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mcp: gracefully handle a nil server in handlers (#164)

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If the getServer function passed to NewSSEHandler or
NewStreamableHTTPHandler returns nil, serve a 400 instead of panicking.

Fixes #161.
This commit is contained in:
Jonathan Amsterdam 2025-07-24 10:35:14 -04:00 • committed by GitHub
commit c879ac3c61
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1 changed files with 17 additions and 6 deletions
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6
mcp/streamable.go
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@ -50,6 +50,7 @@ type StreamableHTTPOptions struct {
//
// The getServer function is used to create or look up servers for new
// sessions. It is OK for getServer to return the same server multiple times.
// If getServer returns nil, a 400 Bad Request will be served.
func NewStreamableHTTPHandler(getServer func(*http.Request) *Server, opts *StreamableHTTPOptions) *StreamableHTTPHandler {
return &StreamableHTTPHandler{
getServer: getServer,
@ -135,6 +136,11 @@ func (h *StreamableHTTPHandler) ServeHTTP(w http.ResponseWriter, req *http.Reque
if session == nil {
s := NewStreamableServerTransport(randText(), nil)
server := h.getServer(req)
if server == nil {
// The getServer argument to NewStreamableHTTPHandler returned nil.
http.Error(w, "no server available", http.StatusBadRequest)
return
}
// Pass req.Context() here, to allow middleware to add context values.
// The context is detached in the jsonrpc2 library when handling the
// long-running stream.