diff --git a/src/services/api/client.test.ts b/src/services/api/client.test.ts
index fae88c55..30df3b93 100644
--- a/src/services/api/client.test.ts
+++ b/src/services/api/client.test.ts
@@ -14,6 +14,7 @@ type ShimClient = {
 const originalFetch = globalThis.fetch
 const originalMacro = (globalThis as Record<string, unknown>).MACRO
 const originalEnv = {
+  CLAUDE_CODE_USE_OPENAI: process.env.CLAUDE_CODE_USE_OPENAI,
   CLAUDE_CODE_USE_GEMINI: process.env.CLAUDE_CODE_USE_GEMINI,
   GEMINI_API_KEY: process.env.GEMINI_API_KEY,
   GEMINI_MODEL: process.env.GEMINI_MODEL,
@@ -25,6 +26,15 @@ const originalEnv = {
   OPENAI_MODEL: process.env.OPENAI_MODEL,
   ANTHROPIC_API_KEY: process.env.ANTHROPIC_API_KEY,
   ANTHROPIC_AUTH_TOKEN: process.env.ANTHROPIC_AUTH_TOKEN,
+  ANTHROPIC_CUSTOM_HEADERS: process.env.ANTHROPIC_CUSTOM_HEADERS,
+}
+
+function restoreEnv(key: string, value: string | undefined): void {
+  if (value === undefined) {
+    delete process.env[key]
+  } else {
+    process.env[key] = value
+  }
 }
 
 beforeEach(() => {
@@ -35,27 +45,31 @@ beforeEach(() => {
   process.env.GEMINI_BASE_URL = 'https://gemini.example/v1beta/openai'
   process.env.GEMINI_AUTH_MODE = 'api-key'
 
+  delete process.env.CLAUDE_CODE_USE_OPENAI
   delete process.env.GOOGLE_API_KEY
   delete process.env.OPENAI_API_KEY
   delete process.env.OPENAI_BASE_URL
   delete process.env.OPENAI_MODEL
   delete process.env.ANTHROPIC_API_KEY
   delete process.env.ANTHROPIC_AUTH_TOKEN
+  delete process.env.ANTHROPIC_CUSTOM_HEADERS
 })
 
 afterEach(() => {
   ;(globalThis as Record<string, unknown>).MACRO = originalMacro
-  process.env.CLAUDE_CODE_USE_GEMINI = originalEnv.CLAUDE_CODE_USE_GEMINI
-  process.env.GEMINI_API_KEY = originalEnv.GEMINI_API_KEY
-  process.env.GEMINI_MODEL = originalEnv.GEMINI_MODEL
-  process.env.GEMINI_BASE_URL = originalEnv.GEMINI_BASE_URL
-  process.env.GEMINI_AUTH_MODE = originalEnv.GEMINI_AUTH_MODE
-  process.env.GOOGLE_API_KEY = originalEnv.GOOGLE_API_KEY
-  process.env.OPENAI_API_KEY = originalEnv.OPENAI_API_KEY
-  process.env.OPENAI_BASE_URL = originalEnv.OPENAI_BASE_URL
-  process.env.OPENAI_MODEL = originalEnv.OPENAI_MODEL
-  process.env.ANTHROPIC_API_KEY = originalEnv.ANTHROPIC_API_KEY
-  process.env.ANTHROPIC_AUTH_TOKEN = originalEnv.ANTHROPIC_AUTH_TOKEN
+  restoreEnv('CLAUDE_CODE_USE_OPENAI', originalEnv.CLAUDE_CODE_USE_OPENAI)
+  restoreEnv('CLAUDE_CODE_USE_GEMINI', originalEnv.CLAUDE_CODE_USE_GEMINI)
+  restoreEnv('GEMINI_API_KEY', originalEnv.GEMINI_API_KEY)
+  restoreEnv('GEMINI_MODEL', originalEnv.GEMINI_MODEL)
+  restoreEnv('GEMINI_BASE_URL', originalEnv.GEMINI_BASE_URL)
+  restoreEnv('GEMINI_AUTH_MODE', originalEnv.GEMINI_AUTH_MODE)
+  restoreEnv('GOOGLE_API_KEY', originalEnv.GOOGLE_API_KEY)
+  restoreEnv('OPENAI_API_KEY', originalEnv.OPENAI_API_KEY)
+  restoreEnv('OPENAI_BASE_URL', originalEnv.OPENAI_BASE_URL)
+  restoreEnv('OPENAI_MODEL', originalEnv.OPENAI_MODEL)
+  restoreEnv('ANTHROPIC_API_KEY', originalEnv.ANTHROPIC_API_KEY)
+  restoreEnv('ANTHROPIC_AUTH_TOKEN', originalEnv.ANTHROPIC_AUTH_TOKEN)
+  restoreEnv('ANTHROPIC_CUSTOM_HEADERS', originalEnv.ANTHROPIC_CUSTOM_HEADERS)
   globalThis.fetch = originalFetch
 })
 
@@ -122,3 +136,135 @@ test('routes Gemini provider requests through the OpenAI-compatible shim', async
     model: 'gemini-2.0-flash',
   })
 })
+
+test('strips Anthropic-specific custom headers before sending OpenAI-compatible shim requests', async () => {
+  let capturedHeaders: Headers | undefined
+
+  process.env.CLAUDE_CODE_USE_OPENAI = '1'
+  process.env.OPENAI_API_KEY = 'openai-test-key'
+  process.env.OPENAI_BASE_URL = 'http://example.test/v1'
+  process.env.OPENAI_MODEL = 'gpt-4o'
+  process.env.ANTHROPIC_CUSTOM_HEADERS = [
+    'anthropic-version: 2023-06-01',
+    'anthropic-beta: prompt-caching-2024-07-31',
+    'x-anthropic-additional-protection: true',
+    'x-claude-remote-session-id: remote-123',
+    'x-app: cli',
+    'x-safe-header: keep-me',
+  ].join('\n')
+
+  globalThis.fetch = (async (_input, init) => {
+    capturedHeaders = new Headers(init?.headers)
+
+    return new Response(
+      JSON.stringify({
+        id: 'chatcmpl-openai',
+        model: 'gpt-4o',
+        choices: [
+          {
+            message: {
+              role: 'assistant',
+              content: 'ok',
+            },
+            finish_reason: 'stop',
+          },
+        ],
+        usage: {
+          prompt_tokens: 8,
+          completion_tokens: 3,
+          total_tokens: 11,
+        },
+      }),
+      {
+        headers: {
+          'Content-Type': 'application/json',
+        },
+      },
+    )
+  }) as FetchType
+
+  const client = (await getAnthropicClient({
+    maxRetries: 0,
+    model: 'gpt-4o',
+  })) as unknown as ShimClient
+
+  await client.beta.messages.create({
+    model: 'gpt-4o',
+    system: 'test system',
+    messages: [{ role: 'user', content: 'hello' }],
+    max_tokens: 64,
+    stream: false,
+  })
+
+  expect(capturedHeaders?.get('anthropic-version')).toBeNull()
+  expect(capturedHeaders?.get('anthropic-beta')).toBeNull()
+  expect(capturedHeaders?.get('x-anthropic-additional-protection')).toBeNull()
+  expect(capturedHeaders?.get('x-claude-remote-session-id')).toBeNull()
+  expect(capturedHeaders?.get('x-app')).toBeNull()
+  expect(capturedHeaders?.get('x-safe-header')).toBe('keep-me')
+  expect(capturedHeaders?.get('authorization')).toBe('Bearer openai-test-key')
+})
+
+test('strips Anthropic-specific custom headers on providerOverride shim requests too', async () => {
+  let capturedHeaders: Headers | undefined
+
+  process.env.ANTHROPIC_CUSTOM_HEADERS = [
+    'anthropic-version: 2023-06-01',
+    'anthropic-beta: prompt-caching-2024-07-31',
+    'x-claude-remote-session-id: remote-123',
+    'x-safe-header: keep-me',
+  ].join('\n')
+
+  globalThis.fetch = (async (_input, init) => {
+    capturedHeaders = new Headers(init?.headers)
+
+    return new Response(
+      JSON.stringify({
+        id: 'chatcmpl-provider-override',
+        model: 'gpt-4o',
+        choices: [
+          {
+            message: {
+              role: 'assistant',
+              content: 'ok',
+            },
+            finish_reason: 'stop',
+          },
+        ],
+        usage: {
+          prompt_tokens: 8,
+          completion_tokens: 3,
+          total_tokens: 11,
+        },
+      }),
+      {
+        headers: {
+          'Content-Type': 'application/json',
+        },
+      },
+    )
+  }) as FetchType
+
+  const client = (await getAnthropicClient({
+    maxRetries: 0,
+    providerOverride: {
+      model: 'gpt-4o',
+      baseURL: 'http://example.test/v1',
+      apiKey: 'provider-test-key',
+    },
+  })) as unknown as ShimClient
+
+  await client.beta.messages.create({
+    model: 'unused',
+    system: 'test system',
+    messages: [{ role: 'user', content: 'hello' }],
+    max_tokens: 64,
+    stream: false,
+  })
+
+  expect(capturedHeaders?.get('anthropic-version')).toBeNull()
+  expect(capturedHeaders?.get('anthropic-beta')).toBeNull()
+  expect(capturedHeaders?.get('x-claude-remote-session-id')).toBeNull()
+  expect(capturedHeaders?.get('x-safe-header')).toBe('keep-me')
+  expect(capturedHeaders?.get('authorization')).toBe('Bearer provider-test-key')
+})
diff --git a/src/services/api/openaiShim.test.ts b/src/services/api/openaiShim.test.ts
index db0c9c2e..4889b6d3 100644
--- a/src/services/api/openaiShim.test.ts
+++ b/src/services/api/openaiShim.test.ts
@@ -7,6 +7,10 @@ const originalEnv = {
   OPENAI_BASE_URL: process.env.OPENAI_BASE_URL,
   OPENAI_API_KEY: process.env.OPENAI_API_KEY,
   OPENAI_MODEL: process.env.OPENAI_MODEL,
+  CLAUDE_CODE_USE_GITHUB: process.env.CLAUDE_CODE_USE_GITHUB,
+  GITHUB_TOKEN: process.env.GITHUB_TOKEN,
+  GH_TOKEN: process.env.GH_TOKEN,
+  CLAUDE_CODE_USE_OPENAI: process.env.CLAUDE_CODE_USE_OPENAI,
   CLAUDE_CODE_USE_GEMINI: process.env.CLAUDE_CODE_USE_GEMINI,
   GEMINI_API_KEY: process.env.GEMINI_API_KEY,
   GOOGLE_API_KEY: process.env.GOOGLE_API_KEY,
@@ -15,6 +19,7 @@ const originalEnv = {
   GEMINI_BASE_URL: process.env.GEMINI_BASE_URL,
   GEMINI_MODEL: process.env.GEMINI_MODEL,
   GOOGLE_CLOUD_PROJECT: process.env.GOOGLE_CLOUD_PROJECT,
+  ANTHROPIC_CUSTOM_HEADERS: process.env.ANTHROPIC_CUSTOM_HEADERS,
 }
 
 const originalFetch = globalThis.fetch
@@ -70,6 +75,10 @@ beforeEach(() => {
   process.env.OPENAI_BASE_URL = 'http://example.test/v1'
   process.env.OPENAI_API_KEY = 'test-key'
   delete process.env.OPENAI_MODEL
+  delete process.env.CLAUDE_CODE_USE_GITHUB
+  delete process.env.GITHUB_TOKEN
+  delete process.env.GH_TOKEN
+  delete process.env.CLAUDE_CODE_USE_OPENAI
   delete process.env.CLAUDE_CODE_USE_GEMINI
   delete process.env.GEMINI_API_KEY
   delete process.env.GOOGLE_API_KEY
@@ -78,12 +87,17 @@ beforeEach(() => {
   delete process.env.GEMINI_BASE_URL
   delete process.env.GEMINI_MODEL
   delete process.env.GOOGLE_CLOUD_PROJECT
+  delete process.env.ANTHROPIC_CUSTOM_HEADERS
 })
 
 afterEach(() => {
   restoreEnv('OPENAI_BASE_URL', originalEnv.OPENAI_BASE_URL)
   restoreEnv('OPENAI_API_KEY', originalEnv.OPENAI_API_KEY)
   restoreEnv('OPENAI_MODEL', originalEnv.OPENAI_MODEL)
+  restoreEnv('CLAUDE_CODE_USE_GITHUB', originalEnv.CLAUDE_CODE_USE_GITHUB)
+  restoreEnv('GITHUB_TOKEN', originalEnv.GITHUB_TOKEN)
+  restoreEnv('GH_TOKEN', originalEnv.GH_TOKEN)
+  restoreEnv('CLAUDE_CODE_USE_OPENAI', originalEnv.CLAUDE_CODE_USE_OPENAI)
   restoreEnv('CLAUDE_CODE_USE_GEMINI', originalEnv.CLAUDE_CODE_USE_GEMINI)
   restoreEnv('GEMINI_API_KEY', originalEnv.GEMINI_API_KEY)
   restoreEnv('GOOGLE_API_KEY', originalEnv.GOOGLE_API_KEY)
@@ -92,9 +106,227 @@ afterEach(() => {
   restoreEnv('GEMINI_BASE_URL', originalEnv.GEMINI_BASE_URL)
   restoreEnv('GEMINI_MODEL', originalEnv.GEMINI_MODEL)
   restoreEnv('GOOGLE_CLOUD_PROJECT', originalEnv.GOOGLE_CLOUD_PROJECT)
+  restoreEnv('ANTHROPIC_CUSTOM_HEADERS', originalEnv.ANTHROPIC_CUSTOM_HEADERS)
   globalThis.fetch = originalFetch
 })
 
+test('strips canonical Anthropic headers from direct shim defaultHeaders', async () => {
+  let capturedHeaders: Headers | undefined
+
+  globalThis.fetch = (async (_input, init) => {
+    capturedHeaders = new Headers(init?.headers)
+
+    return new Response(
+      JSON.stringify({
+        id: 'chatcmpl-1',
+        model: 'gpt-4o',
+        choices: [
+          {
+            message: {
+              role: 'assistant',
+              content: 'ok',
+            },
+            finish_reason: 'stop',
+          },
+        ],
+        usage: {
+          prompt_tokens: 8,
+          completion_tokens: 3,
+          total_tokens: 11,
+        },
+      }),
+      {
+        headers: {
+          'Content-Type': 'application/json',
+        },
+      },
+    )
+  }) as FetchType
+
+  const client = createOpenAIShimClient({
+    defaultHeaders: {
+      'anthropic-version': '2023-06-01',
+      'anthropic-beta': 'prompt-caching-2024-07-31',
+      'x-anthropic-additional-protection': 'true',
+      'x-claude-remote-session-id': 'remote-123',
+      'x-app': 'cli',
+      'x-client-app': 'sdk',
+      'x-safe-header': 'keep-me',
+    },
+  }) as OpenAIShimClient
+
+  await client.beta.messages.create({
+    model: 'gpt-4o',
+    system: 'test system',
+    messages: [{ role: 'user', content: 'hello' }],
+    max_tokens: 64,
+    stream: false,
+  })
+
+  expect(capturedHeaders?.get('anthropic-version')).toBeNull()
+  expect(capturedHeaders?.get('anthropic-beta')).toBeNull()
+  expect(capturedHeaders?.get('x-anthropic-additional-protection')).toBeNull()
+  expect(capturedHeaders?.get('x-claude-remote-session-id')).toBeNull()
+  expect(capturedHeaders?.get('x-app')).toBeNull()
+  expect(capturedHeaders?.get('x-client-app')).toBeNull()
+  expect(capturedHeaders?.get('x-safe-header')).toBe('keep-me')
+})
+
+test('strips canonical Anthropic headers from per-request shim headers too', async () => {
+  let capturedHeaders: Headers | undefined
+
+  globalThis.fetch = (async (_input, init) => {
+    capturedHeaders = new Headers(init?.headers)
+
+    return new Response(
+      JSON.stringify({
+        id: 'chatcmpl-1',
+        model: 'gpt-4o',
+        choices: [
+          {
+            message: {
+              role: 'assistant',
+              content: 'ok',
+            },
+            finish_reason: 'stop',
+          },
+        ],
+        usage: {
+          prompt_tokens: 8,
+          completion_tokens: 3,
+          total_tokens: 11,
+        },
+      }),
+      {
+        headers: {
+          'Content-Type': 'application/json',
+        },
+      },
+    )
+  }) as FetchType
+
+  const client = createOpenAIShimClient({}) as OpenAIShimClient
+
+  await client.beta.messages.create(
+    {
+      model: 'gpt-4o',
+      system: 'test system',
+      messages: [{ role: 'user', content: 'hello' }],
+      max_tokens: 64,
+      stream: false,
+    },
+    {
+      headers: {
+        'anthropic-version': '2023-06-01',
+        'anthropic-beta': 'prompt-caching-2024-07-31',
+        'x-safe-header': 'keep-me',
+      },
+    },
+  )
+
+  expect(capturedHeaders?.get('anthropic-version')).toBeNull()
+  expect(capturedHeaders?.get('anthropic-beta')).toBeNull()
+  expect(capturedHeaders?.get('x-safe-header')).toBe('keep-me')
+})
+
+test('strips Anthropic-specific headers on GitHub Codex transport requests', async () => {
+  let capturedHeaders: Headers | undefined
+
+  process.env.CLAUDE_CODE_USE_GITHUB = '1'
+  process.env.OPENAI_API_KEY = 'github-test-key'
+  delete process.env.OPENAI_BASE_URL
+  delete process.env.OPENAI_MODEL
+
+  globalThis.fetch = (async (_input, init) => {
+    capturedHeaders = new Headers(init?.headers)
+
+    return new Response('', {
+      status: 200,
+      headers: {
+        'Content-Type': 'text/event-stream',
+      },
+    })
+  }) as FetchType
+
+  const client = createOpenAIShimClient({}) as OpenAIShimClient
+
+  await client.beta.messages.create(
+    {
+      model: 'github:gpt-5-codex',
+      system: 'test system',
+      messages: [{ role: 'user', content: 'hello' }],
+      max_tokens: 64,
+      stream: true,
+    },
+    {
+      headers: {
+        'anthropic-version': '2023-06-01',
+        'anthropic-beta': 'prompt-caching-2024-07-31',
+        'x-anthropic-additional-protection': 'true',
+        'x-safe-header': 'keep-me',
+      },
+    },
+  )
+
+  expect(capturedHeaders?.get('anthropic-version')).toBeNull()
+  expect(capturedHeaders?.get('anthropic-beta')).toBeNull()
+  expect(capturedHeaders?.get('x-anthropic-additional-protection')).toBeNull()
+  expect(capturedHeaders?.get('x-safe-header')).toBe('keep-me')
+  expect(capturedHeaders?.get('authorization')).toBe('Bearer github-test-key')
+  expect(capturedHeaders?.get('editor-plugin-version')).toBe('copilot-chat/0.26.7')
+})
+
+test('strips Anthropic-specific headers on GitHub Codex transport with providerOverride API key', async () => {
+  let capturedHeaders: Headers | undefined
+
+  process.env.CLAUDE_CODE_USE_GITHUB = '1'
+  process.env.OPENAI_API_KEY = 'env-should-not-win'
+  delete process.env.OPENAI_BASE_URL
+  delete process.env.OPENAI_MODEL
+
+  globalThis.fetch = (async (_input, init) => {
+    capturedHeaders = new Headers(init?.headers)
+
+    return new Response('', {
+      status: 200,
+      headers: {
+        'Content-Type': 'text/event-stream',
+      },
+    })
+  }) as FetchType
+
+  const client = createOpenAIShimClient({
+    providerOverride: {
+      model: 'github:gpt-5-codex',
+      baseURL: 'https://api.githubcopilot.com',
+      apiKey: 'provider-override-key',
+    },
+  }) as OpenAIShimClient
+
+  await client.beta.messages.create(
+    {
+      model: 'ignored',
+      system: 'test system',
+      messages: [{ role: 'user', content: 'hello' }],
+      max_tokens: 64,
+      stream: true,
+    },
+    {
+      headers: {
+        'anthropic-version': '2023-06-01',
+        'x-claude-remote-session-id': 'remote-123',
+        'x-safe-header': 'keep-me',
+      },
+    },
+  )
+
+  expect(capturedHeaders?.get('anthropic-version')).toBeNull()
+  expect(capturedHeaders?.get('x-claude-remote-session-id')).toBeNull()
+  expect(capturedHeaders?.get('x-safe-header')).toBe('keep-me')
+  expect(capturedHeaders?.get('authorization')).toBe('Bearer provider-override-key')
+  expect(capturedHeaders?.get('editor-plugin-version')).toBe('copilot-chat/0.26.7')
+})
+
 test('preserves usage from final OpenAI stream chunk with empty choices', async () => {
   globalThis.fetch = (async (_input, init) => {
     const url = typeof _input === 'string' ? _input : _input.url
diff --git a/src/services/api/openaiShim.ts b/src/services/api/openaiShim.ts
index c72b8c24..04c9450c 100644
--- a/src/services/api/openaiShim.ts
+++ b/src/services/api/openaiShim.ts
@@ -80,6 +80,32 @@ function isGithubModelsMode(): boolean {
   return isEnvTruthy(process.env.CLAUDE_CODE_USE_GITHUB)
 }
 
+function filterAnthropicHeaders(
+  headers: Record<string, string> | undefined,
+): Record<string, string> {
+  if (!headers) return {}
+
+  const filtered: Record<string, string> = {}
+  for (const [key, value] of Object.entries(headers)) {
+    const lower = key.toLowerCase()
+    if (
+      lower.startsWith('x-anthropic') ||
+      lower.startsWith('anthropic-') ||
+      lower.startsWith('x-claude') ||
+      lower === 'x-app' ||
+      lower === 'x-client-app' ||
+      lower === 'authorization' ||
+      lower === 'x-api-key' ||
+      lower === 'api-key'
+    ) {
+      continue
+    }
+    filtered[key] = value
+  }
+
+  return filtered
+}
+
 function hasGeminiApiHost(baseUrl: string | undefined): boolean {
   if (!baseUrl) return false
 
@@ -989,7 +1015,7 @@ class OpenAIShimMessages {
   private providerOverride?: { model: string; baseURL: string; apiKey: string }
 
   constructor(defaultHeaders: Record<string, string>, reasoningEffort?: 'low' | 'medium' | 'high' | 'xhigh', providerOverride?: { model: string; baseURL: string; apiKey: string }) {
-    this.defaultHeaders = defaultHeaders
+    this.defaultHeaders = filterAnthropicHeaders(defaultHeaders)
     this.reasoningEffort = reasoningEffort
     this.providerOverride = providerOverride
   }
@@ -1099,7 +1125,7 @@ class OpenAIShimMessages {
         params,
         defaultHeaders: {
           ...this.defaultHeaders,
-          ...(options?.headers ?? {}),
+          ...filterAnthropicHeaders(options?.headers),
           ...COPILOT_HEADERS,
         },
         signal: options?.signal,
@@ -1131,7 +1157,7 @@ class OpenAIShimMessages {
         params,
         defaultHeaders: {
           ...this.defaultHeaders,
-          ...(options?.headers ?? {}),
+          ...filterAnthropicHeaders(options?.headers),
         },
         signal: options?.signal,
       })
@@ -1223,7 +1249,7 @@ class OpenAIShimMessages {
     const headers: Record<string, string> = {
       'Content-Type': 'application/json',
       ...this.defaultHeaders,
-      ...(options?.headers ?? {}),
+      ...filterAnthropicHeaders(options?.headers),
     }
 
     const isGemini = isGeminiMode()