diff --git a/src/utils/model/agent.test.ts b/src/utils/model/agent.test.ts
new file mode 100644
index 00000000..9b979ab0
--- /dev/null
+++ b/src/utils/model/agent.test.ts
@@ -0,0 +1,261 @@
+import { describe, test, expect, beforeEach, afterEach, mock } from 'bun:test'
+
+describe('getAgentModel provider-aware fallback', () => {
+  // Restore all mocks after each test
+  afterEach(() => {
+    mock.restore()
+  })
+
+  describe('Claude-native providers', () => {
+    test('haiku alias resolves to haiku model for official Anthropic API', async () => {
+      // Mock providers to return firstParty with official URL
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'firstParty',
+        isFirstPartyAnthropicBaseUrl: () => true,
+      }))
+
+      // Import after mock is set up
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('haiku', 'claude-sonnet-4-6', undefined, 'default')
+
+      // Should resolve haiku alias, not inherit parent
+      expect(result).toContain('haiku')
+      expect(result).not.toBe('claude-sonnet-4-6')
+    })
+
+    test('haiku alias resolves for Bedrock provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'bedrock',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('haiku', 'claude-sonnet-4-6', undefined, 'default')
+
+      // Should resolve haiku alias for Bedrock
+      expect(result).toContain('haiku')
+    })
+
+    test('haiku alias resolves for Vertex provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'vertex',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('haiku', 'claude-sonnet-4-6', undefined, 'default')
+
+      // Should resolve haiku alias for Vertex
+      expect(result).toContain('haiku')
+    })
+
+    test('haiku alias resolves for Foundry provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'foundry',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('haiku', 'claude-sonnet-4-6', undefined, 'default')
+
+      // Should resolve haiku alias for Foundry
+      expect(result).toContain('haiku')
+    })
+  })
+
+  describe('Non-Claude-native providers', () => {
+    test('haiku alias inherits parent model for OpenAI provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'openai',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('haiku', 'gpt-4o-mini', undefined, 'default')
+
+      // Should inherit parent model for OpenAI (no haiku concept)
+      expect(result).toBe('gpt-4o-mini')
+    })
+
+    test('haiku alias inherits parent model for Gemini provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'gemini',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('haiku', 'gemini-2.5-pro', undefined, 'default')
+
+      // Should inherit parent model for Gemini
+      expect(result).toBe('gemini-2.5-pro')
+    })
+
+    test('haiku alias inherits parent model for custom Anthropic-compatible URL', async () => {
+      // firstParty provider but with custom URL (not official Anthropic)
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'firstParty',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('haiku', 'claude-sonnet-4-6', undefined, 'default')
+
+      // Should inherit parent for custom Anthropic-compatible URL
+      expect(result).toBe('claude-sonnet-4-6')
+    })
+
+    test('sonnet alias inherits parent model for OpenAI provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'openai',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('sonnet', 'gpt-4o-mini', undefined, 'default')
+
+      // Should inherit parent model for OpenAI
+      expect(result).toBe('gpt-4o-mini')
+    })
+
+    test('haiku alias inherits parent model for Mistral provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'mistral',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('haiku', 'mistral-small-latest', undefined, 'default')
+
+      // Should inherit parent model for Mistral (no haiku concept)
+      expect(result).toBe('mistral-small-latest')
+    })
+
+    test('haiku alias inherits parent model for GitHub Copilot provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'github',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('haiku', 'gpt-4o-mini', undefined, 'default')
+
+      // Should inherit parent model for GitHub Copilot
+      expect(result).toBe('gpt-4o-mini')
+    })
+
+    test('haiku alias inherits parent model for NVIDIA NIM provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'nvidia-nim',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('haiku', 'meta/llama-3.1-8b-instruct', undefined, 'default')
+
+      // Should inherit parent model for NVIDIA NIM (no haiku concept)
+      expect(result).toBe('meta/llama-3.1-8b-instruct')
+    })
+
+    test('haiku alias inherits parent model for MiniMax provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'minimax',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('haiku', 'MiniMax-M2.5-highspeed', undefined, 'default')
+
+      // Should inherit parent model for MiniMax (no haiku concept)
+      expect(result).toBe('MiniMax-M2.5-highspeed')
+    })
+
+    test('haiku alias inherits parent model for Codex provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'codex',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('haiku', 'gpt-5.5-mini', undefined, 'default')
+
+      // Should inherit parent model for Codex provider (no haiku concept)
+      expect(result).toBe('gpt-5.5-mini')
+    })
+  })
+
+  describe('inherit behavior unchanged', () => {
+    test('inherit always returns parent model regardless of provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'openai',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { getAgentModel } = await import('./agent.js')
+      const result = getAgentModel('inherit', 'gpt-4o', undefined, 'default')
+
+      expect(result).toBe('gpt-4o')
+    })
+  })
+
+  describe('checkIsClaudeNativeProvider helper', () => {
+    test('returns true for official Anthropic API', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'firstParty',
+        isFirstPartyAnthropicBaseUrl: () => true,
+      }))
+
+      const { checkIsClaudeNativeProvider } = await import('./agent.js')
+      expect(checkIsClaudeNativeProvider()).toBe(true)
+    })
+
+    test('returns true for Bedrock provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'bedrock',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { checkIsClaudeNativeProvider } = await import('./agent.js')
+      expect(checkIsClaudeNativeProvider()).toBe(true)
+    })
+
+    test('returns true for Vertex provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'vertex',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { checkIsClaudeNativeProvider } = await import('./agent.js')
+      expect(checkIsClaudeNativeProvider()).toBe(true)
+    })
+
+    test('returns true for Foundry provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'foundry',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { checkIsClaudeNativeProvider } = await import('./agent.js')
+      expect(checkIsClaudeNativeProvider()).toBe(true)
+    })
+
+    test('returns false for OpenAI provider', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'openai',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { checkIsClaudeNativeProvider } = await import('./agent.js')
+      expect(checkIsClaudeNativeProvider()).toBe(false)
+    })
+
+    test('returns false for custom Anthropic URL', async () => {
+      mock.module('./providers.js', () => ({
+        getAPIProvider: () => 'firstParty',
+        isFirstPartyAnthropicBaseUrl: () => false,
+      }))
+
+      const { checkIsClaudeNativeProvider } = await import('./agent.js')
+      expect(checkIsClaudeNativeProvider()).toBe(false)
+    })
+  })
+})
\ No newline at end of file
diff --git a/src/utils/model/agent.ts b/src/utils/model/agent.ts
index 7bccfeba..904873e5 100644
--- a/src/utils/model/agent.ts
+++ b/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)'