fix(routing): Refactor tool turn handling for the conversation history in NumericalClassifierStrategy to prevent 400 Bad Request

packages/core/src/routing/strategies/numericalClassifierStrategy.test.ts

@@ -5,7 +5,10 @@
  */
 
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
-import { NumericalClassifierStrategy } from './numericalClassifierStrategy.js';
+import {
+  NumericalClassifierStrategy,
+  HISTORY_TURNS_FOR_CONTEXT,
+} from './numericalClassifierStrategy.js';
 import type { RoutingContext } from '../routingStrategy.js';
 import type { Config } from '../../config/config.js';
 import type { BaseLlmClient } from '../../core/baseLlmClient.js';
@@ -423,18 +426,71 @@ describe('NumericalClassifierStrategy', () => {
     expect(consoleWarnSpy).toHaveBeenCalled();
   });
 
-  it('should include tool-related history when sending to classifier', async () => {
-    mockContext.history = [
-      { role: 'user', parts: [{ text: 'call a tool' }] },
-      { role: 'model', parts: [{ functionCall: { name: 'test_tool' } }] },
+  it('should strip leading tool turns when history starts with tool calls', async () => {
+    const history: Content[] = [
+      { role: 'model', parts: [{ functionCall: { name: 'leading_tool' } }] },
+      {
+        role: 'user',
+        parts: [
+          {
+            functionResponse: { name: 'leading_tool', response: { ok: true } },
+          },
+        ],
+      },
+      { role: 'model', parts: [{ text: 'text response 1' }] },
+      { role: 'user', parts: [{ text: 'text request 2' }] },
+    ];
+    mockContext.history = history;
+    const mockApiResponse = {
+      complexity_reasoning: 'Simple.',
+      complexity_score: 10,
+    };
+    vi.mocked(mockBaseLlmClient.generateJson).mockResolvedValue(
+      mockApiResponse,
+    );
+
+    await strategy.route(
+      mockContext,
+      mockConfig,
+      mockBaseLlmClient,
+      mockLocalLiteRtLmClient,
+    );
+
+    const generateJsonCall = vi.mocked(mockBaseLlmClient.generateJson).mock
+      .calls[0][0];
+    const contents = generateJsonCall.contents;
+
+    // Expect leading tool turns (index 0 and 1) to be stripped, keeping only text turns (index 2 and 3)
+    const expectedContents = [
+      ...history.slice(2),
+      {
+        role: 'user',
+        parts: [{ text: 'simple task' }],
+      },
+    ];
+
+    expect(contents).toEqual(expectedContents);
+  });
+
+  it('should preserve tool turns when they appear after a non-tool turn in the middle of history', async () => {
+    const history: Content[] = [
+      { role: 'user', parts: [{ text: 'turn 0 (before)' }] },
+      { role: 'model', parts: [{ text: 'turn 1 (before)' }] },
+      { role: 'user', parts: [{ text: 'turn 2 (before)' }] },
+      { role: 'model', parts: [{ text: 'turn 3 (before)' }] },
+      { role: 'model', parts: [{ functionCall: { name: 'middle_tool' } }] },
       {
         role: 'user',
         parts: [
-          { functionResponse: { name: 'test_tool', response: { ok: true } } },
+          { functionResponse: { name: 'middle_tool', response: { ok: true } } },
         ],
       },
-      { role: 'user', parts: [{ text: 'another user turn' }] },
+      { role: 'model', parts: [{ text: 'turn 6 (after)' }] },
+      { role: 'user', parts: [{ text: 'turn 7 (after)' }] },
+      { role: 'model', parts: [{ text: 'turn 8 (after)' }] },
+      { role: 'user', parts: [{ text: 'turn 9 (after)' }] },
     ];
+    mockContext.history = history;
     const mockApiResponse = {
       complexity_reasoning: 'Simple.',
       complexity_score: 10,
@@ -454,9 +510,9 @@ describe('NumericalClassifierStrategy', () => {
       .calls[0][0];
     const contents = generateJsonCall.contents;
 
+    // Expect all 8 sliced turns (starting from non-tool turn 2) to be preserved
     const expectedContents = [
-      ...mockContext.history,
-      // The last user turn is the request part
+      ...history.slice(2),
       {
         role: 'user',
         parts: [{ text: 'simple task' }],
@@ -466,12 +522,110 @@ describe('NumericalClassifierStrategy', () => {
     expect(contents).toEqual(expectedContents);
   });
 
-  it('should respect HISTORY_TURNS_FOR_CONTEXT', async () => {
-    const longHistory: Content[] = [];
-    for (let i = 0; i < 30; i++) {
-      longHistory.push({ role: 'user', parts: [{ text: `Message ${i}` }] });
+  it('should preserve tool turns when they appear at the very end of history following a non-tool turn', async () => {
+    const history: Content[] = [
+      { role: 'user', parts: [{ text: 'turn 0' }] },
+      { role: 'model', parts: [{ text: 'turn 1' }] },
+      { role: 'user', parts: [{ text: 'turn 2' }] },
+      { role: 'model', parts: [{ text: 'turn 3' }] },
+      { role: 'user', parts: [{ text: 'turn 4' }] },
+      { role: 'model', parts: [{ text: 'turn 5' }] },
+      { role: 'user', parts: [{ text: 'turn 6' }] },
+      { role: 'model', parts: [{ text: 'turn 7' }] },
+      { role: 'model', parts: [{ functionCall: { name: 'end_tool' } }] },
+      {
+        role: 'user',
+        parts: [
+          { functionResponse: { name: 'end_tool', response: { ok: true } } },
+        ],
+      },
+    ];
+    mockContext.history = history;
+    const mockApiResponse = {
+      complexity_reasoning: 'Simple.',
+      complexity_score: 10,
+    };
+    vi.mocked(mockBaseLlmClient.generateJson).mockResolvedValue(
+      mockApiResponse,
+    );
+
+    await strategy.route(
+      mockContext,
+      mockConfig,
+      mockBaseLlmClient,
+      mockLocalLiteRtLmClient,
+    );
+
+    const generateJsonCall = vi.mocked(mockBaseLlmClient.generateJson).mock
+      .calls[0][0];
+    const contents = generateJsonCall.contents;
+
+    // Expect all 8 sliced turns to be preserved because index 2 is a non-tool turn
+    const expectedContents = [
+      ...history.slice(2),
+      {
+        role: 'user',
+        parts: [{ text: 'simple task' }],
+      },
+    ];
+
+    expect(contents).toEqual(expectedContents);
+  });
+
+  it('should send only the new request prompt if the entire history consists of tool-related turns', async () => {
+    const history: Content[] = [
+      { role: 'model', parts: [{ functionCall: { name: 'tool_A' } }] },
+      {
+        role: 'user',
+        parts: [
+          { functionResponse: { name: 'tool_A', response: { ok: true } } },
+        ],
+      },
+      { role: 'model', parts: [{ functionCall: { name: 'tool_B' } }] },
+      {
+        role: 'user',
+        parts: [
+          { functionResponse: { name: 'tool_B', response: { ok: true } } },
+        ],
+      },
+    ];
+    mockContext.history = history;
+    const mockApiResponse = {
+      complexity_reasoning: 'Simple standalone task.',
+      complexity_score: 10,
+    };
+    vi.mocked(mockBaseLlmClient.generateJson).mockResolvedValue(
+      mockApiResponse,
+    );
+
+    await strategy.route(
+      mockContext,
+      mockConfig,
+      mockBaseLlmClient,
+      mockLocalLiteRtLmClient,
+    );
+
+    const generateJsonCall = vi.mocked(mockBaseLlmClient.generateJson).mock
+      .calls[0][0];
+    const contents = generateJsonCall.contents;
+
+    // Expect all history turns to be filtered out, leaving exactly just the new request
+    const expectedContents = [
+      {
+        role: 'user',
+        parts: [{ text: 'simple task' }],
+      },
+    ];
+
+    expect(contents).toEqual(expectedContents);
+  });
+
+  it('should respect HISTORY_TURNS_FOR_CONTEXT correctly when history has only text turns', async () => {
+    const history: Content[] = [];
+    for (let i = 0; i < HISTORY_TURNS_FOR_CONTEXT + 2; i++) {
+      history.push({ role: 'user', parts: [{ text: `Message ${i}` }] });
     }
-    mockContext.history = longHistory;
+    mockContext.history = history;
     const mockApiResponse = {
       complexity_reasoning: 'Simple.',
       complexity_score: 10,
@@ -491,18 +645,53 @@ describe('NumericalClassifierStrategy', () => {
       .calls[0][0];
     const contents = generateJsonCall.contents;
 
-    // Manually calculate what the history should be
-    const HISTORY_TURNS_FOR_CONTEXT = 8;
-    const finalHistory = longHistory.slice(-HISTORY_TURNS_FOR_CONTEXT);
+    // Expect exactly the last 8 turns (history.slice(2))
+    expect(contents).toEqual([
+      ...history.slice(2),
+      { role: 'user', parts: [{ text: 'simple task' }] },
+    ]);
+    expect(contents).toHaveLength(HISTORY_TURNS_FOR_CONTEXT + 1);
+  });
 
-    // Last part is the request
-    const requestPart = {
-      role: 'user',
-      parts: [{ text: 'simple task' }],
+  it('should respect HISTORY_TURNS_FOR_CONTEXT correctly when history starts with tool calls', async () => {
+    const history: Content[] = [
+      { role: 'model', parts: [{ functionCall: { name: 'tool_0' } }] },
+      {
+        role: 'user',
+        parts: [
+          { functionResponse: { name: 'tool_0', response: { ok: true } } },
+        ],
+      },
+    ];
+    for (let i = 0; i < HISTORY_TURNS_FOR_CONTEXT; i++) {
+      history.push({ role: 'user', parts: [{ text: `Message ${i}` }] });
+    }
+    mockContext.history = history;
+    const mockApiResponse = {
+      complexity_reasoning: 'Simple.',
+      complexity_score: 10,
     };
+    vi.mocked(mockBaseLlmClient.generateJson).mockResolvedValue(
+      mockApiResponse,
+    );
+
+    await strategy.route(
+      mockContext,
+      mockConfig,
+      mockBaseLlmClient,
+      mockLocalLiteRtLmClient,
+    );
+
+    const generateJsonCall = vi.mocked(mockBaseLlmClient.generateJson).mock
+      .calls[0][0];
+    const contents = generateJsonCall.contents;
 
-    expect(contents).toEqual([...finalHistory, requestPart]);
-    expect(contents).toHaveLength(9);
+    // Expect exactly the last 8 text turns (history.slice(2))
+    expect(contents).toEqual([
+      ...history.slice(2),
+      { role: 'user', parts: [{ text: 'simple task' }] },
+    ]);
+    expect(contents).toHaveLength(HISTORY_TURNS_FOR_CONTEXT + 1);
   });
 
   it('should use a fallback promptId if not found in context', async () => {

packages/core/src/routing/strategies/numericalClassifierStrategy.ts

@@ -15,12 +15,16 @@ import type {
 import { resolveClassifierModel, isGemini3Model } from '../../config/models.js';
 import { createUserContent, Type } from '@google/genai';
 import type { Config } from '../../config/config.js';
+import {
+  isFunctionCall,
+  isFunctionResponse,
+} from '../../utils/messageInspectors.js';
 import { debugLogger } from '../../utils/debugLogger.js';
 import type { LocalLiteRtLmClient } from '../../core/localLiteRtLmClient.js';
 import { LlmRole } from '../../telemetry/types.js';
 
 // The number of recent history turns to provide to the router for context.
-const HISTORY_TURNS_FOR_CONTEXT = 8;
+export const HISTORY_TURNS_FOR_CONTEXT = 8;
 
 const FLASH_MODEL = 'flash';
 const PRO_MODEL = 'pro';
@@ -115,7 +119,22 @@ export class NumericalClassifierStrategy implements RoutingStrategy {
 
       const promptId = getPromptIdWithFallback('classifier-router');
 
-      const finalHistory = context.history.slice(-HISTORY_TURNS_FOR_CONTEXT);
+      const candidateSlice = context.history.slice(-HISTORY_TURNS_FOR_CONTEXT);
+
+      // Find the first non-tool turn. The server cannot always handle tool-related
+      // turns in the first slots of the contents array, so we strip them if they appear at the start.
+      let firstTextIndex = -1;
+      for (let i = 0; i < candidateSlice.length; i++) {
+        if (
+          !isFunctionCall(candidateSlice[i]) &&
+          !isFunctionResponse(candidateSlice[i])
+        ) {
+          firstTextIndex = i;
+          break;
+        }
+      }
+      const finalHistory =
+        firstTextIndex === -1 ? [] : candidateSlice.slice(firstTextIndex);
 
       // Wrap the user's request in tags to prevent prompt injection
       const requestParts = Array.isArray(context.request)