MCP Protocol Compliance - Phase 5 Complete ✅

This document summarizes the protocol compliance improvements made to pluggedin-mcp to ensure 100% adherence to the MCP specification.

Latest Updates (v1.11.1)

Protocol Version Support Enhanced

Date: 2025-11-10 Issue: Smithery Cloud deployment failing due to protocol version mismatch

The server now supports multiple MCP protocol versions for backward compatibility:

Supported versions: 2024-11-05, 2025-06-18
Latest version: 2025-06-18 (sent in response headers)
Behavior: Accepts requests with either protocol version, always responds with latest

Changes Made:

Added SUPPORTED_MCP_PROTOCOL_VERSIONS constant array (src/constants.ts)
Updated versionMiddleware to validate against array instead of single version (src/middleware.ts)
Updated Docker HEALTHCHECK for better container readiness detection (Dockerfile)
Added test coverage for protocol version 2025-06-18 (tests/streamable-http.test.ts)

Impact:

✅ Smithery Cloud scanner can now successfully connect (uses protocol version 2025-06-18)
✅ Backward compatibility maintained for clients using 2024-11-05
✅ Health checks ensure container is fully initialized before accepting traffic

Changes Made (Previous Phases)

1. CORS Headers (Lines 30-36)

Before:

res.header('Access-Control-Allow-Headers', 'Content-Type, Authorization, mcp-session-id');
// Missing: Access-Control-Expose-Headers

After:

res.header('Access-Control-Allow-Headers', 'Content-Type, Authorization, Mcp-Session-Id, Mcp-Protocol-Version');
// ✅ Added: Expose custom headers so clients can read them
res.header('Access-Control-Expose-Headers', 'Mcp-Session-Id, Mcp-Protocol-Version');

Impact: Clients can now read custom MCP headers in responses (required per MCP spec).

2. Protocol Version Validation (Lines 43-65)

Added new middleware:

// MCP Protocol Version Validation
app.use((req: any, res: any, next: any) => {
  if ((req.path === '/mcp' || req.path === '/') && req.method === 'POST') {
    const version = req.headers['mcp-protocol-version'];

    // Protocol version is optional but if provided, validate it
    if (version && version !== '2024-11-05') {
      return res.status(400).json({
        jsonrpc: '2.0',
        error: {
          code: -32600, // Invalid Request
          message: `Unsupported MCP protocol version: ${version}. Supported: 2024-11-05`
        },
        id: null
      });
    }

    // Always send our protocol version in response
    res.setHeader('Mcp-Protocol-Version', '2024-11-05');
  }
  next();
});

Impact:

Server validates protocol version if provided by client
Server always returns its supported version
Rejects incompatible protocol versions with proper JSON-RPC error

3. Session Header Casing (Line 160, 174)

Before:

sessionId = req.headers['mcp-session-id'] as string || randomUUID();
res.setHeader('mcp-session-id', sessionId);

After:

// MCP spec: Use title case for custom headers
sessionId = req.headers['mcp-session-id'] as string || randomUUID();
res.setHeader('Mcp-Session-Id', sessionId); // ✅ Title case per MCP spec

Impact: Response headers use correct title-case format per MCP specification.

4. JSON-RPC Error Codes

Before:

code: -32000,  // Generic app error
message: 'Unauthorized: Invalid or missing API key'

After:

code: -32001,  // JSON-RPC 2.0: Server error (unauthorized)
message: 'Unauthorized: Invalid or missing API key'

Error Code Standards Applied:

-32600: Invalid Request (malformed request, unsupported protocol version)
-32601: Method not found (HTTP method not allowed)
-32603: Internal error (server-side exception)
-32001: Server error - Unauthorized (auth failure)
-32000: Server error - Generic application error (session not found, etc.)

Impact: Error codes now follow JSON-RPC 2.0 specification precisely.

5. Documentation Header (Lines 1-16)

Added comprehensive documentation at the top of streamable-http.ts:

/**
 * Streamable HTTP Server Transport for MCP Proxy
 *
 * MCP Protocol Compliance:
 * - Headers use Title-Case per MCP spec (Mcp-Session-Id, Mcp-Protocol-Version)
 * - CORS headers expose custom headers to clients
 * - Protocol version validation (2024-11-05)
 * - JSON-RPC 2.0 compliant error codes
 *
 * JSON-RPC Error Codes Used:
 * - -32600: Invalid Request (malformed request, unsupported protocol version)
 * - -32601: Method not found (HTTP method not allowed)
 * - -32603: Internal error (server-side exception)
 * - -32001: Server error - Unauthorized (auth failure)
 * - -32000: Server error - Generic application error (session not found, etc.)
 */

6. Comprehensive Test Coverage

Added 13 new protocol compliance tests to tests/streamable-http.test.ts:

CORS Headers Tests (3):

✅ Should include Access-Control-Expose-Headers
✅ Should allow MCP headers in CORS
✅ Should handle OPTIONS preflight correctly

Protocol Version Tests (4):

✅ Should accept requests without protocol version
✅ Should accept valid protocol version (2024-11-05)
✅ Should reject unsupported protocol version
✅ Should send protocol version in response

Session Header Casing Tests (1):

✅ Should return Mcp-Session-Id with title case

JSON-RPC Error Code Tests (3):

✅ Should return -32601 for unsupported HTTP method
✅ Should return -32001 for authentication failures
✅ Should return -32603 for internal errors

Test Results:

✓ tests/streamable-http.test.ts (32 tests) 78ms
  ✓ MCP Protocol Compliance (13 tests)
    ✓ CORS Headers (3 tests)
    ✓ Protocol Version (4 tests)
    ✓ Session Header Casing (1 test)
    ✓ JSON-RPC Error Codes (3 tests)

Test Files  3 passed (3)
     Tests  79 passed (79)

MCP Specification Compliance Checklist

✅ HTTP Transport Requirements

Support POST requests for JSON-RPC messages
Support GET requests for Server-Sent Events (SSE)
Support DELETE requests for session termination
Handle OPTIONS for CORS preflight

✅ Headers

Use title-case for custom headers (Mcp-Session-Id, not mcp-session-id)
Send Mcp-Protocol-Version in all responses
Validate Mcp-Protocol-Version if provided by client
Expose custom headers via Access-Control-Expose-Headers

✅ CORS

Allow cross-origin requests (Access-Control-Allow-Origin: *)
Allow MCP-specific headers (Mcp-Session-Id, Mcp-Protocol-Version)
Expose custom headers to JavaScript clients
Support preflight OPTIONS requests

✅ Session Management

Generate session IDs for stateful connections
Return session ID in response header
Accept session ID in request header
Reuse transport for same session ID
Support session termination via DELETE

✅ Error Handling

Return valid JSON-RPC 2.0 error responses
Use standard error codes (-32xxx range)
Include error message and optional data
Return proper HTTP status codes (400, 401, 405, 500)

✅ JSON-RPC 2.0

Support jsonrpc: "2.0" field
Support id field for request correlation
Return errors in standard format
Handle batch requests (via SDK)

Breaking Changes

None!

All changes are backwards compatible:

✅ Protocol version validation is optional (clients without the header still work)
✅ Session header reads from both lowercase and title-case (reads mcp-session-id, writes Mcp-Session-Id)
✅ Error codes changed but remain in JSON-RPC 2.0 spec range
✅ CORS headers are additive (added Expose-Headers, didn't remove existing)

Files Modified

src/streamable-http.ts
- Added documentation header
- Fixed CORS headers
- Added protocol version validation
- Fixed session header casing
- Updated error codes
- Lines changed: ~30
tests/streamable-http.test.ts
- Added 13 new protocol compliance tests
- Fixed 1 existing test for new error message
- Lines added: ~195

Deployment Notes

No Configuration Changes Required

All changes are code-only. No environment variables or deployment configuration changes needed.

Test Before Deploy

# Run all tests
npm test

# Should see:
# ✓ Test Files  3 passed (3)
# ✓ Tests  79 passed (79)

Verification After Deploy

Check Protocol Version Header:

curl -I https://mcp.plugged.in/mcp
# Should include: Mcp-Protocol-Version: 2024-11-05

Check CORS Expose Headers:

curl -I https://mcp.plugged.in/health
# Should include: Access-Control-Expose-Headers: Mcp-Session-Id, Mcp-Protocol-Version

Check Error Codes:

# Unsupported protocol version
curl -X POST https://mcp.plugged.in/mcp \
  -H "Mcp-Protocol-Version: 2023-01-01" \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","method":"test","id":1}'
# Should return: {"error":{"code":-32600,"message":"Unsupported MCP protocol version..."}}

# Unsupported HTTP method
curl -X PUT https://mcp.plugged.in/mcp
# Should return: {"error":{"code":-32601,"message":"HTTP method PUT not allowed"}}

Performance Impact

None. All changes are lightweight:

Header checks: O(1)
Protocol validation: Single string comparison
Error code changes: No performance impact

Security Improvements

Protocol Version Validation: Prevents clients from using unsupported/outdated protocol versions
Proper Error Codes: Better error categorization for monitoring and debugging
Standard Compliance: Reduces attack surface by following established specifications

Next Steps (Optional)

Future Enhancements (Not Required for Spec Compliance)

Add protocol version negotiation (support multiple versions)
Add rate limiting per session
Add detailed metrics for protocol violations
Add OpenAPI/Swagger documentation for HTTP endpoints

References

MCP Specification: https://spec.modelcontextprotocol.io/
JSON-RPC 2.0 Specification: https://www.jsonrpc.org/specification
CORS Specification: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS
HTTP Status Codes: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status

Changelog

v1.10.6 (Upcoming)

✅ Added Access-Control-Expose-Headers for MCP compliance
✅ Added protocol version validation (2024-11-05)
✅ Fixed session header casing (Mcp-Session-Id)
✅ Standardized JSON-RPC error codes
✅ Added 13 new protocol compliance tests
✅ All 79 tests passing

Status: ✅ Phase 4 (Protocol Compliance) Complete MCP Spec Compliance: 100% Test Coverage: 79 passing tests Breaking Changes: None Ready for Production: Yes

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MCP Protocol Compliance - Phase 5 Complete ✅

Latest Updates (v1.11.1)

Protocol Version Support Enhanced

Changes Made (Previous Phases)

1. CORS Headers (Lines 30-36)

2. Protocol Version Validation (Lines 43-65)

3. Session Header Casing (Line 160, 174)

4. JSON-RPC Error Codes

5. Documentation Header (Lines 1-16)

6. Comprehensive Test Coverage

MCP Specification Compliance Checklist

✅ HTTP Transport Requirements

✅ Headers

✅ CORS

✅ Session Management

✅ Error Handling

✅ JSON-RPC 2.0

Breaking Changes

None!

Files Modified

Deployment Notes

No Configuration Changes Required

Test Before Deploy

Verification After Deploy

Performance Impact

Security Improvements

Next Steps (Optional)

Future Enhancements (Not Required for Spec Compliance)

References

Changelog

v1.10.6 (Upcoming)

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MCP Protocol Compliance - Phase 5 Complete ✅

Latest Updates (v1.11.1)

Protocol Version Support Enhanced

Changes Made (Previous Phases)

1. CORS Headers (Lines 30-36)

2. Protocol Version Validation (Lines 43-65)

3. Session Header Casing (Line 160, 174)

4. JSON-RPC Error Codes

5. Documentation Header (Lines 1-16)

6. Comprehensive Test Coverage

MCP Specification Compliance Checklist

✅ HTTP Transport Requirements

✅ Headers

✅ CORS

✅ Session Management

✅ Error Handling

✅ JSON-RPC 2.0

Breaking Changes

None!

Files Modified

Deployment Notes

No Configuration Changes Required

Test Before Deploy

Verification After Deploy

Performance Impact

Security Improvements

Next Steps (Optional)

Future Enhancements (Not Required for Spec Compliance)

References

Changelog

v1.10.6 (Upcoming)