modelcontextprotocol · guglielmo-san · Aug 3, 2026 · Aug 1, 2026 · Aug 3, 2026
diff --git a/mcp/client.go b/mcp/client.go
@@ -245,11 +245,12 @@ func (e unsupportedProtocolVersionError) Error() string {
 	return fmt.Sprintf("unsupported protocol version: %q", e.version)
 }
 
-// ClientSessionOptions is reserved for future use.
+// ClientSessionOptions configures a client session created by [Client.Connect].
 type ClientSessionOptions struct {
-	// protocolVersion overrides the protocol version sent in the initialize
-	// request, for testing. If empty, latestProtocolVersion is used.
-	protocolVersion string
+	// ProtocolVersion is the protocol version sent in the initialize (or
+	// discover) request. If empty, the latest supported version is used.
+	// The server may negotiate a different mutually supported version.
+	ProtocolVersion string
 }
 
 func (c *Client) capabilities(protocolVersion string) *ClientCapabilities {
@@ -311,8 +312,8 @@ func (c *Client) Connect(ctx context.Context, t Transport, opts *ClientSessionOp
 	}
 
 	protocolVersion := latestProtocolVersion
-	if opts != nil && opts.protocolVersion != "" {
-		protocolVersion = opts.protocolVersion
+	if opts != nil && opts.ProtocolVersion != "" {
+		protocolVersion = opts.ProtocolVersion
 	}
 
 	if protocolVersion >= protocolVersion20260728 {

diff --git a/mcp/client_test.go b/mcp/client_test.go
@@ -740,7 +740,7 @@ func TestClientConnectDiscover(t *testing.T) {
 			defer ss.Close()
 
 			c := NewClient(testImpl, nil)
-			cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+			cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 			if err != nil {
 				t.Fatalf("Connect: %v", err)
 			}
@@ -805,7 +805,7 @@ func TestClientConnectDiscover_RequestContents(t *testing.T) {
 			return nil, nil
 		},
 	})
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("Connect: %v", err)
 	}
@@ -851,7 +851,7 @@ func TestInMemory_E2E_DiscoverSuccess(t *testing.T) {
 	defer ss.Close()
 
 	client := NewClient(&Implementation{Name: "stdio-like-client", Version: "v1"}, nil)
-	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client.Connect: %v", err)
 	}
@@ -908,7 +908,7 @@ func TestInMemory_E2E_DiscoverFallback_NoOverlap(t *testing.T) {
 	defer ss.Close()
 
 	client := NewClient(&Implementation{Name: "new-client", Version: "v1"}, nil)
-	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client.Connect: %v", err)
 	}
@@ -955,7 +955,7 @@ func TestInMemory_E2E_DiscoverFallback_MethodNotFound(t *testing.T) {
 	defer ss.Close()
 
 	client := NewClient(&Implementation{Name: "new-client", Version: "v1"}, nil)
-	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client.Connect: %v", err)
 	}
@@ -1005,7 +1005,7 @@ func TestInMemory_E2E_DiscoverFallback_UnsupportedProtocolVersion(t *testing.T)
 	defer ss.Close()
 
 	client := NewClient(&Implementation{Name: "new-client", Version: "v1"}, nil)
-	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client.Connect: %v", err)
 	}
@@ -1068,7 +1068,7 @@ func TestClientConnectDiscover_UnsupportedVersionNegotiation(t *testing.T) {
 	defer ss.Close()
 
 	c := NewClient(testImpl, nil)
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: unsupportedClientVersion})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: unsupportedClientVersion})
 	if err != nil {
 		t.Fatalf("Connect: %v", err)
 	}

diff --git a/mcp/elicitation_test.go b/mcp/elicitation_test.go
@@ -114,7 +114,7 @@ func TestElicitationURLMode(t *testing.T) {
 				},
 				ElicitationHandler: tc.handler,
 			})
-			cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+			cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -172,7 +172,7 @@ func TestElicitationCompleteNotification(t *testing.T) {
 			t.Fatal(err)
 		}
 		defer ss.Close()
-		cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+		cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -255,7 +255,7 @@ func TestElicitationNoValidationWithoutAccept(t *testing.T) {
 					return &ElicitResult{Action: tc.action, Content: tc.content}, nil
 				},
 			})
-			cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+			cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 			if err != nil {
 				t.Fatal(err)
 			}

diff --git a/mcp/mcp_test.go b/mcp/mcp_test.go
@@ -174,7 +174,7 @@ func TestEndToEnd(t *testing.T) {
 		//
 		// Pin the session to 2025-11-25 so the legacy
 		// semantics apply.
-		cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+		cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -764,7 +764,7 @@ func TestMiddleware(t *testing.T) {
 	// Pin to 2025-11-25 because the test's expected wire sequence asserts
 	// the legacy initialize / notifications/initialized handshake, which
 	// 2026-07-28 replaces with server/discover.
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -854,7 +854,7 @@ func TestNoJSONNull(t *testing.T) {
 	}
 
 	c := NewClient(testImpl, nil)
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -1023,7 +1023,7 @@ func TestElicitationUnsupportedMethod(t *testing.T) {
 			return &CreateMessageResult{Model: "aModel", Content: &TextContent{}}, nil
 		},
 	})
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -1072,7 +1072,7 @@ func TestElicitationSchemaValidation(t *testing.T) {
 			return &ElicitResult{Action: "accept", Content: map[string]any{"test": "value"}}, nil
 		},
 	})
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -1558,7 +1558,7 @@ func TestElicitContentValidation(t *testing.T) {
 			return &ElicitResult{Action: "accept", Content: map[string]any{"test": "potato"}}, nil
 		},
 	})
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -1641,7 +1641,7 @@ func TestElicitationProgressToken(t *testing.T) {
 			return &ElicitResult{Action: "accept"}, nil
 		},
 	})
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -1683,7 +1683,7 @@ func TestElicitationCapabilityDeclaration(t *testing.T) {
 		}
 		defer ss.Close()
 
-		cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+		cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -1720,7 +1720,7 @@ func TestElicitationCapabilityDeclaration(t *testing.T) {
 		}
 		defer ss.Close()
 
-		cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+		cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -1757,7 +1757,7 @@ func TestElicitationDefaultValues(t *testing.T) {
 			return &ElicitResult{Action: "accept", Content: map[string]any{"default": "response"}}, nil
 		},
 	})
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -1912,7 +1912,7 @@ func TestKeepAliveFailure_Logged(t *testing.T) {
 		// Pin to 2025-11-25: KeepAlive uses the ping RPC, which is removed
 		// in 2026-07-28, so keepalive is only meaningful on legacy protocol
 		// versions.
-		cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+		cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -2087,7 +2087,7 @@ func TestSynchronousNotifications(t *testing.T) {
 			t.Fatal(err)
 		}
 		t.Cleanup(func() { _ = ss.Close() })
-		cs, err := client.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+		cs, err := client.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -2501,7 +2501,7 @@ func runSubscriptionsListenTest(t *testing.T, client *Client, server *Server, ct
 	ctx, topCancel := context.WithTimeout(context.Background(), 30*time.Second)
 	defer topCancel()
 
-	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client connect: %v", err)
 	}
@@ -2659,7 +2659,7 @@ func TestSubscriptionsListen_NoHandlersNoListen(t *testing.T) {
 			return next(ctx, method, req)
 		}
 	})
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client connect: %v", err)
 	}
@@ -2782,7 +2782,7 @@ func TestResourceSubscriptions_Streamable(t *testing.T) {
 		},
 	})
 	cs, err := c.Connect(ctx, &StreamableClientTransport{Endpoint: httpServer.URL},
-		&ClientSessionOptions{protocolVersion: protocolVersion20260728})
+		&ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client connect: %v", err)
 	}
@@ -2851,7 +2851,7 @@ func TestResourceSubscriptions_InMemory(t *testing.T) {
 			events <- resourceSubEvent{uri: req.Params.URI, id: id}
 		},
 	})
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client connect: %v", err)
 	}
@@ -2928,7 +2928,7 @@ func TestResourceSubscriptions_Subscribe_Idempotent(t *testing.T) {
 	c := NewClient(testImpl, &ClientOptions{
 		ResourceUpdatedHandler: func(context.Context, *ResourceUpdatedNotificationRequest) {},
 	})
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client connect: %v", err)
 	}
@@ -3000,7 +3000,7 @@ func TestResourceSubscriptions_MultipleURIs(t *testing.T) {
 			events <- resourceSubEvent{uri: req.Params.URI, id: id}
 		},
 	})
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client connect: %v", err)
 	}
@@ -3105,7 +3105,7 @@ func TestSubscriptionsListen_MultipleSessions(t *testing.T) {
 		}
 		c := newSubListenClient(events)
 		cs, err := c.Connect(context.Background(), ct,
-			&ClientSessionOptions{protocolVersion: protocolVersion20260728})
+			&ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 		if err != nil {
 			t.Fatalf("client connect: %v", err)
 		}
@@ -3205,7 +3205,7 @@ func TestSubscriptionsListen_ResourceListChanged(t *testing.T) {
 
 	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
 	defer cancel()
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client connect: %v", err)
 	}
@@ -3249,7 +3249,7 @@ func TestSubscriptionsListen_DisconnectScrubsMaps(t *testing.T) {
 	}
 	c := newSubListenClient(events)
 	cs, err := c.Connect(context.Background(), ct,
-		&ClientSessionOptions{protocolVersion: protocolVersion20260728})
+		&ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client connect: %v", err)
 	}
@@ -3428,7 +3428,7 @@ func TestCallCustomMethodTypedNilParams(t *testing.T) {
 	if err := AddSendingCustomMethod[*pingParams, *pingResult](c, "acme/ping"); err != nil {
 		t.Fatal(err)
 	}
-	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatal(err)
 	}

diff --git a/mcp/mrtr_test.go b/mcp/mrtr_test.go
@@ -312,7 +312,7 @@ func TestMultiRoundTrip_ServerMiddleware(t *testing.T) {
 				},
 			})
 			c.AddRoots(&Root{URI: "file:///workspace", Name: "workspace"})
-			cs, err := c.Connect(t.Context(), ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+			cs, err := c.Connect(t.Context(), ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 			if err != nil {
 				t.Fatalf("client.Connect() error = %v", err)
 			}
@@ -516,7 +516,7 @@ func mustConnect(t *testing.T, s *Server, clientOpts *ClientOptions) *ClientSess
 	})
 
 	c := NewClient(testImpl, clientOpts)
-	cs, err := c.Connect(t.Context(), ct, &ClientSessionOptions{protocolVersion: protocolVersion20260728})
+	cs, err := c.Connect(t.Context(), ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20260728})
 	if err != nil {
 		t.Fatalf("client.Connect() error = %v", err)
 	}

diff --git a/mcp/sampling_test.go b/mcp/sampling_test.go
@@ -53,7 +53,7 @@ func TestSamplingWithTools_ToolUse(t *testing.T) {
 	}
 	defer ss.Close()
 
-	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -124,7 +124,7 @@ func TestSamplingWithTools_ToolResult(t *testing.T) {
 	}
 	defer ss.Close()
 
-	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -288,7 +288,7 @@ func TestSamplingWithTools_ToolResultWithError(t *testing.T) {
 	}
 	defer ss.Close()
 
-	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -350,7 +350,7 @@ func TestSamplingWithTools_ParallelToolCalls(t *testing.T) {
 	}
 	defer ss.Close()
 
-	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -420,7 +420,7 @@ func TestCreateMessage_MultipleContentError(t *testing.T) {
 	}
 	defer ss.Close()
 
-	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protocolVersion20251125})
+	cs, err := client.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protocolVersion20251125})
 	if err != nil {
 		t.Fatal(err)
 	}

diff --git a/mcp/server_test.go b/mcp/server_test.go
@@ -1663,7 +1663,7 @@ func TestServerSession_RejectsServerInitiated(t *testing.T) {
 					},
 				})
 				c.AddRoots(&Root{URI: "file:///workspace"})
-				cs, err := c.Connect(ctx, ct, &ClientSessionOptions{protocolVersion: protoVer})
+				cs, err := c.Connect(ctx, ct, &ClientSessionOptions{ProtocolVersion: protoVer})
 				if err != nil {
 					t.Fatal(err)
 				}