plat 349

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PLAT-349 — Chat delivery has no browser-side observability

Field	Value
Status	implemented locally; deployment and live acceptance pending
Priority	P2 observability
Owner	frontend chat delivery / server telemetry
Reported	2026-09-22
Related	PLAT-292

Gap

When chat delivery misbehaves — a stuck stream, a late paint, SSE-vs-poll disagreement — only the server side is measurable. The browser milestones (when an event arrived, over which transport, when it was processed and painted) never leave the client, so delivery investigations guess from server logs and the Network tab.

Fix

frontend/src/utils/chatDeliveryTelemetry.ts records content-free milestones for visible chat events (user_message, conversation_thinking, llm_generation_end, unified_completion, conversation_error, agent_error) at four phases: sse_received, poll_received, catchup_received, processed. Call sites in ChatArea.tsx (SSE, poll, foreground catchup, event processor) and TerminalEventTranscript.tsx enqueue into a 100ms-batched, 100-event queue; the batch POSTs to POST /api/client-telemetry/chat-delivery fire-and-forget with no retry, so diagnostics can never delay delivery.

The server (agent_go/cmd/server/client_chat_telemetry.go) accepts a fixed schema with DisallowUnknownFields, allowlisted phases, and bounded fields. Message text, tool arguments, and arbitrary metadata are never accepted or logged; each event lands as one [CLIENT_CHAT_TIMELINE] JSON log line. The endpoint is excluded from request tracing to avoid noise.

The painted phase is reserved in the schema but not recorded yet; the first skepticism to resolve live is whether the endpoint needs authentication or rate limiting, since an unauthenticated log sink is a volume vector (payloads are JSON-escaped, so no log forging).

Verification

• TestClientChatTelemetryAcceptsContentFreeTimeline, TestClientChatTelemetryRejectsUnknownFieldsOfMeaning, TestClientChatTelemetryRejectsMessageContent pass.
• chatDeliveryTelemetry.test.ts passes.
• Live acceptance pending: confirm timeline lines correlate with a real slow-delivery report.