OpenCode’s instructions <env> used to start with Current conversation session ID: ses_…. I fetched v2.0.19 packages/core/src/instructions/builtins.ts; that line is gone. Date and env now sit after Code Mode, MCP, skills, and AGENTS.md. I didn’t run OpenCode.

They replayed 2,372 sessions. Moving the ID to the end of the prefix left sol block-cache at 0% and Anthropic 5 min at 29.0%. Dropping it took those to 45.3% and 59.8%. Prefix-caching OpenAI barely moved once date/env were already last (44.8% to 45.4%).

v2.0.19 still writes OPENCODE_SESSION_ID into the shell child, overwriting a stale outer value. AI_AGENT is only set if the outer environment left it empty. That copy is PR #51975.