clanker-agent/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

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

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

Deleting Sessions

Sessions can be removed by deleting their .jsonl files under ~/.clanker/agent/sessions/.

Clanker also supports deleting sessions interactively from /resume (select a session and press Ctrl+D, then confirm). When available, clanker uses the trash CLI to avoid permanent deletion.

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
• Version 3: Renamed hookMessage role to custom (extensions unification)

Existing sessions are automatically migrated to the current version (v3) when loaded.

Source Files

Source on GitHub (clanker-agent):

• packages/coding-agent/src/core/session-manager.ts - Session entry types and SessionManager
• packages/coding-agent/src/core/messages.ts - Extended message types (BashExecutionMessage, CustomMessage, etc.)
• packages/ai/src/types.ts - Base message types (UserMessage, AssistantMessage, ToolResultMessage)
• packages/agent/src/types.ts - AgentMessage union type

For TypeScript definitions in your project, inspect node_modules/@mariozechner/clanker-coding-agent/dist/ and node_modules/@mariozechner/clanker-ai/dist/.

Message Types

Session entries contain AgentMessage objects. Understanding these types is essential for parsing sessions and writing extensions.

Content Blocks

Messages contain arrays of typed content blocks:

interface TextContent {
  type: "text";
  text: string;
}

interface ImageContent {
  type: "image";
  data: string; // base64 encoded
  mimeType: string; // e.g., "image/jpeg", "image/png"
}

interface ThinkingContent {
  type: "thinking";
  thinking: string;
}

interface ToolCall {
  type: "toolCall";
  id: string;
  name: string;
  arguments: Record<string, any>;
}

Base Message Types (from clanker-ai)

interface UserMessage {
  role: "user";
  content: string | (TextContent | ImageContent)[];
  timestamp: number; // Unix ms
}

interface AssistantMessage {
  role: "assistant";
  content: (TextContent | ThinkingContent | ToolCall)[];
  api: string;
  provider: string;
  model: string;
  usage: Usage;
  stopReason: "stop" | "length" | "toolUse" | "error" | "aborted";
  errorMessage?: string;
  timestamp: number;
}

interface ToolResultMessage {
  role: "toolResult";
  toolCallId: string;
  toolName: string;
  content: (TextContent | ImageContent)[];
  details?: any; // Tool-specific metadata
  isError: boolean;
  timestamp: number;
}

interface Usage {
  input: number;
  output: number;
  cacheRead: number;
  cacheWrite: number;
  totalTokens: number;
  cost: {
    input: number;
    output: number;
    cacheRead: number;
    cacheWrite: number;
    total: number;
  };
}

Extended Message Types (from clanker-coding-agent)

interface BashExecutionMessage {
  role: "bashExecution";
  command: string;
  output: string;
  exitCode: number | undefined;
  cancelled: boolean;
  truncated: boolean;
  fullOutputPath?: string;
  excludeFromContext?: boolean; // true for !! prefix commands
  timestamp: number;
}

interface CustomMessage {
  role: "custom";
  customType: string; // Extension identifier
  content: string | (TextContent | ImageContent)[];
  display: boolean; // Show in TUI
  details?: any; // Extension-specific metadata
  timestamp: number;
}

interface BranchSummaryMessage {
  role: "branchSummary";
  summary: string;
  fromId: string; // Entry we branched from
  timestamp: number;
}

interface CompactionSummaryMessage {
  role: "compactionSummary";
  summary: string;
  tokensBefore: number;
  timestamp: number;
}

AgentMessage Union

type AgentMessage =
  | UserMessage
  | AssistantMessage
  | ToolResultMessage
  | BashExecutionMessage
  | CustomMessage
  | BranchSummaryMessage
  | CompactionSummaryMessage;

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": 3,
  "id": "uuid",
  "timestamp": "2024-12-03T14:00:00.000Z",
  "cwd": "/path/to/project"
}

For sessions with a parent (created via /fork or newSession({ parentSession })):

{
  "type": "session",
  "version": 3,
  "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: Implementation-specific data (e.g., { readFiles: string[], modifiedFiles: string[] } for default, or custom data for extensions)
• fromHook: true if generated by an extension, false/undefined if clanker-generated (legacy field name)

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, or custom data for extensions
• fromHook: true if generated by an extension, false/undefined if clanker-generated (legacy field name)

CustomEntry

Extension state persistence. Does NOT participate in LLM context.

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

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

CustomMessageEntry

Extension-injected messages that DO participate in LLM context.

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

Fields:

• content: String or (TextContent | ImageContent)[] (same as UserMessage)
• display: true = show in TUI with distinct styling, false = hidden
• details: Optional extension-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.

SessionInfoEntry

Session metadata (e.g., user-defined display name). Set via /name command or clanker.setSessionName() in extensions.

{
  "type": "session_info",
  "id": "k1l2m3n4",
  "parentId": "j0k1l2m3",
  "timestamp": "2024-12-03T14:35:00.000Z",
  "name": "Refactor auth module"
}

The session name is displayed in the session selector (/resume) instead of the first message when set.

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}] Extension 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.

Static Creation Methods

• SessionManager.create(cwd, sessionDir?) - New session
• SessionManager.open(path, sessionDir?) - Open existing session file
• SessionManager.continueRecent(cwd, sessionDir?) - Continue most recent or create new
• SessionManager.inMemory(cwd?) - No file persistence
• SessionManager.forkFrom(sourcePath, targetCwd, sessionDir?) - Fork session from another project

Static Listing Methods

• SessionManager.list(cwd, sessionDir?, onProgress?) - List sessions for a directory
• SessionManager.listAll(onProgress?) - List all sessions across all projects

Instance Methods - Session Management

• newSession(options?) - Start a new session (options: { parentSession?: string })
• setSessionFile(path) - Switch to a different session file
• createBranchedSession(leafId) - Extract branch to new session file

Instance Methods - 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?) - Extension state (not in context)
• appendSessionInfo(name) - Set session display name
• appendCustomMessageEntry(customType, content, display, details?) - Extension message (in context)
• appendLabelChange(targetId, label) - Set/clear label

Instance Methods - Tree Navigation

• getLeafId() - Current position
• getLeafEntry() - Get current leaf entry
• getEntry(id) - Get entry by ID
• getBranch(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
• resetLeaf() - Reset leaf to null (before any entries)
• branchWithSummary(entryId, summary, details?, fromHook?) - Branch with context summary

Instance Methods - Context & Info

• buildSessionContext() - Get messages, thinkingLevel, and model for LLM
• getEntries() - All entries (excluding header)
• getHeader() - Session header metadata
• getSessionName() - Get display name from latest session_info entry
• getCwd() - Working directory
• getSessionDir() - Session storage directory
• getSessionId() - Session UUID
• getSessionFile() - Session file path (undefined for in-memory)
• isPersisted() - Whether session is saved to disk