feat(studio): add Stop run button + graceful CLI interrupt — pairs with eval resume

[christso]

Objective

Add a "Stop run" affordance — both a UI button on /jobs/:runId and graceful CLI signal handling — so users can interrupt a long-running eval without orphaning the subprocess or losing partial results. Today there is no programmatic stop:

• Studio: the launch endpoint stores process: ChildProcess per run but exposes no DELETE/stop route. Closing the browser tab leaves the CLI subprocess running until it completes naturally.
• CLI: no top-level SIGINT/SIGTERM handler. Ctrl+C hard-kills the eval mid-test. The only child.kill() calls in the codebase live inside agent providers (claude-cli, codex-cli, pi-cli) terminating their own per-test subprocess on timeout — not the orchestrator handling user interrupt.

This pairs naturally with the resume feature shipped in #1220: today the workflow for "I want to bail on this run" is kill the terminal → resume in Studio. With a Stop button it becomes click Stop → click Resume, all without leaving the browser.

Current state — what already works

• Per-test results are flushed row-by-row into index.jsonl as tests complete, so any partial state is durable on disk and resumable. The "stop" feature does not need to invent persistence — only graceful termination.
• eval-runner.ts already retains a process: ChildProcess reference per Studio-launched run, so the server can process.kill('SIGTERM') once an endpoint is added.

Proposed changes

1. CLI signal handler

Register SIGINT / SIGTERM handlers at the top of apps/cli/src/commands/eval/run-eval.ts (or wherever the orchestrator entry point lives):

• On first signal: set a stopRequested flag, allow in-flight tests to finish (they're already isolated), then exit cleanly with a non-zero code distinguishable from "crashed."
• On second signal: hard exit (so users can still escape if a test is hung).
• Print a concise message: Stop requested — waiting for N in-flight test(s) to finish (Ctrl+C again to force-quit).

2. Studio API: DELETE /api/eval/run/:id

Add a route that:

• 404s if the run id is unknown.
• 403s in read-only mode (matches the existing guard on POST).
• 409s (or 200 with {stopped: false}) if the run is already terminal.
• Otherwise calls run.process?.kill('SIGTERM'), sets run.status = 'stopping', returns 202.

The existing child.on('close') handler will flip the status to failed/finished when the CLI exits.

Add benchmark-scoped variant DELETE /api/benchmarks/:benchmarkId/eval/run/:id matching the existing pattern.

3. UI: "Stop run" button on /jobs/:runId

In apps/studio/src/routes/jobs/$runId.tsx:

• Render a destructive-style button (red outline) when status === 'starting' or 'running' and not in read-only mode.
• On click: DELETE /api/eval/run/:id, optimistic-flip the status indicator to "Stopping…".
• After the run hits a terminal state, the existing UI already updates correctly.
• Disable in read-only mode (UI-level, the API also 403s).

4. Tests

• Server: in apps/cli/test/commands/results/serve.test.ts, add cases for unknown id (404), read-only (403), and a happy-path stop using a fake long-running child.
• CLI: a small test that sends SIGINT to a multi-test eval run and asserts (a) exit code is the "stopped" sentinel and (b) index.jsonl contains the rows for tests completed before the signal.
• UI: pure helper for "should the stop button render?" — shouldShowStopButton(status, isReadOnly).

Acceptance signals

• CLI: SIGINT during a multi-test eval produces a clean exit and a partial index.jsonl containing all tests completed before the signal.
• CLI: a second SIGINT within 1s force-quits.
• Server: DELETE /api/eval/run/:id exists and is 403-guarded in read-only mode; benchmark-scoped variant works the same.
• UI: a "Stop run" button renders on /jobs/:runId while running, hidden when terminal, hidden in read-only.
• UI: clicking Stop, then navigating to the originating /runs/:runId, shows the partial run with at least one execution_status: ok row and the Resume run button from feat(studio): expose eval resumability — API + Resume action on run detail #1220 visible.
• Manual red/green: red = on main, killing terminal mid-eval is the only way to stop; green = on this branch, the Stop button on /jobs/<id> terminates cleanly and the partial run is resumable in one click.

Non-goals

• No "Pause" semantics. Stop fully terminates; resume is the way to continue.
• No queue management. This is for one running job at a time — multi-job orchestration is out of scope.
• No SIGINT-to-grader translation. If a grader is mid-flight when the signal arrives, let it finish or time out per existing rules.

Related

• Parent feature (resume): feat(studio): expose eval resumability — API + Resume action on run detail #1219
• Implementing PR (resume): feat(studio): expose eval resumability — API + Resume action on run detail #1220
• Existing per-provider kill patterns to mirror style: packages/core/src/evaluation/providers/{claude-cli,codex-cli,pi-cli}.ts

Estimate

~half a day. CLI signal handling is the biggest unknown (need to thread the flag through the worker pool); the UI + API changes are small.