[Bugfix][Structured Outputs] Reject empty JSON schemas

[fyrsta7]

Purpose

Fix #52011 by rejecting an empty JSON Schema before structured-output backend selection. The empty schema {} is a valid universal JSON Schema, but it leaves number generation unconstrained. If decoding enters the number branch, it may continue producing digits until max_tokens and return an unusable response with finish_reason="length".

This intentionally changes empty schemas from an unconstrained request to an HTTP 400 validation error. It covers dictionary input (json={}), serialized input (json="{}"), and serialized input with surrounding whitespace. Callers should provide a concrete non-empty schema, or use json_object=True only when any JSON object matches the intended output.

The Python and Rust frontends now apply the same check before backend compilation or model execution. The Rust request boundary covers raw structured_outputs, OpenAI response_format, and the gRPC structured-output path. Non-empty schemas continue to pass request validation, and a failed request does not prevent subsequent requests from completing.

This does not duplicate #40099, #51450, #45346, or #47176: those changes respectively address general repetition detection, whitespace-only Rust frontend strings, json=""/json_object=False, and other Rust validation parity gaps such as null schemas and mutually exclusive constraints. Open PR #51450 touches the same Rust protocol file, but its check only uses value.trim().is_empty(); it neither rejects an object {} nor parses serialized "{}". A duplicate check on 2026-08-13 found no assignee, claim comment, linked pull request, or open pull request implementing empty-schema rejection.

AI assistance from OpenAI Codex was used to investigate the issue, implement the validation, add tests and documentation, and prepare this description. I reviewed every changed line, validated the behavior end to end, and ran the tests listed below.

Test Plan

• .venv/bin/python -m pytest tests/v1/structured_output/test_validation.py -v
• .venv/bin/python -m pytest tests/v1/structured_output -m cpu_test -q
• .venv/bin/python -m pytest tests/entrypoints/openai/chat_completion/test_chat_completion.py::test_empty_json_schema_returns_bad_request -v -s
• cd rust && cargo fmt --all --check
• cd rust && cargo test -p vllm-engine-core-client
• cd rust && cargo test -p vllm-server
• cd rust && cargo clippy -p vllm-engine-core-client -p vllm-server --tests -- -D warnings
• git diff --name-only -z origin/main...HEAD | xargs -0 .venv/bin/pre-commit run --files
• Manual OpenAI API smoke tests against both the Python and Rust frontends with Qwen2.5-1.5B-Instruct, covering dictionary and serialized empty schemas followed by a normal request.
• git diff --check origin/main...HEAD

Test Result

• On unmodified main, a direct validation probe accepted {}, "{}", and a whitespace-padded "{}".
• Focused validation tests: 9 passed, 14 warnings in 3.91s.
• Structured-output CPU tests: 25 passed, 23 deselected, 14 warnings in 9.38s.
• OpenAI API regression test: passed.
• Rust engine-core-client tests: 99 passed; Rust server tests: 342 passed; Clippy with warnings denied and cargo fmt --check both passed.
• Manual OpenAI API smoke tests with Qwen2.5-1.5B-Instruct produced the same result on both frontends: dictionary {} returned HTTP 400, serialized "{}" returned HTTP 400, and the immediately following normal request returned HTTP 200.
• All applicable pre-commit hooks passed, including Ruff, markdownlint, typos, mypy for Python 3.10, SPDX, forbidden-import checks, and configuration validation, including Rust formatting.
• git diff --check origin/main...HEAD passed.

Model evaluation is not applicable because the affected requests fail during validation before model execution. Successful request generation is not modified. User-facing behavior and the compatibility tradeoff are documented in docs/features/structured_outputs.md.

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR and the issue it resolves are described above.
• Reproducible unit, integration, lint, and manual test steps are listed.
• Before-and-after validation and HTTP results are listed.
• The necessary structured-output documentation is updated.

[claude]

Claude Code Review

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[mergify]

Documentation preview: https://vllm--52020.org.readthedocs.build/en/52020/

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @fyrsta7.

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

[fyrsta7]

Hi, I rebased this PR onto the latest main and resolved the conflict introduced by #51796, preserving both the upstream NUL-regex validation tests and this PR's empty JSON schema tests. The PR is now conflict-free.

Post-rebase validation:

• tests/v1/structured_output/test_validation.py: 12 passed
• structured-output CPU suite: 28 passed, 23 deselected
• Rust engine-core-client: 99 passed
• Rust server: 342 passed
• cargo fmt, Clippy with warnings denied, changed-files pre-commit, and git diff --check: passed

Thank you!

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @fyrsta7.

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

[fyrsta7]

Hi, I rebased this PR onto the latest main and resolved the conflict introduced by #52394, preserving its structured-output client-error and fallback tests alongside this PR's empty JSON schema validation tests. The concurrent gRPC skip_special_tokens change from #52384 is also preserved. The PR is now conflict-free.

Post-rebase validation:

• tests/v1/structured_output/test_validation.py: 23 passed
• structured-output CPU suite: 39 passed, 23 deselected
• Rust engine-core-client: 99 passed
• Rust server: 343 passed
• Rust fmt, Clippy with warnings denied, changed-files pre-commit, and git diff --check: passed

Thank you!

[BugenZhao]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

Reviewed commit: d27fb41a1b

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[arpera]

@fyrsta7, first of all thanks for a PR! I have several questions about your PR:

1. In this PR you decided to reject requests that send empty json schema, but at the same time initial bug report [bug]: empty JSON schema {} causes unbounded number death-loop garbled output #52011 proposes two different solutions to this problem: (1) to reject such requests; (2) to normalize empty json schema to {"type": "object"}. Any motivation why you decided to follow (1) approach and not (2)?

2. I think the problem reported is not limited only to empty json schema but also it might be reproduced by {"type": "number"}, {"type": "integer"}, {"type": ["object", "number"]}, {"title": "x"}, {"description": "any"}, and so on. So, by rejecting empty json schema we still do not resolve the root cause of the issue and it can be reproduced many other ways. In my opinion the better way would be not to reject such schemas including empty but to set manually max_tokens constraint for such a requests to not allow them consume max_tokens budget. What do you think about this proposal?

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @fyrsta7.

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