[waleedlatif1]

Summary

• Workflow MCP servers answered some JSON-RPC errors to well-formed requests with HTTP 404 (unknown method, unknown tool) or 400 (invalid tools/call params, undeployed workflow, and workflow-service 400/404 failures passed through). In Streamable HTTP those statuses mean a lost or bad session: the MCP SDK client turns any non-2xx into a StreamableHTTPError and drops the JSON-RPC body, and Sim's own MCP client treats 400/404 as a dead session, retires the pooled connection, and retries a call that can never succeed
• New jsonRpcErrorResponse decides the status for every request-scoped JSON-RPC error in one place: 400/401/404 go out at 200 (as the SDK's reference server sends protocol errors); 408/409/413/429/499/500/503 keep their retry, limit, and alarm meaning. data.httpStatus still records the workflow service's status
• Malformed messages (parse error, invalid JSON-RPC envelope) keep their 400, matching the SDK server
• Clients that accept text/event-stream already received every tools/call error at 200 inside the stream; this makes the JSON path agree
• Removes an unreachable Tool name required branch (the params schema already requires a non-empty name)

Type of Change

• Bug fix

Testing

• route.test.ts: drives the route with the real SDK Client + StreamableHTTPClientTransport (MethodNotFound now arrives as McpError), plus a table over the JSON path for invalid params, unknown tool, undeployed workflow, and workflow-service 404/400 failures
• All 6 new cases fail against the current staging route; the 39 existing cases (which pin the 408/409/413/429/499/403 statuses) pass both before and after — 45/45 green
• bun run lint, check:test-patterns, check:comment-hygiene, check:source-conventions, check:explicit-any; full type-check and audits left to CI

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing (new tests pass the test-audit authoring gate)
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

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1 Skipped Deployment

Project	Deployment	Actions	Updated
docs	Skipped		Nov 2, 2026 7:52am UTC

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Turn on auto-fix | Re-trigger cubic}

[waleedlatif1]

@greptile

[waleedlatif1]

@cubic-dev-ai review this PR

[cubic-dev-ai]

@cubic-dev-ai review this PR

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[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Turn on auto-fix | Re-trigger cubic}

[waleedlatif1]

@greptile review

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge; no actionable issues were found.

Summary

Workflow MCP requests now receive JSON-RPC errors over HTTP 200 instead of misleading HTTP 400/401/404 responses.

• Workflow MCP clients can read well-formed request errors as JSON-RPC replies.

_{Reviews (1) · Last reviewed commit: "fix(mcp): build the routed test request ..." · Reviewed by Greptile}