fix(llm): never give a const field the null default sentinel

[u9g]

Summary

A defaulted field gets a null branch in strict schemas so the model can decline a value, which _prepare_function_arguments swaps back for the Python default. A const field has exactly one legal value, so nulling it gains nothing and the emitted schema contradicts itself:

{"const": "celsius", "type": ["string", "null"]}

It also loses information. Strict mode has no discriminator keyword, so the const each variant of a discriminated union pins on its tag is the only thing that resolves the variant on the way back in. Tags are normally written with a default (unit: Literal["celsius"] = "celsius"), so they were taking the sentinel — and a returned null leaves variants that differ only in their tag indistinguishable:

{"temp": {"unit": None, "value": 70.0}}  ->  Celsius(unit='celsius', value=70.0)

The model said Fahrenheit; the tool got Celsius. Covers callable discriminators (Discriminator(fn) + Tag(...)) too, which emit no discriminator keyword to key off.

Testing

• pytest tests/test_tools.py (105 passed), plus -k "strict or schema or tool" across tests/ (211 passed)
• tag emitted as {"const": ..., "type": "string"}, required and non-nullable, and the named variant round-trips, for: bare arg, optional-only arg, list element, nested model field, str-Enum-valued tag, callable discriminator, and to_openai_response_format
• live tool call accepted with the correct tag returned by openai/gpt-4.1-mini and google/gemini-2.5-flash via the inference gateway

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.

[Copilot]

Pull request overview

Fixes strict tool-schema generation so defaulted const fields (notably discriminated-union tags) are never made nullable via the “null default sentinel”, preventing contradictory schemas and incorrect variant resolution during tool argument parsing.

Changes:

• Update strict-schema default handling to skip adding a null sentinel for const-constrained fields.
• Add targeted tests ensuring defaulted Literal[...] tags remain pinned ({"const": ..., "type": "string"}) and discriminated unions round-trip into the correct variant.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/test_tools.py	Adds regression tests for defaulted const fields and discriminated-union tag stability under strict schemas.
livekit-agents/livekit/agents/llm/_strict.py	Adjusts null-sentinel injection to explicitly exempt const schemas while preserving existing handling for enums/unions/refs.

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