Agent tool: spawned sub-agents go idle immediately without executing prompt

Summary

Sub-agents spawned via the Agent tool go idle immediately after spawn without executing the given prompt. The parent agent receives only idle_notification signals — the sub-agent never processes its prompt, never responds to SendMessage follow-ups, and never shuts down cleanly.

Environment

• Claude Code version: latest (observed on v2.1.119+)
• Platform: macOS (darwin)
• Shell: zsh
• Reproducible: consistently, across multiple sessions

Steps to Reproduce

1. In a session with a project open, invoke the Agent tool with a prompt targeting work in a different repo/directory (e.g. a sibling repo).
2. Observe the spawned agent in the Tasks panel.
3. Wait.

Expected Behavior

Sub-agent receives the prompt, uses its tools (Read, Edit, Bash, etc.), completes work, and returns a result to the parent.

Actual Behavior

Sub-agent immediately goes idle. The parent receives repeated idle_notification signals. Sending follow-up messages via SendMessage produces no response. The sub-agent never executes a single tool call — not even the first Read or Bash to start the task.

The agent appears to be silently blocked at a permission gate. It cannot escalate the permission prompt to the parent, so it simply stalls.

Root Cause Hypothesis

Sub-agents spawned via Agent tool do not inherit the parent's permission grants. When the sub-agent attempts its first tool call, it hits a permission gate that:

• Does not bubble up to the parent agent for approval
• Does not time out
• Does not produce any visible error

The result is a permanently stalled sub-agent with no recovery path short of killing the session.

Session Evidence

On 2026-05-22, multiple sub-agents were spawned to work on a sibling repo (sagep-df-api). All agents:

• Appeared in the Tasks panel as active
• Sent only idle_notification signals
• Never processed any tool calls
• Did not respond to SendMessage messages
• Did not terminate after receiving shutdown requests

The work had to be abandoned and done manually in the parent session.

Impact

This makes the Agent tool effectively unusable for cross-repo or multi-agent workflows — which is a primary advertised use case. The silent failure mode (no error, no timeout, just idle) makes it very hard to diagnose.

Related Issues

• #61315 — sub-agent stalls at MCP/tool permission gate (same symptom)
• #56860 — background agents spin indefinitely without timeout
• #54626 — zombie processes without cleanup

Suggested Fix

Sub-agents should either:

1. Inherit parent's permission grants at spawn time, or
2. Escalate permission prompts to the parent agent, or
3. Fail fast with a clear error when a required permission is not available (instead of silently stalling)

A timeout on idle sub-agents (with a visible error) would at minimum make the failure mode recoverable.