fix: implement EnableSampling() to properly declare sampling capability

Previously, EnableSampling() was a no-op that didn't actually enable the
sampling capability in the server's declared capabilities.

Changes:
- Add Sampling field to mcp.ServerCapabilities struct
- Add sampling field to internal serverCapabilities struct
- Update EnableSampling() to set the sampling capability flag
- Update handleInitialize() to include sampling in capability response
- Add test to verify sampling capability is properly declared

Now when EnableSampling() is called, the server will properly declare
sampling capability during initialization, allowing clients to know
that the server supports sending sampling requests.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: andig

mcp/types.go

@@ -472,6 +472,8 @@ type ServerCapabilities struct {
 		// list.
 		ListChanged bool `json:"listChanged,omitempty"`
 	} `json:"resources,omitempty"`
+	// Present if the server supports sending sampling requests to clients.
+	Sampling *struct{} `json:"sampling,omitempty"`
 	// Present if the server offers any tools to call.
 	Tools *struct {
 		// Whether this server supports notifications for changes to the tool list.

server/sampling.go

@@ -12,6 +12,9 @@ import (
 func (s *MCPServer) EnableSampling() {
 	s.capabilitiesMu.Lock()
 	defer s.capabilitiesMu.Unlock()
+	
+	enabled := true
+	s.capabilities.sampling = &enabled
 }
 
 // RequestSampling sends a sampling request to the client.

server/sampling_test.go

@@ -113,3 +113,42 @@ func TestMCPServer_RequestSampling_Success(t *testing.T) {
 		t.Errorf("expected model %q, got %q", "test-model", result.Model)
 	}
 }
+
+func TestMCPServer_EnableSampling_SetsCapability(t *testing.T) {
+	server := NewMCPServer("test", "1.0.0")
+	
+	// Verify sampling capability is not set initially
+	ctx := context.Background()
+	initRequest := mcp.InitializeRequest{
+		Params: mcp.InitializeParams{
+			ProtocolVersion: "2025-03-26",
+			ClientInfo: mcp.Implementation{
+				Name:    "test-client",
+				Version: "1.0.0",
+			},
+			Capabilities: mcp.ClientCapabilities{},
+		},
+	}
+	
+	result, err := server.handleInitialize(ctx, 1, initRequest)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	
+	if result.Capabilities.Sampling != nil {
+		t.Error("sampling capability should not be set before EnableSampling() is called")
+	}
+	
+	// Enable sampling
+	server.EnableSampling()
+	
+	// Verify sampling capability is now set
+	result, err = server.handleInitialize(ctx, 2, initRequest)
+	if err != nil {
+		t.Fatalf("unexpected error after EnableSampling(): %v", err)
+	}
+	
+	if result.Capabilities.Sampling == nil {
+		t.Error("sampling capability should be set after EnableSampling() is called")
+	}
+}

server/server.go

@@ -181,6 +181,7 @@ type serverCapabilities struct {
 	resources *resourceCapabilities
 	prompts   *promptCapabilities
 	logging   *bool
+	sampling  *bool
 }
 
 // resourceCapabilities defines the supported resource-related features
@@ -580,6 +581,10 @@ func (s *MCPServer) handleInitialize(
 		capabilities.Logging = &struct{}{}
 	}
 
+	if s.capabilities.sampling != nil && *s.capabilities.sampling {
+		capabilities.Sampling = &struct{}{}
+	}
+
 	result := mcp.InitializeResult{
 		ProtocolVersion: s.protocolVersion(request.Params.ProtocolVersion),
 		ServerInfo: mcp.Implementation{