diff --git a/src/chiplog/sinks/local_file.py b/src/chiplog/sinks/local_file.py
index cf3c541..82bf300 100644
--- a/src/chiplog/sinks/local_file.py
+++ b/src/chiplog/sinks/local_file.py
@@ -63,8 +63,16 @@ class _DailyFileState:
 
     The rolling hash is only equal to the file's real SHA-256 if the context is
     fed the lines in the SAME ORDER the file received them. Callers must not let
-    two appends interleave — `LocalFileSink` guarantees that with `_write_lock`,
-    which is why `append_line` does no locking of its own.
+    two appends interleave — `LocalFileSink` serialises them with `_write_lock`,
+    which is why `append_line` does no locking of its own. That guarantee stops
+    at the process boundary: `_write_lock` is a `threading.Lock`, so it orders
+    appends from threads and recorders sharing ONE process and nothing else. A
+    second process appending to the same log is not serialised by this sink, and
+    the result is a forked chain that `verify` reports as CHAIN_BREAK over
+    evidence that is in fact intact. `emit.py` states the same limit for the
+    recorder; the only cross-process serialisation in the library is the `flock`
+    in the `chiplog hook-record` subprocess, which covers the Claude Code hook
+    path and nothing else.
     """
 
     def __init__(self, path: Path) -> None:
diff --git a/tests/test_sink_lock_scope_guard.py b/tests/test_sink_lock_scope_guard.py
new file mode 100644
index 0000000..a9d66f3
--- /dev/null
+++ b/tests/test_sink_lock_scope_guard.py
@@ -0,0 +1,74 @@
+"""Structural: where the sink claims to serialise appends, it must name the boundary.
+
+`_DailyFileState` tells its caller that interleaved appends would corrupt the
+rolling hash, and then reassures them: "`LocalFileSink` guarantees that with
+`_write_lock`, which is why `append_line` does no locking of its own." Read
+that and you conclude serialisation is handled. It is a `threading.Lock`. It
+handles nothing across processes.
+
+`emit.py` states the truth in the same codebase — "What is NOT guaranteed: ...
+two processes appending to the same log" — so the library knows. It documents
+the limit accurately in one file and promises the opposite in another, and a
+caller reading the sink's own docstring to decide whether they need their own
+locking gets the wrong answer.
+
+That is not hypothetical. Two bosun workers appending to one chain produced 7
+forked chains across 5825 records (forensics, 2026-07-17). Nothing was
+corrupted — 0 dangling `prev_hash`, every resolvable signature verified — but
+`verify` reported CHAIN_BREAK over intact evidence, which is worse than a clean
+failure: it teaches the operator to disregard the verifier.
+
+This guard is prose-shaped because the defect is prose-shaped, the same way
+`test_report_claims_guard.py` is. A docstring that makes a concurrency promise
+has to say where the promise stops; the word this checks for is the cheapest
+proxy for that, and if a rewrite drops it the test should fail and make someone
+re-read the claim.
+
+It does NOT pin that appends are cross-process safe — they are not. Making them
+so is a real change to the sink's write path, and it is deliberately not this.
+"""
+
+from __future__ import annotations
+
+import threading
+from pathlib import Path
+
+from chiplog.sinks import local_file
+
+
+def _the_promise() -> str:
+    """The paragraph that makes the serialisation claim, not the whole docstring.
+
+    Anchoring to the paragraph matters: the docstring elsewhere says "previous
+    process wrote to it today", so a naive search for the word across the whole
+    text passes while the claim itself stays unqualified — which is how the
+    first draft of this guard went green against a docstring that still lied.
+    """
+    doc = local_file._DailyFileState.__doc__
+    assert doc is not None, "_DailyFileState lost its docstring"
+    paras = [p for p in doc.split("\n\n") if "_write_lock" in p]
+    assert len(paras) == 1, (
+        "this guard is anchored to the one paragraph that promises "
+        f"serialisation; found {len(paras)}. If the claim moved or split, "
+        "move the guard with it rather than loosening it"
+    )
+    return paras[0]
+
+
+def test_the_lock_backing_the_promise_is_process_local(tmp_path: Path) -> None:
+    # The premise of this guard. If this ever becomes a cross-process lock,
+    # the docstring rule below should be revisited, not silenced.
+    sink = local_file.LocalFileSink(dir=tmp_path / "audit")
+    assert isinstance(sink._write_lock, type(threading.Lock()))
+
+
+def test_serialisation_promise_names_its_boundary() -> None:
+    promise = _the_promise()
+    assert "process" in promise.lower(), (
+        "_DailyFileState tells callers LocalFileSink serialises appends for them, "
+        "and _write_lock is a threading.Lock — so the promise holds only within "
+        "one process. Say so in the paragraph that makes it. A caller who "
+        "believes the unqualified version writes from a second process and gets "
+        "a forked chain that verify reports as CHAIN_BREAK over intact evidence. "
+        "See emit.py's accurate statement of the same limit.\n\n" + promise
+    )