Fixed Translate streamablehttp

[kevalmahajan]

🐛 Bug-fix PR

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📌 Summary

Closes #728
In Translate feature (stdio server to streamable-http server):
Implemented a fix that forwards requests/responses from the Streamable HTTP server to the stdio server using pub/sub, enabling full support for the connection.

🐞 Root Cause

The root cause was that responses from the Streamable HTTP server were not being forwarded to the stdio server, resulting in incomplete communication. Without this forwarding, the connection was established via StreamableHTTPSessionManager but subsequent interactions such as listing tools and handling tool calls failed.

💡 Fix Description

Previously, the multi-protocol server only implemented SSE (/message) and lacked proper handling for the Streamable HTTP protocol’s /mcp POST endpoint. As a result, HTTP clients could not send JSON-RPC requests/notifications or receive responses per the MCP spec.

This fix:

1. Adds correct /mcp POST handling for Streamable HTTP protocol.
2. Uses the same pubsub from StdIOEndpoint to correlate stdout JSON-RPC responses to incoming requests.
3. Implements a bounded wait for matching stdout messages before returning the response.
4. Maintains existing SSE behavior without regression.

This ensures full compliance with the MCP specification for both SSE and Streamable HTTP modes.

Testing 🛠️

1. Convert a stdio server to Streamable HTTP and add it to the gateway or MCP Inspector. Verify that all functionalities including listing tools, resources, prompts, and tool invocation work correctly.

python3 -m mcpgateway.translate --stdio "npx @modelcontextprotocol/server-everything" --port 9000 --expose-streamable-http

2. Test with a multi-protocol server supporting both Streamable HTTP and SSE simultaneously:

python3 -m mcpgateway.translate --stdio "npx @modelcontextprotocol/server-everything" --port 9000 --expose-streamable-http --expose-sse

🧪 Verification

Check	Command	Status
Lint suite	make lint
Unit tests	make test
Coverage ≥ 90 %	make coverage
Manual regression no longer fails	steps / screenshots

📐 MCP Compliance (if relevant)

• Matches current MCP spec
• No breaking change to MCP clients

✅ Checklist

• Code formatted (make black isort pre-commit)
• No secrets/credentials committed

[rakdutta]

PR Testing Summary:

• make serve – Pass

• make test – Pass (80% coverage, 1626 passed, 10 skipped, 57 warnings)

• make autoflake isort black flake8 – No errors

• make pylint – Error:

  Module mcpgateway.services.resource_service
  mcpgateway/services/resource_service.py:417:19: W0212: Access to a protected member _initialized of a client class (protected-access)
  
  --------------------------------------------------------------------
  Your code has been rated at 10.00/10 (previous run: 10.00/10, -0.00)
  make: *** [Makefile:719: pylint] Error 4
  

• make smoketest – Pass (✅ Smoketest passed!)

• make doctest – All pass (53% coverage, 467 passed, 6 skipped, 1 warning)

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Testing Steps Performed:

1. Convert a stdio server to Streamable HTTP and add it to the gateway:

   python3 -m mcpgateway.translate --stdio "npx @modelcontextprotocol/server-everything" --port 9000 --expose-streamable-http

   • Successfully registered gateway server via UI and API:

     curl -s -X POST -H "Authorization: Bearer $MCPGATEWAY_BEARER_TOKEN" \
          -H "Content-Type: application/json" \
          -d '{"name":"fast_time_mcp","url":"http://127.0.0.1:9000/mcp","transport":"STREAMABLEHTTP"}' \
          http://localhost:4444/gateways

2. Test with a multi-protocol server supporting both Streamable HTTP and SSE:

   python3 -m mcpgateway.translate --stdio "npx @modelcontextprotocol/server-everything" --port 9000 --expose-streamable-http --expose-sse

   • Successfully registered gateway server via UI and API for and Streamable HTTP and SEE

     curl -s -X POST -H "Authorization: Bearer $MCPGATEWAY_BEARER_TOKEN" \
          -H "Content-Type: application/json" \
          -d '{"name":"fast_time","url":"http://localhost:9000/sse"}' \
          http://localhost:4444/gateways

     curl -s -X POST -H "Authorization: Bearer $MCPGATEWAY_BEARER_TOKEN" \
          -H "Content-Type: application/json" \
          -d '{"name":"fast_time_mcp","url":"http://127.0.0.1:9000/mcp","transport":"STREAMABLEHTTP"}' \
          http://localhost:4444/gateways

[madhav165]

Added sse and mcp servers from translate and both worked from gateway.

• Connect to gateway
• Tools work

[crivetimihai]

✅ PR #729 Review Complete

The PR successfully fixes the Streamable HTTP support in the translate feature.

Code Review

• The changes correctly implement request/response forwarding between Streamable HTTP and stdio servers
• Proper use of shared pubsub for both SSE and Streamable HTTP protocols
• Clean implementation with good error handling and timeout management

Testing Results

✅ Rebased against main branch✅ Linting - All checks pass (flake8, pylint)✅ Doctests - 469 passed✅ Unit tests - 1646 passed, translate-specific tests working⚠️ Coverage - 79% (below 90% target, but acceptable for
translate.py)✅ Manual testing - Both single and multi-protocol servers work correctly:

• Streamable HTTP /mcp endpoint handles initialize, tools/list, and tools/call
• Multi-protocol server successfully exposes both SSE and Streamable HTTP endpoints
• Proper JSON-RPC request/response correlation

The implementation properly addresses issue #728 and enables full MCP compliance for both SSE and Streamable HTTP modes. The PR is ready for merge.

Manual testing

pip install .
python3 -m mcpgateway.translate --stdio "npx -y kubernetes-mcp-server@latest" --port 8105 --expose-streamable-http

Works in gateway