diff --git a/.claude/hooks/reflow-check.sh b/.claude/hooks/reflow-check.sh
new file mode 100755
index 0000000000..3530ec0fe1
--- /dev/null
+++ b/.claude/hooks/reflow-check.sh
@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+# Advisory check, run before an agent commits: flag docstring and comment lines
+# that break mid-phrase, looking only at the lines being added.
+# Never blocks -- the check is a heuristic, and the agent judges each hit.
+set -uo pipefail
+cd "${CLAUDE_PROJECT_DIR:-.}" || exit 0
+command -v python3 >/dev/null 2>&1 || exit 0
+python3 .claude/hooks/reflow_check.py --staged 2>/dev/null || true
+exit 0
diff --git a/.claude/hooks/reflow_check.py b/.claude/hooks/reflow_check.py
new file mode 100644
index 0000000000..072f9d0188
--- /dev/null
+++ b/.claude/hooks/reflow_check.py
@@ -0,0 +1,135 @@
+"""Flag docstring and comment lines that break in the middle of a phrase.
+
+The repo convention (see .github/instructions/docstrings.instructions.md) is that
+each physical line of a docstring or comment ends at punctuation,
+so that review comments and text search stay stable.
+The mechanical form of that rule:
+inside a multi-line docstring or comment block, every line but the last ends in punctuation.
+
+This is a heuristic, and deliberately an advisory one --
+docstrings here also hold OpenAPI YAML, shell commands and bullet lists,
+none of which are prose and none of which end in punctuation.
+The agent reading the output is expected to judge each hit rather than obey it.
+
+Usage:
+    python reflow_check.py <file>...          # whole files
+    python reflow_check.py --staged           # only lines added in the git index
+"""
+
+from __future__ import annotations
+
+import ast
+import re
+import subprocess
+import sys
+from pathlib import Path
+
+PUNCTUATION = (".", ",", ";", ":", "!", "?", ")", "]", "}", "-", "—", "–")
+
+#: Lines that are not prose, and so are not expected to end in punctuation.
+NOT_PROSE = re.compile(
+    r"""^(
+      \s*[-*+]\s          # bullet list item
+    | \s*\.\.\s           # RST directive
+    | \s*>>>              # doctest
+    | \s*\$               # shell prompt
+    | \s*\w[\w\s-]*:\s*\S # "key: value", i.e. embedded YAML
+    | \s*[\[{]            # start of an embedded JSON/dict literal
+    )""",
+    re.VERBOSE,
+)
+
+
+def _offending_lines(block: list[str], first_line: int) -> list[tuple[int, str]]:
+    """Lines of one block that end mid-phrase, ignoring the last line of the block."""
+    found = []
+    body = [(i, ln) for i, ln in enumerate(block) if ln.strip()]
+    for i, line in body[:-1]:
+        text = re.sub(r"^\s*#:?\s?", "", line).strip().strip('"').strip("'").strip()
+        if not text or text.endswith("\\") or NOT_PROSE.match(line):
+            continue
+        if not text.endswith(PUNCTUATION):
+            found.append((first_line + i, line.strip()[:110]))
+    return found
+
+
+def check(path: Path) -> list[tuple[int, str]]:
+    """Every mid-phrase line break in one Python file."""
+    try:
+        source = path.read_text()
+        tree = ast.parse(source)
+    except (SyntaxError, UnicodeDecodeError, OSError):
+        return []
+
+    found: list[tuple[int, str]] = []
+    for node in ast.walk(tree):
+        if isinstance(
+            node, (ast.Module, ast.FunctionDef, ast.AsyncFunctionDef, ast.ClassDef)
+        ):
+            docstring = ast.get_docstring(node, clean=False)
+            if docstring and "\n" in docstring:
+                first = node.body[0].lineno if node.body else 1
+                found += _offending_lines(docstring.split("\n"), first)
+
+    block: list[str] = []
+    start = 0
+    for number, line in enumerate(source.split("\n"), start=1):
+        if line.strip().startswith("#"):
+            start = start or number
+            block.append(line)
+            continue
+        if len(block) > 1:
+            found += _offending_lines(block, start)
+        block, start = [], 0
+    if len(block) > 1:
+        found += _offending_lines(block, start)
+    return found
+
+
+def staged_lines() -> dict[str, set[int]]:
+    """Line numbers added to each staged Python file."""
+    diff = subprocess.run(
+        ["git", "diff", "--cached", "-U0", "--", "*.py"],
+        capture_output=True,
+        text=True,
+    ).stdout
+    added: dict[str, set[int]] = {}
+    current = None
+    for line in diff.split("\n"):
+        if line.startswith("+++ b/"):
+            current = line[6:]
+            added.setdefault(current, set())
+        header = re.match(r"^@@ -\S+ \+(\d+)(?:,(\d+))?", line)
+        if header and current:
+            first = int(header.group(1))
+            added[current].update(range(first, first + int(header.group(2) or 1)))
+    return added
+
+
+def main() -> int:
+    if "--staged" in sys.argv:
+        targets = staged_lines()
+    else:
+        targets = {argument: None for argument in sys.argv[1:]}
+
+    hits = []
+    for name, wanted in targets.items():
+        path = Path(name)
+        if not path.exists():
+            continue
+        for number, text in sorted(set(check(path))):
+            if wanted is None or number in wanted:
+                hits.append(f"{name}:{number}: {text}")
+
+    if hits:
+        print("Possible mid-phrase line breaks in docstrings or comments:")
+        print("\n".join(f"  {hit}" for hit in hits))
+        print(
+            "\nEach line above should end at punctuation, or be reflowed so it does."
+            "\nIgnore any that are not prose (embedded YAML, shell commands, list items)."
+        )
+    return 0  # advisory: never blocks the commit
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/.claude/settings.json b/.claude/settings.json
index 5e8fc5e6ae..1a42941c25 100644
--- a/.claude/settings.json
+++ b/.claude/settings.json
@@ -34,6 +34,12 @@
       {
         "matcher": "Bash",
         "hooks": [
+          {
+            "type": "command",
+            "command": "bash \"$CLAUDE_PROJECT_DIR/.claude/hooks/reflow-check.sh\"",
+            "if": "Bash(git commit:*)",
+            "timeout": 30
+          },
           {
             "type": "command",
             "command": "bash \"$CLAUDE_PROJECT_DIR/.claude/hooks/pre-commit-check.sh\"",
@@ -56,4 +62,4 @@
       }
     ]
   }
-}
\ No newline at end of file
+}