diff --git a/mcp/sse.go b/mcp/sse.go
index 0a1f9b1b..d1b52599 100644
--- a/mcp/sse.go
+++ b/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
 	}
diff --git a/mcp/sse_test.go b/mcp/sse_test.go
index e1df9536..153185d3 100644
--- a/mcp/sse_test.go
+++ b/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)
+}