diff --git a/tests/integration/providers/test_bedrock_mantle_project_wire.py b/tests/integration/providers/test_bedrock_mantle_project_wire.py
new file mode 100644
index 00000000000..d065604e7d6
--- /dev/null
+++ b/tests/integration/providers/test_bedrock_mantle_project_wire.py
@@ -0,0 +1,79 @@
+import json
+import uuid
+from collections.abc import Callable
+from typing import Final
+
+import pytest
+from integration._support.client import Gateway
+from integration._support.wire import Reply, Request, wire_server
+
+MODEL: Final = "bedrock/mantle/anthropic.claude-sonnet-5"
+PROJECT_ID: Final = "synthetic-mantle-project"
+TOKEN: Final = "synthetic-mantle-bearer"
+RESPONSE: Final = json.dumps(
+    {
+        "id": "msg_mantle_project_control",
+        "type": "message",
+        "role": "assistant",
+        "model": "anthropic.claude-sonnet-5",
+        "content": [{"type": "text", "text": "mantle project control"}],
+        "stop_reason": "end_turn",
+        "stop_sequence": None,
+        "usage": {"input_tokens": 9, "output_tokens": 3},
+    }
+).encode()
+
+
+def project_scoped_peer(prompt: str) -> Callable[[Request], Reply]:
+    def respond(request: Request) -> Reply:
+        assert request.method == "POST" and request.target == "/anthropic/v1/messages"
+        assert request.headers["authorization"] == f"Bearer {TOKEN}"
+        assert request.headers.get("anthropic-workspace-id") == PROJECT_ID, dict(request.headers)
+        assert "anthropic-workspace" not in request.headers, dict(request.headers)
+        body: Final = json.loads(request.body)
+        assert body["model"] == "anthropic.claude-sonnet-5"
+        assert body["messages"] == [{"role": "user", "content": [{"type": "text", "text": prompt}]}]
+        assert body["max_tokens"] == 16
+        assert not {"aws_bedrock_project_id", "timeout", "litellm_params", "litellm_metadata", "api_key"}.intersection(
+            body
+        )
+        return Reply(body=RESPONSE)
+
+    return respond
+
+
+@pytest.mark.covers("other.provider_wire.bedrock.mantle_project_id_reaches_provider_as_workspace_id_header")
+def test_mantle_project_id_is_sent_as_anthropic_workspace_id_header_on_chat_and_messages(gateway: Gateway) -> None:
+    prompt: Final = f"synthetic project request {uuid.uuid4().hex}"
+    with wire_server(project_scoped_peer(prompt)) as wire, gateway.scenario() as scenario:
+        model: Final = scenario.model(
+            model=MODEL,
+            api_key=TOKEN,
+            api_base=wire.url,
+            aws_region_name="us-east-1",
+            aws_bedrock_project_id=PROJECT_ID,
+        )
+        chat: Final = gateway.request(
+            "POST",
+            "/v1/chat/completions",
+            {
+                "model": model,
+                "messages": [{"role": "user", "content": prompt}],
+                "max_tokens": 16,
+            },
+        )
+        assert chat.status_code == 200, chat.text
+        assert chat.json()["choices"][0]["message"]["content"] == "mantle project control"
+        assert len(wire.drain()) == 1, "Expected exactly one chat completions call to reach the Mantle peer"
+        messages: Final = gateway.request(
+            "POST",
+            "/v1/messages",
+            {
+                "model": model,
+                "max_tokens": 16,
+                "messages": [{"role": "user", "content": [{"type": "text", "text": prompt}]}],
+            },
+        )
+        assert messages.status_code == 200, messages.text
+        assert messages.json()["content"] == [{"type": "text", "text": "mantle project control"}]
+        assert len(wire.drain()) == 1, "Expected exactly one messages call to reach the Mantle peer"