Add ModelDefinitions to ModelConfigService (#22302)

packages/core/src/agents/browser/browserAgentDefinition.ts

@@ -122,7 +122,7 @@ export const BrowserAgentDefinition = (
 ): LocalAgentDefinition<typeof BrowserTaskResultSchema> => {
   // Use Preview Flash model if the main model is any of the preview models.
   // If the main model is not a preview model, use the default flash model.
-  const model = isPreviewModel(config.getModel())
+  const model = isPreviewModel(config.getModel(), config)
     ? PREVIEW_GEMINI_FLASH_MODEL
     : DEFAULT_GEMINI_FLASH_MODEL;
 

packages/core/src/availability/policyHelpers.ts

@@ -54,19 +54,21 @@ export function resolvePolicyChain(
     useCustomToolModel,
     hasAccessToPreview,
   );
-  const isAutoPreferred = preferredModel ? isAutoModel(preferredModel) : false;
-  const isAutoConfigured = isAutoModel(configuredModel);
+  const isAutoPreferred = preferredModel
+    ? isAutoModel(preferredModel, config)
+    : false;
+  const isAutoConfigured = isAutoModel(configuredModel, config);
 
   if (resolvedModel === DEFAULT_GEMINI_FLASH_LITE_MODEL) {
     chain = getFlashLitePolicyChain();
   } else if (
-    isGemini3Model(resolvedModel) ||
+    isGemini3Model(resolvedModel, config) ||
     isAutoPreferred ||
     isAutoConfigured
   ) {
     if (hasAccessToPreview) {
       const previewEnabled =
-        isGemini3Model(resolvedModel) ||
+        isGemini3Model(resolvedModel, config) ||
         preferredModel === PREVIEW_GEMINI_MODEL_AUTO ||
         configuredModel === PREVIEW_GEMINI_MODEL_AUTO;
       chain = getModelPolicyChain({

packages/core/src/config/config.ts

@@ -609,6 +609,7 @@ export interface ConfigParameters {
   disableAlwaysAllow?: boolean;
   rawOutput?: boolean;
   acceptRawOutputRisk?: boolean;
+  dynamicModelConfiguration?: boolean;
   modelConfigServiceConfig?: ModelConfigServiceConfig;
   enableHooks?: boolean;
   enableHooksUI?: boolean;
@@ -810,6 +811,7 @@ export class Config implements McpContext, AgentLoopContext {
   private readonly disableAlwaysAllow: boolean;
   private readonly rawOutput: boolean;
   private readonly acceptRawOutputRisk: boolean;
+  private readonly dynamicModelConfiguration: boolean;
   private pendingIncludeDirectories: string[];
   private readonly enableHooks: boolean;
   private readonly enableHooksUI: boolean;
@@ -957,6 +959,40 @@ export class Config implements McpContext, AgentLoopContext {
     this.disabledSkills = params.disabledSkills ?? [];
     this.adminSkillsEnabled = params.adminSkillsEnabled ?? true;
     this.modelAvailabilityService = new ModelAvailabilityService();
+    this.dynamicModelConfiguration = params.dynamicModelConfiguration ?? false;
+
+    // HACK: The settings loading logic doesn't currently merge the default
+    // generation config with the user's settings. This means if a user provides
+    // any `generation` settings (e.g., just `overrides`), the default `aliases`
+    // are lost. This hack manually merges the default aliases back in if they
+    // are missing from the user's config.
+    // TODO(12593): Fix the settings loading logic to properly merge defaults and
+    // remove this hack.
+    let modelConfigServiceConfig = params.modelConfigServiceConfig;
+    if (modelConfigServiceConfig) {
+      // Ensure user-defined model definitions augment, not replace, the defaults.
+      const mergedModelDefinitions = {
+        ...DEFAULT_MODEL_CONFIGS.modelDefinitions,
+        ...modelConfigServiceConfig.modelDefinitions,
+      };
+
+      modelConfigServiceConfig = {
+        // Preserve other user settings like customAliases
+        ...modelConfigServiceConfig,
+        // Apply defaults for aliases and overrides if they are not provided
+        aliases:
+          modelConfigServiceConfig.aliases ?? DEFAULT_MODEL_CONFIGS.aliases,
+        overrides:
+          modelConfigServiceConfig.overrides ?? DEFAULT_MODEL_CONFIGS.overrides,
+        // Use the merged model definitions
+        modelDefinitions: mergedModelDefinitions,
+      };
+    }
+
+    this.modelConfigService = new ModelConfigService(
+      modelConfigServiceConfig ?? DEFAULT_MODEL_CONFIGS,
+    );
+
     this.experimentalJitContext = params.experimentalJitContext ?? false;
     this.topicUpdateNarration = params.topicUpdateNarration ?? false;
     this.modelSteering = params.modelSteering ?? false;
@@ -1013,7 +1049,7 @@ export class Config implements McpContext, AgentLoopContext {
     this.truncateToolOutputThreshold =
       params.truncateToolOutputThreshold ??
       DEFAULT_TRUNCATE_TOOL_OUTPUT_THRESHOLD;
-    this.useWriteTodos = isPreviewModel(this.model)
+    this.useWriteTodos = isPreviewModel(this.model, this)
       ? false
       : (params.useWriteTodos ?? true);
     this.workspacePoliciesDir = params.workspacePoliciesDir;
@@ -1131,33 +1167,6 @@ export class Config implements McpContext, AgentLoopContext {
     this._sandboxManager = createSandboxManager(params.toolSandboxing ?? false);
     this.shellExecutionConfig.sandboxManager = this._sandboxManager;
     this.modelRouterService = new ModelRouterService(this);
-
-    // HACK: The settings loading logic doesn't currently merge the default
-    // generation config with the user's settings. This means if a user provides
-    // any `generation` settings (e.g., just `overrides`), the default `aliases`
-    // are lost. This hack manually merges the default aliases back in if they
-    // are missing from the user's config.
-    // TODO(12593): Fix the settings loading logic to properly merge defaults and
-    // remove this hack.
-    let modelConfigServiceConfig = params.modelConfigServiceConfig;
-    if (modelConfigServiceConfig) {
-      if (!modelConfigServiceConfig.aliases) {
-        modelConfigServiceConfig = {
-          ...modelConfigServiceConfig,
-          aliases: DEFAULT_MODEL_CONFIGS.aliases,
-        };
-      }
-      if (!modelConfigServiceConfig.overrides) {
-        modelConfigServiceConfig = {
-          ...modelConfigServiceConfig,
-          overrides: DEFAULT_MODEL_CONFIGS.overrides,
-        };
-      }
-    }
-
-    this.modelConfigService = new ModelConfigService(
-      modelConfigServiceConfig ?? DEFAULT_MODEL_CONFIGS,
-    );
   }
 
   get config(): Config {
@@ -1355,7 +1364,10 @@ export class Config implements McpContext, AgentLoopContext {
 
     // Only reset when we have explicit "no access" (hasAccessToPreviewModel === false).
     // When null (quota not fetched) or true, we preserve the saved model.
-    if (isPreviewModel(this.model) && this.hasAccessToPreviewModel === false) {
+    if (
+      isPreviewModel(this.model, this) &&
+      this.hasAccessToPreviewModel === false
+    ) {
       this.setModel(DEFAULT_GEMINI_MODEL_AUTO);
     }
 
@@ -1627,7 +1639,7 @@ export class Config implements McpContext, AgentLoopContext {
 
     const isPreview =
       model === PREVIEW_GEMINI_MODEL_AUTO ||
-      isPreviewModel(this.getActiveModel());
+      isPreviewModel(this.getActiveModel(), this);
     const proModel = isPreview ? PREVIEW_GEMINI_MODEL : DEFAULT_GEMINI_MODEL;
     const flashModel = isPreview
       ? PREVIEW_GEMINI_FLASH_MODEL
@@ -1825,8 +1837,9 @@ export class Config implements McpContext, AgentLoopContext {
       }
 
       const hasAccess =
-        quota.buckets?.some((b) => b.modelId && isPreviewModel(b.modelId)) ??
-        false;
+        quota.buckets?.some(
+          (b) => b.modelId && isPreviewModel(b.modelId, this),
+        ) ?? false;
       this.setHasAccessToPreviewModel(hasAccess);
       return quota;
     } catch (e) {
@@ -2226,6 +2239,10 @@ export class Config implements McpContext, AgentLoopContext {
     return this.acceptRawOutputRisk;
   }
 
+  getExperimentalDynamicModelConfiguration(): boolean {
+    return this.dynamicModelConfiguration;
+  }
+
   getPendingIncludeDirectories(): string[] {
     return this.pendingIncludeDirectories;
   }

packages/core/src/config/defaultModelConfigs.ts

@@ -249,4 +249,94 @@ export const DEFAULT_MODEL_CONFIGS: ModelConfigServiceConfig = {
       },
     },
   ],
+  modelDefinitions: {
+    // Concrete Models
+    'gemini-3.1-pro-preview': {
+      tier: 'pro',
+      family: 'gemini-3',
+      isPreview: true,
+      dialogLocation: 'manual',
+      features: { thinking: true, multimodalToolUse: true },
+    },
+    'gemini-3.1-pro-preview-customtools': {
+      tier: 'pro',
+      family: 'gemini-3',
+      isPreview: true,
+      features: { thinking: true, multimodalToolUse: true },
+    },
+    'gemini-3-pro-preview': {
+      tier: 'pro',
+      family: 'gemini-3',
+      isPreview: true,
+      dialogLocation: 'manual',
+      features: { thinking: true, multimodalToolUse: true },
+    },
+    'gemini-3-flash-preview': {
+      tier: 'flash',
+      family: 'gemini-3',
+      isPreview: true,
+      dialogLocation: 'manual',
+      features: { thinking: false, multimodalToolUse: true },
+    },
+    'gemini-2.5-pro': {
+      tier: 'pro',
+      family: 'gemini-2.5',
+      isPreview: false,
+      dialogLocation: 'manual',
+      features: { thinking: false, multimodalToolUse: false },
+    },
+    'gemini-2.5-flash': {
+      tier: 'flash',
+      family: 'gemini-2.5',
+      isPreview: false,
+      dialogLocation: 'manual',
+      features: { thinking: false, multimodalToolUse: false },
+    },
+    'gemini-2.5-flash-lite': {
+      tier: 'flash-lite',
+      family: 'gemini-2.5',
+      isPreview: false,
+      dialogLocation: 'manual',
+      features: { thinking: false, multimodalToolUse: false },
+    },
+    // Aliases
+    auto: {
+      tier: 'auto',
+      isPreview: true,
+      features: { thinking: true, multimodalToolUse: false },
+    },
+    pro: {
+      tier: 'pro',
+      isPreview: false,
+      features: { thinking: true, multimodalToolUse: false },
+    },
+    flash: {
+      tier: 'flash',
+      isPreview: false,
+      features: { thinking: false, multimodalToolUse: false },
+    },
+    'flash-lite': {
+      tier: 'flash-lite',
+      isPreview: false,
+      features: { thinking: false, multimodalToolUse: false },
+    },
+    'auto-gemini-3': {
+      displayName: 'Auto (Gemini 3)',
+      tier: 'auto',
+      isPreview: true,
+      dialogLocation: 'main',
+      dialogDescription:
+        'Let Gemini CLI decide the best model for the task: gemini-3.1-pro, gemini-3-flash',
+      features: { thinking: true, multimodalToolUse: false },
+    },
+    'auto-gemini-2.5': {
+      displayName: 'Auto (Gemini 2.5)',
+      tier: 'auto',
+      isPreview: false,
+      dialogLocation: 'main',
+      dialogDescription:
+        'Let Gemini CLI decide the best model for the task: gemini-2.5-pro, gemini-2.5-flash',
+      features: { thinking: false, multimodalToolUse: false },
+    },
+  },
 };

packages/core/src/config/models.test.ts

@@ -31,6 +31,96 @@ import {
   isPreviewModel,
   isProModel,
 } from './models.js';
+import type { Config } from './config.js';
+import { ModelConfigService } from '../services/modelConfigService.js';
+import { DEFAULT_MODEL_CONFIGS } from './defaultModelConfigs.js';
+
+const modelConfigService = new ModelConfigService(DEFAULT_MODEL_CONFIGS);
+
+const dynamicConfig = {
+  getExperimentalDynamicModelConfiguration: () => true,
+  modelConfigService,
+} as unknown as Config;
+
+const legacyConfig = {
+  getExperimentalDynamicModelConfiguration: () => false,
+  modelConfigService,
+} as unknown as Config;
+
+describe('Dynamic Configuration Parity', () => {
+  const modelsToTest = [
+    GEMINI_MODEL_ALIAS_AUTO,
+    GEMINI_MODEL_ALIAS_PRO,
+    GEMINI_MODEL_ALIAS_FLASH,
+    PREVIEW_GEMINI_MODEL_AUTO,
+    DEFAULT_GEMINI_MODEL_AUTO,
+    PREVIEW_GEMINI_MODEL,
+    DEFAULT_GEMINI_MODEL,
+    'custom-model',
+  ];
+
+  it('getDisplayString should match legacy behavior', () => {
+    for (const model of modelsToTest) {
+      const legacy = getDisplayString(model, legacyConfig);
+      const dynamic = getDisplayString(model, dynamicConfig);
+      expect(dynamic).toBe(legacy);
+    }
+  });
+
+  it('isPreviewModel should match legacy behavior', () => {
+    const allModels = [
+      ...modelsToTest,
+      PREVIEW_GEMINI_3_1_MODEL,
+      PREVIEW_GEMINI_3_1_CUSTOM_TOOLS_MODEL,
+      PREVIEW_GEMINI_FLASH_MODEL,
+    ];
+    for (const model of allModels) {
+      const legacy = isPreviewModel(model, legacyConfig);
+      const dynamic = isPreviewModel(model, dynamicConfig);
+      expect(dynamic).toBe(legacy);
+    }
+  });
+
+  it('isProModel should match legacy behavior', () => {
+    for (const model of modelsToTest) {
+      const legacy = isProModel(model, legacyConfig);
+      const dynamic = isProModel(model, dynamicConfig);
+      expect(dynamic).toBe(legacy);
+    }
+  });
+
+  it('isGemini3Model should match legacy behavior', () => {
+    for (const model of modelsToTest) {
+      const legacy = isGemini3Model(model, legacyConfig);
+      const dynamic = isGemini3Model(model, dynamicConfig);
+      expect(dynamic).toBe(legacy);
+    }
+  });
+
+  it('isCustomModel should match legacy behavior', () => {
+    for (const model of modelsToTest) {
+      const legacy = isCustomModel(model, legacyConfig);
+      const dynamic = isCustomModel(model, dynamicConfig);
+      expect(dynamic).toBe(legacy);
+    }
+  });
+
+  it('supportsModernFeatures should match legacy behavior', () => {
+    for (const model of modelsToTest) {
+      const legacy = supportsModernFeatures(model);
+      const dynamic = supportsModernFeatures(model);
+      expect(dynamic).toBe(legacy);
+    }
+  });
+
+  it('supportsMultimodalFunctionResponse should match legacy behavior', () => {
+    for (const model of modelsToTest) {
+      const legacy = supportsMultimodalFunctionResponse(model, legacyConfig);
+      const dynamic = supportsMultimodalFunctionResponse(model, dynamicConfig);
+      expect(dynamic).toBe(legacy);
+    }
+  });
+});
 
 describe('isPreviewModel', () => {
   it('should return true for preview models', () => {

packages/core/src/config/models.ts

@@ -4,6 +4,33 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+/**
+ * Interface for the ModelConfigService to break circular dependencies.
+ */
+export interface IModelConfigService {
+  getModelDefinition(modelId: string):
+    | {
+        tier?: string;
+        family?: string;
+        isPreview?: boolean;
+        displayName?: string;
+        features?: {
+          thinking?: boolean;
+          multimodalToolUse?: boolean;
+        };
+      }
+    | undefined;
+}
+
+/**
+ * Interface defining the minimal configuration required for model capability checks.
+ * This helps break circular dependencies between Config and models.ts.
+ */
+export interface ModelCapabilityContext {
+  readonly modelConfigService: IModelConfigService;
+  getExperimentalDynamicModelConfiguration(): boolean;
+}
+
 export const PREVIEW_GEMINI_MODEL = 'gemini-3-pro-preview';
 export const PREVIEW_GEMINI_3_1_MODEL = 'gemini-3.1-pro-preview';
 export const PREVIEW_GEMINI_3_1_CUSTOM_TOOLS_MODEL =
@@ -139,7 +166,17 @@ export function resolveClassifierModel(
   }
   return resolveModel(requestedModel, useGemini3_1, useCustomToolModel);
 }
-export function getDisplayString(model: string) {
+export function getDisplayString(
+  model: string,
+  config?: ModelCapabilityContext,
+) {
+  if (config?.getExperimentalDynamicModelConfiguration?.() === true) {
+    const definition = config.modelConfigService.getModelDefinition(model);
+    if (definition?.displayName) {
+      return definition.displayName;
+    }
+  }
+
   switch (model) {
     case PREVIEW_GEMINI_MODEL_AUTO:
       return 'Auto (Gemini 3)';
@@ -160,9 +197,19 @@ export function getDisplayString(model: string) {
  * Checks if the model is a preview model.
  *
  * @param model The model name to check.
+ * @param config Optional config object for dynamic model configuration.
  * @returns True if the model is a preview model.
  */
-export function isPreviewModel(model: string): boolean {
+export function isPreviewModel(
+  model: string,
+  config?: ModelCapabilityContext,
+): boolean {
+  if (config?.getExperimentalDynamicModelConfiguration?.() === true) {
+    return (
+      config.modelConfigService.getModelDefinition(model)?.isPreview === true
+    );
+  }
+
   return (
     model === PREVIEW_GEMINI_MODEL ||
     model === PREVIEW_GEMINI_3_1_MODEL ||
@@ -177,9 +224,16 @@ export function isPreviewModel(model: string): boolean {
  * Checks if the model is a Pro model.
  *
  * @param model The model name to check.
+ * @param config Optional config object for dynamic model configuration.
  * @returns True if the model is a Pro model.
  */
-export function isProModel(model: string): boolean {
+export function isProModel(
+  model: string,
+  config?: ModelCapabilityContext,
+): boolean {
+  if (config?.getExperimentalDynamicModelConfiguration?.() === true) {
+    return config.modelConfigService.getModelDefinition(model)?.tier === 'pro';
+  }
   return model.toLowerCase().includes('pro');
 }
 
@@ -187,9 +241,22 @@ export function isProModel(model: string): boolean {
  * Checks if the model is a Gemini 3 model.
  *
  * @param model The model name to check.
+ * @param config Optional config object for dynamic model configuration.
  * @returns True if the model is a Gemini 3 model.
  */
-export function isGemini3Model(model: string): boolean {
+export function isGemini3Model(
+  model: string,
+  config?: ModelCapabilityContext,
+): boolean {
+  if (config?.getExperimentalDynamicModelConfiguration?.() === true) {
+    // Legacy behavior resolves the model first.
+    const resolved = resolveModel(model);
+    return (
+      config.modelConfigService.getModelDefinition(resolved)?.family ===
+      'gemini-3'
+    );
+  }
+
   const resolved = resolveModel(model);
   return /^gemini-3(\.|-|$)/.test(resolved);
 }
@@ -201,6 +268,8 @@ export function isGemini3Model(model: string): boolean {
  * @returns True if the model is a Gemini-2.x model.
  */
 export function isGemini2Model(model: string): boolean {
+  // This is legacy behavior, will remove this when gemini 2 models are no
+  // longer needed.
   return /^gemini-2(\.|$)/.test(model);
 }
 
@@ -208,9 +277,20 @@ export function isGemini2Model(model: string): boolean {
  * Checks if the model is a "custom" model (not Gemini branded).
  *
  * @param model The model name to check.
+ * @param config Optional config object for dynamic model configuration.
  * @returns True if the model is not a Gemini branded model.
  */
-export function isCustomModel(model: string): boolean {
+export function isCustomModel(
+  model: string,
+  config?: ModelCapabilityContext,
+): boolean {
+  if (config?.getExperimentalDynamicModelConfiguration?.() === true) {
+    const resolved = resolveModel(model);
+    return (
+      config.modelConfigService.getModelDefinition(resolved)?.tier ===
+        'custom' || !resolved.startsWith('gemini-')
+    );
+  }
   const resolved = resolveModel(model);
   return !resolved.startsWith('gemini-');
 }
@@ -231,9 +311,16 @@ export function supportsModernFeatures(model: string): boolean {
  * Checks if the model is an auto model.
  *
  * @param model The model name to check.
+ * @param config Optional config object for dynamic model configuration.
  * @returns True if the model is an auto model.
  */
-export function isAutoModel(model: string): boolean {
+export function isAutoModel(
+  model: string,
+  config?: ModelCapabilityContext,
+): boolean {
+  if (config?.getExperimentalDynamicModelConfiguration?.() === true) {
+    return config.modelConfigService.getModelDefinition(model)?.tier === 'auto';
+  }
   return (
     model === GEMINI_MODEL_ALIAS_AUTO ||
     model === PREVIEW_GEMINI_MODEL_AUTO ||
@@ -248,7 +335,16 @@ export function isAutoModel(model: string): boolean {
  * @param model The model name to check.
  * @returns True if the model supports multimodal function responses.
  */
-export function supportsMultimodalFunctionResponse(model: string): boolean {
+export function supportsMultimodalFunctionResponse(
+  model: string,
+  config?: ModelCapabilityContext,
+): boolean {
+  if (config?.getExperimentalDynamicModelConfiguration?.() === true) {
+    return (
+      config.modelConfigService.getModelDefinition(model)?.features
+        ?.multimodalToolUse === true
+    );
+  }
   return model.startsWith('gemini-3-');
 }
 

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

@@ -36,7 +36,7 @@ export class ApprovalModeStrategy implements RoutingStrategy {
     const model = context.requestedModel ?? config.getModel();
 
     // This strategy only applies to "auto" models.
-    if (!isAutoModel(model)) {
+    if (!isAutoModel(model, config)) {
       return null;
     }
 

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

@@ -139,7 +139,7 @@ export class ClassifierStrategy implements RoutingStrategy {
       const model = context.requestedModel ?? config.getModel();
       if (
         (await config.getNumericalRoutingEnabled()) &&
-        isGemini3Model(model)
+        isGemini3Model(model, config)
       ) {
         return null;
       }

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

@@ -109,7 +109,7 @@ export class NumericalClassifierStrategy implements RoutingStrategy {
         return null;
       }
 
-      if (!isGemini3Model(model)) {
+      if (!isGemini3Model(model, config)) {
         return null;
       }
 

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

@@ -29,7 +29,7 @@ export class OverrideStrategy implements RoutingStrategy {
     const overrideModel = context.requestedModel ?? config.getModel();
 
     // If the model is 'auto' we should pass to the next strategy.
-    if (isAutoModel(overrideModel)) {
+    if (isAutoModel(overrideModel, config)) {
       return null;
     }
 

packages/core/src/scheduler/tool-executor.ts

@@ -296,6 +296,7 @@ export class ToolExecutor {
         call.request.callId,
         output,
         this.config.getActiveModel(),
+        this.config,
       );
 
       // Inject the cancellation error into the response object
@@ -352,6 +353,7 @@ export class ToolExecutor {
       callId,
       content,
       this.config.getActiveModel(),
+      this.config,
     );
 
     const successResponse: ToolCallResponseInfo = {

packages/core/src/services/modelConfigService.ts

@@ -51,11 +51,34 @@ export interface ModelConfigAlias {
   modelConfig: ModelConfig;
 }
 
+// A model definition is a mapping from a model name to a list of features
+// that the model supports. Model names can be either direct model IDs
+// (gemini-2.5-pro) or aliases (auto).
+export interface ModelDefinition {
+  displayName?: string;
+  tier?: string; // 'pro' | 'flash' | 'flash-lite' | 'custom' | 'auto'
+  family?: string; // The gemini family, e.g. 'gemini-3' | 'gemini-2'
+  isPreview?: boolean;
+  // Specifies which view the model should appear in. If unset, the model will
+  // not appear in the dialog.
+  dialogLocation?: 'main' | 'manual';
+  /** A short description of the model for the dialog. */
+  dialogDescription?: string;
+  features?: {
+    // Whether the model supports thinking.
+    thinking?: boolean;
+    // Whether the model supports mutlimodal function responses. This is
+    // supported in Gemini 3.
+    multimodalToolUse?: boolean;
+  };
+}
+
 export interface ModelConfigServiceConfig {
   aliases?: Record<string, ModelConfigAlias>;
   customAliases?: Record<string, ModelConfigAlias>;
   overrides?: ModelConfigOverride[];
   customOverrides?: ModelConfigOverride[];
+  modelDefinitions?: Record<string, ModelDefinition>;
 }
 
 const MAX_ALIAS_CHAIN_DEPTH = 100;
@@ -76,6 +99,28 @@ export class ModelConfigService {
   // TODO(12597): Process config to build a typed alias hierarchy.
   constructor(private readonly config: ModelConfigServiceConfig) {}
 
+  getModelDefinition(modelId: string): ModelDefinition | undefined {
+    const definition = this.config.modelDefinitions?.[modelId];
+    if (definition) {
+      return definition;
+    }
+
+    // For unknown models, return an implicit custom definition to match legacy behavior.
+    if (!modelId.startsWith('gemini-')) {
+      return {
+        tier: 'custom',
+        family: 'custom',
+        features: {},
+      };
+    }
+
+    return undefined;
+  }
+
+  getModelDefinitions(): Record<string, ModelDefinition> {
+    return this.config.modelDefinitions ?? {};
+  }
+
   registerRuntimeModelConfig(aliasName: string, alias: ModelConfigAlias): void {
     this.runtimeAliases[aliasName] = alias;
   }

packages/core/src/utils/generateContentResponseUtilities.ts

@@ -13,6 +13,7 @@ import type {
 import { getResponseText } from './partUtils.js';
 import { supportsMultimodalFunctionResponse } from '../config/models.js';
 import { debugLogger } from './debugLogger.js';
+import type { Config } from '../config/config.js';
 
 /**
  * Formats tool output for a Gemini FunctionResponse.
@@ -48,6 +49,7 @@ export function convertToFunctionResponse(
   callId: string,
   llmContent: PartListUnion,
   model: string,
+  config?: Config,
 ): Part[] {
   if (typeof llmContent === 'string') {
     return [createFunctionResponsePart(callId, toolName, llmContent)];
@@ -96,7 +98,10 @@ export function convertToFunctionResponse(
     },
   };
 
-  const isMultimodalFRSupported = supportsMultimodalFunctionResponse(model);
+  const isMultimodalFRSupported = supportsMultimodalFunctionResponse(
+    model,
+    config,
+  );
   const siblingParts: Part[] = [...fileDataParts];
 
   if (inlineDataParts.length > 0) {