Bug: prepare_request() mutates caller's messages list via shallow copy

[truecallerabreham]

Describe the bug

prepare_request() methods in Cohere, Mistral, and xAI providers mutate the caller's messages list on the first call (no retry needed).

To reproduce

import instructor
from openai import OpenAI

client = instructor.from_openai(OpenAI())

messages = [{"role": "user", "content": "hello"}]

try:
    client.chat.completions.create(
        model="gpt-4o",
        messages=messages,
        response_model=str,
    )
except Exception:
    pass

print(len(messages))  # Expected: 1, Actual: 2+ (system message prepended)

Expected behavior

messages should remain unchanged after prepare_request() runs.

Root cause

prepare_request() uses kwargs.copy() (shallow copy) or _convert_messages(kwargs) which returns a dict with a shared messages list. Then .insert(), .append(), or direct dict key assignment mutates the caller's data.

Affected providers:

• Cohere: _convert_messages_to_cohere_v2 (shallow copy)
• Mistral: tools, json_schema, json_instructions modes (shallow copy + .insert())
• xAI: tools, parallel_tools, json_schema, json_instructions modes (shallow copy + .insert())

Fix

new_kwargs = {**kwargs, 'messages': [dict(m) for m in kwargs.get('messages', [])]}

[ErenAta16]

Traced this against current main before writing a fix, and it looks like this is already resolved.

_convert_messages_to_cohere_v2 (Cohere) already routes through copy_messages_for_mutation rather than a shallow copy. Mistral and xAI's TOOLS/PARALLEL_TOOLS/JSON_SCHEMA handlers do new_kwargs["messages"] = list(kwargs.get("messages", [])), a fresh list, and never mutate the message dicts inside it, only add new keys like tools/tool_choice. Their MD_JSON handlers also use copy_messages_for_mutation.

Ran tests/v2/test_messages_not_mutated.py locally (which covers exactly this, both the initial aliasing check and the reask/retry mutation path via test_reask_does_not_mutate_caller_messages) against current main: 66 passed, 0 failed, across Cohere, Mistral, xAI, Writer, and OpenRouter.

If you hit this on a pinned/older version rather than current main, that would explain the discrepancy, worth confirming which version you tested against. Otherwise this looks safe to close as already fixed.