Subagent tool: sessionID not clearly marked as optional in serialized JSON schema #55503

Problem

The subagent tool's sessionID parameter is correctly defined as optional in the TypeScript source (Schema.optionalKey(SessionSchema.ID)), but some models do not reliably infer optionality from the serialized JSON schema. They see sessionID in properties and treat it as required, filling it with garbage values like "sesh" or "sesh_omit_should_not_be_here".

This results in Subagent session not found errors that confuse the model, which then retries with more garbage values in a loop.

Evidence

• Source: packages/core/src/tool/plugin/subagent.ts line 40 — Schema.optionalKey(SessionSchema.ID)
• Serialization: packages/ai/src/tool.ts — Schema.toJsonSchemaDocument() produces a JSON schema where sessionID is in properties but not in required
• The serialized schema is technically correct, but some models do not check the required array and instead pattern-match "fill in all properties"

Expected Behavior

Models should omit sessionID when starting a new subagent. The tool description already says "Calls without a sessionID start a new conversation," but models ignore this.

Actual Behavior

Models fill sessionID with fabricated values, causing Subagent session not found errors. Explicit user prompts to omit the field do not reliably fix the behavior.

Suggested Fix

Add explicit "optional": true to the serialized JSON schema for sessionID, background, model, and agent in the subagent tool definition. Alternatively, add a stronger hint in the tool description (e.g., "IMPORTANT: Do NOT include sessionID unless continuing a previous subagent").

Environment

• OpenCode version: latest (desktop)
• Model: third-party model with weaker JSON schema adherence