SendMessage tool not available despite being documented as replacement for Agent.resume

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Since v2.1.77, the Agent tool's resume parameter was removed and SendMessage({to: agentId}) was introduced as the replacement for continuing a previously spawned subagent. However, SendMessage is not injected into the tool list and attempting to call it returns "No such tool available: SendMessage".

The tool is also absent from <available-deferred-tools> and cannot be fetched via ToolSearch (No matching deferred tools found).

What Should Happen?

SendMessage should be available as a tool (either always-loaded or deferred) to allow resuming a previously spawned agent, as documented in the v2.1.77

changelog:

The Agent tool no longer accepts a resume parameter — use SendMessage({to: agentId}) to continue a previously spawned agent.

Error Messages/Logs

No such tool available: SendMessage

Steps to Reproduce

1. Tell Claude to spawn agent
2. Tell Claude to resume it
3. Observe Claude runs a new Agent with fresh context instead

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.76

Claude Code Version

2.1.81

Platform

Anthropic API

Operating System

Windows 11 Pro

Terminal/Shell

Windows Terminal

Additional Information

There is currently no way to resume a subagent:

• Old mechanism (Agent tool resume parameter) — removed in v2.1.77
• New mechanism (SendMessage) — not available

Agent results still instruct the model to use SendMessage, making the guidance misleading.