diff --git a/litellm/llms/anthropic/chat/transformation.py b/litellm/llms/anthropic/chat/transformation.py
index 5b1b663e85..86378b97d2 100644
--- a/litellm/llms/anthropic/chat/transformation.py
+++ b/litellm/llms/anthropic/chat/transformation.py
@@ -934,8 +934,15 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
             )
         return tools
 
-    def _ensure_context_management_beta_header(self, headers: dict) -> None:
-        beta_value = ANTHROPIC_BETA_HEADER_VALUES.CONTEXT_MANAGEMENT_2025_06_27.value
+    def _ensure_beta_header(self, headers: dict, beta_value: str) -> None:
+        """
+        Ensure a beta header value is present in the anthropic-beta header.
+        Merges with existing values instead of overriding them.
+        
+        Args:
+            headers: Dictionary of headers to update
+            beta_value: The beta header value to add
+        """
         existing_beta = headers.get("anthropic-beta")
         if existing_beta is None:
             headers["anthropic-beta"] = beta_value
@@ -944,6 +951,10 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
         if beta_value not in existing_values:
             headers["anthropic-beta"] = f"{existing_beta}, {beta_value}"
 
+    def _ensure_context_management_beta_header(self, headers: dict) -> None:
+        beta_value = ANTHROPIC_BETA_HEADER_VALUES.CONTEXT_MANAGEMENT_2025_06_27.value
+        self._ensure_beta_header(headers, beta_value)
+
     def update_headers_with_optional_anthropic_beta(
         self, headers: dict, optional_params: dict
     ) -> dict:
@@ -960,20 +971,20 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
             if tool.get("type", None) and tool.get("type").startswith(
                 ANTHROPIC_HOSTED_TOOLS.WEB_FETCH.value
             ):
-                headers["anthropic-beta"] = (
-                    ANTHROPIC_BETA_HEADER_VALUES.WEB_FETCH_2025_09_10.value
+                self._ensure_beta_header(
+                    headers, ANTHROPIC_BETA_HEADER_VALUES.WEB_FETCH_2025_09_10.value
                 )
             elif tool.get("type", None) and tool.get("type").startswith(
                 ANTHROPIC_HOSTED_TOOLS.MEMORY.value
             ):
-                headers["anthropic-beta"] = (
-                    ANTHROPIC_BETA_HEADER_VALUES.CONTEXT_MANAGEMENT_2025_06_27.value
+                self._ensure_beta_header(
+                    headers, ANTHROPIC_BETA_HEADER_VALUES.CONTEXT_MANAGEMENT_2025_06_27.value
                 )
         if optional_params.get("context_management") is not None:
             self._ensure_context_management_beta_header(headers)
         if optional_params.get("output_format") is not None:
-            headers["anthropic-beta"] = (
-                ANTHROPIC_BETA_HEADER_VALUES.STRUCTURED_OUTPUT_2025_09_25.value
+            self._ensure_beta_header(
+                headers, ANTHROPIC_BETA_HEADER_VALUES.STRUCTURED_OUTPUT_2025_09_25.value
             )
         return headers
 
diff --git a/tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py b/tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py
index e7a123aa8c..7e96c4634f 100644
--- a/tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py
+++ b/tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py
@@ -681,6 +681,73 @@ def test_anthropic_chat_headers_add_context_management_beta():
     assert headers["anthropic-beta"] == "context-management-2025-06-27"
 
 
+def test_anthropic_beta_header_merging_with_output_format():
+    """
+    Test that anthropic-beta headers from extra_headers are merged with
+    output_format beta headers instead of being overridden.
+    
+    This is a regression test for: https://github.com/BerriAI/litellm/issues/...
+    When using response_format with a Pydantic model AND extra_headers with
+    anthropic-beta (e.g., for context-1m extension), both beta headers should
+    be present in the final request.
+    """
+    config = AnthropicConfig()
+    
+    # Simulate headers that already have the context-1m beta header from extra_headers
+    headers = {"anthropic-beta": "context-1m-2025-08-07"}
+    
+    # Simulate output_format being set (happens when using response_format with Sonnet 4.5)
+    optional_params = {
+        "output_format": {
+            "type": "json_schema",
+            "schema": {"type": "object", "properties": {}}
+        }
+    }
+    
+    result_headers = config.update_headers_with_optional_anthropic_beta(
+        headers, optional_params
+    )
+    
+    # Both beta headers should be present
+    beta_value = result_headers["anthropic-beta"]
+    assert "context-1m-2025-08-07" in beta_value, \
+        f"User's context-1m beta header missing from: {beta_value}"
+    assert "structured-outputs-2025-11-13" in beta_value, \
+        f"Structured output beta header missing from: {beta_value}"
+
+
+def test_anthropic_beta_header_merging_with_multiple_features():
+    """
+    Test that multiple beta headers can be merged when using multiple features.
+    """
+    config = AnthropicConfig()
+    
+    # Start with a user-provided beta header
+    headers = {"anthropic-beta": "context-1m-2025-08-07"}
+    
+    # Use multiple features that require beta headers
+    optional_params = {
+        "output_format": {
+            "type": "json_schema",
+            "schema": {"type": "object", "properties": {}}
+        },
+        "context_management": _sample_context_management_payload(),
+        "tools": [{"type": "web_fetch_20250910", "name": "web_fetch"}]
+    }
+    
+    result_headers = config.update_headers_with_optional_anthropic_beta(
+        headers, optional_params
+    )
+    
+    beta_value = result_headers["anthropic-beta"]
+    
+    # All beta headers should be present
+    assert "context-1m-2025-08-07" in beta_value
+    assert "structured-outputs-2025-11-13" in beta_value
+    assert "context-management-2025-06-27" in beta_value
+    assert "web-fetch-2025-09-10" in beta_value
+
+
 def test_anthropic_chat_transform_request_includes_context_management():
     config = AnthropicConfig()
     headers = {}