The documented self-location recipe is unexecutable: filters={"is_caller": true} silently returns [] #92

Summary

The server instructions tell every agent, verbatim, how to answer "which pane am I in?":

Tool results mark is_caller=true; filter list_panes for it to answer 'which pane am I in?' (no whoami tool).

That instruction cannot work. _apply_filters applies filters to the raw libtmux QueryList of Pane objects before serialization, but is_caller is a serialization-time computation — it does not exist as an attribute on libtmux.Pane. libtmux's keygetter returns None for the unknown attribute and filter_lookup then returns False for every row, so the call returns an empty list.

It fails silently: no error, no warning, just []. That is the worst possible failure shape for an LLM — the agent concludes "I'm not in any pane" or, more commonly, falls back to pulling the full unbounded listing and post-filtering in its own context. Which is precisely the expensive path the instruction appears to be steering it away from.

Evidence

The instruction, in _BASE_INSTRUCTIONS:

• src/libtmux_mcp/server.py#L208

". Tool results mark is_caller=true; filter list_panes for it to answer "
"'which pane am I in?' (no whoami tool)."

Filters are applied to un-serialized libtmux objects:

• src/libtmux_mcp/_utils.py#L841 — _apply_filters calls items.filter(**coerced) on the QueryList, then serializes the survivors.

But is_caller is only produced during serialization:

• src/libtmux_mcp/_utils.py#L977 — is_caller=_compute_is_caller(pane) inside _serialize_pane
• src/libtmux_mcp/_utils.py#L153 — _compute_is_caller

And the lookup silently degrades to False rather than raising:

• libtmux/_internal/query_list.py#L535 — keygetter returns None for an unknown attribute; filter_lookup then evaluates False for every pane.

Reproduction

Environment: libtmux-mcp==0.1.0a17, MCP server launched from inside a tmux pane on the default socket.

1. Call list_panes(filters={"is_caller": true}).
2. Observed: [] — an empty list, no error.
3. Expected: exactly one PaneInfo (the caller's own pane); or, failing that, an explicit error stating that is_caller is not a filterable field.
4. Control: call list_panes() with no filters and observe that exactly one row does carry is_caller: true. The field is real; it just cannot be filtered on.

Suggested fix

Two independent changes; both are worth making.

1. Make the advertised filter actually work. is_caller is a synthetic, serialization-time field, so it must be filtered after serialization. Split the filter set in _apply_filters:

_SYNTHETIC_PANE_FIELDS = {"is_caller"}

def _apply_filters(items, filters, serializer):
    coerced = _coerce_dict_arg("filters", filters)
    if not coerced:
        return [serializer(item) for item in items]
    synthetic = {k: v for k, v in coerced.items() if k in _SYNTHETIC_PANE_FIELDS}
    native = {k: v for k, v in coerced.items() if k not in _SYNTHETIC_PANE_FIELDS}
    rows = [serializer(item) for item in (items.filter(**native) if native else items)]
    for key, want in synthetic.items():
        rows = [r for r in rows if getattr(r, key, None) == want]
    return rows

Alternatively, reject unknown filter keys with an ExpectedToolError rather than returning [] — a silent empty result is strictly worse than a loud failure.

2. Stop routing self-location through a full listing at all. Even a working filters={"is_caller": true} still pays a full list-panes -a server-side; it only saves tokens, not tmux work (_apply_filters materializes the whole QueryList and filters in Python). The right primitive is a dedicated self-location tool (where_am_i) — see the companion issue. Once it exists, delete the filter list_panes for it sentence from _BASE_INSTRUCTIONS entirely.

Tests

In tests/test_pane_tools.py (or tests/test_utils.py):

1. Monkeypatch _get_caller_identity to return a deterministic CallerIdentity pointing at a known pane on the test server.
2. Assert list_panes(filters={"is_caller": True}) returns exactly that one pane — this fails today, returning [].
3. Assert list_panes() (unfiltered) contains exactly one row with is_caller is True, proving the field is populated and the bug is in the filter path only.
4. Add a regression test that an unknown/synthetic filter key does not silently yield [].

Related

• is_caller annotation gives false positives across tmux sockets #19 — is_caller annotation gives false positives across tmux sockets (same annotation, adjacent defect)
• Companion issue: add a whoami primitive so the instructions no longer have to advertise this recipe at all.

Environment

• libtmux-mcp: 0.1.0a17
• libtmux: 0.61.0
• fastmcp: 3.4.3 / mcp: 1.28.1
• tmux: 3.7b
• Python: 3.14.0

Cross-references — filed together from a single audit of v0.1.0a17. The self-location gap is the root cause; the rest are what it exposed.

• The documented self-location recipe is unexecutable: filters={"is_caller": true} silently returns [] #92 — [bug] documented self-location recipe is unexecutable (filters={"is_caller": true} → [])
• Add a self-location primitive (where_am_i): agents must dump every pane on the server to learn which pane they are in #93 — [feat] where_am_i self-location primitive + self-first relational routing (root fix)
• The caller's tmux socket is advertised in the instructions but never targeted: an MCP inside tmux -L work queries default #94 — [bug] caller's socket advertised but never targeted; TMUX env race vs. the threadpool
• The list_* family is unbounded, unscoped, unprojected — and explicitly excluded from the response limiter #95 — [perf] list_* unbounded, unscoped, unprojected, excluded from the response limiter
• Surface invocation context (in tmux? server running? which socket?) and make tmux-server auto-start explicit #96 — [feat] invocation context (in tmux? server running?) + explicit tmux-server auto-start
• No self-location API (TMUX_PANE), and the socket_path derivation in Server.__init__ is unreachable dead code libtmux#704 — upstream: no self-location API (TMUX_PANE); unreachable socket_path derivation

(← this issue: #92)