[Frontend] Support image inputs in the Decisions API

[Txoka]

Overview

Preserve ordered text and image content through the Decisions API's native multimodal renderer. Stacked on #60713 (Winnow decision support), itself dependent on #60465.

Review only the image-support changes. The full diff includes the unmerged predecessors; this branch will be rebased after they merge.

Claims

• Support ordered input_image content for generic and Winnow read strategies using existing native model processors.
• Preserve expanded image tokens, hashes, placeholders and multimodal kwargs while appending private question suffixes.
• Reject unsupported image models and expanded-context overflow through tested API errors.

Validation

October 10 rebase: updated onto upstream 2bea220 and refreshed the pending #60465 implementation from a8867eab, retaining upstream DiffusionGemma routing and shared tracing. The image branch remains stacked on the text branch. All 91 affected CPU/HTTP tests and all contribution-file pre-commit hooks passed. GPU/model evaluations were not rerun for this rebase.

Restacked on the rebased text branch (ecd4029) and upstream 9adba10 on October 9. Preserved native image metadata and passed cache salt through the native renderer. All 90 affected CPU/HTTP tests and contribution-file pre-commit hooks, including mypy, passed. Earlier GPU measurements were not rerun for this rebase.

75 focused CPU/HTTP tests pass after restacking onto the refreshed dependency across the structured Decisions question-types, strategies and endpoint suites. Changed-file pre-commit hooks pass. Actual native OnlineRenderer checks on the refreshed branch pass for Gemma and Qwen: stable/distinct image hashes, reversed hashes when image order swaps, exact repeated prompts and shared prefixes covering every expanded image span.

Post-refresh native BF16 GPU checks passed five requests/15 decisions per model, covering all three answer types, repeated inputs and image ordering. Gemma passed with prefix caching enabled; Qwen passed the unchanged 1e-6 repeat gate with caching disabled. With caching enabled, Qwen served all requests and correct colors/order but showed a repeat probability delta up to 0.00128; batch-invariant mode is unsupported by its GDN_ATTN backend in this runtime. These checks used CPU offload and establish functional behavior, not visual benchmark quality or resident throughput. Quantized vision support remains unverified.

The full API generation area passed 83 tests with 18 explicit GPU-memory skips. Broader launcher testing reached 227 passes with 18 failures and 3 skips; failing GPU cases reported CUDA device busy/unavailable, reproduced by a one-element PyTorch allocation outside vLLM. Complete upstream hardware CI has not been run. Earlier native GPU checks below predate the October 9 rebase; the affected CPU/HTTP tests and contribution-file hooks were rerun after it.

Reproduction and bound runtime configurations are recorded in benchmarks/decision_models/vision-verification.json; generated-color API client: benchmarks/decision_models/check_images.py.

Details

The previous API flattened input to text, discarding image content before rendering. This change retains media order and uses existing OnlineRenderer/native multimodal processing and media restrictions. Rendering remains serial as required by the parent API implementation. No scheduler, model runner, attention kernel or custom downloader is introduced.

This differs from #60676, which adds LiquidAI image support to the Jev/systemone interface with a separate images field; this contribution handles ordered OpenAI Decisions content for multiple native model families. AI assistance was used to implement, test and document this contribution. The submitting human confirmed review of the changes and execution of relevant tests.

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Pull Request Checklist

• I used vLLM's /pr-checklist skill.
• AI assistance was used during the creation of this PR.
• Design Fit: Uses existing rendering/media/backend infrastructure.
• Testing and Validation: CPU/native renderer and post-refresh GPU evidence complete with documented Qwen cache limitation; human review and relevant test execution confirmed.
• Code Quality and Style: Changed-file hooks pass; documentation and reproduction client included.
• Pull Request Contents: Human review confirmed; dependencies and limitations disclosed.

[claude]

Claude Code Review

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[mergify]

Documentation preview: https://vllm--60745.org.readthedocs.build/en/60745/

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Txoka.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Txoka.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Txoka.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork