next-devtools-mcp

Hi! Just a quick heads-up: Next.js DevTools MCP is listed on AgentTask Atlas, a small directory of MCP servers and Agent Skills for coding agents. Entry: https://agenttaskatlas.com/entry/next-devtools-mcp The entry is built from your official README/docs and shows: tasks (Debugging & observability, Frontend & UI), documented compatible agents, permissions, credentials, install steps with a link to your install docs, maintenance (latest release/commit), license, source link, and the date it was last verified (2026-10-17). If you'd like, there's an optional README badge that links back to the entry: No action needed, so feel free to close this. If anything on the entry is wrong or out of date, just comment here and I'll correct it. Thanks for building Next.js DevTools MCP! Giuliano

Summary Passing an explicit for an optional argument fails the call with a JSON-RPC error instead of being treated as omitted. Some MCP clients send for optional arguments they leave unset, so those clients can't call or without the argument. The advertised schemas aren't nullable, so this may be working as intended. Question: would you accept either nullable schemas for optional arguments, or null-tolerant parsing that treats as omitted? Repro At (0.4.1), built and run over stdio: Observed Both return a JSON-RPC error with the full zod dump: Expected for an optional argument behaves the same as leaving it out: runs auto-discovery, and calls the tool with no arguments. The description asks callers to "OMIT the 'args' parameter entirely", which a client that always sends for unset fields can't do. Cause in checks , so is passed to , which rejects it, and line 112 throws . Suggested fix Either: treat as omitted for optional keys: at (a required key with would then hit the existing "Missing required argument" error), or make optional input schemas so the advertised JSON Schema allows .

What's wrong 1. with an explicit port hides "cannot connect". With nothing listening on port 4999: returns on the same port correctly returns So the agent is told to check the Next.js version when the dev server simply isn't running. A 404, an or an invalid SSE response get the same "No MCP tools found" text. 2. Auto-discovery ignores in its output. With , reports for a server on 3007, while reports . Cause 1. () catches every error and returns . () reads as "not Next.js 16+", so its own ("Failed to connect to port ...") never runs. 2. hard-codes . The explicit-port path (line 99) uses . Fix 1. Split into (rejects with the reason) and (same as before: logs and returns ). calls , so an explicit port now returns . Auto-discovery still uses , so one failing candidate still drops out without failing the whole listing (#154). 2. Use in the auto-discovery URL. Includes a patch changeset. Tests New : calls on a port with nothing listening and expects the "Cannot connect" reason. No mocks, no network beyond loopback. : the module mock now sets to and provides . A new case checks that an auto-discovered server's URL uses that host and matches the explicit-port URL. The existing explicit-port case now mocks . Both new tests fail on ( and ) and pass with the fix. and pass. : 50 passed, 1 skipped (: 48 passed, 1 skipped). In both runs fails in setup locally because it shells out to for its fixture app and isn't on my PATH. That's unrelated to this change. I didn't run the e2e tests.