[Bugfix] Fix AttributeError when passing StructuredOutputsParams to CompletionRequest

[pks]

Summary

The check_structured_outputs_count model validator on CompletionRequest and ChatCompletionRequest uses mode="before" and calls .get() on the structured_outputs value, assuming it is always a dict. However, the field is typed as StructuredOutputsParams | None, so passing a dataclass instance is a valid usage but crashes with:

  AttributeError: 'StructuredOutputsParams' object has no attribute 'get'

This PR handles both dict and StructuredOutputsParams inputs by checking the type and using getattr for dataclass instances.

Reproduction

from vllm.entrypoints.openai.completion.protocol import CompletionRequest
from vllm.sampling_params import StructuredOutputsParams

# Crashes on v0.12.0 through v0.15.1
CompletionRequest(
    model="my-model",
    prompt="hello",
    max_tokens=100,
    structured_outputs=StructuredOutputsParams(regex=r"^ [\s\S]*"),  # could be fixed w/ structured_outputs={"regex": " [\s\S]*"}
)
# AttributeError: 'StructuredOutputsParams' object has no attribute 'get'

Test plan

• Verified CompletionRequest accepts StructuredOutputsParams object (was crashing)
• Verified CompletionRequest still accepts dict (existing behavior)
• Verified CompletionRequest still accepts None
• Verified multi-constraint validation still rejects for both dict and object inputs
• Verified ChatCompletionRequest same as above

---

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
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• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request addresses an AttributeError in check_structured_outputs_count for CompletionRequest and ChatCompletionRequest. The error occurred when structured_outputs was a StructuredOutputsParams object instead of a dictionary. The submitted change corrects this by checking the type of structured_outputs and using either getattr for objects or .get() for dictionaries to access its properties. The fix is applied consistently in both chat_completion/protocol.py and completion/protocol.py. The changes correctly resolve the described bug.

[mergify]

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