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

@@ -505,6 +505,112 @@ describe('NumericalClassifierStrategy', () => {
     expect(contents).toHaveLength(9);
   });
 
+  it('should adjust slice boundary to avoid severing a functionResponse from its preceding functionCall', async () => {
+    const history: Content[] = [
+      { role: 'user', parts: [{ text: 'initial request' }] },
+      { role: 'model', parts: [{ functionCall: { name: 'test_tool' } }] },
+      {
+        role: 'user',
+        parts: [
+          { functionResponse: { name: 'test_tool', response: { ok: true } } },
+        ],
+      },
+      { role: 'model', parts: [{ text: 'tool output analyzed' }] },
+      { role: 'user', parts: [{ text: 'next step' }] },
+      { role: 'model', parts: [{ text: 'working on it' }] },
+      { role: 'user', parts: [{ text: 'almost done?' }] },
+      { role: 'model', parts: [{ text: 'yes' }] },
+      { role: 'user', parts: [{ text: 'final check' }] },
+      { role: 'model', parts: [{ text: 'all good' }] },
+    ];
+    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 it to start at index 1 (functionCall) rather than index 2 (functionResponse)
+    const expectedContents = [
+      ...history.slice(1),
+      {
+        role: 'user',
+        parts: [{ text: 'simple task' }],
+      },
+    ];
+
+    expect(contents).toEqual(expectedContents);
+  });
+
+  it('should adjust slice boundary correctly even when the preceding functionCall turn contains mixed parts (text + functionCall)', async () => {
+    const history: Content[] = [
+      { role: 'user', parts: [{ text: 'initial request' }] },
+      {
+        role: 'model',
+        parts: [
+          { text: 'thinking about which tool to call...' },
+          { functionCall: { name: 'test_tool' } },
+        ],
+      },
+      {
+        role: 'user',
+        parts: [
+          { functionResponse: { name: 'test_tool', response: { ok: true } } },
+        ],
+      },
+      { role: 'model', parts: [{ text: 'tool output analyzed' }] },
+      { role: 'user', parts: [{ text: 'next step' }] },
+      { role: 'model', parts: [{ text: 'working on it' }] },
+      { role: 'user', parts: [{ text: 'almost done?' }] },
+      { role: 'model', parts: [{ text: 'yes' }] },
+      { role: 'user', parts: [{ text: 'final check' }] },
+      { role: 'model', parts: [{ text: 'all good' }] },
+    ];
+    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 it to start at index 1 (mixed functionCall turn) rather than index 2 (functionResponse)
+    const expectedContents = [
+      ...history.slice(1),
+      {
+        role: 'user',
+        parts: [{ text: 'simple task' }],
+      },
+    ];
+
+    expect(contents).toEqual(expectedContents);
+  });
+
   it('should use a fallback promptId if not found in context', async () => {
     const consoleWarnSpy = vi
       .spyOn(debugLogger, 'warn')

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

@@ -115,7 +115,22 @@ export class NumericalClassifierStrategy implements RoutingStrategy {
 
       const promptId = getPromptIdWithFallback('classifier-router');
 
-      const finalHistory = context.history.slice(-HISTORY_TURNS_FOR_CONTEXT);
+      let startIndex = Math.max(
+        0,
+        context.history.length - HISTORY_TURNS_FOR_CONTEXT,
+      );
+      // Ensure we don't sever a functionResponse from its preceding functionCall
+      while (
+        startIndex > 0 &&
+        startIndex < context.history.length &&
+        context.history[startIndex].role === 'user' &&
+        context.history[startIndex].parts?.some((p) => !!p.functionResponse) &&
+        context.history[startIndex - 1].role === 'model' &&
+        context.history[startIndex - 1].parts?.some((p) => !!p.functionCall)
+      ) {
+        startIndex--;
+      }
+      const finalHistory = context.history.slice(startIndex);
 
       // Wrap the user's request in tags to prevent prompt injection
       const requestParts = Array.isArray(context.request)