fix(agent): provider-aware fallback for haiku/sonnet aliases (#908)

* fix(agent): provider-aware fallback for haiku/sonnet aliases

Explore agent fails on custom providers (Z.AI GLM, Alibaba Anthropic-compatible,
local OpenAI endpoints) because 'haiku' alias resolves to a non-existent model.

Changes:
- Add isClaudeNativeProvider check (Bedrock, Vertex, Foundry, official Anthropic)
- For non-Claude-native providers, haiku/sonnet aliases inherit parent model
- Add 8 tests for provider-aware fallback behavior

Fixes Explore agent "model not found" errors on custom Anthropic-compatible APIs.

* test(agent): use Bun mock.module() for provider tests

Replace env manipulation with proper Bun mock.module() to reliably
mock getAPIProvider() and isFirstPartyAnthropicBaseUrl() functions.
This ensures tests work correctly on CI where module caching caused
false negatives.

---------

Co-authored-by: Ali Alakbarli <ali.alakbarli@users.noreply.github.com>

src/utils/model/agent.ts

@@ -7,7 +7,7 @@ import {
   getRuntimeMainLoopModel,
   parseUserSpecifiedModel,
 } from './model.js'
-import { getAPIProvider } from './providers.js'
+import { getAPIProvider, isFirstPartyAnthropicBaseUrl } from './providers.js'
 
 export const AGENT_MODEL_OPTIONS = [...MODEL_ALIASES, 'inherit'] as const
 export type AgentModelAlias = (typeof AGENT_MODEL_OPTIONS)[number]
@@ -77,6 +77,26 @@ export function getAgentModel(
 
   const agentModelWithExp = agentModel ?? getDefaultSubagentModel()
 
+  // Provider-aware model alias fallback for agents.
+  // Claude-native providers (Bedrock, Vertex, Foundry, official Anthropic API)
+  // have guaranteed haiku/sonnet model availability. Custom Anthropic-compatible
+  // endpoints, OpenAI-shim, Gemini, Mistral, and other providers may not have
+  // equivalent models, causing "model not found" errors when resolving aliases.
+  // For haiku/sonnet aliases on non-Claude-native providers, inherit parent model.
+  // Note: 'opus' is NOT included here because it's handled separately by
+  // aliasMatchesParentTier() which checks if parent's tier matches the alias.
+  if (
+    (agentModelWithExp === 'haiku' || agentModelWithExp === 'sonnet') &&
+    !checkIsClaudeNativeProvider()
+  ) {
+    // Non-Claude-native provider → inherit parent model
+    return getRuntimeMainLoopModel({
+      permissionMode: permissionMode ?? 'default',
+      mainLoopModel: parentModel,
+      exceeds200kTokens: false,
+    })
+  }
+
   if (agentModelWithExp === 'inherit') {
     // Apply runtime model resolution for inherit to get the effective model
     // This ensures agents using 'inherit' get opusplan→Opus resolution in plan mode
@@ -121,6 +141,22 @@ function aliasMatchesParentTier(alias: string, parentModel: string): boolean {
   }
 }
 
+/**
+ * Check if the current provider is Claude-native (has guaranteed haiku/sonnet models).
+ * Claude-native providers: Bedrock, Vertex, Foundry, official Anthropic API.
+ * Non-Claude-native: OpenAI, Gemini, Mistral, GitHub, NVIDIA NIM, MiniMax,
+ * and custom Anthropic-compatible endpoints (proxies, self-hosted).
+ */
+export function checkIsClaudeNativeProvider(): boolean {
+  const provider = getAPIProvider()
+  return (
+    provider === 'bedrock' ||
+    provider === 'vertex' ||
+    provider === 'foundry' ||
+    (provider === 'firstParty' && isFirstPartyAnthropicBaseUrl())
+  )
+}
+
 export function getAgentModelDisplay(model: string | undefined): string {
   // When model is omitted, getDefaultSubagentModel() returns 'inherit' at runtime
   if (!model) return 'Inherit from parent (default)'