Support MCP servers with partial capabilities in vmcp (#2416)

The vmcp was rejecting backends that don't implement all three MCP
capabilities (tools, resources, prompts). This violated the MCP
specification, which explicitly makes all capabilities optional.

The oci-registry MCP server only implements tools, causing vmcp to
fail during aggregation when it tried to unconditionally query
resources and prompts.

Changes:
- Query server capabilities during MCP initialization handshake
- Conditionally query only advertised capabilities
- Return empty results for unsupported capabilities
- Add tests for backends with partial capability support

This allows vmcp to successfully aggregate backends that implement
any subset of tools, resources, and prompts, as intended by the
MCP specification.

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

Co-authored-by: Claude <noreply@anthropic.com>

Author: JAORMX

pkg/vmcp/client/client.go

@@ -207,18 +207,15 @@ func (h *httpBackendClient) defaultClientFactory(ctx context.Context, target *vm
 		return nil, fmt.Errorf("failed to start client connection: %w", err)
 	}
 
-	// Initialize the MCP connection
-	if err := initializeClient(ctx, c); err != nil {
-		_ = c.Close()
-		return nil, fmt.Errorf("failed to initialize MCP connection: %w", err)
-	}
-
+	// Note: Initialization is deferred to the caller (e.g., ListCapabilities)
+	// so that ServerCapabilities can be captured and used for conditional querying
 	return c, nil
 }
 
-// initializeClient performs MCP protocol initialization handshake.
-func initializeClient(ctx context.Context, c *client.Client) error {
-	_, err := c.Initialize(ctx, mcp.InitializeRequest{
+// initializeClient performs MCP protocol initialization handshake and returns server capabilities.
+// This allows the caller to determine which optional features the server supports.
+func initializeClient(ctx context.Context, c *client.Client) (*mcp.ServerCapabilities, error) {
+	result, err := c.Initialize(ctx, mcp.InitializeRequest{
 		Params: mcp.InitializeParams{
 			ProtocolVersion: mcp.LATEST_PROTOCOL_VERSION,
 			ClientInfo: mcp.Implementation{
@@ -235,37 +232,88 @@ func initializeClient(ctx context.Context, c *client.Client) error {
 			},
 		},
 	})
-	return err
+	if err != nil {
+		return nil, err
+	}
+	return &result.Capabilities, nil
+}
+
+// queryTools queries tools from a backend if the server advertises tool support.
+func queryTools(ctx context.Context, c *client.Client, supported bool, backendID string) (*mcp.ListToolsResult, error) {
+	if supported {
+		result, err := c.ListTools(ctx, mcp.ListToolsRequest{})
+		if err != nil {
+			return nil, fmt.Errorf("failed to list tools from backend %s: %w", backendID, err)
+		}
+		return result, nil
+	}
+	logger.Debugf("Backend %s does not advertise tools capability, skipping tools query", backendID)
+	return &mcp.ListToolsResult{Tools: []mcp.Tool{}}, nil
+}
+
+// queryResources queries resources from a backend if the server advertises resource support.
+func queryResources(ctx context.Context, c *client.Client, supported bool, backendID string) (*mcp.ListResourcesResult, error) {
+	if supported {
+		result, err := c.ListResources(ctx, mcp.ListResourcesRequest{})
+		if err != nil {
+			return nil, fmt.Errorf("failed to list resources from backend %s: %w", backendID, err)
+		}
+		return result, nil
+	}
+	logger.Debugf("Backend %s does not advertise resources capability, skipping resources query", backendID)
+	return &mcp.ListResourcesResult{Resources: []mcp.Resource{}}, nil
+}
+
+// queryPrompts queries prompts from a backend if the server advertises prompt support.
+func queryPrompts(ctx context.Context, c *client.Client, supported bool, backendID string) (*mcp.ListPromptsResult, error) {
+	if supported {
+		result, err := c.ListPrompts(ctx, mcp.ListPromptsRequest{})
+		if err != nil {
+			return nil, fmt.Errorf("failed to list prompts from backend %s: %w", backendID, err)
+		}
+		return result, nil
+	}
+	logger.Debugf("Backend %s does not advertise prompts capability, skipping prompts query", backendID)
+	return &mcp.ListPromptsResult{Prompts: []mcp.Prompt{}}, nil
 }
 
 // ListCapabilities queries a backend for its MCP capabilities.
 // Returns tools, resources, and prompts exposed by the backend.
+// Only queries capabilities that the server advertises during initialization.
 func (h *httpBackendClient) ListCapabilities(ctx context.Context, target *vmcp.BackendTarget) (*vmcp.CapabilityList, error) {
 	logger.Debugf("Querying capabilities from backend %s (%s)", target.WorkloadName, target.BaseURL)
 
-	// Create a client for this backend
+	// Create a client for this backend (not yet initialized)
 	c, err := h.clientFactory(ctx, target)
 	if err != nil {
 		return nil, fmt.Errorf("failed to create client for backend %s: %w", target.WorkloadID, err)
 	}
 	defer c.Close()
 
-	// Query tools
-	toolsResp, err := c.ListTools(ctx, mcp.ListToolsRequest{})
+	// Initialize the client and get server capabilities
+	serverCaps, err := initializeClient(ctx, c)
 	if err != nil {
-		return nil, fmt.Errorf("failed to list tools from backend %s: %w", target.WorkloadID, err)
+		return nil, fmt.Errorf("failed to initialize client for backend %s: %w", target.WorkloadID, err)
 	}
 
-	// Query resources
-	resourcesResp, err := c.ListResources(ctx, mcp.ListResourcesRequest{})
+	logger.Debugf("Backend %s capabilities: tools=%v, resources=%v, prompts=%v",
+		target.WorkloadID, serverCaps.Tools != nil, serverCaps.Resources != nil, serverCaps.Prompts != nil)
+
+	// Query each capability type based on server advertisement
+	// Check for nil BEFORE passing to functions to avoid interface{} nil pointer issues
+	toolsResp, err := queryTools(ctx, c, serverCaps.Tools != nil, target.WorkloadID)
 	if err != nil {
-		return nil, fmt.Errorf("failed to list resources from backend %s: %w", target.WorkloadID, err)
+		return nil, err
 	}
 
-	// Query prompts
-	promptsResp, err := c.ListPrompts(ctx, mcp.ListPromptsRequest{})
+	resourcesResp, err := queryResources(ctx, c, serverCaps.Resources != nil, target.WorkloadID)
 	if err != nil {
-		return nil, fmt.Errorf("failed to list prompts from backend %s: %w", target.WorkloadID, err)
+		return nil, err
+	}
+
+	promptsResp, err := queryPrompts(ctx, c, serverCaps.Prompts != nil, target.WorkloadID)
+	if err != nil {
+		return nil, err
 	}
 
 	// Convert MCP types to vmcp types
@@ -355,6 +403,11 @@ func (h *httpBackendClient) CallTool(
 	}
 	defer c.Close()
 
+	// Initialize the client
+	if _, err := initializeClient(ctx, c); err != nil {
+		return nil, fmt.Errorf("failed to initialize client for backend %s: %w", target.WorkloadID, err)
+	}
+
 	// Call the tool
 	result, err := c.CallTool(ctx, mcp.CallToolRequest{
 		Params: mcp.CallToolParams{
@@ -426,6 +479,11 @@ func (h *httpBackendClient) ReadResource(ctx context.Context, target *vmcp.Backe
 	}
 	defer c.Close()
 
+	// Initialize the client
+	if _, err := initializeClient(ctx, c); err != nil {
+		return nil, fmt.Errorf("failed to initialize client for backend %s: %w", target.WorkloadID, err)
+	}
+
 	// Read the resource
 	result, err := c.ReadResource(ctx, mcp.ReadResourceRequest{
 		Params: mcp.ReadResourceParams{
@@ -476,6 +534,11 @@ func (h *httpBackendClient) GetPrompt(
 	}
 	defer c.Close()
 
+	// Initialize the client
+	if _, err := initializeClient(ctx, c); err != nil {
+		return "", fmt.Errorf("failed to initialize client for backend %s: %w", target.WorkloadID, err)
+	}
+
 	// Get the prompt
 	// Convert map[string]any to map[string]string
 	stringArgs := make(map[string]string)

pkg/vmcp/client/client_test.go

@@ -51,6 +51,37 @@ func TestHTTPBackendClient_ListCapabilities_WithMockFactory(t *testing.T) {
 	})
 }
 
+func TestQueryHelpers_PartialCapabilities(t *testing.T) {
+	t.Parallel()
+
+	t.Run("queryTools with unsupported capability returns empty slice", func(t *testing.T) {
+		t.Parallel()
+
+		result, err := queryTools(context.Background(), nil, false, "test-backend")
+		require.NoError(t, err)
+		assert.NotNil(t, result)
+		assert.Empty(t, result.Tools)
+	})
+
+	t.Run("queryResources with unsupported capability returns empty slice", func(t *testing.T) {
+		t.Parallel()
+
+		result, err := queryResources(context.Background(), nil, false, "test-backend")
+		require.NoError(t, err)
+		assert.NotNil(t, result)
+		assert.Empty(t, result.Resources)
+	})
+
+	t.Run("queryPrompts with unsupported capability returns empty slice", func(t *testing.T) {
+		t.Parallel()
+
+		result, err := queryPrompts(context.Background(), nil, false, "test-backend")
+		require.NoError(t, err)
+		assert.NotNil(t, result)
+		assert.Empty(t, result.Prompts)
+	})
+}
+
 func TestDefaultClientFactory_UnsupportedTransport(t *testing.T) {
 	t.Parallel()