#``) keeps a
 # section with its title; a blank line keeps a paragraph intact; a bare newline
diff --git a/tests/test_oversized_document_slicing.py b/tests/test_oversized_document_slicing.py
new file mode 100644
index 0000000000..04e151bbbd
--- /dev/null
+++ b/tests/test_oversized_document_slicing.py
@@ -0,0 +1,114 @@
+"""Every document type that reaches the semantic pass must be sliceable.
+
+`_read_files` caps each unit at `_FILE_CHAR_CAP` (20,000 characters) before
+joining it into the user message. `expand_oversized_files` exists so an
+oversized document is split into contiguous `FileSlice`s that together cover the
+whole file — but it only splits suffixes listed in `_SPLITTABLE_TEXT_SUFFIXES`.
+
+That list and `detect.DOC_EXTENSIONS` drifted. `.qmd`, `.skill`, `.html`,
+`.yaml` and `.yml` were classified as documents, so they reached the model, and
+were not splittable, so a 38,411-character one arrived as its first 20,000
+characters — no warning, no `_partial_files` marker, nothing in the report
+(#2900).
+
+The contract test below is the point of this file: it fails when a new suffix is
+added to `DOC_EXTENSIONS` without deciding how it gets sliced, so the two lists
+cannot drift apart again silently.
+"""
+from pathlib import Path
+
+import pytest
+
+from graphify.detect import DOC_EXTENSIONS
+from graphify.file_slice import (
+    _SPLITTABLE_TEXT_SUFFIXES,
+    expand_oversized_files,
+    is_splittable_text,
+    read_slice_text,
+    slice_boundaries,
+)
+from graphify.llm import _FILE_CHAR_CAP, _read_files
+
+# Document suffixes whose bytes are NOT what the model is shown, so a character
+# range over the raw file would be meaningless. `_file_to_text` routes these
+# through a converter; slicing them needs that converter threaded through
+# `read_slice_text` first, which is deliberately out of scope here.
+BINARY_DOC_SUFFIXES = frozenset({".pdf"})
+
+BIG = "# Heading\n\n" + ("The parser calls the tokenizer. " * 1200)
+
+
+# ---------------------------------------------------------------------------
+# The contract
+# ---------------------------------------------------------------------------
+
+def test_every_text_document_type_is_splittable():
+    """The guard against re-drift: a plain-text document type that reaches the
+    semantic pass must be sliceable, or an oversized one loses its tail."""
+    missing = sorted(
+        (DOC_EXTENSIONS - BINARY_DOC_SUFFIXES) - _SPLITTABLE_TEXT_SUFFIXES
+    )
+    assert not missing, (
+        f"these document types reach the LLM but are never sliced, so anything "
+        f"over {_FILE_CHAR_CAP} characters is silently truncated: {missing}"
+    )
+
+
+@pytest.mark.parametrize(
+    "ext", sorted(DOC_EXTENSIONS - BINARY_DOC_SUFFIXES)
+)
+def test_an_oversized_document_reaches_the_model_whole(tmp_path, ext):
+    f = tmp_path / f"doc{ext}"
+    f.write_text(BIG, encoding="utf-8")
+    units = expand_oversized_files([f], _FILE_CHAR_CAP)
+    assert len(units) > 1, f"{ext} was not sliced; its tail never reaches the model"
+    # The slices tile the file exactly — no gap, no overlap, nothing dropped.
+    assert "".join(read_slice_text(u) for u in units) == BIG
+
+
+# ---------------------------------------------------------------------------
+# Slicing itself is unchanged for the types that already worked
+# ---------------------------------------------------------------------------
+
+def test_a_small_document_is_still_passed_through_whole(tmp_path):
+    f = tmp_path / "small.qmd"
+    f.write_text("# tiny\n\nnothing to slice\n", encoding="utf-8")
+    units = expand_oversized_files([f], _FILE_CHAR_CAP)
+    assert units == [f], "a file under the cap must pass through as a plain Path"
+
+
+def test_binary_documents_are_still_not_sliced(tmp_path):
+    """A PDF's bytes are not what the model is shown, so slicing the raw file
+    would be wrong. It must keep passing through untouched."""
+    f = tmp_path / "paper.pdf"
+    f.write_bytes(b"%PDF-1.4\n" + b"x" * 40_000)
+    assert not is_splittable_text(f)
+    assert expand_oversized_files([f], _FILE_CHAR_CAP) == [f]
+
+
+def test_code_files_are_still_not_sliced(tmp_path):
+    f = tmp_path / "mod.py"
+    f.write_text("def f():\n    pass\n" * 4000, encoding="utf-8")
+    assert not is_splittable_text(f)
+    assert expand_oversized_files([f], _FILE_CHAR_CAP) == [f]
+
+
+def test_slices_stay_within_the_cap(tmp_path):
+    for start, end in slice_boundaries(BIG, _FILE_CHAR_CAP):
+        assert end - start <= _FILE_CHAR_CAP
+
+
+# ---------------------------------------------------------------------------
+# End to end through the prompt builder
+# ---------------------------------------------------------------------------
+
+@pytest.mark.parametrize("ext", [".qmd", ".html", ".yaml", ".yml", ".skill"])
+def test_the_tail_of_a_big_document_reaches_the_prompt(tmp_path, ext):
+    """The symptom a user would notice: content past 20k was invisible to the
+    semantic pass, so nothing in the tail could ever become a node."""
+    marker = "UNIQUE_TAIL_MARKER_XYZZY"
+    f = tmp_path / f"doc{ext}"
+    f.write_text(BIG + "\n\n" + marker + "\n", encoding="utf-8")
+    units = expand_oversized_files([f], _FILE_CHAR_CAP)
+    joined = "".join(_read_files([u], tmp_path) for u in units)
+    assert marker in joined