pi-auto-compressor/docs/spec/2026-04-23_fix-tool-output-pruning.md

Fix Tool Output Pruning

Goal

Prevent pi-auto-compressor from removing valid Pi tool results from the model context.

Scope

In: Pi native assistant tool-call content blocks, tool-result pairing, token estimates, summary serialization, protected-tail tool sweeping, Hermes compaction override status. Out: Changing compression thresholds or persistent session storage.

Requirements

• R1: Preserve valid tool results. Done means: an assistant content block with type: "toolCall" and matching toolResult.toolCallId survives applyPruning(). VERIFY: focused script reports the matching tool result is present.
• R2: Preserve pair safety during compression. Done means: boundary alignment recognizes toolResult messages. VERIFY: TypeScript compiles and the helper paths use Pi message roles.
• R3: Keep legacy compatibility. Done means: existing OpenAI-style tool_calls fallback still works when present. VERIFY: helper handles both shapes.
• R4: Manual Hermes compaction runs through Pi's compaction lifecycle. Done means: /acp compress refuses active/queued runs, calls ctx.compact(), and session_before_compact supplies a custom Hermes CompactionResult or cancels. VERIFY: TypeScript compiles and subagent review finds no blocker.
• R5: Tool sweeping is simple and tail-safe. Done means: old, large tool outputs outside the protected tail are tombstoned while recent tail outputs are preserved. VERIFY: focused scripts show an old 250-char result is swept and a tail 250-char result is kept.

Tasks

• T1 (R1-R3): Patch pruner.ts.
  • verify: npx tsc --noEmit
  • success: no TypeScript errors.
  • likely_fail: tool result still filtered because assistant IDs are not collected.
  • sneaky_fail: assistant tool-call block is kept without a corresponding result after pruning.
• T2 (R1): Run a focused regression script against applyPruning().
  • verify: script prints preserved tool result count and content.
  • success: toolResults=1 and output text is visible.
  • likely_fail: toolResults=0.
  • sneaky_fail: result exists but text is the pruning placeholder.
• T3 (R4): Move Hermes compression into session_before_compact.
  • verify: npx tsc --noEmit; focused scripts for forced too-short and summary-failed paths.
  • success: /acp compress calls Pi compaction and the extension overrides it with Hermes summary.
  • likely_fail: hook returns nothing and Pi default compaction runs.
  • sneaky_fail: truncated/error summary is accepted as successful compaction.
• T4 (R5): Add protected-tail tool sweeping.
  • verify: focused old-output and tail-output scripts.
  • success: old output prints a tombstone; tail output length remains 250.
  • likely_fail: recent tool output is swept.
  • sneaky_fail: swept tool result is deleted instead of tombstoned.

Log

• Pi messages use assistant content blocks with type: "toolCall"; they do not use msg.tool_calls as the primary shape.
• npx tsc --noEmit passed.
• Regression script output: toolResults=1, assistantCalls=1, text README.md\npruner.ts\n.
• Hermes compaction now uses Pi's session_before_compact hook. The context hook only performs tool sweeping.
• The compaction override cancels on no model, auth failure, thrown errors, empty summaries, or non-stop summary responses; it does not intentionally fall back to Pi default compaction.
• Tool sweep script output: old 250-char bash result becomes [Tool output swept: ...]; protected-tail 250-char result remains length 250.
• Subagent review found and fixes addressed: reject truncated/error summary responses, avoid tombstoning currently protected-tail messages even when ID was previously marked, and wrap compaction hook with cancel-on-error.