[BUG] Agent Teams: peer message body not delivered to recipient context

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?

With experimental Agent Teams enabled (CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1, teammateMode: in-process), SendMessage between two teammates reports success ("sent to X's inbox") but the message body is not reliably injected into the recipient teammate's context. The recipient can see that a reply happened but not its content. Lead↔teammate delivery was mostly reliable; teammate↔teammate content delivery was the consistent failure.

What Should Happen?

SendMessage from teammate A to teammate B delivers the message body into B's context (or B can retrieve it), so B can act on the content — not merely observe that a message arrived.

Error Messages/Logs

Verbatim symptom from the run (one teammate to the lead):

"DevLead's three feasibility answers are not reaching my context — I can see your message saying he replied, but not the content of his reply, and nothing is recorded on the tasks."

Steps to Reproduce

1. Start a session with CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1, teammateMode: in-process.
2. Lead spawns two teammates, A and B; let both go idle (no pending work).
3. Have A SendMessage a distinctive payload to B.
4. Inspect B's context/transcript for the body (not just an arrival notice).
5. Repeat several times, varying whether B is idle vs mid-turn when the message lands, to probe the timing correlation.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.183

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

Impact

Repeated resends (same content sent 2–3×), idle-loops, and the lead having to hand-reconstruct/relay a teammate's answers to keep progress. A large coordination tax; not a crash or data loss, but it undermines the core value of peer messaging.

Actual

SendMessage returns success ("sent to B's inbox").
B's context shows a notification that A replied, but not the body.
Relaying the same content via the lead also sometimes failed to surface in B's context.
Nothing was recorded on the shared task list for the exchange.

Frequency / reproducibility

Intermittent. Observed repeatedly within one session but not on every message.
Correlated with timing — appears when the recipient is idle / mid-turn-transition at delivery time.
Direction-dependent — teammate↔teammate failed consistently; lead↔teammate was mostly fine.
Not yet reduced to a deterministic repro.

Current workaround

Route any decision-gating content through durable files (write to the spec/plan; the other agent reads it) or lead relay with explicit read-back confirmation; use the shared task list for acknowledgements rather than prose messages.