Subagent spawn fails with 'No reasoning level selected' (reasoning-level-missing) — no default applied when caller omits reasoning level

[zestybaron]

Environment

• OS: Windows 11 (build 10.0.26200)
• ZCode: desktop client
• Provider/account: zai coding plan, model GLM-5.3-Flash

Symptom

Spawning a subagent (Agent tool) fails with:

Cannot start subagent: No reasoning level selected
[reason=reasoning-level-missing]

Observed on 2026-09-13, 09-14, and 09-17 across different sessions on the same
account. Within an affected period it is persistent across retries — not
transient — and blocks all subagent use; the workaround is performing the work
directly in the main agent.

Analysis

The subagent request path validates the reasoning level before dispatch, and
when the caller did not supply one the validation fails with
reasoning-level-missing rather than falling back to anything. Providers and
models in this setup already declare reasoning variants with a default variant,
so a sensible default exists — it just isn't applied on this code path. It also
appears account/provider-selective (same client version worked at other times).

Suggested fix

Either:

1. Fall back to the model's declared default reasoning variant when the caller
   supplies no reasoning level, or
2. Inherit the current session's reasoning level for subagents, or
3. Expose a setting (e.g. a default subagent reasoning level) so users can pin
   it.

Any of the three removes a hard blocker that currently makes subagent use
unreliable on this provider.