Make Go MCPServerConfig type-safe with interface + marker method

Change MCPServerConfig from map[string]any to an interface with a
private marker method, matching the type-safety approach used for C#.
MCPStdioServerConfig and MCPHTTPServerConfig implement the interface
and use MarshalJSON to auto-inject the type discriminator.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: stephentoub

docs/features/mcp.md

@@ -113,15 +113,13 @@ func main() {
     }
     defer client.Stop()
 
-    // MCPServerConfig is map[string]any for flexibility
     session, err := client.CreateSession(ctx, &copilot.SessionConfig{
         Model: "gpt-5",
         MCPServers: map[string]copilot.MCPServerConfig{
-            "my-local-server": {
-                "type":    "local",
-                "command": "node",
-                "args":    []string{"./mcp-server.js"},
-                "tools":   []string{"*"},
+            "my-local-server": copilot.MCPStdioServerConfig{
+                Command: "node",
+                Args:    []string{"./mcp-server.js"},
+                Tools:   []string{"*"},
             },
         },
     })

go/internal/e2e/mcp_and_agents_test.go

@@ -18,11 +18,10 @@ func TestMCPServers(t *testing.T) {
 		ctx.ConfigureForTest(t)
 
 		mcpServers := map[string]copilot.MCPServerConfig{
-			"test-server": {
-				"type":    "local",
-				"command": "echo",
-				"args":    []string{"hello"},
-				"tools":   []string{"*"},
+			"test-server": copilot.MCPStdioServerConfig{
+				Command: "echo",
+				Args:    []string{"hello"},
+				Tools:   []string{"*"},
 			},
 		}
 
@@ -75,11 +74,10 @@ func TestMCPServers(t *testing.T) {
 
 		// Resume with MCP servers
 		mcpServers := map[string]copilot.MCPServerConfig{
-			"test-server": {
-				"type":    "local",
-				"command": "echo",
-				"args":    []string{"hello"},
-				"tools":   []string{"*"},
+			"test-server": copilot.MCPStdioServerConfig{
+				Command: "echo",
+				Args:    []string{"hello"},
+				Tools:   []string{"*"},
 			},
 		}
 
@@ -117,13 +115,12 @@ func TestMCPServers(t *testing.T) {
 		mcpServerDir := filepath.Dir(mcpServerPath)
 
 		mcpServers := map[string]copilot.MCPServerConfig{
-			"env-echo": {
-				"type":    "local",
-				"command": "node",
-				"args":    []string{mcpServerPath},
-				"tools":   []string{"*"},
-				"env":     map[string]string{"TEST_SECRET": "hunter2"},
-				"cwd":     mcpServerDir,
+			"env-echo": copilot.MCPStdioServerConfig{
+				Command: "node",
+				Args:    []string{mcpServerPath},
+				Tools:   []string{"*"},
+				Env:     map[string]string{"TEST_SECRET": "hunter2"},
+				Cwd:     mcpServerDir,
 			},
 		}
 
@@ -157,17 +154,15 @@ func TestMCPServers(t *testing.T) {
 		ctx.ConfigureForTest(t)
 
 		mcpServers := map[string]copilot.MCPServerConfig{
-			"server1": {
-				"type":    "local",
-				"command": "echo",
-				"args":    []string{"server1"},
-				"tools":   []string{"*"},
+			"server1": copilot.MCPStdioServerConfig{
+				Command: "echo",
+				Args:    []string{"server1"},
+				Tools:   []string{"*"},
 			},
-			"server2": {
-				"type":    "local",
-				"command": "echo",
-				"args":    []string{"server2"},
-				"tools":   []string{"*"},
+			"server2": copilot.MCPStdioServerConfig{
+				Command: "echo",
+				Args:    []string{"server2"},
+				Tools:   []string{"*"},
 			},
 		}
 
@@ -327,11 +322,10 @@ func TestCustomAgents(t *testing.T) {
 				Description: "An agent with its own MCP servers",
 				Prompt:      "You are an agent with MCP servers.",
 				MCPServers: map[string]copilot.MCPServerConfig{
-					"agent-server": {
-						"type":    "local",
-						"command": "echo",
-						"args":    []string{"agent-mcp"},
-						"tools":   []string{"*"},
+					"agent-server": copilot.MCPStdioServerConfig{
+						Command: "echo",
+						Args:    []string{"agent-mcp"},
+						Tools:   []string{"*"},
 					},
 				},
 			},
@@ -399,11 +393,10 @@ func TestCombinedConfiguration(t *testing.T) {
 		ctx.ConfigureForTest(t)
 
 		mcpServers := map[string]copilot.MCPServerConfig{
-			"shared-server": {
-				"type":    "local",
-				"command": "echo",
-				"args":    []string{"shared"},
-				"tools":   []string{"*"},
+			"shared-server": copilot.MCPStdioServerConfig{
+				Command: "echo",
+				Args:    []string{"shared"},
+				Tools:   []string{"*"},
 			},
 		}
 

go/types.go

@@ -382,29 +382,57 @@ type SessionHooks struct {
 	OnErrorOccurred       ErrorOccurredHandler
 }
 
-// MCPStdioServerConfig configures a local/stdio MCP server
+// MCPServerConfig is implemented by MCP server configuration types.
+// Only MCPStdioServerConfig and MCPHTTPServerConfig implement this interface.
+type MCPServerConfig interface {
+	mcpServerConfig()
+}
+
+// MCPStdioServerConfig configures a local/stdio MCP server.
 type MCPStdioServerConfig struct {
 	Tools   []string          `json:"tools"`
-	Type    string            `json:"type,omitempty"` // "local" or "stdio"
 	Timeout int               `json:"timeout,omitempty"`
 	Command string            `json:"command"`
 	Args    []string          `json:"args"`
 	Env     map[string]string `json:"env,omitempty"`
 	Cwd     string            `json:"cwd,omitempty"`
 }
 
-// MCPHTTPServerConfig configures a remote MCP server (HTTP or SSE)
+func (MCPStdioServerConfig) mcpServerConfig() {}
+
+// MarshalJSON implements json.Marshaler, injecting the "type" discriminator.
+func (c MCPStdioServerConfig) MarshalJSON() ([]byte, error) {
+	type alias MCPStdioServerConfig
+	return json.Marshal(struct {
+		Type string `json:"type"`
+		alias
+	}{
+		Type:  "stdio",
+		alias: alias(c),
+	})
+}
+
+// MCPHTTPServerConfig configures a remote MCP server (HTTP or SSE).
 type MCPHTTPServerConfig struct {
 	Tools   []string          `json:"tools"`
-	Type    string            `json:"type"` // "http" or "sse"
 	Timeout int               `json:"timeout,omitempty"`
 	URL     string            `json:"url"`
 	Headers map[string]string `json:"headers,omitempty"`
 }
 
-// MCPServerConfig can be either MCPStdioServerConfig or MCPHTTPServerConfig
-// Use a map[string]any for flexibility, or create separate configs
-type MCPServerConfig map[string]any
+func (MCPHTTPServerConfig) mcpServerConfig() {}
+
+// MarshalJSON implements json.Marshaler, injecting the "type" discriminator.
+func (c MCPHTTPServerConfig) MarshalJSON() ([]byte, error) {
+	type alias MCPHTTPServerConfig
+	return json.Marshal(struct {
+		Type string `json:"type"`
+		alias
+	}{
+		Type:  "http",
+		alias: alias(c),
+	})
+}
 
 // CustomAgentConfig configures a custom agent
 type CustomAgentConfig struct {

test/scenarios/tools/mcp-servers/go/main.go

@@ -30,10 +30,10 @@ func main() {
 		if argsStr := os.Getenv("MCP_SERVER_ARGS"); argsStr != "" {
 			args = strings.Split(argsStr, " ")
 		}
-		mcpServers["example"] = copilot.MCPServerConfig{
-			"type":    "stdio",
-			"command": cmd,
-			"args":    args,
+		mcpServers["example"] = copilot.MCPStdioServerConfig{
+			Command: cmd,
+			Args:    args,
+			Tools:   []string{"*"},
 		}
 	}