diff --git a/src/anthropic/lib/foundry.py b/src/anthropic/lib/foundry.py
index fde5be54..3ce866eb 100644
--- a/src/anthropic/lib/foundry.py
+++ b/src/anthropic/lib/foundry.py
@@ -126,6 +126,10 @@ def __init__(
         resource: str | None = None,
         api_key: str | None = None,
         azure_ad_token_provider: AzureADTokenProvider | None = None,
+        # Passed through to parent but not used for Foundry auth; accepted so
+        # the parent's `copy()` (which forwards `auth_token=self.auth_token`)
+        # doesn't raise TypeError when called on a Foundry client.
+        auth_token: str | None = None,
         webhook_key: str | None = None,
         base_url: str | None = None,
         timeout: float | Timeout | None | NotGiven = NOT_GIVEN,
@@ -166,6 +170,7 @@ def __init__(
 
         super().__init__(
             api_key=api_key,
+            auth_token=auth_token,
             webhook_key=webhook_key,
             base_url=base_url,
             timeout=timeout,
@@ -309,6 +314,10 @@ def __init__(
         resource: str | None = None,
         api_key: str | None = None,
         azure_ad_token_provider: AsyncAzureADTokenProvider | None = None,
+        # Passed through to parent but not used for Foundry auth; accepted so
+        # the parent's `copy()` (which forwards `auth_token=self.auth_token`)
+        # doesn't raise TypeError when called on a Foundry client.
+        auth_token: str | None = None,
         webhook_key: str | None = None,
         base_url: str | None = None,
         timeout: float | Timeout | None | NotGiven = NOT_GIVEN,
@@ -349,6 +358,7 @@ def __init__(
 
         super().__init__(
             api_key=api_key,
+            auth_token=auth_token,
             webhook_key=webhook_key,
             base_url=base_url,
             timeout=timeout,
diff --git a/tests/lib/test_azure.py b/tests/lib/test_azure.py
index ec3ef7b1..f95a0750 100644
--- a/tests/lib/test_azure.py
+++ b/tests/lib/test_azure.py
@@ -65,6 +65,20 @@ def test_missing_resource_error(self) -> None:
                 api_key="test-key",
             )
 
+    def test_copy_does_not_crash(self) -> None:
+        """Regression: ``copy()`` forwards ``auth_token`` via the parent client,
+        so ``AnthropicFoundry.__init__`` must accept it (passed through but
+        unused), mirroring ``AnthropicAWS``. Previously raised
+        ``TypeError: ... got an unexpected keyword argument 'auth_token'``.
+        """
+        client = AnthropicFoundry(api_key="test-key", resource="example-resource")
+        copied = client.copy()
+        assert isinstance(copied, AnthropicFoundry)
+        assert copied.api_key == "test-key"
+
+        copied_with_override = client.copy(timeout=30)
+        assert copied_with_override.timeout == 30
+
 
 class TestAsyncAnthropicFoundry:
     @pytest.mark.asyncio
@@ -104,3 +118,17 @@ async def test_initialization_from_environment_variables(self, monkeypatch: pyte
 
         assert client.api_key == "env-key"
         assert "env-resource.services.ai.azure.com" in str(client.base_url)
+
+    def test_copy_does_not_crash(self) -> None:
+        """Regression: ``copy()`` forwards ``auth_token`` via the parent client,
+        so ``AsyncAnthropicFoundry.__init__`` must accept it (passed through
+        but unused), mirroring ``AsyncAnthropicAWS``. Previously raised
+        ``TypeError: ... got an unexpected keyword argument 'auth_token'``.
+        """
+        client = AsyncAnthropicFoundry(api_key="test-key", resource="example-resource")
+        copied = client.copy()
+        assert isinstance(copied, AsyncAnthropicFoundry)
+        assert copied.api_key == "test-key"
+
+        copied_with_override = client.copy(timeout=30)
+        assert copied_with_override.timeout == 30