From 070fb63a32366acfe514b43351d989d1e5f75c16 Mon Sep 17 00:00:00 2001
From: Charan Rathore <180254320+charan-rathore@users.noreply.github.com>
Date: Fri, 23 Oct 2026 08:28:00 +0530
Subject: [PATCH] fix(bedrock): keep compatible message API version

Fixes #2031

Signed-off-by: Charan Rathore <180254320+charan-rathore@users.noreply.github.com>
---
 src/anthropic/lib/bedrock/_client.py |  4 ++--
 tests/lib/test_bedrock.py            | 19 +++++++++++++++++--
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/src/anthropic/lib/bedrock/_client.py b/src/anthropic/lib/bedrock/_client.py
index 5409532fe..49f0328c8 100644
--- a/src/anthropic/lib/bedrock/_client.py
+++ b/src/anthropic/lib/bedrock/_client.py
@@ -14,7 +14,6 @@
 from ..._client import _keep_client_header_params
 from ..._compat import model_copy
 from ..._version import __version__
-from ..._constants import ANTHROPIC_API_VERSION
 from ..._streaming import Stream, AsyncStream
 from ..._exceptions import AnthropicError, APIStatusError
 from ..._middleware import MiddlewareInput
@@ -32,13 +31,14 @@
 
 _HttpxClientT = TypeVar("_HttpxClientT", bound=Union[httpx2.Client, httpx2.AsyncClient])
 _DefaultStreamT = TypeVar("_DefaultStreamT", bound=Union[Stream[Any], AsyncStream[Any]])
+DEFAULT_VERSION = "bedrock-2023-05-31"
 
 
 def _prepare_options(input_options: FinalRequestOptions) -> FinalRequestOptions:
     options = model_copy(_keep_client_header_params(input_options), deep=True)
 
     if is_dict(options.json_data):
-        options.json_data.setdefault("anthropic_version", ANTHROPIC_API_VERSION)
+        options.json_data.setdefault("anthropic_version", DEFAULT_VERSION)
 
         if is_given(options.headers):
             betas = build_headers(options.headers)[0].get("anthropic-beta")
diff --git a/tests/lib/test_bedrock.py b/tests/lib/test_bedrock.py
index 901c93aec..df6c67537 100644
--- a/tests/lib/test_bedrock.py
+++ b/tests/lib/test_bedrock.py
@@ -16,7 +16,6 @@
 from anthropic import APIStatusError, AnthropicBedrock, AsyncAnthropicBedrock, beta_tool, beta_async_tool
 from anthropic._compat import PYDANTIC_V1
 from anthropic._models import FinalRequestOptions
-from anthropic._constants import ANTHROPIC_API_VERSION
 from anthropic.lib.bedrock._client import _prepare_options
 from anthropic.lib.bedrock._stream_decoder import _chunk_bytes_to_sse
 
@@ -203,6 +202,22 @@ def test_extra_body_anthropic_version_wins(respx_mock: MockRouter) -> None:
     assert json.loads(request.content)["anthropic_version"] == "2000-01-01"
 
 
+@pytest.mark.respx()
+def test_default_anthropic_version_is_bedrock_compatible(respx_mock: MockRouter) -> None:
+    respx_mock.post(re.compile(r"https://bedrock-runtime\.us-east-1\.amazonaws\.com/model/.*/invoke")).mock(
+        return_value=httpx2.Response(200, json={"foo": "bar"}),
+    )
+
+    sync_client.messages.create(
+        max_tokens=16,
+        messages=[{"role": "user", "content": "Say ok"}],
+        model="anthropic.claude-sonnet-4-5-20250929-v1:0",
+    )
+
+    request = cast("list[MockRequestCall]", respx_mock.calls)[0].request
+    assert json.loads(request.content)["anthropic_version"] == "bedrock-2023-05-31"
+
+
 @pytest.mark.respx()
 @pytest.mark.asyncio()
 async def test_extra_body_anthropic_version_wins_async(respx_mock: MockRouter) -> None:
@@ -582,7 +597,7 @@ def _assert_tool_runner_calls(calls: t.List[MockRequestCall]) -> None:
         assert "anthropic-version" not in call.request.headers
         body = json.loads(call.request.content)
         assert "model" not in body and "stream" not in body and "output_format" not in body
-        assert body["anthropic_version"] == ANTHROPIC_API_VERSION
+        assert body["anthropic_version"] == "bedrock-2023-05-31"
     second = json.loads(calls[1].request.content)
     assert [m["role"] for m in second["messages"]] == ["user", "assistant", "user"]
     assert second["messages"][1]["content"][0]["type"] == "tool_use"