[Bug] Calls with json_format=True yields double sets of curly braces

[evdevdev]

Is this a new bug?

• I believe this is a new bug
• I have searched the existing Github issues and Community Forum, and I could not find an existing post for this bug

Describe the bug
I am attempting to use json_format=True. About one in every 20 or so calls will yield invalid JSON. Specifically it gives double curly braces.

Example code:

msg = Message(role="user", content=content)
response = assistant.chat(messages=[msg], model=model, json_response=True)
# vanilla use of pydantic; the error occurs here
evidence_evaluation = EvidenceEvaluation.model_validate_json(
    response.message.content
)

Expected response:
{"first_key": "Example", "second_key": "Example again."}

Actual response (about one in every 20 times):
{{"first_key": "Example", "second_key": "Example again."}}

Error information

Invalid JSON: key must be a string at line 1 column 2 [type=json_invalid, input_value='{{"first_key": "Example", "second_key": "Example again."}}`

It's worth noting that the double curlies happen at start and end.

Steps to reproduce the issue locally
Call the above code about 20-30 times. It will randomly happen.

Environment

• OS Version: Sequoia 15.3
• Python version: 3.13.1
• Python SDK version: 5.4.2

Additional context
Add any other context about the problem here.