vllm-project/vllm

[Feature]: Improve DCP error messages

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#28407 opened on Nov 10, 2025

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 (4 comments) (2 reactions) (0 assignees)Python (80,034 stars) (16,816 forks)batch import
feature requestgood first issuestale

Description

🚀 The feature, motivation and pitch

Currently if a backend doesn't support DCP we get the following error message

AssertionError: DCP requires attention impls to return the softmax lse for decode, but the impl FlashInferImpl does not return the softmax lse for decode.

It would good to suggest to the user to try an alternative backend using VLLM_ATTENTION_BACKEND

Alternatives

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Additional context

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Contributor guide

Tech stack
python
Domain
backenddeveloper experience
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
under 1 hour
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Pythonvllm framework familiarity
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
85
Research direction
Locate the file where the AssertionError about DCP attention is raised (likely in vllm/attention/backends/ or vllm/attention/selector.py). Modify the error message to include a suggestion to set the VLLM ATTENTION BACKEND environment variable to an alternative backend (e.g., 'FLASH ATTN' or 'FLASHINFER'). Ensure the change follows the project's error message conventions. Optionally, add a test to verify the new message appears when expected.