diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3ff39a63..6c4a6082 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -52,6 +52,12 @@ All notable changes to vouch are documented here. Format follows
 - demo image build: `hatch_build.py` (the build hook pyproject.toml
   declares for console bundling) is copied into the docker build context;
   the image had been unbuildable since the hook landed (#474).
+- `vouch capture finalize-all` reads the session id off the SessionStart
+  hook payload on stdin, the same wire `capture finalize` reads at
+  SessionEnd. the shipped hook passes no `--session-id` and nothing sets
+  `VOUCH_SESSION_ID`, so the sweep silently no-opped on every session
+  start and buffers orphaned by sessions that ended without SessionEnd
+  were never rolled up into pending proposals (#492).
 
 ## [1.3.0] — 2026-07-14
 
diff --git a/src/vouch/cli.py b/src/vouch/cli.py
index 5a8bd3ce..b2cf7f23 100644
--- a/src/vouch/cli.py
+++ b/src/vouch/cli.py
@@ -2428,11 +2428,34 @@ def capture_answer_cmd(session_id: str | None) -> None:
 
 
 @capture.command("finalize-all")
-@click.option("--session-id", default=None, help="Current session id (else env VOUCH_SESSION_ID).")
+@click.option(
+    "--session-id", default=None,
+    help="Current session id (else stdin payload, else env VOUCH_SESSION_ID).",
+)
 @click.option("--max-age-seconds", type=float, default=3600.0, help="Max age in seconds.")
 def capture_finalize_all_cmd(session_id: str | None, max_age_seconds: float) -> None:
-    """Finalize all capture buffers except current session (SessionStart cleanup)."""
-    sid = session_id or os.environ.get("VOUCH_SESSION_ID") or ""
+    """Finalize all buffers except current session (SessionStart hook payload on stdin)."""
+    # the shipped SessionStart hook passes no --session-id and nothing sets
+    # VOUCH_SESSION_ID, so the current session id has to come off the hook
+    # payload on stdin — the same wire `capture finalize` reads at SessionEnd.
+    # without it sid is always "" and the sweep no-ops, leaving the buffers of
+    # sessions that died without SessionEnd on disk forever.
+    payload: dict[str, Any] = {}
+    if not sys.stdin.isatty():
+        raw = sys.stdin.read()
+        if raw.strip():
+            try:
+                loaded = json.loads(raw)
+                if isinstance(loaded, dict):
+                    payload = loaded
+            except json.JSONDecodeError:
+                payload = {}
+    sid = (
+        session_id
+        or str(payload.get("session_id") or "")
+        or os.environ.get("VOUCH_SESSION_ID")
+        or ""
+    )
     if not sid:
         # No session ID provided; silently succeed
         _emit_json({"finalized": [], "skipped_recent": [], "skipped_current": []})
diff --git a/tests/test_capture.py b/tests/test_capture.py
index e40ca0ed..9d8144e1 100644
--- a/tests/test_capture.py
+++ b/tests/test_capture.py
@@ -413,6 +413,7 @@ def commands(event: str) -> list[str]:
     assert any("capture observe" in c for c in commands("PostToolUse"))
     assert any("capture finalize" in c for c in commands("SessionEnd"))
     assert any("capture banner" in c for c in commands("SessionStart"))
+    assert any("capture finalize-all" in c for c in commands("SessionStart"))
 
 
 def test_capture_finalize_all_cmd_with_old_buffers(tmp_path: Path, monkeypatch) -> None:
@@ -477,6 +478,70 @@ def test_capture_finalize_all_cmd_reads_session_from_env(tmp_path: Path, monkeyp
     assert current_sess in output["skipped_current"]
 
 
+def test_capture_finalize_all_cmd_reads_session_from_stdin(tmp_path: Path) -> None:
+    """The shipped SessionStart hook passes no --session-id, so the current
+    session id has to come off the hook payload on stdin."""
+    import os
+    import time as time_mod
+
+    store = _make_store(tmp_path)
+    current_sess = "current-from-stdin"
+    old_sess = "old-session"
+
+    old_path = cap.buffer_path(store, old_sess)
+    old_path.parent.mkdir(parents=True, exist_ok=True)
+    old_path.write_text(
+        '{"ts": 1.0, "tool": "Read", "summary": "test1"}\n'
+        '{"ts": 2.0, "tool": "Read", "summary": "test2"}\n'
+        '{"ts": 3.0, "tool": "Read", "summary": "test3"}\n'
+    )
+    old_mtime = time_mod.time() - 7200
+    os.utime(old_path, (old_mtime, old_mtime))
+
+    curr_path = cap.buffer_path(store, current_sess)
+    curr_path.write_text('{"ts": 1.0, "tool": "Read", "summary": "test"}\n')
+
+    result = CliRunner().invoke(
+        cli, ["capture", "finalize-all"],
+        input=_json.dumps({"session_id": current_sess, "cwd": str(tmp_path)}),
+        env={"VOUCH_KB_PATH": str(store.kb_dir)},
+    )
+
+    assert result.exit_code == 0
+    output = _json.loads(result.output)
+    assert old_sess in output["finalized"]
+    assert current_sess in output["skipped_current"]
+
+
+def test_capture_finalize_all_cmd_survives_malformed_stdin(tmp_path: Path) -> None:
+    """A hook must never fail the host turn on an unparseable payload."""
+    store = _make_store(tmp_path)
+    result = CliRunner().invoke(
+        cli, ["capture", "finalize-all"],
+        input="not json at all",
+        env={"VOUCH_KB_PATH": str(store.kb_dir)},
+    )
+    assert result.exit_code == 0
+    assert _json.loads(result.output)["finalized"] == []
+
+
+def test_capture_finalize_all_cmd_flag_beats_stdin(tmp_path: Path) -> None:
+    """An explicit --session-id wins over the stdin payload."""
+    store = _make_store(tmp_path)
+    curr_path = cap.buffer_path(store, "from-flag")
+    curr_path.parent.mkdir(parents=True, exist_ok=True)
+    curr_path.write_text('{"ts": 1.0, "tool": "Read", "summary": "test"}\n')
+
+    result = CliRunner().invoke(
+        cli, ["capture", "finalize-all", "--session-id", "from-flag"],
+        input=_json.dumps({"session_id": "from-stdin"}),
+        env={"VOUCH_KB_PATH": str(store.kb_dir)},
+    )
+
+    assert result.exit_code == 0
+    assert "from-flag" in _json.loads(result.output)["skipped_current"]
+
+
 def test_capture_finalize_all_cmd_silent_on_no_kb(tmp_path: Path, monkeypatch) -> None:
     """CLI command should silently succeed if KB not found."""
     runner = CliRunner()