mcp: don't break the streamable client connection for transient errors #723
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When POST requests in the streamableClientConn return a transient error, return this error to the caller rather than permanently breaking the connection. This is achieved by using the special sentinel ErrRejected error to the jsonrpc2 layer. In doing so, the change revealed a pre-existing bug: ErrRejected had the same code as ErrConnectionClosing, and jsonrpc2.WireError implements errors.Is, so the two sentinel values could be conflated. This is fixed by using a new internal code. The new test required some additional machinery in our fake server: the ability to handle multiple requests to the same logical key. There's more to do for modelcontextprotocol#683: we should also retry transient errors in handleSSE. For modelcontextprotocol#683
jba
approved these changes
Dec 12, 2025
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When POST requests in the streamableClientConn return a transient error, return this error to the caller rather than permanently breaking the connection.
This is achieved by using the special sentinel ErrRejected error to the jsonrpc2 layer. In doing so, the change revealed a pre-existing bug: ErrRejected had the same code as ErrConnectionClosing, and jsonrpc2.WireError implements errors.Is, so the two sentinel values could be conflated. This is fixed by using a new internal code.
There's more to do for #683: we should also retry transient errors in handleSSE.
For #683