co-mono/packages/coding-agent/docs/session.md

Session File Format

Sessions are stored as JSONL (JSON Lines) files. Each line is a JSON object with a type field. Session entries form a tree structure via id/parentId fields, enabling in-place branching without creating new files.

File Location

~/.pi/agent/sessions/--<path>--/<timestamp>_<uuid>.jsonl

Where <path> is the working directory with / replaced by -.

Session Version

Sessions have a version field in the header:

• Version 1: Linear entry sequence (legacy, auto-migrated on load)
• Version 2: Tree structure with id/parentId linking

Existing v1 sessions are automatically migrated to v2 when loaded.

Type Definitions

• src/core/session-manager.ts - Session entry types
• packages/agent/src/types.ts - AgentMessage, Attachment, ThinkingLevel
• packages/ai/src/types.ts - UserMessage, AssistantMessage, ToolResultMessage, Usage, ToolCall

Entry Base

All entries (except SessionHeader) extend SessionEntryBase:

interface SessionEntryBase {
  type: string;
  id: string;           // 8-char hex ID
  parentId: string | null;  // Parent entry ID (null for first entry)
  timestamp: string;    // ISO timestamp
}

Entry Types

SessionHeader

First line of the file. Metadata only, not part of the tree (no id/parentId).

{"type":"session","version":2,"id":"uuid","timestamp":"2024-12-03T14:00:00.000Z","cwd":"/path/to/project"}

For branched sessions (created via /branch command):

{"type":"session","version":2,"id":"uuid","timestamp":"2024-12-03T14:00:00.000Z","cwd":"/path/to/project","branchedFrom":"/path/to/original/session.jsonl"}

SessionMessageEntry

A message in the conversation. The message field contains an AgentMessage.

{"type":"message","id":"a1b2c3d4","parentId":"prev1234","timestamp":"2024-12-03T14:00:01.000Z","message":{"role":"user","content":"Hello"}}
{"type":"message","id":"b2c3d4e5","parentId":"a1b2c3d4","timestamp":"2024-12-03T14:00:02.000Z","message":{"role":"assistant","content":[{"type":"text","text":"Hi!"}],"provider":"anthropic","model":"claude-sonnet-4-5","usage":{...},"stopReason":"stop"}}
{"type":"message","id":"c3d4e5f6","parentId":"b2c3d4e5","timestamp":"2024-12-03T14:00:03.000Z","message":{"role":"toolResult","toolCallId":"call_123","toolName":"bash","content":[{"type":"text","text":"output"}],"isError":false}}

ModelChangeEntry

Emitted when the user switches models mid-session.

{"type":"model_change","id":"d4e5f6g7","parentId":"c3d4e5f6","timestamp":"2024-12-03T14:05:00.000Z","provider":"openai","modelId":"gpt-4o"}

ThinkingLevelChangeEntry

Emitted when the user changes the thinking/reasoning level.

{"type":"thinking_level_change","id":"e5f6g7h8","parentId":"d4e5f6g7","timestamp":"2024-12-03T14:06:00.000Z","thinkingLevel":"high"}

CompactionEntry

Created when context is compacted. Stores a summary of earlier messages.

{"type":"compaction","id":"f6g7h8i9","parentId":"e5f6g7h8","timestamp":"2024-12-03T14:10:00.000Z","summary":"User discussed X, Y, Z...","firstKeptEntryId":"c3d4e5f6","tokensBefore":50000}

Optional fields:

• details: Compaction-implementation specific data (e.g., file operations for default implementation, or custom data for custom hook implementations)
• fromHook: true if generated by a hook, false/undefined if pi-generated

BranchSummaryEntry

Created when switching branches via /tree with an LLM generated summary of the left branch up to the common ancestor. Captures context from the abandoned path.

{"type":"branch_summary","id":"g7h8i9j0","parentId":"a1b2c3d4","timestamp":"2024-12-03T14:15:00.000Z","fromId":"f6g7h8i9","summary":"Branch explored approach A..."}

Optional fields:

• details: File tracking data ({ readFiles: string[], modifiedFiles: string[] }) for default implementation, arbitrary for custom implementation
• fromHook: true if generated by a hook

CustomEntry

Hook state persistence. Does NOT participate in LLM context.

{"type":"custom","id":"h8i9j0k1","parentId":"g7h8i9j0","timestamp":"2024-12-03T14:20:00.000Z","customType":"my-hook","data":{"count":42}}

Use customType to identify your hook's entries on reload.

CustomMessageEntry

Hook-injected messages that DO participate in LLM context.

{"type":"custom_message","id":"i9j0k1l2","parentId":"h8i9j0k1","timestamp":"2024-12-03T14:25:00.000Z","customType":"my-hook","content":"Injected context...","display":true}

Fields:

• content: String or (TextContent | ImageContent)[] (same as UserMessage)
• display: true = show in TUI with purple styling, false = hidden
• details: Optional hook-specific metadata (not sent to LLM)

LabelEntry

User-defined bookmark/marker on an entry.

{"type":"label","id":"j0k1l2m3","parentId":"i9j0k1l2","timestamp":"2024-12-03T14:30:00.000Z","targetId":"a1b2c3d4","label":"checkpoint-1"}

Set label to undefined to clear a label.

Tree Structure

Entries form a tree:

• First entry has parentId: null
• Each subsequent entry points to its parent via parentId
• Branching creates new children from an earlier entry
• The "leaf" is the current position in the tree

[user msg] ─── [assistant] ─── [user msg] ─── [assistant] ─┬─ [user msg] ← current leaf
                                                            │
                                                            └─ [branch_summary] ─── [user msg] ← alternate branch

Context Building

buildSessionContext() walks from the current leaf to the root, producing the message list for the LLM:

1. Collects all entries on the path
2. Extracts current model and thinking level settings
3. If a CompactionEntry is on the path:
   • Emits the summary first
   • Then messages from firstKeptEntryId to compaction
   • Then messages after compaction
4. Converts BranchSummaryEntry and CustomMessageEntry to appropriate message formats

Parsing Example

import { readFileSync } from "fs";

const lines = readFileSync("session.jsonl", "utf8").trim().split("\n");

for (const line of lines) {
  const entry = JSON.parse(line);

  switch (entry.type) {
    case "session":
      console.log(`Session v${entry.version ?? 1}: ${entry.id}`);
      break;
    case "message":
      console.log(`[${entry.id}] ${entry.message.role}: ${JSON.stringify(entry.message.content)}`);
      break;
    case "compaction":
      console.log(`[${entry.id}] Compaction: ${entry.tokensBefore} tokens summarized`);
      break;
    case "branch_summary":
      console.log(`[${entry.id}] Branch from ${entry.fromId}`);
      break;
    case "custom":
      console.log(`[${entry.id}] Custom (${entry.customType}): ${JSON.stringify(entry.data)}`);
      break;
    case "custom_message":
      console.log(`[${entry.id}] Hook message (${entry.customType}): ${entry.content}`);
      break;
    case "label":
      console.log(`[${entry.id}] Label "${entry.label}" on ${entry.targetId}`);
      break;
    case "model_change":
      console.log(`[${entry.id}] Model: ${entry.provider}/${entry.modelId}`);
      break;
    case "thinking_level_change":
      console.log(`[${entry.id}] Thinking: ${entry.thinkingLevel}`);
      break;
  }
}

SessionManager API

Key methods for working with sessions programmatically:

Creation

• SessionManager.create(cwd, sessionDir?) - New session
• SessionManager.open(path, sessionDir?) - Open existing
• SessionManager.continueRecent(cwd, sessionDir?) - Continue most recent or create new
• SessionManager.inMemory(cwd?) - No file persistence

Appending (all return entry ID)

• appendMessage(message) - Add message
• appendThinkingLevelChange(level) - Record thinking change
• appendModelChange(provider, modelId) - Record model change
• appendCompaction(summary, firstKeptEntryId, tokensBefore, details?, fromHook?) - Add compaction
• appendCustomEntry(customType, data?) - Hook state (not in context)
• appendCustomMessageEntry(customType, content, display, details?) - Hook message (in context)
• appendLabelChange(targetId, label) - Set/clear label

Tree Navigation

• getLeafId() - Current position
• getEntry(id) - Get entry by ID
• getPath(fromId?) - Walk from entry to root
• getTree() - Get full tree structure
• getChildren(parentId) - Get direct children
• getLabel(id) - Get label for entry
• branch(entryId) - Move leaf to earlier entry
• branchWithSummary(entryId, summary, details?, fromHook?) - Branch with context summary

Context

• buildSessionContext() - Get messages for LLM
• getEntries() - All entries (excluding header)
• getHeader() - Session metadata