Preserve uncertainty in context limits #23

Problem

When PAIR omits context_length (uncertain capacity), the client substitutes /api/show metadata or a 32,768 fallback, which may exceed the actual runtime capacity.

Evidence

• codex-rs/local/models/src/lib.rs:46-47 FALLBACK_CONTEXT_WINDOW = 32_768; lib.rs:104 calls /api/show for Ollama-shaped entries lacking context_length; v1.rs:40 parses context_length as optional.

Proposal

Preserve the uncertainty: use an explicit safe limit or validated runtime capacity instead of the metadata/32k substitute when PAIR omits the field.

Acceptance criteria

• Test: PAIR omits context_length while the loaded window is smaller than 32,768; the client does not advertise a window larger than the safe/validated limit.
• Behaviour with context_length present is unchanged.

Refs: codex #18-#21; PAIR #6, #8.

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https://claude.ai/code/session_01RPCzgwU247gMtjybR5v4iR