diff --git a/agent/agent_runtime_helpers.py b/agent/agent_runtime_helpers.py
index 43e862e52e..5dc9a30f4d 100644
--- a/agent/agent_runtime_helpers.py
+++ b/agent/agent_runtime_helpers.py
@@ -1739,6 +1739,15 @@ def create_openai_client(agent, client_kwargs: dict, *, reason: str, shared: boo
     # gets its OWN fresh ``httpx.Client`` whose lifetime is tied to the OpenAI client it is passed to. When
     # the OpenAI client is closed (rebuild, teardown, credential rotation), the paired ``httpx.Client``
     # closes with it, and the next call constructs a fresh one — no stale closed transport can be reused.
+    # Bedrock Mantle: the ``aws-sdk`` placeholder is a sentinel for IAM-chain auth, not a bearer token.
+    # Every rebuild from bare ``{api_key, base_url}`` kwargs (switch_model, fallback restore, credential
+    # rotation, request-scoped clients) must reinstall the SigV4 http_client or Mantle answers 401.
+    if "bedrock-mantle." in str(client_kwargs.get("base_url") or ""):
+        from agent.bedrock_adapter import configure_bedrock_openai_client_kwargs
+        timeout = client_kwargs.get("timeout")
+        configure_bedrock_openai_client_kwargs(
+            client_kwargs, timeout=timeout if isinstance(timeout, (int, float)) else None,
+        )
     if "http_client" not in client_kwargs:
         keepalive_http = agent._build_keepalive_http_client(client_kwargs.get("base_url", ""), verify=httpx_verify)
         if keepalive_http is not None:
diff --git a/tests/run_agent/test_switch_model_bedrock_sigv4.py b/tests/run_agent/test_switch_model_bedrock_sigv4.py
new file mode 100644
index 0000000000..7fe24f29bb
--- /dev/null
+++ b/tests/run_agent/test_switch_model_bedrock_sigv4.py
@@ -0,0 +1,39 @@
+"""A /model switch onto a Bedrock Mantle (OpenAI-wire) model must rebuild the client with SigV4
+auth. ``aws-sdk`` is a Hermes sentinel for IAM-chain auth; agent_init installed the SigV4
+http_client but switch_model rebuilt bare ``{api_key, base_url}`` kwargs, so the SDK sent
+``Authorization: Bearer aws-sdk`` and Mantle answered 401."""
+
+from unittest.mock import MagicMock, patch
+
+from agent.bedrock_adapter import BedrockOpenAISigV4Auth
+from agent.context_compressor import ContextCompressor
+from run_agent import AIAgent
+
+MANTLE = "https://bedrock-mantle.us-east-1.api.aws/openai/v1"
+
+
+def _agent() -> AIAgent:
+    agent = AIAgent.__new__(AIAgent)
+    agent.model, agent.provider = "claude-opus-4.8", "anthropic"
+    agent.base_url, agent.api_key, agent.api_mode = "https://api.anthropic.com", "sk-ant", "anthropic_messages"
+    agent.client, agent._anthropic_client, agent._client_kwargs = None, MagicMock(), {}
+    agent.quiet_mode, agent._config_context_length, agent._primary_runtime = True, None, {}
+    agent.context_compressor = ContextCompressor(
+        model=agent.model, threshold_percent=0.5, base_url=agent.base_url, api_key="sk-ant",
+        provider="anthropic", quiet_mode=True, config_context_length=None,
+    )
+    return agent
+
+
+@patch("agent.model_metadata.get_model_context_length", return_value=272_000)
+def test_switch_to_bedrock_mantle_installs_sigv4_http_client(_ctx, monkeypatch):
+    monkeypatch.setenv("AWS_ACCESS_KEY_ID", "AKIAFAKEFAKEFAKEFAKE")
+    monkeypatch.setenv("AWS_SECRET_ACCESS_KEY", "fake-secret")
+    monkeypatch.delenv("AWS_BEARER_TOKEN_BEDROCK", raising=False)
+    agent = _agent()
+
+    agent.switch_model("openai.gpt-5.6-terra", "bedrock", api_key="aws-sdk", base_url=MANTLE, api_mode="codex_responses")
+
+    auth = getattr(agent.client._client, "auth", None)
+    assert isinstance(auth, BedrockOpenAISigV4Auth), f"switched client would send Bearer aws-sdk (auth={auth!r})"
+    assert auth.region == "us-east-1"