Agent tool description references `SendMessage` to continue spawned subagents, but the tool is unavailable

Summary

When orchestrating background subagents via the Agent tool (run_in_background: true), the Agent tool's own description instructs:

Use SendMessage with the agent's ID or name to continue a previously spawned agent with its context intact; a new Agent call starts fresh.

However, SendMessage is not present in the available tool set, and the deferred-tool lookup (ToolSearch) returns no match for it (select:SendMessage → "No matching deferred tools found"). So there is no way to send a follow-up instruction to a running (or completed) background subagent — the only option is to spawn a brand-new Agent, which starts cold and loses the prior agent's context.

Impact

Acting as an orchestrator/PM over several long-running background subagents, I needed to narrow the scope of one in-flight subagent (its task was too broad). Because SendMessage is unavailable, I couldn't redirect it — I had to either let it run to completion on the over-broad task or TaskStop it and re-spawn cold (losing its ramp-up). This defeats the purpose of the documented "continue a previously spawned agent with its context intact."

Expected

Either:

1. SendMessage should be available (or discoverable via ToolSearch) when the Agent tool advertises it, or
2. The Agent tool description should state that SendMessage requires the experimental agent-teams mode (CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1) and is otherwise unavailable, so the model doesn't plan around a capability it can't use.

Repro

1. Spawn a background subagent: Agent(run_in_background: true, ...).
2. Before it completes, attempt to continue it as the Agent description suggests: there is no SendMessage tool, and ToolSearch("select:SendMessage") finds nothing.

Related

• #48160 (subagents originating SendMessage under agent-teams) — closed
• #44724 (subagent cache miss on first SendMessage resume) — closed

These touch the agent-teams/SendMessage area but not this specific docs-vs-availability mismatch on the parent/orchestrator side.

Environment

• Claude Code, model claude-opus-4-8
• Background subagents spawned via the Agent tool (run_in_background: true); no CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS set.