feat(core): fallback to chat-base when using unrecognized models for chat (#19016)

packages/core/src/core/client.test.ts

@@ -892,7 +892,7 @@ ${JSON.stringify(
       // Assert
       expect(ideContextStore.get).toHaveBeenCalled();
       expect(mockTurnRunFn).toHaveBeenCalledWith(
-        { model: 'default-routed-model' },
+        { model: 'default-routed-model', isChatModel: true },
         initialRequest,
         expect.any(AbortSignal),
         undefined,
@@ -1722,7 +1722,7 @@ ${JSON.stringify(
         expect(mockConfig.getModelRouterService).toHaveBeenCalled();
         expect(mockRouterService.route).toHaveBeenCalled();
         expect(mockTurnRunFn).toHaveBeenCalledWith(
-          { model: 'routed-model' },
+          { model: 'routed-model', isChatModel: true },
           [{ text: 'Hi' }],
           expect.any(AbortSignal),
           undefined,
@@ -1740,7 +1740,7 @@ ${JSON.stringify(
 
         expect(mockRouterService.route).toHaveBeenCalledTimes(1);
         expect(mockTurnRunFn).toHaveBeenCalledWith(
-          { model: 'routed-model' },
+          { model: 'routed-model', isChatModel: true },
           [{ text: 'Hi' }],
           expect.any(AbortSignal),
           undefined,
@@ -1758,7 +1758,7 @@ ${JSON.stringify(
         expect(mockRouterService.route).toHaveBeenCalledTimes(1);
         // Should stick to the first model
         expect(mockTurnRunFn).toHaveBeenCalledWith(
-          { model: 'routed-model' },
+          { model: 'routed-model', isChatModel: true },
           [{ text: 'Continue' }],
           expect.any(AbortSignal),
           undefined,
@@ -1776,7 +1776,7 @@ ${JSON.stringify(
 
         expect(mockRouterService.route).toHaveBeenCalledTimes(1);
         expect(mockTurnRunFn).toHaveBeenCalledWith(
-          { model: 'routed-model' },
+          { model: 'routed-model', isChatModel: true },
           [{ text: 'Hi' }],
           expect.any(AbortSignal),
           undefined,
@@ -1798,7 +1798,7 @@ ${JSON.stringify(
         expect(mockRouterService.route).toHaveBeenCalledTimes(2);
         // Should use the newly routed model
         expect(mockTurnRunFn).toHaveBeenCalledWith(
-          { model: 'new-routed-model' },
+          { model: 'new-routed-model', isChatModel: true },
           [{ text: 'A new topic' }],
           expect.any(AbortSignal),
           undefined,
@@ -1826,7 +1826,7 @@ ${JSON.stringify(
         expect(mockRouterService.route).toHaveBeenCalledTimes(1);
         expect(mockTurnRunFn).toHaveBeenNthCalledWith(
           1,
-          { model: 'original-model' },
+          { model: 'original-model', isChatModel: true },
           [{ text: 'Hi' }],
           expect.any(AbortSignal),
           undefined,
@@ -1849,7 +1849,7 @@ ${JSON.stringify(
         expect(mockRouterService.route).toHaveBeenCalledTimes(2);
         expect(mockTurnRunFn).toHaveBeenNthCalledWith(
           2,
-          { model: 'fallback-model' },
+          { model: 'fallback-model', isChatModel: true },
           [{ text: 'Continue' }],
           expect.any(AbortSignal),
           undefined,
@@ -1935,7 +1935,7 @@ ${JSON.stringify(
       // First call with original request
       expect(mockTurnRunFn).toHaveBeenNthCalledWith(
         1,
-        { model: 'default-routed-model' },
+        { model: 'default-routed-model', isChatModel: true },
         initialRequest,
         expect.any(AbortSignal),
         undefined,
@@ -1944,7 +1944,7 @@ ${JSON.stringify(
       // Second call with "Please continue."
       expect(mockTurnRunFn).toHaveBeenNthCalledWith(
         2,
-        { model: 'default-routed-model' },
+        { model: 'default-routed-model', isChatModel: true },
         [{ text: 'System: Please continue.' }],
         expect.any(AbortSignal),
         undefined,

packages/core/src/core/client.ts

@@ -656,7 +656,10 @@ export class GeminiClient {
     }
 
     // availability logic
-    const modelConfigKey: ModelConfigKey = { model: modelToUse };
+    const modelConfigKey: ModelConfigKey = {
+      model: modelToUse,
+      isChatModel: true,
+    };
     const { model: finalModel } = applyModelSelection(
       this.config,
       modelConfigKey,

packages/core/src/services/modelConfigService.test.ts

@@ -729,6 +729,71 @@ describe('ModelConfigService', () => {
     });
   });
 
+  describe('fallback behavior', () => {
+    it('should fallback to chat-base if the requested model is completely unknown', () => {
+      const config: ModelConfigServiceConfig = {
+        aliases: {
+          'chat-base': {
+            modelConfig: {
+              model: 'default-fallback-model',
+              generateContentConfig: {
+                temperature: 0.99,
+              },
+            },
+          },
+        },
+      };
+      const service = new ModelConfigService(config);
+      const resolved = service.getResolvedConfig({
+        model: 'my-custom-model',
+        isChatModel: true,
+      });
+
+      // It preserves the requested model name, but inherits the config from chat-base
+      expect(resolved.model).toBe('my-custom-model');
+      expect(resolved.generateContentConfig).toEqual({
+        temperature: 0.99,
+      });
+    });
+
+    it('should return empty config if requested model is unknown and chat-base is not defined', () => {
+      const config: ModelConfigServiceConfig = {
+        aliases: {},
+      };
+      const service = new ModelConfigService(config);
+      const resolved = service.getResolvedConfig({
+        model: 'my-custom-model',
+        isChatModel: true,
+      });
+
+      expect(resolved.model).toBe('my-custom-model');
+      expect(resolved.generateContentConfig).toEqual({});
+    });
+
+    it('should NOT fallback to chat-base if the requested model is completely unknown but isChatModel is false', () => {
+      const config: ModelConfigServiceConfig = {
+        aliases: {
+          'chat-base': {
+            modelConfig: {
+              model: 'default-fallback-model',
+              generateContentConfig: {
+                temperature: 0.99,
+              },
+            },
+          },
+        },
+      };
+      const service = new ModelConfigService(config);
+      const resolved = service.getResolvedConfig({
+        model: 'my-custom-model',
+        isChatModel: false,
+      });
+
+      expect(resolved.model).toBe('my-custom-model');
+      expect(resolved.generateContentConfig).toEqual({});
+    });
+  });
+
   describe('unrecognized models', () => {
     it('should apply overrides to unrecognized model names', () => {
       const unregisteredModelName = 'my-unregistered-model-v1';

packages/core/src/services/modelConfigService.ts

@@ -26,6 +26,10 @@ export interface ModelConfigKey {
   // This allows overrides to specify different settings (e.g., higher temperature)
   // specifically for retry scenarios.
   isRetry?: boolean;
+
+  // Indicates whether this request originates from the primary interactive chat model.
+  // Enables the default fallback configuration to `chat-base` when unknown.
+  isChatModel?: boolean;
 }
 
 export interface ModelConfig {
@@ -122,6 +126,7 @@ export class ModelConfigService {
     const { aliasChain, baseModel, resolvedConfig } = this.resolveAliasChain(
       context.model,
       allAliases,
+      context.isChatModel,
     );
 
     const modelToLevel = this.buildModelLevelMap(aliasChain, baseModel);
@@ -159,6 +164,7 @@ export class ModelConfigService {
   private resolveAliasChain(
     requestedModel: string,
     allAliases: Record<string, ModelConfigAlias>,
+    isChatModel?: boolean,
   ): {
     aliasChain: string[];
     baseModel: string | undefined;
@@ -206,6 +212,21 @@ export class ModelConfigService {
       };
     }
 
+    if (isChatModel) {
+      const fallbackAlias = 'chat-base';
+      if (allAliases[fallbackAlias]) {
+        const fallbackResolution = this.resolveAliasChain(
+          fallbackAlias,
+          allAliases,
+        );
+        return {
+          aliasChain: [...fallbackResolution.aliasChain, requestedModel],
+          baseModel: requestedModel,
+          resolvedConfig: fallbackResolution.resolvedConfig,
+        };
+      }
+    }
+
     return {
       aliasChain: [requestedModel],
       baseModel: requestedModel,