Prompt Queue Backpressure E2E Test Plan

Scope

Validate per-session prompt admission backpressure for qwen serve, REST clients, ACP HTTP clients, and the TypeScript SDK.

Baseline

1. Start qwen serve with defaults.
2. Create a session.
3. Send one prompt.
4. Expected: prompt is accepted and the session emits normal turn events.

Full Queue

1. Start qwen serve with defaults.
2. Create a session.
3. Hold one prompt active and enqueue four more prompts for the same session.
4. Send the sixth prompt.
5. Expected: the sixth request returns HTTP 503, Retry-After: 5, and code: "prompt_queue_full". The body includes sessionId, limit: 5, and pendingCount: 5. The response does not include promptId.

Release Then Recover

1. Fill the default five pending prompt slots.
2. Let the active prompt complete or fail.
3. Send another prompt.
4. Expected: the new prompt is accepted after the previous slot is released.

ACP HTTP

1. Send session/prompt through /acp while the same session has five pending prompts.
2. Expected: JSON-RPC returns stable error data with errorKind: "prompt_queue_full", limit, pendingCount, and sessionId.

SDK Local Guard

1. Construct DaemonClient with maxPendingPromptsPerSession: 1.
2. Use a daemon or fetch mock that accepts the first prompt with 202 and keeps its SSE stream pending.
3. Call prompt() again for the same session.
4. Expected: the SDK throws DaemonPendingPromptLimitError and does not issue the second fetch.

Disabled Cap

1. Start qwen serve --max-pending-prompts-per-session 0.
2. Create a session.
3. Enqueue more than five prompts for the same session.
4. Expected: admission is not rejected by the prompt queue cap. /capabilities.limits.maxPendingPromptsPerSession is null.

Verification Commands

cd packages/acp-bridge && npx vitest run src/bridge.test.ts
cd packages/cli && npx vitest run src/serve/server.test.ts src/serve/acpHttp/transport.test.ts
cd packages/sdk-typescript && npx vitest run test/unit/DaemonClient.test.ts test/unit/DaemonSessionClient.test.ts
npm run build && npm run typecheck