ADR 0194: Optional subagent thinking override

• Status: Accepted for implementation
• Date: 2026-09-09
• Deciders: PI-Desktop runtime and UX maintainers
• Related: D356, E2E-203, ADR 0062, ADR 0063, ADR 0144

Context

The subagent editor previously offered inheritance and explicit thinking levels, but no way to leave the provider's default untouched. The normal provider-neutral stream path can translate the agent's off state into an explicit provider setting. That is not equivalent to omitting a thinking parameter for endpoints whose own default should remain in control. The model configuration thinking chips also used a raised dark-theme fill with too little contrast against their track.

Decision

1. Subagent frontmatter and input accept thinkingLevel: omit in addition to the empty inherit value and the seven canonical levels. The value is stored in frontmatter and is scoped to subagents; session and model thinking-level lists remain canonical.
2. The runtime represents omit as the agent's bookkeeping off state, but uses pi-ai's low-level stream for the provider request so no provider-neutral thinking override is synthesized. Explicit off continues through the normal simple stream path.
3. Selected model-configuration thinking chips use the accent background and inverted primary text in both themes.
4. This is an additive frontmatter/API value. It requires no SQLite migration, protocol version bump, or change to capability enablement storage.

Consequences

• Users can distinguish inherit, explicit off, and no provider override.
• Existing definitions remain valid and retain their current behavior.
• Provider adapters remain responsible for deciding their default behavior when the subagent selects omit.
• The selected chip state is more legible in dark mode while staying aligned with the existing accent tokens.