mcp: fail when streamable http client encounters unsupported content (#130)

The streamable HTTP client is not fully implemented, but did not fail
with meaningful error messages when encountering unsupported content
types from other servers. This may be straightforward to fix, but for
now at least provide a meaningful error message.

For modelcontextprotocol/go-sdk#129.

mcp/streamable.go

@@ -645,8 +645,7 @@ type streamableClientConn struct {
 	mu              sync.Mutex
 	protocolVersion string
 	_sessionID      string
-	// bodies map[*http.Response]io.Closer
-	err error
+	err             error
 }
 
 func (c *streamableClientConn) setProtocolVersion(s string) {
@@ -741,10 +740,16 @@ func (s *streamableClientConn) postMessage(ctx context.Context, sessionID string
 	}
 
 	sessionID = resp.Header.Get(sessionIDHeader)
-	if resp.Header.Get("Content-Type") == "text/event-stream" {
+	switch ct := resp.Header.Get("Content-Type"); ct {
+	case "text/event-stream":
 		go s.handleSSE(resp)
-	} else {
+	case "application/json":
+		// TODO: read the body and send to s.incoming (in a select that also receives from s.done).
 		resp.Body.Close()
+		return "", fmt.Errorf("streamable HTTP client does not yet support raw JSON responses")
+	default:
+		resp.Body.Close()
+		return "", fmt.Errorf("unsupported content type %q", ct)
 	}
 	return sessionID, nil
 }
@@ -760,7 +765,11 @@ func (s *streamableClientConn) handleSSE(resp *http.Response) {
 				// TODO: surface this error; possibly break the stream
 				return
 			}
-			s.incoming <- evt.data
+			select {
+			case <-s.done:
+				return
+			case s.incoming <- evt.data:
+			}
 		}
 	}()