What is the MCP 424 error in OpenAI Agent Builder?

HTTP 424 Failed Dependency means Agent Builder reached your MCP server but got an unexpected response. Root causes include stateful session lifecycle conflicts and strict SSE transport requirements.

How do I fix MCP 424 errors in Agent Builder?

Set stateless_http=True in FastMCP as a workaround. Ensure your server uses Streamable HTTP transport with correct SSE format. The 2026 MCP stateless spec should resolve this structurally.

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clawsmith.com/signal/openai-agent-builder-mcp-424-tool-connection-failure

⚠ IssueUnderservedai_agent_mcpLive

OpenAI Agent Builder throws 424 Failed Dependency when connecting custom MCP servers blocking agent deployment

Developers connecting compliant MCP servers to OpenAI Agent Builder consistently get HTTP 424 errors with 'Unable to load tools for this server'. The server passes all JSON-RPC tests and works via direct API calls but fails in Agent Builder. Root causes include stateful session lifecycle conflicts, strict SSE transport requirements, and Accept header mismatches that the client silently converts to 424. The error cluster spans 6+ OpenAI community threads from Oct-Feb 2026 and a GitHub issue on the MCP python-sdk. No clear fix was shipped; workaround is stateless_http=True.

Product Idea from this Signal

A CLI tool that validates custom MCP server compatibility against OpenAI Agent Builder before deployment

4.6k ▲

Developers building custom MCP servers routinely pass every standard JSON-RPC compliance test yet still get 424 Failed Dependency errors when connecting to OpenAI's Agent Builder. The gap is that generic JSON-RPC testers do not simulate OpenAI's actual session lifecycle, stateless HTTP handshake behavior, or timeout thresholds. This tool runs a local pre-flight check that faithfully reproduces the Agent Builder connection sequence end-to-end, reports exactly which phase fails, and prescribes the fix.

mcpdeveloper-toolsopenai-agentsapi-debuggingai-agentscompatibility-testing

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