How Claude Code 2.1.221‑2.1.226 Enables Multi‑Agent Collaboration

Problem: multiple Claude Code sessions lack shared awareness

When developers open separate sessions for backend, frontend, and tests, they cannot see each other’s changes. Overlapping edits on uncommitted files cause compilation errors or failing tests.

1. Cross‑session messaging

Version 2.1.224 introduced SendMessage. A session can call ListAgents to discover other sessions and send plain‑text change notifications. The CLI shortcut is: /list-agents Version 2.1.225 extended the feature so a message can be sent proactively to a Remote Control session on another machine (e.g., a home Mac and a work PC) without waiting for the remote side to issue a request. The capability works on macOS and Linux.

The messages are text only; they do not merge sessions, share files, or transfer permissions. Each receiver keeps its own working directory and permission rules.

2. Isolating code with independent worktrees

Even with messaging, two sessions could edit the same uncommitted file. Version 2.1.221 changed /fork to create an independent Git worktree instead of sharing the original directory.

A worktree is a separate checkout of the same repository. Executing /fork spawns a new session that has its own file state, branch, and uncommitted changes, preventing interference with the original session.

Version 2.1.222 patched a vulnerability: agents inside a worktree can no longer execute destructive Git commands in the main work area.

3. Integrated collaboration flow

The three pieces form a complete workflow:

Message → describe change
Worktree → isolate code
Git → merge at the end

Example: the main session updates an API, sends the new field name and commit hash to the frontend session, which updates its types and builds inside its own worktree. After verification, the frontend session merges the changes back. If both agents edit the same line, Git reports a conflict during merge, preserving the conflict for manual resolution.

4. Security and reliability fixes

Credential masking added in 2.1.221 for Linux and WSL; later extended (2.1.224) to mask JWT, AWS credentials, and SigV4 rules.

Background sessions now automatically commit and push their work; a Draft PR is created only when the task truly requires it.

5. Usability enhancements

Focus View (2.1.221) collapses search, file reads, and command execution into a per‑round summary. Shortcut: Ctrl + Alt + F. Version 2.1.225 fixed over‑aggressive folding.

From 2.1.223, /review is an alias of /code-review. Running /review shows the current diff; high‑risk changes can be sent to a deeper cloud review with /code-review ultra. The default level is sufficient for ordinary edits.

6. Version highlights (2.1.221 – 2.1.226)

2.1.221 : Focus View, credential masking, background session auto‑commit/push, /fork uses independent worktree.

2.1.222 : Strengthened worktree isolation, blocked agents from executing destructive Git ops in the main worktree.

2.1.223 : /review unified to /code-review, multiple permission‑bypass fixes.

2.1.224 : Cross‑session messaging, finer‑grained credential masking.

2.1.225 : Proactive cross‑machine messaging, Focus View bug fixes.

2.1.226 : General bug fixes and reliability improvements.

7. Quick verification steps for upgraded installations

Open two sessions and run /list-agents to confirm they discover each other.

Run /fork in a new session and verify it entered an independent worktree.

Select a real diff and execute /review to observe the change flow.

Illustrations