diff --git a/src/anthropic/lib/bedrock/_client.py b/src/anthropic/lib/bedrock/_client.py
index cda0690df..d2dfe889a 100644
--- a/src/anthropic/lib/bedrock/_client.py
+++ b/src/anthropic/lib/bedrock/_client.py
@@ -35,6 +35,30 @@
 _DefaultStreamT = TypeVar("_DefaultStreamT", bound=Union[Stream[Any], AsyncStream[Any]])
 
 
+def _strip_null_caller_from_tool_use(json_data: dict[object, object]) -> None:
+    # Bedrock rejects `"caller": null` in tool_use / server_tool_use content blocks
+    # with HTTP 400.  ToolUseBlock (response) has `caller: Optional[Caller]`, so
+    # `.to_dict()` includes the key when the value is None.  ToolUseBlockParam
+    # (request) treats `caller` as absent-or-valid, never null, so drop the key
+    # before the payload reaches the Bedrock endpoint.
+    messages = json_data.get("messages")
+    if not isinstance(messages, list):
+        return
+    for message in messages:
+        if not isinstance(message, dict):
+            continue
+        content = message.get("content")
+        if not isinstance(content, list):
+            continue
+        for block in content:
+            if (
+                isinstance(block, dict)
+                and block.get("type") in {"tool_use", "server_tool_use"}
+                and block.get("caller") is None
+            ):
+                block.pop("caller", None)
+
+
 def _prepare_options(input_options: FinalRequestOptions) -> FinalRequestOptions:
     options = model_copy(input_options, deep=True)
 
@@ -58,6 +82,8 @@ def _prepare_options(input_options: FinalRequestOptions) -> FinalRequestOptions:
         else:
             options.url = f"/model/{model}/invoke"
 
+        _strip_null_caller_from_tool_use(options.json_data)
+
     if options.url.startswith("/v1/messages/batches"):
         raise AnthropicError("The Batch API is not supported in Bedrock yet")
 
diff --git a/tests/lib/test_bedrock.py b/tests/lib/test_bedrock.py
index 6e45c27f7..4b26da80b 100644
--- a/tests/lib/test_bedrock.py
+++ b/tests/lib/test_bedrock.py
@@ -1,4 +1,5 @@
 import re
+import json
 import typing as t
 import tempfile
 from typing import TypedDict, cast
@@ -275,3 +276,78 @@ def test_region_infer_from_specified_profile(
     client = AnthropicBedrock()
 
     assert client.aws_region == next(profile for profile in profiles if profile["name"] == aws_profile)["region"]
+
+
+@pytest.mark.filterwarnings("ignore::DeprecationWarning")
+@pytest.mark.respx()
+def test_tool_use_null_caller_stripped(respx_mock: MockRouter) -> None:
+    """Bedrock rejects `"caller": null` in tool_use blocks; verify it is dropped."""
+    respx_mock.post(re.compile(r"https://bedrock-runtime\.us-east-1\.amazonaws\.com/model/.*/invoke")).mock(
+        return_value=httpx.Response(200, json={"foo": "bar"}),
+    )
+
+    sync_client.messages.create(
+        max_tokens=1024,
+        messages=[
+            {
+                "role": "assistant",
+                "content": [
+                    {
+                        "type": "tool_use",
+                        "id": "toolu_01",
+                        "name": "get_weather",
+                        "input": {"location": "SF"},
+                        "caller": None,
+                    }
+                ],
+            }
+        ],
+        model="anthropic.claude-3-5-sonnet-20241022-v2:0",
+    )
+
+    calls = cast("list[MockRequestCall]", respx_mock.calls)
+    assert len(calls) == 1
+
+    body = json.loads(calls[0].request.content)
+    tool_use_block = body["messages"][0]["content"][0]
+    assert "caller" not in tool_use_block, (
+        "Bedrock request must not contain 'caller: null' in tool_use blocks"
+    )
+
+
+@pytest.mark.filterwarnings("ignore::DeprecationWarning")
+@pytest.mark.respx()
+@pytest.mark.asyncio()
+async def test_tool_use_null_caller_stripped_async(respx_mock: MockRouter) -> None:
+    """Bedrock rejects `"caller": null` in tool_use blocks; verify it is dropped (async)."""
+    respx_mock.post(re.compile(r"https://bedrock-runtime\.us-east-1\.amazonaws\.com/model/.*/invoke")).mock(
+        return_value=httpx.Response(200, json={"foo": "bar"}),
+    )
+
+    await async_client.messages.create(
+        max_tokens=1024,
+        messages=[
+            {
+                "role": "assistant",
+                "content": [
+                    {
+                        "type": "tool_use",
+                        "id": "toolu_01",
+                        "name": "get_weather",
+                        "input": {"location": "SF"},
+                        "caller": None,
+                    }
+                ],
+            }
+        ],
+        model="anthropic.claude-3-5-sonnet-20241022-v2:0",
+    )
+
+    calls = cast("list[MockRequestCall]", respx_mock.calls)
+    assert len(calls) == 1
+
+    body = json.loads(calls[0].request.content)
+    tool_use_block = body["messages"][0]["content"][0]
+    assert "caller" not in tool_use_block, (
+        "Bedrock request must not contain 'caller: null' in tool_use blocks"
+    )