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Fix SSE JSON-RPC parsing for HTTP error paths #1085

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Merged
lpcox merged 1 commit into from
Feb 19, 2026
Merged

Fix SSE JSON-RPC parsing for HTTP error paths #1085

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35 changes: 18 additions & 17 deletions internal/mcp/connection.go
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Original file line number Diff line number Diff line change
Expand Up @@ -69,6 +69,16 @@ func parseSSEResponse(body []byte) ([]byte, error) {
// - error: Detailed parsing error with body preview on failure
func parseJSONRPCResponseWithSSE(body []byte, statusCode int, contextDesc string) (*Response, error) {
var rpcResponse Response
httpErrorResponse := func() *Response {
return &Response{
JSONRPC: "2.0",
Error: &ResponseError{
Code: -32603, // Internal error
Message: fmt.Sprintf("HTTP %d: %s", statusCode, http.StatusText(statusCode)),
Data: json.RawMessage(body),
},
}
}

// Try parsing as standard JSON first
if err := json.Unmarshal(body, &rpcResponse); err != nil {
Expand All @@ -80,21 +90,14 @@ func parseJSONRPCResponseWithSSE(body []byte, statusCode int, contextDesc string
// construct a JSON-RPC error response with HTTP error details
if statusCode != http.StatusOK {
logConn.Printf("HTTP error status=%d, body cannot be parsed as JSON-RPC", statusCode)
return &Response{
JSONRPC: "2.0",
Error: &ResponseError{
Code: -32603, // Internal error
Message: fmt.Sprintf("HTTP %d: %s", statusCode, http.StatusText(statusCode)),
Data: json.RawMessage(body),
},
}, nil
return httpErrorResponse(), nil
}
// Include the response body to help debug what the server actually returned
bodyPreview := string(body)
if len(bodyPreview) > 500 {
bodyPreview = bodyPreview[:500] + "... (truncated)"
}
return nil, fmt.Errorf("failed to parse %s (received non-JSON or malformed response): %w\nResponse body: %s", contextDesc, err, bodyPreview)
return nil, fmt.Errorf("failed to parse %s (received non-JSON or malformed response): %w\nResponse body: %s", contextDesc, sseErr, bodyPreview)
}

// Successfully extracted JSON from SSE, now parse it
Expand All @@ -103,20 +106,18 @@ func parseJSONRPCResponseWithSSE(body []byte, statusCode int, contextDesc string
// construct a JSON-RPC error response with HTTP error details
if statusCode != http.StatusOK {
logConn.Printf("HTTP error status=%d, SSE data cannot be parsed as JSON-RPC", statusCode)
return &Response{
JSONRPC: "2.0",
Error: &ResponseError{
Code: -32603, // Internal error
Message: fmt.Sprintf("HTTP %d: %s", statusCode, http.StatusText(statusCode)),
Data: json.RawMessage(body),
},
}, nil
return httpErrorResponse(), nil
}
return nil, fmt.Errorf("failed to parse JSON data extracted from SSE response: %w\nJSON data: %s", err, string(sseData))
}
logConn.Printf("Successfully parsed SSE-formatted response")
}

if statusCode != http.StatusOK {
logConn.Printf("HTTP error status=%d, returning synthetic JSON-RPC error response", statusCode)
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Copilot AI Feb 19, 2026

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The unconditional non-200 handling at the end returns a synthetic JSON-RPC error even when the body was successfully parsed as a valid JSON-RPC response. This discards backend-provided JSON-RPC errors (code/message/data) and conflicts with sendHTTPRequest()’s documented behavior of passing through valid JSON-RPC on HTTP error statuses. Consider removing this final statusCode check, or only synthesizing an error when the parsed response is not a valid JSON-RPC response (e.g., missing jsonrpc/id and no error), and otherwise return the parsed rpcResponse unchanged.

Suggested change
logConn.Printf("HTTP error status=%d, returning synthetic JSON-RPC error response", statusCode)
// If we successfully parsed what looks like a JSON-RPC response, prefer the
// backend-provided payload (including any error object) over a synthetic one.
if rpcResponse.JSONRPC != "" || rpcResponse.Error != nil {
logConn.Printf("HTTP error status=%d, but response contains JSON-RPC payload; returning backend response", statusCode)
return &rpcResponse, nil
}
logConn.Printf("HTTP error status=%d with no valid JSON-RPC payload, returning synthetic JSON-RPC error response", statusCode)

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return httpErrorResponse(), nil
}

return &rpcResponse, nil
}

Expand Down