internal/mcp: implement roots

Add support for roots.

Change-Id: Ia50abc88f0047238272d698f30ce615b1a8fd486
Reviewed-on: https://go-review.googlesource.com/c/tools/+/671360
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

mcp/protocol/protocol.go

@@ -70,14 +70,16 @@ type CancelledParams struct {
 // additional capabilities.
 type ClientCapabilities struct {
 	// Experimental, non-standard capabilities that the client supports.
-	Experimental map[string]map[string]json.RawMessage `json:"experimental,omitempty"`
+	Experimental map[string]struct {
+	} `json:"experimental,omitempty"`
 	// Present if the client supports listing roots.
 	Roots *struct {
 		// Whether the client supports notifications for changes to the roots list.
 		ListChanged bool `json:"listChanged,omitempty"`
 	} `json:"roots,omitempty"`
 	// Present if the client supports sampling from an LLM.
-	Sampling map[string]json.RawMessage `json:"sampling,omitempty"`
+	Sampling struct {
+	} `json:"sampling,omitempty"`
 }
 
 type GetPromptParams struct {
@@ -131,7 +133,11 @@ type InitializeResult struct {
 	ServerInfo      Implementation `json:"serverInfo"`
 }
 
-type InitializedParams map[string]json.RawMessage
+type InitializedParams struct {
+	// This parameter name is reserved by MCP to allow clients and servers to attach
+	// additional metadata to their notifications.
+	Meta map[string]json.RawMessage `json:"_meta,omitempty"`
+}
 
 type ListPromptsParams struct {
 	// An opaque token representing the current pagination position. If provided,
@@ -150,6 +156,26 @@ type ListPromptsResult struct {
 	Prompts    []Prompt `json:"prompts"`
 }
 
+type ListRootsParams struct {
+	Meta *struct {
+		// If specified, the caller is requesting out-of-band progress notifications for
+		// this request (as represented by notifications/progress). The value of this
+		// parameter is an opaque token that will be attached to any subsequent
+		// notifications. The receiver is not obligated to provide these notifications.
+		ProgressToken *any `json:"progressToken,omitempty"`
+	} `json:"_meta,omitempty"`
+}
+
+// The client's response to a roots/list request from the server. This result
+// contains an array of Root objects, each representing a root directory or file
+// that the server can operate on.
+type ListRootsResult struct {
+	// This result property is reserved by the protocol to allow clients and servers
+	// to attach additional metadata to their responses.
+	Meta  map[string]json.RawMessage `json:"_meta,omitempty"`
+	Roots []Root                     `json:"roots"`
+}
+
 type ListToolsParams struct {
 	// An opaque token representing the current pagination position. If provided,
 	// the server should return results starting after this cursor.
@@ -213,16 +239,31 @@ type ResourceCapabilities struct {
 // The sender or recipient of messages and data in a conversation.
 type Role string
 
+// Represents a root directory or file that the server can operate on.
+type Root struct {
+	// An optional name for the root. This can be used to provide a human-readable
+	// identifier for the root, which may be useful for display purposes or for
+	// referencing the root in other parts of the application.
+	Name string `json:"name,omitempty"`
+	// The URI identifying the root. This *must* start with file:// for now. This
+	// restriction may be relaxed in future versions of the protocol to allow other
+	// URI schemes.
+	URI string `json:"uri"`
+}
+
 // Capabilities that a server may support. Known capabilities are defined here,
 // in this schema, but this is not a closed set: any server can define its own,
 // additional capabilities.
 type ServerCapabilities struct {
 	// Present if the server supports argument autocompletion suggestions.
-	Completions map[string]json.RawMessage `json:"completions,omitempty"`
+	Completions struct {
+	} `json:"completions,omitempty"`
 	// Experimental, non-standard capabilities that the server supports.
-	Experimental map[string]map[string]json.RawMessage `json:"experimental,omitempty"`
+	Experimental map[string]struct {
+	} `json:"experimental,omitempty"`
 	// Present if the server supports sending log messages to the client.
-	Logging map[string]json.RawMessage `json:"logging,omitempty"`
+	Logging struct {
+	} `json:"logging,omitempty"`
 	// Present if the server offers any prompt templates.
 	Prompts *PromptCapabilities `json:"prompts,omitempty"`
 	// Present if the server offers any resources to read.