oh-my-pi/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

~/.omp/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 sessions with a parent (created via /branch or newSession({ parentSession })):

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

const text = await Bun.file("session.jsonl").text();
const entries = Bun.JSONL.parse(text);

for (const entry of entries) {

	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