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mcp: Enable legacy initialize fallback on any error (#1014)

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To guarantee a better compatibility with legacy server, enable the
fallback to the legacy initialize rpc in case of any error returned by
the server, in case of `server/discover` request.
This commit is contained in:
Guglielmo Colombo 2026-06-18 15:52:11 +02:00 • committed by GitHub
commit b85574f685
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1 changed files with 84 additions and 148 deletions
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26
mcp/streamable.go
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@ -2103,12 +2103,14 @@ func (c *streamableClientConn) Write(ctx context.Context, msg jsonrpc.Message) e
}
var requestSummary string
var requestMethod string
var forCall *jsonrpc.Request
switch msg := msg.(type) {
case *jsonrpc.Request:
requestSummary = fmt.Sprintf("sending %q", msg.Method)
if msg.IsCall() {
forCall = msg
requestMethod = msg.Method
}
case *jsonrpc.Response:
requestSummary = fmt.Sprintf("sending jsonrpc response #%d", msg.ID)
@ -2184,10 +2186,14 @@ func (c *streamableClientConn) Write(ctx context.Context, msg jsonrpc.Message) e
}
if err := c.checkResponse(ctx, requestSummary, resp); err != nil {
// Only fail the connection for non-transient errors.
// Transient errors (wrapped with ErrRejected) should not break the connection.
// ErrMethodNotFound and ErrUnsupportedProtocolVersion should not break the connection as they trigger the initialize fallback.
if !errors.Is(err, jsonrpc2.ErrRejected) && !errors.Is(err, jsonrpc2.ErrMethodNotFound) && !errors.Is(err, jsonrpc2.ErrUnsupportedProtocolVersion) {
if requestMethod == methodDiscover {
// Wrap the discover failure with ErrRejected so the jsonrpc2 layer
// doesn't set writeErr, which would prevent the legacy initialize
// fallback from succeeding on the same connection.
err = fmt.Errorf("%w: %w", err, jsonrpc2.ErrRejected)
} else if !errors.Is(err, jsonrpc2.ErrRejected) {
// Only fail the connection for non-transient errors.
// Transient errors (wrapped with ErrRejected) should not break the connection.
c.fail(err)
}
return err
@ -2400,10 +2406,8 @@ func (c *streamableClientConn) checkResponse(ctx context.Context, requestSummary
}
if resp.StatusCode < 200 || resp.StatusCode >= 300 {
// By default, always try to decode the body and surface the underlying
// JSON-RPC error (or detect vPre servers that reject "server/discover"
// as an unsupported method with a plain HTTP 400) regardless of the
// negotiated protocol version. Setting MCPGODEBUG=noprotocolerrorbody=1
// restores the previous behavior.
// JSON-RPC error.
// Setting MCPGODEBUG=noprotocolerrorbody=1 restores the previous behavior.
if noprotocolerrorbody == "1" {
return fmt.Errorf("%s: %v", requestSummary, http.StatusText(resp.StatusCode))
}
@ -2412,12 +2416,6 @@ func (c *streamableClientConn) checkResponse(ctx context.Context, requestSummary
if response, ok := msg.(*jsonrpc.Response); ok && response.Error != nil {
return fmt.Errorf("%s: %w: %v", requestSummary, response.Error, http.StatusText(resp.StatusCode))
}
if strings.Contains(string(body), fmt.Sprintf("%s: %q unsupported", jsonrpc2.ErrNotHandled, methodDiscover)) {
return fmt.Errorf("%s: %w: %v", requestSummary, jsonrpc2.ErrMethodNotFound, http.StatusText(resp.StatusCode))
}
if strings.Contains(string(body), "Unsupported protocol version") {
return fmt.Errorf("%s: %w: %v", requestSummary, jsonrpc2.ErrUnsupportedProtocolVersion, http.StatusText(resp.StatusCode))
}
return fmt.Errorf("%s: %v", requestSummary, http.StatusText(resp.StatusCode))
}
return nil