ADR 0136: Preserve the active task boundary across context compaction

• Status: Accepted
• Date: 2026-08-31
• Deciders: PI-Desktop core
• Amends: ADR 0064 / D203

Context

Codex-shaped compaction kept several recent user prompts while dropping the assistant and tool messages that established whether those prompts were completed. After a completed task, the next prompt could therefore look like another bare user message and the model could resume an old task (issue #22).

Decision

1. Checkpoints carry opaque details.retainedTailMode, either active_turn or completed_turn.
2. An active-turn checkpoint is used when the provider must continue after a tool result, toolUse, or overflow recovery. It retains only the latest user message, subject to the existing 20,000-token cap and truncation.
3. A completed-turn checkpoint is used at a terminal turn boundary, before a new prompt, and for manual compaction. Its retained tail is empty; the summary is authoritative for completed work and the next user prompt is the sole new task.
4. The fresh_window family remains the ADR 0064 no-summary exception and always carries an empty tail.
5. Legacy checkpoints without this field are normalized to their latest user message only. The visible transcript, durable checkpoint chain, host ownership, and protocol shape do not change.

Consequences

• Completed user requests cannot be replayed as naked historical context after compaction or restart.
• Active tool loops retain the one prompt needed to continue the current task, while assistant/tool messages remain represented by the summary.
• Existing checkpoints remain readable without a migration, with a conservative latest-user fallback.

Alternatives

• Retaining multiple recent user messages preserves more literal prompt text, but loses the task boundary when completion messages are compacted away.
• Retaining assistant and tool messages would preserve more context but can strand incomplete tool calls and violates the Codex-shaped checkpoint form.