fix(core): clean up mcp event surface (#35221)

packages/core/src/mcp/client.ts

@@ -46,7 +46,11 @@ const TolerantListPromptsResult = ListPromptsResultSchema.extend({
 
 export class NeedsAuthError extends Schema.TaggedErrorClass<NeedsAuthError>()("MCP.NeedsAuthError", {
   server: Schema.String,
-}) {}
+}) {
+  override get message() {
+    return `MCP server requires authentication: ${this.server}`
+  }
+}
 
 export class ConnectError extends Schema.TaggedErrorClass<ConnectError>()("MCP.ConnectError", {
   server: Schema.String,

packages/core/src/mcp/index.ts

@@ -127,7 +127,11 @@ export class ResourceContent extends Schema.Class<ResourceContent>("MCP.Resource
 
 export class NotFoundError extends Schema.TaggedErrorClass<NotFoundError>()("MCP.NotFoundError", {
   server: ServerName,
-}) {}
+}) {
+  override get message() {
+    return `MCP server not found: ${this.server}`
+  }
+}
 
 export class ToolCallError extends Schema.TaggedErrorClass<ToolCallError>()("MCP.ToolCallError", {
   server: ServerName,