fix(cli): preserve default subagent model and reasoning

[marius-kilocode]

What Problem This Solves

With experimental Task model selection enabled, providers that require every advertised field have no null value for leaving model, provider, and reasoning unchanged. The tool instructions also encourage autonomous model choices based on complexity, cost, and latency, which can replace the intended defaults with an unsuitable or older model.

Why This Change Was Made

Keep the normal delegation path unchanged unless an override is explicitly requested. Accept omitted or null selectors as no override, and tell the agent to use catalog lookup only for requested overrides rather than guessing model names. Existing default precedence, resume behavior, and validation of real overrides remain unchanged. This corrects the optional-selection behavior introduced in #13557.

User Impact

Subagents retain their normal model and reasoning settings without requiring the caller to name either. Explicit model selection and reasoning-only overrides still work. The experiment remains opt-in.

Evidence

• Reproduced null-argument rejection and the non-nullable advertised schema before the fix.
• 177 focused tests pass across Task model resolution, parameter schemas, config overlays, and existing Task behavior. The 146-test selection/schema/config set also passes on the pinned Bun 1.3.14.
• CLI typecheck and lint pass; lint reports existing warnings only. No shared upstream source changes are needed.
• A fresh isolated backend with a local fake provider completed omitted-selector, all-null, and reasoning-only override flows. Verified child state, Task results, outgoing HTTP requests, SSE completion, and preservation of parent reasoning.
• No VS Code UI or real-model instruction-following test was performed.

Manual check: enable Task model selection, delegate without an override, then request only a reasoning change. Confirm the child keeps its normal model and only the requested reasoning changes.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (4 files)

• .changeset/fix-subagent-selection-defaults.md
• packages/opencode/src/kilocode/tool/task.ts
• packages/opencode/test/kilocode/tool-task-model.test.ts
• packages/opencode/test/tool/__snapshots__/parameters.test.ts.snap

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[WebReflection]

I wonder when/if TS will stop this madness:

{ field?: type | null }

and introduce something like:

{ field??: type }

which allows both undefined and null as type, following the TC39 Nullish coalescing operator (??) standard https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing