Add local MCP dogfood testing workflow

[SPerekrestova]

Summary

Adds a deterministic dogfood path that runs the built MCP server exactly like a client would: npm run dogfood:local builds build/index.js, spawns it over stdio with StdioClientTransport, injects the existing e2e nock preload fixture, isolates HOME, and drives a user-like flow through:

listTools → runner_doctor → start_problem → request_hint → run_local_tests → get_session_state

The smoke covers available local runners (python3, go, java) without requiring LeetCode credentials or live network access, so Devin/CI-like environments can validate the MCP behavior directly. It also documents a second path for a real local Claude/agent setup, including the MCP config command and a copy-paste prompt for testing the server as a practicing user while avoiding accidental live submissions.

Verified locally with npm run build, npm run test:types, npx prettier --check ., npm run dogfood:local, npm test, and npm run test:e2e.

Link to Devin session: https://app.devin.ai/sessions/3e6f3e63f6e54852b9bf6da1c792774f
Requested by: @SPerekrestova

[devin-ai-integration]

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📥 Commits

Reviewing files that changed from the base of the PR and between 4d44985 and 1655a2b.

📒 Files selected for processing (1)

• scripts/dogfood-local.ts

🚧 Files skipped from review as they are similar to previous changes (1)

• scripts/dogfood-local.ts

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📝 Walkthrough

Walkthrough

Adds a local dogfood testing workflow: a new scripts/dogfood-local.ts runner that spawns the built MCP server over stdio with an isolated HOME and nock fixture preload, drives a sequence of MCP tool calls (runner_doctor, start_problem, request_hint, run_local_tests, get_session_state), and validates results. Wired via dogfood:local npm script and documented in DOGFOOD_TESTING.md and README.md.

Changes

Dogfood Local Testing Runner

Layer / File(s)	Summary
Dogfood runner script implementation scripts/dogfood-local.ts	Defines constants, fixture data, and per-language runner smoke configurations; implements JSON parsing helpers for MCP text responses; spawns the server via StdioClientTransport with isolated HOME and NODE_OPTIONS preload; validates run_local_tests results via assertRunPassed; orchestrates the full tool-call sequence in main() including server lifecycle management and cleanup.
npm script and TypeScript config package.json, tsconfig.test.json	Adds dogfood:local npm script that builds and runs the dogfood runner; expands tsconfig.test.json to include scripts/**/*.ts for TypeScript compilation.
Documentation DOGFOOD_TESTING.md, README.md	DOGFOOD_TESTING.md documents both the deterministic local run and local-agent dogfooding workflows with commands, expected behaviors, and guidance on which approach to use. README.md adds a Dogfood Testing section linking to the full workflow documentation.

✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch devin/1781092086-mcp-dogfood-testing

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[SPerekrestova]

Reviewed for correctness. Two concrete issues to address in scripts/dogfood-local.ts:

1. stderr: "pipe" is set on StdioClientTransport but never drained — risk of the server deadlocking once stderr exceeds the pipe buffer (~64 KB), especially during Go/Java runs.
2. home and fixtureDir are mkdtemp'd before client.connect(), but a connect failure bypasses main's finally { cleanup }, leaving temp dirs (and possibly the child process) leaked.

Rest LGTM — fixture/preload reuse is clean, the runtime-availability gate is sensible, and the typed JSON unwrap is solid.

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Generated by Claude Code

[SPerekrestova]

Deadlock risk: stderr: "pipe" without a consumer.

When StdioClientTransport is configured with stderr: "pipe", the child's stderr is piped to a Readable that the caller is expected to drain. This script never reads from transport.stderr, so once the child writes ~64 KB of stderr the OS pipe buffer fills and the server blocks on its next write. That can hang the smoke (notably on the Go/Java runs, where compilation diagnostics or logger output land on stderr).

The existing e2e harness (tests/e2e/harness/spawn-server.ts:114) uses stderr: "inherit" precisely to avoid this. Recommend matching that, or alternatively piping transport.stderr to process.stderr after connect.

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Generated by Claude Code

[devin-ai-integration]

Fixed in 1655a2b: switched the dogfood transport to stderr: "inherit", matching the e2e harness so server stderr is continuously drained by the parent process.

[SPerekrestova]

Temp directories leak if client.connect fails.

spawnDogfoodServer mkdtemps home and fixtureDir (and writes fixture.json) before calling client.connect(transport). If connect throws (transport spawn failure, handshake error, preload exit-1 from a bad fixture), the function rejects without returning the SpawnedDogfoodServer, so main's finally { await spawned.cleanup() } never runs and both temp dirs are leaked in os.tmpdir() for every failed run. The spawned child process can also leak depending on how the SDK handles a partial start.

The e2e harness handles this with try { await client.connect(transport); } catch (e) { await cleanup(); throw e; } — worth mirroring here.

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Generated by Claude Code

[devin-ai-integration]

Fixed in 1655a2b: spawnDogfoodServer() now builds a cleanup closure before client.connect(), calls it on connect failure, and then rethrows so temp dirs are removed even if spawn/handshake/preload setup fails.