diff --git a/tests/tools/test_read_extract.py b/tests/tools/test_read_extract.py
index 4b656e2de6b7a..038d073bb3e8f 100644
--- a/tests/tools/test_read_extract.py
+++ b/tests/tools/test_read_extract.py
@@ -134,6 +134,65 @@ class TestAnydocExtraction(unittest.TestCase):
         self.assertEqual(text, "hello\n")
 
 
+class TestAnydocSizeCap(unittest.TestCase):
+    """Oversized inputs must be rejected before anydoc converts them.
+    Uses a fake binding so it runs regardless of local install state."""
+
+    def setUp(self):
+        from tools import read_extract
+
+        self.rex = read_extract
+        self._saved_module = read_extract._anydoc_module
+        self._saved_cap = read_extract.MAX_ANYDOC_BYTES
+        self.tmp = tempfile.mkdtemp(prefix="rex_cap_")
+        self.calls = []
+
+        class _FakeAnydoc:
+            def to_markdown(_self, path):
+                self.calls.append(path)
+                return "converted\n"
+
+        read_extract._anydoc_module = _FakeAnydoc()
+
+    def tearDown(self):
+        import shutil
+
+        self.rex._anydoc_module = self._saved_module
+        self.rex.MAX_ANYDOC_BYTES = self._saved_cap
+        shutil.rmtree(self.tmp, ignore_errors=True)
+
+    def _write(self, name, size):
+        p = os.path.join(self.tmp, name)
+        with open(p, "wb") as fh:
+            fh.write(b"x" * size)
+        return p
+
+    def test_oversized_file_rejected_before_conversion(self):
+        from tools.read_extract import _extract_anydoc
+
+        self.rex.MAX_ANYDOC_BYTES = 10
+        p = self._write("big.pdf", 11)
+        with self.assertRaises(ExtractionError) as ctx:
+            _extract_anydoc(p)
+        self.assertIn("too large", str(ctx.exception))
+        self.assertEqual(self.calls, [])
+
+    def test_file_at_limit_converts(self):
+        from tools.read_extract import _extract_anydoc
+
+        self.rex.MAX_ANYDOC_BYTES = 10
+        p = self._write("ok.pdf", 10)
+        self.assertEqual(_extract_anydoc(p), "converted\n")
+        self.assertEqual(self.calls, [p])
+
+    def test_missing_file_raises_extraction_error(self):
+        from tools.read_extract import _extract_anydoc
+
+        with self.assertRaises(ExtractionError):
+            _extract_anydoc(os.path.join(self.tmp, "gone.pdf"))
+        self.assertEqual(self.calls, [])
+
+
 class TestAnydocAbsent(unittest.TestCase):
     """The absent-dep contract, verified regardless of local install state
     by forcing the cached module handle to None."""
diff --git a/tools/read_extract.py b/tools/read_extract.py
index 1230e986adec3..0ec42a17308db 100644
--- a/tools/read_extract.py
+++ b/tools/read_extract.py
@@ -14,6 +14,7 @@ from __future__ import annotations
 
 import importlib
 import json
+import os
 import posixpath
 import threading
 import time
@@ -34,6 +35,10 @@ ANYDOC_EXTENSIONS = frozenset({
     ".rtf", ".epub", ".pdf",
 })
 MAX_XLSX_BYTES = 50 * 1024 * 1024
+# Refuse to convert huge documents. anydoc loads the whole file through its
+# Rust core with no streaming, and the read_file char budget only applies
+# after conversion, so an unbounded input can pin a tool turn and spike RAM.
+MAX_ANYDOC_BYTES = 50 * 1024 * 1024
 _MAX_XLSX_ROWS_PER_SHEET = 5000
 _MAX_XLSX_COLS = 256
 
@@ -120,6 +125,14 @@ def _extract_anydoc(path: str) -> str:
     mod = _anydoc()
     if mod is None:
         raise ExtractionError(f"Unsupported document type: {path!r}")
+    try:
+        size = os.path.getsize(path)
+    except OSError as exc:
+        raise ExtractionError(str(exc)) from exc
+    if size > MAX_ANYDOC_BYTES:
+        raise ExtractionError(
+            f"Document too large to convert ({size:,} bytes, limit is {MAX_ANYDOC_BYTES:,})"
+        )
     try:
         text = mod.to_markdown(path)
     except OSError as exc: