@@ -499,7 +499,7 @@ def _prepare_options(
         # client_kwargs) join the request before the prompt-ARN cleanup so that
         # branch still strips fields Converse rejects, mirroring the other clients.
         run_options.update(kwargs)
-        if ":prompt/" in model:
+        if ":prompt/" in run_options["modelId"]:
             # A Prompt Management ARN takes these fields from the prompt, and Converse rejects requests that set them.
             if run_options["inferenceConfig"] == {"maxTokens": DEFAULT_MAX_TOKENS}:
                 del run_options["inferenceConfig"]  # only the client default, nothing the caller asked for
 
@@ -1031,3 +1031,20 @@ def test_prepare_options_keeps_per_call_client_kwargs() -> None:
 
     assert request["guardrailConfig"] == {"guardrailIdentifier": "gr-1", "guardrailVersion": "DRAFT"}
     assert request["requestMetadata"] == {"channel": "test"}
+
+
+def test_prepare_options_prompt_cleanup_uses_overridden_model_id() -> None:
+    """A prompt ARN arriving through client kwargs must trigger the ARN cleanup (#9344)."""
+    client = _make_client()
+    messages = [Message(role="user", contents=[Content.from_text(text="hello")])]
+
+    request = client._prepare_options(
+        messages,
+        {},
+        modelId="arn:aws:bedrock:us-east-1:123456789012:prompt/PROMPT123",
+    )
+
+    assert ":prompt/" in request["modelId"]
+    assert "inferenceConfig" not in request, "prompt ARN requests must not carry inferenceConfig"
+    assert "system" not in request
+    assert "toolConfig" not in request