fix(mcp): Use SDK web-standard transport for copilot mcp

[TheodoreSpeaks]

Summary

Our mcp copilot was broken, consistently returning 5xx. This was due to upgrading mcp sdk versions. Since now the sdk supports WebStandardStreamableHTTPServerTransport, removed the shims and directly used that class instead.

When it broke

Commit 45bf39696 — "fix(deps): bump drizzle-orm 0.45.2 + adopt MCP SDK 1.25.3 native types (#4252)" — merged 2026-04-21.

That PR touched only apps/sim/package.json (one-line bump @modelcontextprotocol/sdk 1.20.2 → 1.25.3) and apps/sim/lib/copilot/tools/mcp/definitions.ts (cosmetic type cleanup). The route file itself was not modified. The shim there had been working untouched since 2026-02-09.

Why the PR review didn't catch it

The SDK's StreamableHTTPServerTransport was silently re-architected upstream:

• Same export name, same handleRequest(req, res, parsedBody) signature → TypeScript saw nothing.
• 1.20.2: self-contained Node implementation using raw-body + node:crypto. Only ever read req.headers.*. Our minimal shim { method, headers } was sufficient.
• 1.25.3: "a thin wrapper around WebStandardStreamableHTTPServerTransport" using @hono/node-server's getRequestListener. For any non-GET/HEAD request, Hono calls incoming.on("end", …) (listener.js:598).
• Our shim has no .on → TypeError → caught by Hono → routed to handleFetchError → Response(null, { status: 500 }) → written to our NextResponseCapture via writeHead(500, …) → returned by our route as if it were a normal response.
• Our route's catch block never saw the error (which is why the "recategorize" framing of this branch was misleading — there was no fault to recategorize, the route handler never ran).
• No route.test.ts under apps/sim/app/api/mcp/copilot/ — nothing in CI round-tripped a POST through the transport, so the regression slipped to prod.

The fix

Swap the broken Node-style transport for the SDK's web-standard transport, which Next.js can drive directly (NextRequest already extends Web Request)

Also drops the now-unused NextResponseCapture shim (~190 lines) and normalizeRequestHeaders helper. The catch block additionally maps client-aborted requests to 499 (ConnectionClosed) since aborts do legitimately reach it now (unlike everything else, which the SDK handles internally).

Verification (local)

	Before	After
initialize POST	500, empty body, text/plain	200, full JSON-RPC envelope, application/json
tools/list POST	unreachable	200, all 23 tools (list_workspaces, create_workflow, sim_workflow, …)
Catch block fires	never (synthetic 500 from Hono)	only on real exceptions

Confirmed end-to-end with claude mcp add --transport http sim-copilot http://localhost:3000/api/mcp/copilot --header "x-api-key: …" → Connected.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation
• Other: ___________

Testing

• Tested locally. Initialize handshake returns 200 with proper MCP server info; tools/list returns all 23 tools.
• Validated claude mcp add --transport http sim-copilot http://localhost:3000/api/mcp/copilot succeeds with Connected.

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• 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 (UTC)
docs	Skipped		Apr 28, 2026 7:15pm

[cursor]

PR Summary

Medium Risk
Changes request/response handling for the MCP copilot endpoint by swapping transport implementations and altering how responses are produced, which could affect streaming/timeout behavior. Also reclassifies aborted requests to 499, reducing noisy 5xxs but touching error-path behavior.

Overview
Reduces spurious 5xxs from /api/mcp/copilot by detecting client-aborted requests (AbortError/aborted signal) and returning a JSON-RPC ConnectionClosed error with HTTP 499 instead of 500.

Simplifies MCP request handling by switching to WebStandardStreamableHTTPServerTransport and removing the custom NextResponseCapture/header normalization adapter, returning the transport’s Response directly.

^{Reviewed by Cursor Bugbot for commit 71c906a. Bugbot is set up for automated code reviews on this repo. Configure here.}

[greptile-apps]

Greptile Summary

This PR reverts the MCP copilot route to use WebStandardStreamableHTTPServerTransport from the SDK, replacing the custom NextResponseCapture adapter class and normalizeRequestHeaders helper that were introduced in commit 45bf396 and caused 5xx failures. The simplification removes ~185 lines of adapter boilerplate and adds a 499 response for client-cancelled requests in the outer catch.

Confidence Score: 5/5

Safe to merge — straightforward transport swap that removes a broken custom adapter; only finding is a P2 abort edge case.

All findings are P2 (minor inconsistency in abort handling between inner/outer catch). No P0 or P1 issues found. The core change is a clean simplification that delegates to well-tested SDK internals.

No files require special attention.

Important Files Changed

Filename	Overview
apps/sim/app/api/mcp/copilot/route.ts	Replaces the custom StreamableHTTPServerTransport + NextResponseCapture adapter with WebStandardStreamableHTTPServerTransport, greatly simplifying the handler. Adds a 499 abort response in the outer catch. One minor issue: inner try/catch around request.json() still swallows AbortError before the new 499 path.

Sequence Diagram

sequenceDiagram
    participant Client
    participant POST Handler
    participant handleMcpRequestWithSdk
    participant WebStandardTransport
    participant MCP Server

    Client->>POST Handler: POST /api/mcp/copilot
    POST Handler->>POST Handler: Auth check
    POST Handler->>POST Handler: request.json()
    POST Handler->>handleMcpRequestWithSdk: (request, parsedBody)
    handleMcpRequestWithSdk->>MCP Server: buildMcpServer(request.signal)
    handleMcpRequestWithSdk->>WebStandardTransport: new WebStandardStreamableHTTPServerTransport
    handleMcpRequestWithSdk->>MCP Server: server.connect(transport)
    handleMcpRequestWithSdk->>WebStandardTransport: transport.handleRequest(request, {parsedBody})
    WebStandardTransport->>MCP Server: dispatch tool call
    MCP Server-->>WebStandardTransport: tool result
    WebStandardTransport-->>handleMcpRequestWithSdk: Response
    handleMcpRequestWithSdk-->>POST Handler: Response
    POST Handler-->>Client: JSON response
    note over POST Handler: On AbortError → 499
    note over POST Handler: On other error → 500

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_{Reviews (2): Last reviewed commit: "fix(mcp): fix copilot mcp response" | Re-trigger Greptile}

[TheodoreSpeaks]

@greptile review