fix(server): invoke onerror callback for all error responses

[owendevereaux]

Summary

Fixes #1395

Several error responses in WebStandardStreamableHTTPServerTransport were returned without invoking the onerror callback, making it impossible to debug or log these errors.

Problem

As reported in #1395, errors in nested try/catch blocks within handlePostRequest, handleGetRequest, and validation methods were being silently swallowed. The onerror callback was only invoked in the outermost catch block, so errors like:

• Accept header validation failures (406)
• Content-Type validation failures (415)
• JSON parse errors (400)
• Session validation errors (400, 404)

...were never reported via the callback, preventing any form of debugging.

Solution

Added this.onerror?.(new Error(message)) calls before all createJsonErrorResponse returns, matching the existing pattern already used in validateRequestHeaders().

For parse errors, the original exception is preserved via { cause: e }:

} catch (e) {
    const error = 'Parse error: Invalid JSON';
    this.onerror?.(new Error(error, { cause: e }));
    return this.createJsonErrorResponse(400, -32_700, error);
}

Error Types Now Reported

• Accept header validation (406)
• Content-Type validation (415)
• JSON parse errors (400)
• JSON-RPC message parse errors (400)
• Session validation errors (400, 404)
• Protocol version errors (400)
• Initialization errors (400)
• SSE stream conflicts (409)
• Event store errors (400, 409, 500)

Testing

Build passes. The changes are straightforward additions of onerror calls before existing error returns.

[changeset-bot]

🦋 Changeset detected

Latest commit: 305a1d1

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@modelcontextprotocol/server	Patch
@modelcontextprotocol/express	Patch
@modelcontextprotocol/hono	Patch
@modelcontextprotocol/node	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR