# pi-compaction

High-performance unified context projection and compaction for Pi.

## Responsibilities

- incrementally index completed tool calls;
- replace large tool results with deterministic bounded evidence stubs;
- pass small tool results through unchanged whenever a stub would not be shorter;
- refine large stubs with non-blocking semantic summaries;
- expose one effective context projection to normal provider requests and Codex/SubAPI native compaction;
- cancel Pi's raw threshold compaction until the effective lower-bound reaches the configured threshold;
- retain original tool evidence through `context_tree_query`;
- publish read-only effective metrics for the footer and context observer.

Context Mode remains responsible for preventing large external data from entering the session. RTK remains responsible for ordinary tool-output cleanup.

## Configuration

Configuration lives in the installed bundle's `config/pi-compaction.json` (the bundle repository's `config/` directory is the single source of truth; `pi update` refreshes the installed copy). The extension never reads `settings.json` and never writes config at runtime:

```json
{
  "enabled": true,
  "softRatio": 0.72,
  "hardRatio": 0.9,
  "targetRatio": 0.55,
  "deterministic": {
    "enabled": true,
    "maxStubTokens": 1024
  },
  "semantic": {
    "enabled": true,
    "model": "default",
    "thinking": "low",
    "concurrency": 1,
    "minInputChars": 4000,
    "maxInputChars": 24000,
    "maxRetries": 2
  },
  "native": {
    "enabled": true
  }
}
```

Invalid or missing fields are normalized in memory.

## Native providers

Native compaction is enabled by the `native.enabled` config key and uses the model selected in the session when compaction starts. It does not select a separate summarization model.

| Provider | API | Native transport |
| --- | --- | --- |
| `openai-codex` | `openai-codex-responses` | Existing Codex Responses endpoint and JWT/account-ID authentication |
| `sub-api` | `openai-responses` | Public Responses endpoint and resolved gateway headers or API key |

For SubAPI, keep the existing `models.json` API as `openai-responses`. The adapter requires an explicit HTTP(S) `baseUrl`: for example, `https://gateway.example/v1` becomes `https://gateway.example/v1/responses`; an existing `/responses` suffix is not duplicated. It preserves configured headers and merges the native compaction beta feature, without decoding the API key or adding a ChatGPT account ID. No gateway hostname or credentials are bundled.

SubAPI preserves a non-empty explicit `Authorization` header (case-insensitively) ahead of the API key. Without it, a non-blank API key supplies Bearer authentication; if no key is available, a non-empty `cf-aig-authorization` header also supports proxy-managed authentication. Header-only credentials do not create a dummy Bearer token. Missing or blank credentials fail before any network request and return control to Pi. Official Codex still requires its API key/JWT even when explicit authentication headers are present.

Both transports send the current projection as ResponseItems, append `compaction_trigger`, and parse the SSE compaction item. SubAPI must support Sub2API native remote compaction v2 on its public Responses endpoint. This is not a client for the separate `/v1/responses/compact` endpoint; supporting only that endpoint or ordinary Responses chat is insufficient.

Request interception also restores SubAPI native checkpoints into subsequent Responses input. Checkpoints retain a bounded portable summary for switching models/providers, and their version 1 format is unchanged. Native authentication, transport or response failures return control to Pi's normal compaction fallback. Other provider/API pairs and `native.enabled: false` retain the existing portable-summary path. Setting `native.enabled` to false does not discard an already-created native checkpoint.

The gateway must have a compatible deployed version, model and available upstream account. Offline tests cover the protocol and lifecycle; they do not establish live gateway availability. No settings migration, provider rename or Pi core modification is required.

## Session format

The only custom entry type is `pi-compaction`, version 1. Segment summary entries contain hashes, summaries, and source references; they never duplicate complete raw tool output. Native checkpoints use Pi compaction-entry details with the same kind/version.

Projection replaces a tool result only when the resulting summary plus its `Source:` recovery line is strictly shorter than the original text content. Small tool results are therefore passed through byte-for-byte and keep their original content parts, including any non-text parts. Because a stub is only larger than the text it would replace when the original is short, this comparison is the sole gate; no separate size option exists. Every indexed result remains recoverable through `context_tree_query` whether or not the message was replaced.

This implementation intentionally does not read legacy `context-prune-*` or `openai-codex-native-compaction` entries. Start a new session after switching to this extension.

## Interfaces

- `context_tree_query`: recover original bounded tool evidence by source key or tool call ID.
- `/compaction-status`: show raw/effective tokens, reclaimed tokens, confidence, segments, and pending semantic jobs.

## Performance model

The first session hydration scans the branch once. Appended turns use delta projection and immutable cached snapshots. The `context` hook performs no model call and no session write. Semantic summaries run in a bounded background worker and become visible only at later safe event boundaries.