vllm-project/vllm

[Transformers v5] Tarsier2ForConditionalGeneration

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#38736 opened on Apr 1, 2026

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good first issuehelp wanted

Description

This is a sub-issue forming part of the work in https://github.com/vllm-project/vllm/issues/38379, please read the description of this issue before beginning to work on this one.

Which test is failing?

Tarsier 2 has a malformed config.json in its checkpoint and vLLM already has some workarounds to account for this (the Tarsier2Config that gets registered in vLLM). This appears to not be working correctly with the latest Transformers.

$ pytest tests/models/test_initialization.py::test_can_initialize_large_subset[Tarsier2ForConditionalGeneration]
...
[2026-04-01T10:33:31Z]   Value error, The text_config extracted from the model config does not have `num_attention_heads` attribute. This indicates a mismatch between the model config and vLLM's expectations. Please ensure that the model config is compatible with vLLM. [type=value_error, input_value=ArgsKwargs((), {'model': ...nderer_num_workers': 1}), input_type=ArgsKwargs]

How to configure my environment?

It's very important that you install both vLLM and Transformers from source so that your test results reflect the current state of both libraries.

# Or your fork
git clone https://github.com/huggingface/transformers.git
git clone https://github.com/vllm-project/vllm.git

cd vllm
VLLM_USE_PRECOMPILED=1 uv pip install -e .
uv pip install -e ../transformers

Contributor guide

Tech stack
python
Domain
backendtesting
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
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
Python programmingvLLM basicsTransformers library basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55
Research direction
Investigate the failing test `tests/models/test initialization.py::test can initialize large subset[Tarsier2ForConditionalGeneration]`. The issue is a malformed `config.json` in the Tarsier2 checkpoint. Examine the existing workaround in `Tarsier2Config` in vLLM and compare it with the latest Transformers changes. Look at how other models handle similar config mismatches to propose a fix.