[Bug]: Mistral reasoning not correctly parsed

[gpanneti]

Bug Description

When I use a Mistral reasoning model, the thinking part is always included in the response, without any <think> tag. The showThinking parameter does not change anything to this.

So it's impossible to parse the output in order to split reasoning and response parts.

Version

0.14.12

Steps to Reproduce

Use the following piece of code to test this:

from llama_index.core.base.llms.types import (
    ChatMessage,
    MessageRole,
    TextBlock,
)

from llama_index.llms.mistralai import MistralAI

llm = MistralAI(model="magistral-small-latest", api_key=my_api_key, show_thinking=True)
response = llm.chat([ChatMessage(role=MessageRole.USER, blocks=[TextBlock(text="What is the capital of France?")])])
print("response", response)

Relevant Logs/Tracbacks

[dosubot]

Hi @gpanneti! I'm Dosu and I’m helping the llama_index team.

This is a known limitation of the current MistralAI integration in LlamaIndex. The tags are not inserted because the integration uses a hardcoded message formatting function that doesn't separate reasoning and response, and the show_thinking parameter doesn't affect this behavior. This isn't a bug in the Mistral model itself, but rather a design choice in the integration layer, and there's no supported way to override it without patching the integration code directly.

Some users work around similar issues in other models by improving their output parsing logic or monkey-patching the message formatting function, but these are not officially supported and can be brittle. There is ongoing discussion about making reasoning tokens (like thinking blocks) consistently accessible across all LlamaIndex APIs, but this hasn't been implemented yet. You may want to monitor future releases for improvements in this area.
Relevant discussion and workaround details
Feature request for consistent reasoning token access

To reply, just mention @dosu.

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