AE Test Bridge MCP

A fake Adobe After Effects bridge listener — same wire protocol as the real thing — for testing bridge client code without needing real After Effects running. Also exposed as an MCP server, so an AI agent can spin one up, script its responses, and inspect what a client under test sent it, entirely conversationally.

Part of a small set of related repos:

• AE_Bridge_MCP — the real MCP server for live After Effects introspection/eval.

• AE_Eval — the same capability as a plain terminal CLI.

• IPC_Client — the general-purpose transport client this repo's MockAEBridge speaks the same protocol as (this repo re-implements the send/recv framing inline rather than depending on that package, to stay a standalone, zero-dependency install).

This repo exists because none of the three above had anything to test against other than mocking the transport itself — a real fake server closes that gap and lets a client under test exercise its actual socket code, not a stand-in for it.

What it does

• ae_test_bridge_mcp/mock_server.py — MockAEBridge: a real TCP server. Register canned responses with .when(match, response) (matched by a substring in the request's script field, or an arbitrary predicate), a .default(response) fallback for anything unmatched, then point a real client at bridge.host/bridge.port. Every request it receives is recorded in bridge.received_requests for assertions.

• ae_test_bridge_mcp/server.py — an MCP server wrapping one MockAEBridge instance behind 7 tools (below), for an AI agent to drive directly instead of writing Python.

It cannot actually evaluate the ExtendScript in a request's script field — there's no JS engine here, it's a test double. You tell it what to say back; it doesn't compute anything.

Related MCP server: SpyNet

Tools exposed over MCP

Install

pip install -e .
# or, for running tests too:
pip install -e ".[dev]"

Zero runtime dependencies.

Usage as a Python library (pytest, or anywhere else)

from ae_test_bridge_mcp import MockAEBridge

with MockAEBridge() as bridge:
    bridge.when("app.project.activeItem.name", {"status": "OK", "result": "Hero Comp"})
    bridge.default({"status": "OK", "result": None})

    # Point whatever you're testing at 127.0.0.1:bridge.port instead of
    # the real After Effects listener's fixed port (45445) --
    # e.g. ipc_client.execute_job(job, port=bridge.port).

    assert bridge.received_requests[-1]["script"] == "..."

Responses can also be a one-argument callable (request_dict) -> dict for dynamic or stateful behavior (an incrementing counter, echoing part of the request back, simulating a value that changes across calls):

bridge.when("echo", lambda req: {"status": "OK", "result": req["script"]})

Rules are checked in registration order — the first match wins.

Usage as an MCP server

{
  "mcpServers": {
    "ae-test-bridge": {
      "command": "python3",
      "args": ["-m", "ae_test_bridge_mcp"]
    }
  }
}

A typical agent-driven session: call start_mock_bridge, note the returned port, tell the client under test to connect to that port instead of the real one, call queue_response for the scenarios you want to exercise, run the client, then call get_received_requests to confirm it sent what you expected.

Design notes

• Hardened tool dispatch. Every MCP tool call is wrapped so a bad argument (wrong type, missing field) returns a normal isError: true result — it can never raise out of handle_call_tool and kill the whole persistent stdio server process. This was a real bug found in a sibling repo during a review pass; fixed here from the start.

• One bridge at a time, by design. The MCP tool surface manages a single module-level MockAEBridge instance — start_mock_bridge fails loudly if one is already running rather than silently leaking the old one. For multiple concurrent fake bridges in the same process, use the MockAEBridge class directly instead of the MCP tools.

• Each accepted connection runs on its own thread, so a slow or stuck client doesn't block other connections. Rule/state mutation is lock-protected so registering rules from the main thread while a request is mid-dispatch is safe.

• A raising response callable never kills the connection silently. If a .when()/.default() callable raises, the client gets back a clean {"status": "ERROR", "error": "... raised ValueError: ..."} instead of the socket just closing with zero bytes.

• A registered response that isn't JSON-serializable (a set, an arbitrary object) gets the same treatment — a clean error naming the problem, not a silent close.

• Declared message length is capped (max_size, default 10 MiB, same class of protection as IPC_Client's own max_size — built independently here since this repo deliberately has zero dependencies) so a peer claiming a huge body can't make the server block trying to read it.

• stop() force-closes in-flight connections rather than waiting out their own read timeout, so tearing down a bridge mid-test doesn't leave a stray thread appending to received_requests after the caller believes it's gone.

Testing

pip install -e ".[dev]"
pytest tests/

Tests use real sockets throughout — no mocking of the socket module — covering rule matching/ordering, dynamic responses, request recording, malformed input on both the raw-socket layer and the MCP tool-argument layer, and the full MCP JSON-RPC stdio loop.

Changelog

• v1.1.0 — Hardening pass from a first-round review: a raising response/default callable now returns a clean ERROR result instead of silently dropping the connection; a non-JSON-serializable response gets the same treatment instead of closing with zero bytes; incoming messages are capped at max_size (default 10 MiB) so a peer can't make the server block trying to read an oversized declared length; stop() now force-closes any in-flight connections and joins their threads instead of waiting out their own read timeout. Also added the clear_rules tool/method for symmetry with clear_received_requests.

• v1.0.0 — Initial extraction, as a mock-bridge test double inspired by (but not a direct port of) Dimension's own test_ae_bridge_mcp.py patterns.

License

MIT — see LICENSE.