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mcp: accept parameterized Content-Type types (#890)

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Use a shared helper for Content-Type parsing in streamable transport
request validation and client response handling.

Follow up to https://github.com/modelcontextprotocol/go-sdk/pull/853,
where we did this for `Accept` headers.

This allows the streamable mcp server to validate the following header
`Content-Type: application/json;charset=utf-8`

Co-authored-by: Maciej Kisiel <mkisiel@google.com>
This commit is contained in:
Kalvin C 2026-04-23 05:51:47 -07:00 • committed by GitHub
commit 35f1b07e48
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1 changed files with 40 additions and 12 deletions
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21
mcp/streamable.go
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@ -264,12 +264,9 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
return
}
// Validate 'Content-Type' header.
if req.Method == http.MethodPost {
mediaType, _, err := mime.ParseMediaType(req.Header.Get("Content-Type"))
if err != nil || mediaType != "application/json" {
http.Error(w, "Content-Type must be 'application/json'", http.StatusUnsupportedMediaType)
return
}
if req.Method == http.MethodPost && baseMediaType(req.Header.Get("Content-Type")) != "application/json" {
http.Error(w, "Content-Type must be 'application/json'", http.StatusUnsupportedMediaType)
return
}
}
@ -560,6 +557,14 @@ func streamableAccepts(values []string) (jsonOK, streamOK bool) {
return jsonOK, streamOK
}
func baseMediaType(value string) string {
mediaType, _, err := mime.ParseMediaType(value)
if err != nil {
return ""
}
return mediaType
}
// A StreamableServerTransport implements the server side of the MCP streamable
// transport.
//
@ -1671,7 +1676,7 @@ func (c *streamableClientConn) connectStandaloneSSE() {
resp.Body.Close()
return
}
if resp.Header.Get("Content-Type") != "text/event-stream" {
if baseMediaType(resp.Header.Get("Content-Type")) != "text/event-stream" {
// modelcontextprotocol/go-sdk#736: some servers return 200 OK or redirect with
// non-SSE content type instead of text/event-stream for the standalone
// SSE stream.
@ -1855,7 +1860,7 @@ func (c *streamableClientConn) Write(ctx context.Context, msg jsonrpc.Message) e
return nil
}
contentType := strings.TrimSpace(strings.SplitN(resp.Header.Get("Content-Type"), ";", 2)[0])
contentType := baseMediaType(resp.Header.Get("Content-Type"))
switch contentType {
case "application/json":
go c.handleJSON(requestSummary, resp)