Function createFork

src/commands/branch/branch.ts:61–173

* Creates a fork of the current conversation by copying from the transcript file. * Preserves all original metadata (timestamps, gitBranch, etc.) while updating * sessionId and adding forkedFrom traceability.

(customTitle?: string)

 * sessionId and adding forkedFrom traceability.
 */
async function createFork(customTitle?: string): Promise<{
  sessionId: UUID
  title: string | undefined
  forkPath: string
  serializedMessages: SerializedMessage[]
  contentReplacementRecords: ContentReplacementEntry['replacements']
}> {
  const forkSessionId = randomUUID() as UUID
  const originalSessionId = getSessionId()
  const projectDir = getProjectDir(getOriginalCwd())
  const forkSessionPath = getTranscriptPathForSession(forkSessionId)
  const currentTranscriptPath = getTranscriptPath()

  // Ensure project directory exists
  await mkdir(projectDir, { recursive: true, mode: 0o700 })

  // Read current transcript file
  let transcriptContent: Buffer
  try {
    transcriptContent = await readFile(currentTranscriptPath)
  } catch {
    throw new Error('No conversation to branch')
  }

  if (transcriptContent.length === 0) {
    throw new Error('No conversation to branch')
  }

  // Parse all transcript entries (messages + metadata entries like content-replacement)
  const entries = parseJSONL<Entry>(transcriptContent)

  // Filter to only main conversation messages (exclude sidechains and non-message entries)
  const mainConversationEntries = entries.filter(
    (entry): entry is TranscriptMessage =>
      isTranscriptMessage(entry) && !entry.isSidechain,
  )

  // Content-replacement entries for the original session. These record which
  // tool_result blocks were replaced with previews by the per-message budget.
  // Without them in the fork JSONL, `claude -r {forkId}` reconstructs state
  // with an empty replacements Map → previously-replaced results are classified
  // as FROZEN and sent as full content (prompt cache miss + permanent overage).
  // sessionId must be rewritten since loadTranscriptFile keys lookup by the
  // session's messages' sessionId.
  const contentReplacementRecords = entries
    .filter(
      (entry): entry is ContentReplacementEntry =>
        entry.type === 'content-replacement' &&
        entry.sessionId === originalSessionId,
    )
    .flatMap(entry => entry.replacements)

  if (mainConversationEntries.length === 0) {
    throw new Error('No messages to branch')
  }

  // Build forked entries with new sessionId and preserved metadata
  let parentUuid: UUID | null = null