mcp: fix sseClientConn to use custom HTTP client (#105)

When attempting to create an OAuth authentication on top of the SSE
transport I discovered the sseClientConn was using http.DefaultClient
for requests instead of the custom client provided via
SSEClientTransportOptions.

This change stores the HTTP client in the sseClientConn struct and uses
it for all requests. Also adds test to verify the custom client is
properly used.

mcp/sse.go

@@ -391,6 +391,7 @@ func (c *SSEClientTransport) Connect(ctx context.Context) (Connection, error) {
 
 	// From here on, the stream takes ownership of resp.Body.
 	s := &sseClientConn{
+		client:      httpClient,
 		sseEndpoint: c.sseEndpoint,
 		msgEndpoint: msgEndpoint,
 		incoming:    make(chan []byte, 100),
@@ -511,9 +512,10 @@ func scanEvents(r io.Reader) iter.Seq2[event, error] {
 //   - Reads are SSE 'message' events, and pushes them onto a buffered channel.
 //   - Close terminates the GET request.
 type sseClientConn struct {
-	sseEndpoint *url.URL    // SSE endpoint for the GET
-	msgEndpoint *url.URL    // session endpoint for POSTs
-	incoming    chan []byte // queue of incoming messages
+	client      *http.Client // HTTP client to use for requests
+	sseEndpoint *url.URL     // SSE endpoint for the GET
+	msgEndpoint *url.URL     // session endpoint for POSTs
+	incoming    chan []byte  // queue of incoming messages
 
 	mu     sync.Mutex
 	body   io.ReadCloser // body of the hanging GET
@@ -564,7 +566,7 @@ func (c *sseClientConn) Write(ctx context.Context, msg JSONRPCMessage) error {
 		return err
 	}
 	req.Header.Set("Content-Type", "application/json")
-	resp, err := http.DefaultClient.Do(req)
+	resp, err := c.client.Do(req)
 	if err != nil {
 		return err
 	}

mcp/sse_test.go

@@ -10,6 +10,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"strings"
+	"sync/atomic"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -34,7 +35,17 @@ func TestSSEServer(t *testing.T) {
 			httpServer := httptest.NewServer(sseHandler)
 			defer httpServer.Close()
 
-			clientTransport := NewSSEClientTransport(httpServer.URL, nil)
+			var customClientUsed int64
+			customClient := &http.Client{
+				Transport: roundTripperFunc(func(req *http.Request) (*http.Response, error) {
+					atomic.AddInt64(&customClientUsed, 1)
+					return http.DefaultTransport.RoundTrip(req)
+				}),
+			}
+
+			clientTransport := NewSSEClientTransport(httpServer.URL, &SSEClientTransportOptions{
+				HTTPClient: customClient,
+			})
 
 			c := NewClient("testClient", "v1.0.0", nil)
 			cs, err := c.Connect(ctx, clientTransport)
@@ -61,6 +72,11 @@ func TestSSEServer(t *testing.T) {
 				t.Errorf("tools/call 'greet' mismatch (-want +got):\n%s", diff)
 			}
 
+			// Verify that customClient was used
+			if atomic.LoadInt64(&customClientUsed) == 0 {
+				t.Error("Expected custom HTTP client to be used, but it wasn't")
+			}
+
 			// Test that closing either end of the connection terminates the other
 			// end.
 			if closeServerFirst {
@@ -162,3 +178,10 @@ func TestScanEvents(t *testing.T) {
 		})
 	}
 }
+
+// roundTripperFunc is a helper to create a custom RoundTripper
+type roundTripperFunc func(*http.Request) (*http.Response, error)
+
+func (f roundTripperFunc) RoundTrip(req *http.Request) (*http.Response, error) {
+	return f(req)
+}