[V1] Fix json_object support with xgrammar

[russellb]

Our previous behavior with xgrammar was to ensure that the response was
valid JSON. When the json_object output format is requested, the correct
behavior is to ensure the response is a JSON object. Valid JSON could
also be an array, a string, or a number, which is not desired here.

The fix is to explicitly use a json schema of {"type": "object"}. An
upgrade to xgrammar is necessary to fix a bug with xgrammar's support of
this schema, so we upgrade to 0.1.17.

• {"type": "object"} json schema fails mlc-ai/xgrammar#256
• [Fix] Fix primitives for json schema converter mlc-ai/xgrammar#264

Signed-off-by: Russell Bryant rbryant@redhat.com

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[aarnphm]

This overall looks good to me, I just have one follow questions wrt an options for any json schemas.

[aarnphm]

hmm, should we also have an option to compile_builtin_json_schema?

[russellb]

Maybe. You could also have a json schema of {} to get the same behavior. I'm not sure if there's a performance difference between the two.

From a user perspective though, it's still possible to get "give me any valid json", but I seriously doubt most people actually want that.

[aarnphm]

hmm, I always thought that compile_builtin_json_schema is just compiling the default jsonschema spec, then users will have a prompt saying something like:

```Generate me an user for a game that has default 'strength', 'power', and 'abilities'``

[aarnphm]

But you are correct, probably makes it more easier this way then.

[Copilot]

Pull Request Overview

This PR fixes the handling of the json_object output format when using xgrammar by explicitly compiling a JSON schema with {"type": "object"}. It ensures that only a valid JSON object is produced rather than other JSON types.

• Updated backend_xgrammar.py to compile the JSON schema instead of using the built-in JSON grammar.
• Modified xgrammar_decoding.py to use the JSON schema for json_object with proper whitespace handling.
• Removed the fallback error for json_object in guided_decoding/init.py.
• Updated tests to assert that the generated output is a JSON object.

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

File	Description
vllm/v1/structured_output/backend_xgrammar.py	Replaced compile_builtin_json_grammar with compile_json_schema using '{"type": "object"}'.
vllm/model_executor/guided_decoding/xgrammar_decoding.py	Updated json_object branch to compile the proper JSON schema with correct whitespace handling.
vllm/model_executor/guided_decoding/init.py	Removed the error fallback for json_object to support the new behavior.
tests/v1/entrypoints/llm/test_struct_output_generate.py	Changed test assertion to verify the generated JSON is an object.

Files not reviewed (1)

• requirements/common.txt: Language not supported

[mergify]

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

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

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