[BUG] Agent-Teams sub-agents with model: opus cannot call SendMessage / TaskCreate / TaskUpdate etc. despite frontmatter declaration (tmux backend)

Summary

When spawning a sub-agent via the Agent tool with model: opus (claude-opus-4-7 / 1M context) inside an Agent Teams context (CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1, tmux backend), deferred tools listed in the agent's tools: frontmatter — most critically SendMessage and TaskCreate / TaskUpdate / TaskGet / TaskList — are not callable from the spawned process. The identical agent definition spawned with model: sonnet (claude-sonnet-4-6) has all those tools callable and the team communicates correctly.

Concretely confirmed via direct tool-call attempts:

• Sonnet sub-agent: SendMessage, TaskCreate, TaskUpdate, TaskGet, TaskList all succeed.
• Opus sub-agent: self-reports the same tools as absent from its function schema, and the SendMessage back to the team lead never arrives (observed across repeated sessions, including a prior incident on 2026-04-21 that first surfaced the problem).

Self-reports from the Opus sub-agent additionally list Grep and Glob as missing from its schema, but we have not independently confirmed those with forced invocations — treat the core claim as the deferred tools (SendMessage / Task*), and treat Grep/Glob as a secondary self-report.

This is distinct from #35240 / #34750 — those describe the case where CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS is disabled. In this report the flag is enabled and Sonnet sub-agents work fine, so the tool-registration layer is functional overall. The regression is model-scoped.

Environment

• Claude Code: 2.1.117
• Platform: macOS (Darwin 24.6.0)
• Feature flag: CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1 (verified via tmux pane env)
• Teammate backend: tmux
• Affected model: claude-opus-4-7 (1M context), invoked via model: opus in agent frontmatter
• Unaffected model: claude-sonnet-4-6, invoked via model: sonnet

Steps to Reproduce

1. Create .claude/agents/probe-opus.md:

   ---
   name: probe-opus
   description: Test agent
   tools: Read, Grep, Glob, SendMessage, TaskCreate, TaskUpdate, TaskGet, TaskList
   model: opus
   ---

2. Create .claude/agents/probe-sonnet.md with identical frontmatter except model: sonnet.

3. Start a team: TeamCreate({team_name: "debug"})

4. Spawn both:

   Agent({subagent_type: "probe-opus", team_name: "debug", name: "p-opus",
          prompt: "Try calling SendMessage to team-lead with text 'ping'"})
   Agent({subagent_type: "probe-sonnet", team_name: "debug", name: "p-sonnet",
          prompt: "Try calling SendMessage to team-lead with text 'ping'"})

5. Observe:

   • p-sonnet → SendMessage delivers; TaskCreate / TaskUpdate / TaskGet / TaskList all succeed via direct invocation (verified end-to-end in this report)
   • p-opus → no message arrives; tmux pane inspection shows the model self-reporting the deferred tools as absent from its function schema, and a TaskCreate attempt produces no task

Expected Behavior

All tools declared in the agent's tools: frontmatter should be callable from the spawned sub-agent process, regardless of the model.

Actual Behavior

• Confirmed by failed invocation (Opus): SendMessage, TaskCreate, TaskUpdate, TaskGet, TaskList cannot be called from the sub-agent. SendMessage to the team lead never arrives; TaskCreate produces no task.
• Self-reported but not independently verified (Opus): Grep and Glob also reported as absent from the function schema. Read and Bash were confirmed callable, consistent with static tools being registered.
• Control — Sonnet, same frontmatter: all listed tools callable, deferred tools included. Messages and Task CRUD both functioned end-to-end.

Evidence

Self-report captured verbatim from the opus sub-agent's tmux pane:

ツール付与診断の結果を報告します。本プロセスで実際に利用可能なツールは Read のみ でした。

検証結果

• Read: OK
• Grep: FAIL（ツール未付与 — 関数スキーマに存在しない）
• Glob: FAIL（ツール未付与）
• SendMessage: FAIL（ツール未付与 — このためこの報告は team-lead に届けられません）
• TaskCreate: FAIL（ツール未付与）
• TaskUpdate: FAIL（ツール未付与）
• TaskGet: FAIL（ツール未付与）
• TaskList: FAIL（ツール未付与）

Frontmatter（tools: Read, Grep, Glob, SendMessage, TaskCreate, TaskUpdate, TaskGet, TaskList）で宣言されているが、このサブエージェントプロセスに実際に付与されていないツール: Grep, Glob, SendMessage, TaskCreate, TaskUpdate, TaskGet, TaskList

Paired Sonnet agent (same TeamCreate session) with identical frontmatter returned:

All declared tools are available. Read, Bash, Edit, Write, Grep, Glob, SendMessage, TaskCreate, TaskUpdate, TaskGet, TaskList: all ✅

Both panes launched via the same command template (only --model differs):

/Users/<user>/.local/share/claude/versions/2.1.117
  --agent-id <id>
  --team-name <team>
  --agent-color <color>
  --parent-session-id <session>
  --agent-type <type>
  --permission-mode auto
  --model opus     # or --model sonnet

Impact

• Opus sub-agents cannot communicate with the team lead → deliverables are lost
• Multi-turn orchestration breaks for any opus-model role (architect, code-reviewer, etc.)
• Silent failure: no structured error surfaces to the orchestrator. Only observable via tmux pane inspection, or by timing out waiting for an expected SendMessage response
• validate-agents.sh-style static checks can't detect this since the frontmatter is well-formed; the drift is runtime-only

Related

• #35240 — SendMessage gated behind AGENT_TEAMS flag (different scenario: flag-off; here flag is on and Sonnet works)
• #34750 — TeamCreate/TeamDelete/SendMessage unavailable despite flag (closed; predates this regression)
• #51895 — Background sub-agents silent-skip Write for .md (different: run_in_background: true; here tmux backend)
• #51818 — Teammate CLI crashes on permission_response (different failure mode)

The following recent reports may indicate a broader cluster of Opus-4.7 sub-agent instability in Agent Teams mode; they describe different symptoms but share the environment:

• #52139 (2026-04-22) — Ink renderer crashes in tmux teammate panes
• #52223 (2026-04-23) — sub-agent Bash returns minified cli.js source instead of command output
• #52216 (2026-04-23) — Opus 4.7 sub-agents miss trivially greppable items
• #51959 (2026-04-22) — Lead agent requires manual stdin to process teammate notifications

Additional Notes

Repeatedly reproducible across sessions. Same-agent, same-prompt, same-team — only model: opus vs model: sonnet toggles the fault.

Workaround: use model: sonnet for all sub-agents that need to communicate back. This loses opus quality for architecture/review roles but restores team functionality.