[backryun]

Part of #8484. Two widened inference boundaries, three diagnostics, zero new.

Root cause

detectThinkingBudget() built a heterogeneous nested array without a contextual type, so each
destructured pair widened to string | JsonRecord. Separately,
attachReasoningRuleDirective() inferred its spread result as a closed { model: unknown }
object even though the function deliberately appends internal directive fields and returns a
JsonRecord.

This declares the existing contracts at those two boundaries:

• budget-field pairs are [JsonRecord, string]
• the directive-bearing body is a JsonRecord

Runtime values and routing behavior are unchanged.

Diagnostics

Measured independently against release/v3.8.49 at 0eeb8f45c:

• TypeScript 6 open-sse diagnostics: 150 → 147
• TypeScript 7.0.2 native diagnostics: 149 → 146
• Full line-number-agnostic error-set diff: 3 fixed, 0 new

With #8809 and #8810 included, the cumulative projection is TS6 140 / TS7 139.

Validation

• npm run typecheck:core
• Reasoning routing unit, pipeline, and Codex WebSocket tests — 16/16 passed
• ESLint on src/lib/reasoningRouting/policy.ts
• npm run check:file-size
• Prettier and git diff --check

No new runtime test was added because the values and branches are unchanged, while the existing
tests already cover budget-only intent, directive application, Chat Completions, Responses,
Anthropic Messages, combo filtering, connection rules, and the Codex WebSocket path.