pi-codex-compact — remote compaction V2 for Pi

[!WARNING] This extension is experimental. It depends on an undocumented OpenAI Codex Responses wire contract and stores provider-specific opaque checkpoints. Keep backups of important sessions.

@signalridge/pi-codex-compact adds Codex Remote Compaction V2 to the Pi Coding Agent for the built-in openai-codex OAuth provider. It replaces Pi's plaintext summary-generation call with a server-generated opaque compaction item and safely replays that item in later OpenAI Codex Responses requests.

Pi remains responsible for deciding when compaction runs, including automatic thresholds, the built-in /compact command, overflow retries, retained-message selection, and append-only session publication. This package owns only the Remote V2 request, checkpoint persistence, and later payload replay.

Features

• Uses the active openai-codex OAuth credentials without persisting tokens or request headers.
• Handles Pi's manual, threshold, and overflow compaction reasons through the same lifecycle hook.
• Validates one bounded opaque compaction item from a completed Responses SSE stream.
• Persists a versioned checkpoint in CompactionEntry.details and restores it after reload, resume, or a fork that retains the checkpoint.
• Replays the newest compatible checkpoint while preserving later conversation and extension-added context.
• Supports repeated compaction by carrying the previous opaque item into the next Remote V2 request.
• Falls back to Pi's native plaintext compaction after authentication, transport, protocol, or validation failure; user cancellation does not launch the fallback.
• Provides /codex-compact for manual compaction, effective-route visibility, and bounded settings.

Install

Install persistently from npm:

pi install npm:@signalridge/pi-codex-compact

Try the published package without installing:

pi -e npm:@signalridge/pi-codex-compact

Try a local checkout from the repository root:

pi -e ./packages/pi-codex-compact
# or
just try codex-compact

Loading the package enables Remote V2 with safe defaults. Avoid loading a global npm installation and the local workspace at the same time.

Quick start

1. Sign in through Pi's built-in OpenAI Codex OAuth provider.
2. Select a model whose provider is openai-codex and API is openai-codex-responses.
3. Work normally. Pi's automatic compaction and built-in /compact continue to operate.
4. Run /codex-compact to inspect the effective route or choose Compact now.
5. After compaction, continue the session normally; compatible requests replay the opaque checkpoint.

The extension shows an experimental warning at session start in UI-capable modes. When the active model is unsupported, compaction remains entirely Pi-native.

Command

/codex-compact

In TUI mode, the root menu shows whether Remote V2 is enabled, the active model, and whether a manual compact will use Codex Remote V2 or Pi native. It contains:

Compact now
Settings
Close

Compact now closes the menu before asking Pi to compact the active session. Escape or Ctrl+C closes without compacting, and an obsolete menu cannot trigger work after session replacement or shutdown. Settings opens the bounded settings editor. In non-TUI modes, the command reports the manual settings path instead of opening custom UI or compacting.

Pi's built-in /compact remains available and follows the same extension hook when the active model is compatible.

Settings

The extension has one optional, global-only JSON settings file:

<getAgentDir()>/pi-codex-compact.json

The normal path is ~/.pi/agent/pi-codex-compact.json. There is no environment-variable or project-level override.

{
  "enabled": true,
  "requestTimeoutMs": 300000,
  "maxRetries": 2,
  "replacementTokenBudget": 64000,
  "notifyOnFallback": true
}

| Setting | Default | Accepted values | Behavior | Recommendation | | --- | ---: | --- | --- | --- | | enabled | true | Boolean | Attempt Remote V2 for a compatible model. | Keep enabled unless diagnosing provider behavior. | | requestTimeoutMs | 300000 | Integer from 30,000 to 600,000 ms | Bound the entire extension-owned operation, including retries and response-body inspection. | Keep five minutes; increase only for a consistently slow connection. | | maxRetries | 2 | Integer from 0 to 2 | Retry transient provider transport failures before Pi fallback. | Keep two; use zero when diagnosing the first failure. | | replacementTokenBudget | 64000 | Integer from 8,000 to 128,000 tokens | Bound approximate retained user-message text beside the opaque item. | Keep 64K; lower it to reduce session size or raise it only when recent user context is being lost. | | notifyOnFallback | true | Boolean | Warn when Remote V2 fails and Pi-native compaction takes over. | Keep enabled so silent fallback does not hide protocol or entitlement problems. |

Missing fields use defaults. Settings reload on every session_start, including /reload, resume, and fork. Menu writes apply immediately, preserve unknown JSON fields, serialize within the current Pi process, and use a final conflict check plus same-directory atomic rename. On Unix, temporary files use mode 0600.

Malformed, invalid, oversized, or symlinked settings files are never overwritten. Safe defaults stay active, and the menu provides read-only repair guidance until the file is fixed and Pi is reloaded. Separate Pi processes do not share a mutation lock; a detected concurrent edit is rejected so the user can reopen Settings and retry.

Relationship to Codex configuration

This extension does not read ~/.codex/config.toml.

| Codex setting | Extension behavior | | --- | --- | | features.remote_compaction_v2 | Conceptually corresponds to this extension's enabled; it is not imported. | | model_auto_compact_token_limit | Not duplicated. Pi's own compaction threshold remains authoritative. | | model_auto_compact_token_limit_scope | Not supported; Pi extensions do not own Codex's compact-window lineage. | | compact_prompt / experimental_compact_prompt_file | Not used by Remote V2, whose opaque checkpoint is generated by the server. | | features.token_budget | Not supported; token-budget context reset is a different experimental strategy. |

Requirements and compatibility

• Pi APIs compatible with the package's declared peer dependencies.
• The built-in provider openai-codex.
• API openai-codex-responses on the active model.
• A working OpenAI Codex OAuth login and Remote V2 entitlement.

OpenAI API-key, Azure, GitHub Copilot, proxies, and arbitrary Responses-compatible providers are not supported. Switching to another provider or model leaves Pi's visible fallback marker plus retained recent messages in context. Switching back to the checkpoint's original compatible model restores opaque replay.

How it works

1. Pi prepares compaction and selects the recent message suffix it will retain.
2. The extension projects an earlier compatible checkpoint, if present, into the current Responses input.
3. It appends exactly one final compaction_trigger and sends a normal authenticated Codex Responses SSE request with cache retention disabled.
4. It requires a completed response containing exactly one non-empty opaque compaction item.
5. It constructs bounded replacement history from recent raw user-role Responses items followed by the opaque item.
6. It stores that history and fingerprints of Pi's retained suffix in versioned CompactionEntry.details.
7. On later compatible requests, it replaces an exactly validated marker with the persisted replacement history immediately before provider dispatch.

If fingerprints, model identity, payload shape, or marker count do not match exactly, the extension leaves Pi's visible fallback context unchanged instead of guessing.

Loadout hooks can transform model-facing tool descriptions without changing the raw registry. The extension recognizes those transformations only after fresh agent-context preparation and a provider payload whose tool names and descriptions match the persisted declarations. It freezes both public tool surfaces and the serialized wire tools; Remote V2 must reproduce that wire snapshot before sending. Cached provider callbacks cannot approve unsent registry edits, and hidden declarations cause Pi-native fallback rather than restoring hidden tools remotely. Evidence is in-memory only and resets on startup, reload, or tree navigation. A fresh, unchanged normal request can establish it again; prompt, tool, and schema changes remain guarded.

Privacy, storage, and limits

Remote compaction sends the active conversation context, system prompt, and active tool schemas to the same OpenAI Codex backend used by the selected model. The Pi session stores the encrypted compaction item and bounded recent user-role Responses items. It does not store OAuth tokens, authorization headers, or request headers in checkpoint details.

| Boundary | Limit | | --- | ---: | | Observed SSE stream | 8 MiB | | Serialized opaque compaction item | 2 MiB | | Persisted replacement history | 8 MiB | | Retained user text | 64K approximate tokens by default; configurable from 8K to 128K | | Settings file | 64 KiB | | Transport retries | At most 2 | | Request timeout | At most 10 minutes |

An individually oversized media item is dropped rather than making the session entry unbounded. The oldest fitting text item may be partially truncated to preserve newer context. These hard byte ceilings are intentionally not configurable.

Experimental limitations

• The wire contract is undocumented and can change independently of Pi or this package.
• Full older history depends on this extension and the same compatible model. Removing the extension exposes only the portability fallback marker and Pi-retained recent messages.
• The package does not reproduce Codex core's context-window UUID/number lineage, previous-model compatibility fallback, exact pre-turn ordering, or exact mid-turn model-session ownership.
• A live prompt or tool change that has not been written into Pi's transcript causes a native compaction fallback; an opaque checkpoint must match the system state Pi will persist.
• Pi exposes no public effective-loadout snapshot. Observed, unchanged description-only rewrites (including codemode mode: "on") can use Remote V2, but unproven transformations, hidden declarations (including codemode mode: "only"), membership/schema mismatches, or different serialized wire tools use native compaction. Observation does not weaken prompt/tool equality or stale-state checks.
• Pi invokes session_before_compact handlers in registration order. Another extension that mutates prompt, tools, or context after this handler returns can invalidate an already prepared opaque checkpoint before Pi appends it. Load such handlers before this extension or disable Remote V2 in that combination; the extension cannot inspect later handlers' changes.
• Remote failure falls back to Pi's plaintext summary, so a session can contain both remote opaque and native compaction entries over time.
• Settings concurrency is coordinated only within one Pi process; separate processes rely on the final conflict check.

Package layout

src/index.ts          Thin Pi entrypoint
src/codex-compact.ts  Pi lifecycle, command, provider projection, and fallback
src/tool-state.ts     Prepared/observed public tool surfaces and frozen wire evidence
src/remote.ts         Provider stream invocation, auth payload, timeout, and retry controls
src/protocol.ts       Bounded SSE parsing and Remote V2 payload/output validation
src/checkpoint.ts     Replacement history, fingerprints, persistence, and replay projection
src/settings.ts       Global settings validation and atomic persistence
src/settings-menu.ts  Manual compaction and settings TUI

test/                 Protocol, checkpoint, lifecycle, remote, settings, and menu coverage

Development

From the repository root:

npm --workspace @signalridge/pi-codex-compact run check
npm test
just pack codex-compact

The checkpoint and remote protocol boundaries are documented in src/checkpoint.ts and src/remote.ts.

License

MIT