[Deep-Sixed]

Follow-up to #23, covering two gaps it left: the refusal emitted no routing event, and the caller-field rule wasn't documented.

Problem

#23 answers a non-function tool on a dispatch-path alias with 400 unsupported_tool before any upstream call. The check runs in app.py and covers dispatch, free and jev aliases plus direct free models. It emitted no routing event, so the refusal never reached /admin/events or the telemetry sink. Fusion and jev-router record their own caller-field refusals. docs/architecture.md also never described the dispatch-path rule. It documents jev-router's allowlist only.

Change

• router/dispatch.py: new invalid_request_event(...), built like unauthorized_event: status 400, outcome invalid_request, no attempts, no target. invalid_request is the outcome jev-router already uses for the same situation: "the caller sent a field the alias does not accept; nothing went upstream". The admin UI already styles it.
• app.py: the unsupported_tool refusal now emits that event with the caller's identity and the request's config version and checksum. The 400 body gains a request_id that matches the event. This is additive: message and reason are unchanged.
• docs/architecture.md: a new paragraph in Dispatch control plane covers:
  • the body a provider receives is limited to the CALLER_FIELDS allowlist, with model replaced by the target
  • what is dropped silently (models, route, plugins, preset, provider, …) and why
  • the function-tool refusal and its routing event
  • how fusion and jev-router differ

Not changed: fusion still records its own caller-field refusal as upstream_error/400, from before invalid_request existed. The doc doesn't name fusion's outcome, so aligning it later won't require a doc change.

Tests

tests/test_caller_fields.py::test_server_tool_refusal_is_a_routing_event covers an openrouter:fusion entry in tools and an openrouter:fusion tool_choice. Each case checks:

• the response is 400 and nothing reaches the mocked upstream
• exactly one event is emitted, with outcome invalid_request and status 400
• the event's alias, mode, identity and config version/checksum are correct
• the event's request_id equals the one in the response

The test fails on current main and passes with this change. ./scripts/ci.sh (ruff + pytest) passes with 433 tests.

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