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Fix subagent tool calling loops silently switching models #2932

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Fix subagent tool calling loops silently switching models #2932

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Copilot Jan 16, 2026
0259f69
Fix: Remove model fallback in subagent tool calling loops to preserve…
Copilot Jan 16, 2026
efc570d
Address code review feedback: Remove unused variable and use undersco…
Copilot Jan 16, 2026
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6 changes: 1 addition & 5 deletions src/extension/mcp/vscode-node/mcpToolCallingLoop.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -43,11 +43,7 @@ export class McpToolCallingLoop extends ToolCallingLoop<IMcpToolCallingLoopOptio
}

private async getEndpoint(request: ChatRequest) {
let endpoint = await this.endpointProvider.getChatEndpoint(this.options.request);
if (!endpoint.supportsToolCalls) {
endpoint = await this.endpointProvider.getChatEndpoint('gpt-4.1');
}
return endpoint;
return await this.endpointProvider.getChatEndpoint(this.options.request);
}

protected async buildPrompt(buildPromptContext: IBuildPromptContext, progress: Progress<ChatResponseReferencePart | ChatResponseProgressPart>, token: CancellationToken): Promise<IBuildPromptResult> {
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6 changes: 1 addition & 5 deletions src/extension/prompt/node/codebaseToolCalling.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -47,11 +47,7 @@ export class CodebaseToolCallingLoop extends ToolCallingLoop<ICodebaseToolCallin
}

private async getEndpoint(request: ChatRequest) {
let endpoint = await this.endpointProvider.getChatEndpoint(this.options.request);
if (!endpoint.supportsToolCalls) {
endpoint = await this.endpointProvider.getChatEndpoint('gpt-4.1');
}
return endpoint;
return await this.endpointProvider.getChatEndpoint(this.options.request);
}

protected async buildPrompt(buildPromptContext: IBuildPromptContext, progress: Progress<ChatResponseReferencePart | ChatResponseProgressPart>, token: CancellationToken): Promise<IBuildPromptResult> {
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6 changes: 1 addition & 5 deletions src/extension/prompt/node/searchSubagentToolCallingLoop.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -62,11 +62,7 @@ export class SearchSubagentToolCallingLoop extends ToolCallingLoop<ISearchSubage
}

private async getEndpoint(request: ChatRequest) {
let endpoint = await this.endpointProvider.getChatEndpoint(this.options.request);
if (!endpoint.supportsToolCalls) {
endpoint = await this.endpointProvider.getChatEndpoint('gpt-4.1');
}
return endpoint;
return await this.endpointProvider.getChatEndpoint(this.options.request);
}

protected async buildPrompt(buildPromptContext: IBuildPromptContext, progress: Progress<ChatResponseReferencePart | ChatResponseProgressPart>, token: CancellationToken): Promise<IBuildPromptResult> {
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113 changes: 113 additions & 0 deletions src/extension/prompt/node/test/searchSubagentToolCallingLoop.spec.ts
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@@ -0,0 +1,113 @@
/*---------------------------------------------------------------------------------------------
* Copyright (c) Microsoft Corporation. All rights reserved.
* Licensed under the MIT License. See License.txt in the project root for license information.
*--------------------------------------------------------------------------------------------*/

import { beforeEach, describe, expect, it, vi } from 'vitest';
import { IEndpointProvider } from '../../../../platform/endpoint/common/endpointProvider';
import { IChatEndpoint } from '../../../../platform/networking/common/networking';
import { DisposableStore } from '../../../../util/vs/base/common/lifecycle';
import { IInstantiationService } from '../../../../util/vs/platform/instantiation/common/instantiation';
import { createExtensionUnitTestingServices } from '../../../test/node/services';
import { Conversation, Turn } from '../../common/conversation';
import { ISearchSubagentToolCallingLoopOptions, SearchSubagentToolCallingLoop } from '../searchSubagentToolCallingLoop';

describe('SearchSubagentToolCallingLoop', () => {
let store: DisposableStore;
let instantiationService: IInstantiationService;
let endpointProvider: IEndpointProvider;
let mockEndpoint: IChatEndpoint;

beforeEach(() => {
store = new DisposableStore();

// Create testing services
const serviceCollection = store.add(createExtensionUnitTestingServices(store));
const accessor = serviceCollection.createTestingAccessor();
instantiationService = accessor.get(IInstantiationService);

// Create mock endpoint that simulates Gemini 3 Pro with supportsToolCalls = false
mockEndpoint = {
family: 'gemini',
model: 'gemini-3-pro-preview',
name: 'Gemini 3 Pro',
version: '1.0',
maxOutputTokens: 8192,
modelMaxPromptTokens: 100000,
supportsToolCalls: false, // This simulates the bug scenario
supportsVision: true,
supportsPrediction: false,
showInModelPicker: true,
urlOrRequestMetadata: 'https://test-endpoint',
tokenizer: 0,
isDefault: false,
isFallback: false,
isPremium: false,
multiplier: 1,
} as IChatEndpoint;

// Get and override the endpoint provider to return our mock endpoint
endpointProvider = accessor.get(IEndpointProvider);
endpointProvider.getChatEndpoint = vi.fn(async (_requestOrModel: any) => {
// Always return the same endpoint - this ensures we don't fallback to GPT-4.1
return mockEndpoint;
});
});

it('should use the requested model even when supportsToolCalls is false', async () => {
// Arrange
const options: ISearchSubagentToolCallingLoopOptions = {
toolCallLimit: 4,
conversation: new Conversation('test-session', [
new Turn('turn-1', { type: 'user', message: 'test query' })
]),
request: {} as any,
location: 7 as any, // ChatLocation.Panel
promptText: 'test query',
};

const loop = instantiationService.createInstance(SearchSubagentToolCallingLoop, options);

// Act - call the private getEndpoint method via reflection to test the behavior
const getEndpoint = (loop as any).getEndpoint.bind(loop);
const endpoint = await getEndpoint(options.request);

// Assert
// The endpoint should be our mock Gemini endpoint, not GPT-4.1
expect(endpoint.model).toBe('gemini-3-pro-preview');
expect(endpoint.family).toBe('gemini');

// Verify that getChatEndpoint was called with the original request
expect(endpointProvider.getChatEndpoint).toHaveBeenCalledWith(options.request);

// Verify that it was NOT called with 'gpt-4.1' (the old fallback behavior)
const calls = (endpointProvider.getChatEndpoint as any).mock.calls;
expect(calls.every((call: any) => call[0] !== 'gpt-4.1')).toBe(true);
});

it('should use the requested model with supportsToolCalls true', async () => {
// Arrange - Update mock endpoint to support tool calls
mockEndpoint.supportsToolCalls = true;

const options: ISearchSubagentToolCallingLoopOptions = {
toolCallLimit: 4,
conversation: new Conversation('test-session', [
new Turn('turn-1', { type: 'user', message: 'test query' })
]),
request: {} as any,
location: 7 as any, // ChatLocation.Panel
promptText: 'test query',
};

const loop = instantiationService.createInstance(SearchSubagentToolCallingLoop, options);

// Act
const getEndpoint = (loop as any).getEndpoint.bind(loop);
const endpoint = await getEndpoint(options.request);

// Assert
expect(endpoint.model).toBe('gemini-3-pro-preview');
expect(endpoint.family).toBe('gemini');
expect(endpoint.supportsToolCalls).toBe(true);
});
});