[FEATURE] Add /fork (conversation branching) support to VS Code extension

Want to give credit to @ahartzog for creating the original issue that was prematurely closed, which I copied some text from since it was well worded

@ianwieds you _can_ sort of accomplish this with double-pressing escape but it's rather unintuitive and I think adding the /fork command would be very helpful

@ahartzog yeah that method is okay, but it still doesn't have the latest context since it's applied to an older user message, unless I'm missing something?

In the meantime, I made a workaround that works well enough, although it's super hacky. It's a skill that runs a shell command with 2 methods:

1. File-copy mode (default) — Copies the session's JSONL file with sed to swap in a new session ID, appends a title entry, and copies the session directory. Pure filesystem operation — faster, no CLI needed, no extra messages injected.

1. CLI mode (--cli) — Calls claude --resume <id> --fork-session with --model haiku --effort low --safe-mode and a throwaway prompt to create the fork through the official mechanism. Slower and adds a small prompt/response to the new session's history.

In either case, it then opens a new VS Code panel with the forked chat session. Hacky, but it does effectively "fork from here".

SKILL.md

---
name: claude:fork
description: Fork the current conversation into a new Claude Code panel.
when_to_use: Use when the user wants to fork, branch, or split the current conversation into a new window/panel.
user-invocable: true
---

# Fork Conversation

Forks the current session into a new VS Code Claude Code panel with full conversation history.

## Steps

Run the fork script:

`bash ~/.claude/skills/claude:fork/fork.sh "$CLAUDE_CODE_SESSION_ID"`

The script handles everything: generates a UUID, auto-names the fork, forks via the CLI (haiku — cheap/fast), verifies the JSONL, and opens the new panel in VS Code.

Report the output to the user.

fork.sh

#!/usr/bin/env bash
# Fork the current Claude Code session into a new VS Code panel.
# Usage: fork.sh <session-id> [--cli]
#   Default: file-copy mode (instant, no CLI needed, no extra messages)
#   --cli:   use the Claude CLI (official fork, adds a small prompt/response)

set -euo pipefail

LOG_FILE="/tmp/claude-fork.log"

log() { printf '[%s] %s\n' "$(date '+%H:%M:%S')" "$*" | tee -a "$LOG_FILE"; }
die() { log "ERROR: $*" >&2; exit 1; }

log "=== Fork started ==="

SESSION_ID="${1:?Usage: fork.sh <session-id> [--cli]}"
MODE="copy"
[[ "${2:-}" == "--cli" ]] && MODE="cli"
log "Source session: $SESSION_ID (mode: $MODE)"

# Find the source JSONL
SESSION_JSONL=$(find ~/.claude/projects -name "${SESSION_ID}.jsonl" 2>/dev/null | head -1)
log "JSONL: ${SESSION_JSONL:-NOT FOUND}"
[[ -z "$SESSION_JSONL" ]] && die "No JSONL found for session $SESSION_ID"

# Extract current session name
CURRENT_NAME=""
CURRENT_NAME=$(grep -o '"customTitle":"[^"]*"' "$SESSION_JSONL" 2>/dev/null | tail -1 | sed 's/"customTitle":"//;s/"$//' || true)
if [[ -z "$CURRENT_NAME" ]]; then
  CURRENT_NAME=$(grep -o '"aiTitle":"[^"]*"' "$SESSION_JSONL" 2>/dev/null | tail -1 | sed 's/"aiTitle":"//;s/"$//' || true)
fi
log "Current name: ${CURRENT_NAME:-<unnamed>}"

# Build fork name: "X" → "X (fork)", "X (fork)" → "X (fork 2)", etc.
if [[ -z "$CURRENT_NAME" ]]; then
  FORK_NAME="fork"
elif [[ "$CURRENT_NAME" =~ \(fork\ ([0-9]+)\)$ ]]; then
  NEXT=$(( ${BASH_REMATCH[1]} + 1 ))
  FORK_NAME="${CURRENT_NAME% (fork *)} (fork ${NEXT})"
elif [[ "$CURRENT_NAME" =~ \(fork\)$ ]]; then
  FORK_NAME="${CURRENT_NAME% (fork)} (fork 2)"
else
  FORK_NAME="${CURRENT_NAME} (fork)"
fi
log "Fork name: $FORK_NAME"

NEW_SESSION=$(uuidgen | tr '[:upper:]' '[:lower:]')
log "New session: $NEW_SESSION"

PROJECT_DIR=$(dirname "$SESSION_JSONL")
NEW_JSONL="${PROJECT_DIR}/${NEW_SESSION}.jsonl"

if [[ "$MODE" == "copy" ]]; then
  # File-copy fork: replace session IDs, inject the new title
  log "Copying JSONL with replaced session IDs"
  sed "s/${SESSION_ID}/${NEW_SESSION}/g" "$SESSION_JSONL" > "$NEW_JSONL"

  # Inject a custom-title entry so the fork gets its name
  printf '{"type":"custom-title","sessionId":"%s","customTitle":"%s"}\n' \
    "$NEW_SESSION" "$FORK_NAME" >> "$NEW_JSONL"

  # Copy the session directory (subagents, tool-results) if it exists
  SRC_DIR="${PROJECT_DIR}/${SESSION_ID}"
  if [[ -d "$SRC_DIR" ]]; then
    cp -R "$SRC_DIR" "${PROJECT_DIR}/${NEW_SESSION}"
    log "Copied session directory"
  fi
else
  # CLI fork: official mechanism
  CLAUDE_BIN=$(command -v claude 2>/dev/null || echo "$HOME/.local/bin/claude")
  if [[ ! -x "$CLAUDE_BIN" ]]; then
    die "Claude CLI not found at $CLAUDE_BIN (install: curl -fsSL https://claude.ai/install.sh | bash)"
  fi
  log "CLI: $CLAUDE_BIN"

  log "Running: claude --resume ... --fork-session --print"
  if ! FORK_OUTPUT=$("$CLAUDE_BIN" --resume "$SESSION_ID" \
    --fork-session \
    --session-id "$NEW_SESSION" \
    --name "$FORK_NAME" \
    --model haiku \
    --effort low \
    --safe-mode \
    --system-prompt "Respond with exactly: Done!" \
    --print "Fork you!" 2>&1); then
    log "CLI output: $FORK_OUTPUT"
    die "Fork command failed (exit $?)"
  fi
  log "CLI completed"
fi

# Verify
if [[ ! -f "$NEW_JSONL" ]]; then
  die "No session file created at $NEW_JSONL"
fi
log "Verified: $NEW_JSONL"

open "vscode://anthropic.claude-code/open?session=$NEW_SESSION"
log "Opened in VS Code"

echo "SESSION_ID=$NEW_SESSION"
echo "NAME=$FORK_NAME"
echo "FILE=$NEW_JSONL"

log "=== Fork complete ==="

Isn't what you want the same as what /branch command in CLI version do? And yes, I do not find such functionality in VS Code extension version. Now sure why but I use /branch command often. It is appreciated to have this in VS code because I love GUI.

This keeps getting dismissed as "just hit esc twice" when this is a completely different thing. The request is not for rewinding and forking from rewound point. The request is FOR FORKING FROM ACTIVE SESSION, WITHOUT THE REWIND. Also worth noting that claude-cli does not fork when rewinding, while claude for vscode does. Behaviors across these products are extremely inconsistent, with horrendous nomenclature.

@orcinus-evc 100%! Didn't know all that about the inconsistent behavior, but I'm not surprised, lol.