fix(server): call onerror callback for all transport errors

[codewithkenzo]

Summary

Fixes #1395 - Transport errors are silently swallowed due to missing onerror callback usage.

This PR adds this.onerror?.() calls before every createJsonErrorResponse in WebStandardStreamableHTTPServerTransport, ensuring that all transport errors are properly reported to the error callback.

Changes

packages/server/src/server/streamableHttp.ts

Added this.onerror?.(new Error(...)) calls at 15 locations:

• handleGetRequest: Missing Accept header (406), concurrent SSE stream conflict (409)
• replayEvents: Event store not configured (400), invalid event ID format (400), stream conflict (409)
• handlePostRequest: Invalid Accept header (406), unsupported Content-Type (415), JSON parse error (400), invalid JSON-RPC message (400), re-initialization attempt (400), multiple init requests (400)
• validateSession: Server not initialized (400), missing session ID (400), session ID mismatch (404)
• validateProtocolVersion: Unsupported protocol version (400)

packages/server/test/server/streamableHttp.test.ts

Added 10 test cases to verify onerror is called for various error conditions:

• Invalid JSON
• Invalid JSON-RPC message
• Missing Accept header on POST
• Unsupported Content-Type
• Server not initialized
• Invalid session ID
• Re-initialization attempt
• Missing Accept header on GET
• Concurrent SSE streams
• Unsupported protocol version

Testing

# All 91 tests pass (76 existing + 10 new in the describe block)
pnpm -r --filter @modelcontextprotocol/server test

# Build succeeds
pnpm build:all

# Lint passes
pnpm -r --filter @modelcontextprotocol/server lint

Pattern Used

Follows the existing pattern already present at lines 296, 306, etc.:

this.onerror?.(new Error('Error message here'));
return this.createJsonErrorResponse(status, code, 'Error message here');

[changeset-bot]

🦋 Changeset detected

Latest commit: a4e1e5a

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

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commit: a4e1e5a