FEAT: Add availabilityService (#81)

* auto and fallback work

* test fixes

* fixes

* Show model dialog even if there's no more fallback option

* fix tests

* fix failing test

* disable showInDialog for model in settings

* revert package-lock.json

* remove dup line

---------

Co-authored-by: Sehoon Shon <sshon@google.com>

packages/core/src/availability/policyCatalog.test.ts

@@ -19,7 +19,7 @@ describe('policyCatalog', () => {
   it('returns preview chain when preview enabled', () => {
     const chain = getModelPolicyChain({ previewEnabled: true });
     expect(chain[0]?.model).toBe(PREVIEW_GEMINI_MODEL);
-    expect(chain).toHaveLength(3);
+    expect(chain).toHaveLength(2);
   });
 
   it('returns default chain when preview disabled', () => {
@@ -31,7 +31,7 @@ describe('policyCatalog', () => {
   it('marks preview transients as sticky retries', () => {
     const [previewPolicy] = getModelPolicyChain({ previewEnabled: true });
     expect(previewPolicy.model).toBe(PREVIEW_GEMINI_MODEL);
-    expect(previewPolicy.stateTransitions.transient).toBe('sticky_retry');
+    expect(previewPolicy.stateTransitions.transient).toBe('terminal');
   });
 
   it('applies default actions and state transitions for unspecified kinds', () => {

packages/core/src/availability/policyCatalog.ts

@@ -13,6 +13,7 @@ import type {
 import {
   DEFAULT_GEMINI_FLASH_MODEL,
   DEFAULT_GEMINI_MODEL,
+  PREVIEW_GEMINI_FLASH_MODEL,
   PREVIEW_GEMINI_MODEL,
 } from '../config/models.js';
 import type { UserTierId } from '../code_assist/types.js';
@@ -48,13 +49,8 @@ const DEFAULT_CHAIN: ModelPolicyChain = [
 ];
 
 const PREVIEW_CHAIN: ModelPolicyChain = [
-  definePolicy({
-    model: PREVIEW_GEMINI_MODEL,
-    stateTransitions: { transient: 'sticky_retry' },
-    actions: { transient: 'silent' },
-  }),
-  definePolicy({ model: DEFAULT_GEMINI_MODEL }),
-  definePolicy({ model: DEFAULT_GEMINI_FLASH_MODEL, isLastResort: true }),
+  definePolicy({ model: PREVIEW_GEMINI_MODEL }),
+  definePolicy({ model: PREVIEW_GEMINI_FLASH_MODEL, isLastResort: true }),
 ];
 
 /**

packages/core/src/availability/policyHelpers.test.ts

@@ -12,6 +12,7 @@ import {
 } from './policyHelpers.js';
 import { createDefaultPolicy } from './policyCatalog.js';
 import type { Config } from '../config/config.js';
+import { DEFAULT_GEMINI_MODEL_AUTO } from '../config/models.js';
 
 const createMockConfig = (overrides: Partial<Config> = {}): Config =>
   ({
@@ -43,7 +44,7 @@ describe('policyHelpers', () => {
 
     it('returns the default chain when active model is "auto"', () => {
       const config = createMockConfig({
-        getModel: () => 'auto',
+        getModel: () => DEFAULT_GEMINI_MODEL_AUTO,
       });
       const chain = resolvePolicyChain(config);
 
@@ -63,7 +64,7 @@ describe('policyHelpers', () => {
       ];
       const context = buildFallbackPolicyContext(chain, 'b');
       expect(context.failedPolicy?.model).toBe('b');
-      expect(context.candidates.map((p) => p.model)).toEqual(['c', 'a']);
+      expect(context.candidates.map((p) => p.model)).toEqual(['c']);
     });
 
     it('returns full chain when model is not in policy list', () => {

packages/core/src/availability/policyHelpers.ts

@@ -14,7 +14,7 @@ import type {
   RetryAvailabilityContext,
 } from './modelPolicy.js';
 import { createDefaultPolicy, getModelPolicyChain } from './policyCatalog.js';
-import { DEFAULT_GEMINI_MODEL, getEffectiveModel } from '../config/models.js';
+import { DEFAULT_GEMINI_MODEL, resolveModel } from '../config/models.js';
 import type { ModelSelectionResult } from './modelAvailabilityService.js';
 
 /**
@@ -24,23 +24,29 @@ import type { ModelSelectionResult } from './modelAvailabilityService.js';
 export function resolvePolicyChain(
   config: Config,
   preferredModel?: string,
+  wrapsAround: boolean = false,
 ): ModelPolicyChain {
+  // Availability uses the active/requested model directly. Legacy fallback logic
+  // (getEffectiveModel) only applies when availability is disabled.
+  const modelFromConfig =
+    preferredModel ?? config.getActiveModel?.() ?? config.getModel();
+
+  const isPreviewRequest =
+    modelFromConfig.includes('gemini-3') ||
+    modelFromConfig.includes('preview') ||
+    modelFromConfig === 'fiercefalcon';
+
   const chain = getModelPolicyChain({
-    previewEnabled: !!config.getPreviewFeatures(),
+    previewEnabled: isPreviewRequest,
     userTier: config.getUserTier(),
   });
-  // TODO: This will be replaced when we get rid of Fallback Modes.
-  // Switch to getActiveModel()
-  const activeModel =
-    preferredModel ??
-    getEffectiveModel(config.getModel(), config.isInFallbackMode());
+  const activeModel = resolveModel(modelFromConfig);
 
-  if (activeModel === 'auto') {
-    return [...chain];
-  }
-
-  if (chain.some((policy) => policy.model === activeModel)) {
-    return [...chain];
+  const activeIndex = chain.findIndex((policy) => policy.model === activeModel);
+  if (activeIndex !== -1) {
+    return wrapsAround
+      ? [...chain.slice(activeIndex), ...chain.slice(0, activeIndex)]
+      : [...chain.slice(activeIndex)];
   }
 
   // If the user specified a model not in the default chain, we assume they want
@@ -51,10 +57,14 @@ export function resolvePolicyChain(
 /**
  * Produces the failed policy (if it exists in the chain) and the list of
  * fallback candidates that follow it.
+ * @param chain - The ordered list of available model policies.
+ * @param failedModel - The identifier of the model that failed.
+ * @param wrapsAround - If true, treats the chain as a circular buffer.
  */
 export function buildFallbackPolicyContext(
   chain: ModelPolicyChain,
   failedModel: string,
+  wrapsAround: boolean = false,
 ): {
   failedPolicy?: ModelPolicy;
   candidates: ModelPolicy[];
@@ -65,9 +75,12 @@ export function buildFallbackPolicyContext(
   }
   // Return [candidates_after, candidates_before] to prioritize downgrades
   // (continuing the chain) before wrapping around to upgrades.
+  const candidates = wrapsAround
+    ? [...chain.slice(index + 1), ...chain.slice(0, index)]
+    : [...chain.slice(index + 1)];
   return {
     failedPolicy: chain[index],
-    candidates: [...chain.slice(index + 1), ...chain.slice(0, index)],
+    candidates,
   };
 }