feat(mcp): add Model Context Protocol server support

Add MCP client and provider packages that allow integrating external
MCP tool servers into the review loop. Includes config commands for
managing MCP servers, stdio subprocess integration tests, and
comprehensive test coverage.

Author: lizhengfeng101

cmd/opencodereview/config_cmd.go

@@ -94,17 +94,23 @@ func runConfigSet(key, value string) error {
 
 func runConfigUnset(key string) error {
 	parts := strings.SplitN(key, ".", 2)
-	if len(parts) != 2 || parts[0] != "custom_providers" || parts[1] == "" {
-		return fmt.Errorf("unset only supports custom_providers.<name>")
+	if len(parts) != 2 || parts[1] == "" {
+		return fmt.Errorf("unset supports custom_providers.<name> and mcp_servers.<name>")
 	}
-	name := parts[1]
 
 	configPath, err := defaultConfigPath()
 	if err != nil {
 		return err
 	}
 
-	return unsetCustomProvider(configPath, name)
+	switch parts[0] {
+	case "custom_providers":
+		return unsetCustomProvider(configPath, parts[1])
+	case "mcp_servers":
+		return unsetMCPServer(configPath, parts[1])
+	default:
+		return fmt.Errorf("unset supports custom_providers.<name> and mcp_servers.<name>")
+	}
 }
 
 func unsetCustomProvider(configPath, name string) error {
@@ -130,6 +136,32 @@ func unsetCustomProvider(configPath, name string) error {
 	return nil
 }
 
+func unsetMCPServer(configPath, name string) error {
+	cfg, err := loadOrCreateConfig(configPath)
+	if err != nil {
+		return fmt.Errorf("load config: %w", err)
+	}
+
+	if cfg.MCPServers == nil {
+		return fmt.Errorf("MCP server %q not found", name)
+	}
+	if _, exists := cfg.MCPServers[name]; !exists {
+		return fmt.Errorf("MCP server %q not found", name)
+	}
+
+	delete(cfg.MCPServers, name)
+	if len(cfg.MCPServers) == 0 {
+		cfg.MCPServers = nil
+	}
+
+	if err := saveConfig(configPath, cfg); err != nil {
+		return err
+	}
+
+	fmt.Printf("Deleted MCP server %q.\n", name)
+	return nil
+}
+
 // deleteCustomProvider removes a custom provider from cfg in memory.
 // Returns true if the deleted provider was the active one.
 func deleteCustomProvider(cfg *Config, name string) (bool, error) {
@@ -166,15 +198,25 @@ type ProviderEntry struct {
 	ExtraHeaders map[string]string `json:"extra_headers,omitempty"`
 }
 
+// MCPServerConfig holds configuration for a single MCP server (stdio transport).
+type MCPServerConfig struct {
+	Command string   `json:"command"`
+	Args    []string `json:"args,omitempty"`
+	Env     []string `json:"env,omitempty"`
+	Tools   []string `json:"tools,omitempty"`
+	Setup   string   `json:"setup,omitempty"`
+}
+
 // Config represents the user-level configuration file (~/.opencodereview/config.json).
 type Config struct {
-	Provider        string                   `json:"provider,omitempty"`
-	Model           string                   `json:"model,omitempty"`
-	Providers       map[string]ProviderEntry `json:"providers,omitempty"`
-	CustomProviders map[string]ProviderEntry `json:"custom_providers,omitempty"`
-	Llm             LlmConfig                `json:"llm,omitempty"`
-	Language        string                   `json:"language,omitempty"`
-	Telemetry       *TelemetryConfig         `json:"telemetry,omitempty"`
+	Provider        string                     `json:"provider,omitempty"`
+	Model           string                     `json:"model,omitempty"`
+	Providers       map[string]ProviderEntry   `json:"providers,omitempty"`
+	CustomProviders map[string]ProviderEntry   `json:"custom_providers,omitempty"`
+	Llm             LlmConfig                  `json:"llm,omitempty"`
+	Language        string                     `json:"language,omitempty"`
+	Telemetry       *TelemetryConfig           `json:"telemetry,omitempty"`
+	MCPServers      map[string]MCPServerConfig `json:"mcp_servers,omitempty"`
 }
 
 type LlmConfig struct {
@@ -234,6 +276,9 @@ func setConfigValue(cfg *Config, key, value string) error {
 	if strings.HasPrefix(key, "custom_providers.") {
 		return setCustomProviderValue(cfg, key, value)
 	}
+	if strings.HasPrefix(key, "mcp_servers.") {
+		return setMCPServerValue(cfg, key, value)
+	}
 
 	switch key {
 	case "provider":
@@ -329,7 +374,7 @@ func setConfigValue(cfg *Config, key, value string) error {
 		}
 		cfg.Llm.ExtraBody = m
 	default:
-		return fmt.Errorf("unknown config key: %s\nSupported keys: provider, model, providers.<name>.<field>, custom_providers.<name>.<field>, llm.url, llm.auth_token, llm.auth_header, llm.model, llm.use_anthropic, llm.extra_body, llm.extra_headers, language, telemetry.enabled, telemetry.exporter, telemetry.otlp_endpoint, telemetry.content_logging\nProvider fields: api_key, url, protocol, model, models, auth_header, extra_body, extra_headers", key)
+		return fmt.Errorf("unknown config key: %s\nSupported keys: provider, model, providers.<name>.<field>, custom_providers.<name>.<field>, mcp_servers.<name>.<field>, llm.url, llm.auth_token, llm.auth_header, llm.model, llm.use_anthropic, llm.extra_body, llm.extra_headers, language, telemetry.enabled, telemetry.exporter, telemetry.otlp_endpoint, telemetry.content_logging\nProvider fields: api_key, url, protocol, model, models, auth_header, extra_body, extra_headers\nMCP server fields: command, args, env, tools, setup", key)
 	}
 	return nil
 }
@@ -491,6 +536,70 @@ func setCustomProviderField(cfg *Config, name, field, key, value string) error {
 	return nil
 }
 
+func setMCPServerValue(cfg *Config, key, value string) error {
+	parts := strings.SplitN(key, ".", 3)
+	if len(parts) != 3 || parts[1] == "" || parts[2] == "" {
+		return fmt.Errorf("invalid MCP server key %q: expected mcp_servers.<name>.<field>", key)
+	}
+	name, field := parts[1], parts[2]
+
+	if cfg.MCPServers == nil {
+		cfg.MCPServers = make(map[string]MCPServerConfig)
+	}
+	entry := cfg.MCPServers[name]
+
+	switch field {
+	case "command":
+		if value == "" {
+			return fmt.Errorf("MCP server command cannot be empty")
+		}
+		entry.Command = value
+	case "args":
+		var args []string
+		if err := json.Unmarshal([]byte(value), &args); err != nil {
+			return fmt.Errorf("invalid JSON array for %s: %w", key, err)
+		}
+		entry.Args = args
+	case "env":
+		var env []string
+		if err := json.Unmarshal([]byte(value), &env); err != nil {
+			return fmt.Errorf("invalid JSON array for %s: %w", key, err)
+		}
+		for _, e := range env {
+			idx := strings.Index(e, "=")
+			if idx <= 0 {
+				return fmt.Errorf("invalid env entry %q: must be in KEY=VALUE format", e)
+			}
+		}
+		entry.Env = env
+	case "tools":
+		var tools []string
+		if err := json.Unmarshal([]byte(value), &tools); err != nil {
+			return fmt.Errorf("invalid JSON array for %s: %w", key, err)
+		}
+		seen := make(map[string]struct{}, len(tools))
+		filtered := make([]string, 0, len(tools))
+		for _, t := range tools {
+			if t == "" {
+				return fmt.Errorf("tool names in %s must not be empty", key)
+			}
+			if _, dup := seen[t]; dup {
+				continue
+			}
+			seen[t] = struct{}{}
+			filtered = append(filtered, t)
+		}
+		entry.Tools = filtered
+	case "setup":
+		entry.Setup = value
+	default:
+		return fmt.Errorf("unknown MCP server field %q: supported fields are command, args, env, tools, setup", field)
+	}
+
+	cfg.MCPServers[name] = entry
+	return nil
+}
+
 func (c *Config) ensureTelemetry() {
 	if c.Telemetry == nil {
 		c.Telemetry = &TelemetryConfig{}