fix: use prompt token length for advantage group extraction and fix token mask#2176
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fix: use prompt token length for advantage group extraction and fix token mask#2176yfw wants to merge 2 commits into
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The previous role-based extraction (`_extract_prompt_only_messages`) broke on multi-turn prompts containing assistant messages in the conversation history — it would strip them, corrupting the prompt IDs used for advantage estimation. Replace with `extract_initial_prompt_messages()` which uses the `length` field to identify the original prompt boundary. Applied to both sync and async GRPO paths. Closes #1960 Co-Authored-By: Jiaqi Zeng <jiaqiz@nvidia.com> Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com> Signed-off-by: Yi-Fu Wu <yifu.wu@gmail.com>
Signed-off-by: Anish Mahishi <amahishi@cw-dfw-cs-001-vscode-02.cm.cluster>
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This PR fixes two multi-turn GRPO training issues:
The previous role-based extraction (
_extract_prompt_only_messages) broke on multi-turn prompts containing assistant messages in the conversation history — it would strip them, corrupting the prompt IDs used for advantage estimation.Replace with
extract_initial_prompt_messages()which uses thelengthfield to identify the original prompt boundary. Applied to both sync and async GRPO paths.GRPO token loss masks previously unmasked every message with
role == "assistant". In multi-turn data, assistant messages can be part of the prompt history, not generated rollout output, so those tokens should not contribute to the policy loss. This PR updates masking so only assistant messages produced by generation, identified by existinggeneration_logprobs, are trainable. Missinggeneration_logprobsare still filled with zeros for downstream tensorization.Closes #1960 and #1956
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