Parity with VLLM over the reasoning field

[tdene]

What does this PR do ?

⚠️ For major changes (either in lines of code or in its impact), please make sure to first share a design doc with the team. If you're unsure what's the best way to do so, contact the @mcore-oncall.

Contribution process

Pre-checks

• I have added relevant unit tests
• I have added relevant functional tests
• I have added proper typing to my code Typing guidelines
• I have added relevant documentation
• I have run the autoformatter.sh on my PR

Code review

Feel free to message or comment the @mcore-oncall to help accelerate your merge into main. The less complex your PR is, the faster it will be approved and merged!

All PRs start as draft. If you open a non-draft PR, it will be automatically converted to draft.

Step 1: Mark PR as "Ready for Review"

1. When your PR is ready, click Ready for Review.
2. An oncall reviewer is auto-assigned and expert reviewers are notified based on your changes.
   • Some PRs may jump straight to step 2. This is determined by .github/CODEOWNERS.

⚠️ Only mark as ready once merge-conflicts are resolved and the CI is passing.
Final Review might get declined if these requirements are not fulfilled.

Step 2: Final Review

For PRs that change megatron/core, once all expert reviewers have approved, the Final Review label is applied automatically and final reviewers are assigned.

For PRs outside megatron/core, this step is skipped.

Step 3: Approved

Once all required reviewers have approved, the Approved label is applied automatically.

Merge

Any member of mcore-engineers will be able to merge your PR.

For MRs into `dev` branch

The proposed review process for `dev` branch is under active discussion.

MRs are mergable after one approval by either eharper@nvidia.com or zijiey@nvidia.com.

[copy-pr-bot]

Auto-sync is disabled for draft pull requests in this repository. Workflows must be run manually.

Contributors can view more details about this message here.

[tdene]

/claude review

[claude]

No critical issues found. One observation:

• There are no unit tests for _reconstruct_reasoning_content (or any of the other helpers in chat_completions.py). The function is simple, but a couple of test cases (e.g. message with reasoning_content + existing content, message without reasoning_content, non-assistant message with reasoning_content) would help prevent regressions as this logic evolves.

LGTM otherwise.

[tdene]

No critical issues found. One observation:

• There are no unit tests for _reconstruct_reasoning_content (or any of the other helpers in chat_completions.py). The function is simple, but a couple of test cases (e.g. message with reasoning_content + existing content, message without reasoning_content, non-assistant message with reasoning_content) would help prevent regressions as this logic evolves.

LGTM otherwise.

Understood. While unit tests are very important, there is - in general - a lack of unit tests for this particular section of the code. Such unit tests will be added for the entire text_generation_server in a follow-up PR, by no later than March 24th.

[santhnm2]

LGTM but please take a look @i-riyad

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23212926764

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23214453991

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23215938152

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23238234634