fix: point the model back to a search when an MCP tool is withdrawn

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JgzuxyafKNF2uaEeL35XQw

frontend/src/lib/components/copilot/chat/AIChatManager.svelte.ts

@@ -144,6 +144,7 @@ import {
 	createMcpTools,
 	forgetLoadedMcpTools,
 	invalidateMcpRegistrations,
+	isRequestBodyRejection,
 	loadedMcpServers,
 	loadedMcpTools,
 	loadMcpServers,
@@ -2286,14 +2287,11 @@ export class AIChatManager {
 	 * A registered MCP tool carries a schema a third party wrote, and a provider that
 	 * refuses it refuses every later request in the conversation the same way — with an
 	 * error naming the request, not the tool. So a rejection drops the registered tools:
-	 * the next send goes out with the search tool and the wrappers only, and the model
-	 * can register again. A false positive costs one re-search. Statuses about the
-	 * account rather than the body (auth, quota) are left alone.
+	 * the send that follows goes out with the search tool and the wrappers only, and the
+	 * model can register again. A false positive costs one re-search.
 	 */
 	private dropMcpToolsOnRejectedRequest = (err: unknown) => {
-		const status = getErrorStatus(err)
-		if (status === undefined || status < 400 || status >= 500) return
-		if (status === 401 || status === 403 || status === 429) return
+		if (!isRequestBodyRejection(getErrorStatus(err))) return
 		if (loadedMcpTools(this.mcpOwnerId).length === 0) return
 		console.warn('Dropping registered MCP tools after a rejected request', err)
 		forgetLoadedMcpTools(this.mcpOwnerId)

frontend/src/lib/components/copilot/chat/global/mcpTools.test.ts

@@ -36,6 +36,7 @@ import {
 	createMcpTools,
 	forgetLoadedMcpTools,
 	invalidateMcpRegistrations,
+	isRequestBodyRejection,
 	loadedMcpServers,
 	mcpRegistryGeneration,
 	loadedMcpTools,
@@ -459,6 +460,22 @@ describe('loaded remote tools', () => {
 	})
 })
 
+// Dropping the registered tools is how a conversation recovers from a schema the
+// provider refuses, and it must not fire on an account problem: a quota error would
+// then cost the user every tool they had searched for.
+describe('request rejections that withdraw registered tools', () => {
+	it('separates a refused body from a refused account', () => {
+		expect(isRequestBodyRejection(400)).toBe(true)
+		expect(isRequestBodyRejection(422)).toBe(true)
+		expect(isRequestBodyRejection(401)).toBe(false)
+		expect(isRequestBodyRejection(403)).toBe(false)
+		expect(isRequestBodyRejection(429)).toBe(false)
+		expect(isRequestBodyRejection(500)).toBe(false)
+		// An abort carries no status.
+		expect(isRequestBodyRejection(undefined)).toBe(false)
+	})
+})
+
 describe('read/write split', () => {
 	it('refuses a mutating tool on the read path', async () => {
 		const result = await run('call_mcp_read_tool', {

frontend/src/lib/components/copilot/chat/global/mcpTools.ts

@@ -1,7 +1,7 @@
 import { z } from 'zod'
 import { ResourceService, type GetMcpToolsResponse } from '$lib/gen'
 import { createToolDef, type Tool } from '../shared'
-import { normalizeToolParameterSchema } from '../toolSchema'
+import { MCP_TOOL_NAME_PREFIX, normalizeToolParameterSchema } from '../toolSchema'
 import { enabledMcpPaths } from '$lib/components/mcp/enabledServers'
 
 /**
@@ -591,8 +591,19 @@ function shortHash(text: string): string {
  * `list_issues` stay apart, and nothing parses it back: the registry keys on the
  * pair, so truncation only has to stay unique.
  */
+/**
+ * Whether a failed chat request was the provider refusing the body rather than the
+ * account. A remote schema this provider will not accept refuses every later request
+ * in the conversation the same way, so the registered tools are dropped on the first
+ * of these; auth and quota statuses say nothing about the schemas.
+ */
+export function isRequestBodyRejection(status: number | undefined): boolean {
+	if (status === undefined || status < 400 || status >= 500) return false
+	return status !== 401 && status !== 403 && status !== 429
+}
+
 function registeredToolName(serverPath: string, toolName: string): string {
-	const full = `mcp_${sanitizeToolNamePart(serverPath)}__${sanitizeToolNamePart(toolName)}`
+	const full = `${MCP_TOOL_NAME_PREFIX}${sanitizeToolNamePart(serverPath)}__${sanitizeToolNamePart(toolName)}`
 	if (full.length <= MAX_TOOL_NAME_CHARS) return full
 	return `${full.slice(0, MAX_TOOL_NAME_CHARS - 8)}_${shortHash(full)}`
 }

frontend/src/lib/components/copilot/chat/shared.ts

@@ -7,7 +7,7 @@ import type { UserDraftItemKind } from '$lib/gen'
 // The gate's two refusals, from a module that holds prose and one size limit: under the
 // shallow-import rule below, the rest of plan mode is not reachable from here.
 import { PLAN_MODE_MESSAGES } from './planModeMessages'
-import { normalizeToolParameterSchema } from './toolSchema'
+import { MCP_TOOL_NAME_PREFIX, normalizeToolParameterSchema } from './toolSchema'
 // Import-free leaf, so it satisfies the shallow-import rule below.
 import {
 	openItemPreviewAction,
@@ -719,6 +719,15 @@ async function callTool<T>({
 }): Promise<string> {
 	const tool = tools.find((t) => t.def.function.name === functionName)
 	if (!tool) {
+		// A registered MCP tool is withdrawn whenever its registration is dropped — a
+		// server turned off, an eviction, a schema the provider refused — while the search
+		// result that advertised its name stays in the transcript. Searching again is the
+		// way back; the mode advice below would send the model after a tool it has not got.
+		if (functionName.startsWith(MCP_TOOL_NAME_PREFIX)) {
+			throw new Error(
+				`Unknown tool call: ${functionName}. That MCP tool is no longer loaded — call search_mcp_tools again to load it.`
+			)
+		}
 		throw new Error(
 			`Unknown tool call: ${functionName}. Probably not in the correct mode, use the change_mode tool to switch to the correct mode.`
 		)

frontend/src/lib/components/copilot/chat/toolSchema.ts

@@ -1,3 +1,10 @@
+/**
+ * Marks a chat tool name as a remote MCP tool registered for the conversation; built by
+ * `registeredToolName` in `global/mcpTools`. Here rather than there so `shared` can read
+ * it without an import cycle.
+ */
+export const MCP_TOOL_NAME_PREFIX = 'mcp_'
+
 /**
  * Recursively normalizes JSON Schema quirks that specific providers reject.
  *