mcp: accept parameterized Accept media types (#853)

Normalize accept tokens before validation so valid headers with media
type parameters still match their base media types.

This allows the streamable mcp server to validate the following header
`Accept: application/json;charset=utf-8, text/event-stream`

Co-authored-by: Maciej Kisiel <mkisiel@google.com>

mcp/streamable.go

@@ -274,19 +274,7 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 
 	// Allow multiple 'Accept' headers.
 	// https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Accept#syntax
-	accept := strings.Split(strings.Join(req.Header.Values("Accept"), ","), ",")
-	var jsonOK, streamOK bool
-	for _, c := range accept {
-		switch strings.TrimSpace(c) {
-		case "application/json", "application/*":
-			jsonOK = true
-		case "text/event-stream", "text/*":
-			streamOK = true
-		case "*/*":
-			jsonOK = true
-			streamOK = true
-		}
-	}
+	jsonOK, streamOK := streamableAccepts(req.Header.Values("Accept"))
 
 	if req.Method == http.MethodGet {
 		if !streamOK {
@@ -551,6 +539,26 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
 	sessInfo.transport.ServeHTTP(w, req)
 }
 
+func streamableAccepts(values []string) (jsonOK, streamOK bool) {
+	for _, value := range values {
+		for _, raw := range strings.Split(value, ",") {
+			token := strings.TrimSpace(raw)
+			// Ignore Accept parameters like ";charset=utf-8"; match the base media type.
+			base, _, _ := strings.Cut(token, ";")
+			switch strings.ToLower(strings.TrimSpace(base)) {
+			case "application/json", "application/*":
+				jsonOK = true
+			case "text/event-stream", "text/*":
+				streamOK = true
+			case "*/*":
+				jsonOK = true
+				streamOK = true
+			}
+		}
+	}
+	return jsonOK, streamOK
+}
+
 // A StreamableServerTransport implements the server side of the MCP streamable
 // transport.
 //