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<!-- Write all pull request text in Simplified Technical English (ASD-STE100): short sentences, one instruction per sentence, simple approved vocabulary, and the active voice. --> ## Thinking Path > - Paperclip is the open source app people use to manage AI agents for work > - Agents call remote MCP tools through the Paperclip tool gateway > - The gateway keeps a health status for each remote MCP connection, and it hides the tools of a connection that has `healthStatus = "error"` > - One oversized, malformed, or invalid-JSON `tools/call` reply set the whole connection to `error` > - Health goes back to `ok` only after a successful call, but the hidden tools prevent that call, so the connection stayed locked until a person reconnected it > - This pull request makes these reply errors fail only the one call, and it starts a new MCP session for the caller on the next call > - The benefit is that one large or bad reply no longer disconnects a working connection for every agent ## Linked Issues or Issue Description No public issue exists. Related open PRs fix other health downgrades in the same function. They do not overlap with this change: - Refs #11910 (timeouts and JSON-RPC errors) - Refs #15325 (transport errors) **What happened?** An agent called a remote MCP tool that returned more than `MAX_REMOTE_MCP_RESPONSE_BYTES` (1 MB). The gateway returned 502 `mcp_remote_response_too_large`. It also set the connection to `healthStatus = "error"`. Then `connectedMcpConnectionFilter` hid all tools of the connection (except `per_user` connections), and `connections_search` showed the connection as `needs_user_action`. The `malformed_response` and `invalid_json` errors from `readMcpHttpResponse` did the same thing. `readMcpHttpResponse` can also cancel the reply stream before the end. The remote server can then close its MCP session. The gateway kept the cached `mcp-session-id` for up to 30 minutes and cleared it only after a 404. The next calls then failed with "Remote MCP session expired". **Expected behavior** A reply that is too large or not correct fails only that call. The connection stays healthy, and its tools stay visible. The next call uses a new MCP session. **Steps to reproduce** 1. Add a remote MCP connection with `mcpSessionRequired: true`. 2. Call a tool that returns a reply larger than 1 MB. 3. Look at the connection: `healthStatus` is `error`. 4. Start a new gateway session: the tools of the connection are not in the list. **Paperclip version or commit** `master` at `99a9de9940bf5974352d9dbfbb2f21e62e89689f` **Deployment mode** All modes. The fault is in the server tool gateway. ## What Changed - `server/src/services/tool-gateway.ts`: `too_large`, `malformed_response`, and `invalid_json` from `readMcpHttpResponse` now fail only the call. The caller gets the same 502 reason code as before. The gateway does not call `markRemoteConnectionHealth(…, "error")` for these errors. The `invalid_json` branch after `JSON.parse(body)` also does not change health now, so all `invalid_json` paths are the same. - The `mcp_remote_response_too_large` message now tells the caller to request a smaller result, for example a narrower query or a smaller page size. The error details now include `maxBytes`. - `server/src/services/mcp-http.ts`: new `forgetMcpHttpSession()`. It removes only the cached session for one scope and one credential set, and only while that entry still holds the session ID that failed. It does not touch other agents, sessions that are initializing, or a newer session that replaced the failed one. - The gateway calls `forgetMcpHttpSession()` after these reply errors, so the next call from that caller initializes a new session. The existing 404 "session expired" path now uses the same function. Before, both paths cleared all sessions and all pending initializations on the connection. That made a concurrent initialization by another agent fail with "MCP connection changed while initializing", which the gateway reported as a fetch failure and marked as a connection `error`. - `forgetMcpHttpSessions(connectionId)` is not changed. Disconnect and revocation still clear the full connection. Why `malformed_response` and `invalid_json` are also per-call errors: - Each error is about one reply body. The server was reachable, accepted the credentials, and sent HTTP 2xx. A reply can be too large or bad because of the tool and its arguments. That is not a fault of the connection. - The other malformed-reply checks in the same function (payload is not an object, or has no `result`) already throw `remote_mcp_malformed_response` and do not change health. Only the reader-level errors changed health. - Health recovers only after a successful call. An `error` status for one bad reply therefore locks the connection until a person reconnects it. - Other failures (HTTP errors, fetch failures, timeouts, JSON-RPC errors) still change health. This PR does not change them. #11910 and #15325 address some of them. ## Verification - `server/src/__tests__/tool-gateway.test.ts`: the recovery test now runs for three replies: oversized, invalid JSON, and a reply without the requested message ID. Each run uses a fake HTTP MCP server with `mcpSessionRequired: true` that gives `session-N` for each `initialize`. Each run checks that: - the call fails with the correct 502 reason code (and, for the oversized reply, the smaller-page hint and `maxBytes: 1000000`) - `healthStatus` stays `ok` - a new gateway session still lists the tool and can call it - the second `tools/call` uses `session-2`, not `session-1` - New test: agent A gets an oversized reply while agent B initializes a session on the same connection. Agent B's call completes, health stays `ok`, and the two calls use `session-1` and `session-2`. With the old connection-wide reset, this test fails: agent B gets 502 `mcp_remote_fetch_failed`. - `server/src/__tests__/remote-mcp-protocol.test.ts`: new unit test. `forgetMcpHttpSession()` keeps the session of a different identity, and a late failure from an old session does not remove the newer session. - Each regression check fails without its fix. Without the health change, the test fails on `healthStatus: 'error'`. Without the session reset, it fails with `['session-1', 'session-1']`. ``` cd server npx vitest run src/__tests__/tool-gateway.test.ts src/__tests__/tool-gateway-service.test.ts src/__tests__/remote-mcp-protocol.test.ts Test Files 3 passed (3) Tests 132 passed (132) ``` - I did not run the full server `tsc --noEmit` locally because the sandbox does not have sufficient memory. The CI typecheck covers it. ## Risks - Low risk. The change affects only the error path of remote MCP `tools/call`. - A remote server that always sends bad replies now keeps `healthStatus = "ok"`. Each call still fails with a clear 502 reason code and an audit record, so the failure stays visible. Only the connection-wide hiding of tools stops. - After one of these errors, the next call from the same caller sends one more `initialize` request. This adds one round trip. - A 404 "session expired" now clears only the session of the caller that got the 404. Before, it cleared the cached sessions of all agents on the connection. If the remote server restarts, each agent now gets its own "session expired" error one time and then initializes again. A 404 is about one session, and the old connection-wide clear also cancelled other agents' initializations. - #11910 and #15325 change the same catch block. The PR that merges last can have a small merge conflict. > For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and discuss it in `#dev` before opening the PR. Feature PRs that overlap with planned core work may need to be redirected — check the roadmap first. See `CONTRIBUTING.md`. ## Model Used - Claude Opus 5.5 (Anthropic), model ID `claude-opus-5-5`, 1M-token context window. - Run as an agent in Claude Code with tool use (shell, file edit, and local test runs). ## Checklist - [x] I have included a thinking path that traces from project context to this change - [x] I have specified the model used (with version and capability details) - [x] I have checked ROADMAP.md and confirmed this PR does not duplicate planned core work - [x] I have searched GitHub for duplicate or related PRs and linked them above - [x] I have either (a) linked existing issues with `Fixes: #` / `Closes #` / `Refs #` OR (b) described the issue in-PR following the relevant issue template - [x] I have not referenced internal/instance-local Paperclip issues or links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip` URLs) - [x] My branch name describes the change (e.g. `docs/...`, `fix/...`) and contains no internal Paperclip ticket id or instance-derived details - [x] I have run tests locally and they pass - [x] I have added or updated tests where applicable - [x] I have updated relevant documentation to reflect my changes (no user-facing documentation changes are necessary) - [x] I have considered and documented any risks above - [ ] All Paperclip CI gates are green - [ ] Greptile is 5/5 with no open P2s, recommendations, or follow-ups - [x] I will address all Greptile and reviewer comments before requesting merge This PR replaces #15418. It keeps the same commits on a branch name without an internal ticket ID, and adds a fix for the Greptile review on #15418. 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: devinfoley <139239+devinfoley@users.noreply.github.com> Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>