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,

packages/core/test/mcp.test.ts

@@ -0,0 +1,14 @@
+import { describe, expect, test } from "bun:test"
+import { MCP } from "@opencode-ai/core/mcp/index"
+import { MCPClient } from "@opencode-ai/core/mcp/client"
+
+describe("MCP errors", () => {
+  test("expose useful messages", () => {
+    expect(new MCP.NotFoundError({ server: MCP.ServerName.make("demo") }).message).toBe("MCP server not found: demo")
+    expect(new MCP.ToolCallError({ server: MCP.ServerName.make("demo"), tool: "search", message: "failed" }).message).toBe(
+      "failed",
+    )
+    expect(new MCPClient.NeedsAuthError({ server: "demo" }).message).toBe("MCP server requires authentication: demo")
+    expect(new MCPClient.ConnectError({ server: "demo", message: "offline" }).message).toBe("offline")
+  })
+})