Validated on a 3-PR combined board: openai-responses-opencode-subagent-sessionid within the board's 20/20, typecheck:core + check:open-sse-typecheck clean, gates within baseline. Uses null as the omission sentinel for OpenCode's optional default-less subagent.sessionID under Responses/Codex strict mode (which required every declared property) — models can now omit the field to spawn a new child instead of inventing filler IDs; the null is stripped before OpenCode sees the call, real child IDs pass through unchanged. Thank you @ofonseca-pyming!
