Feature request: set distinct CLAUDE_CODE_ENTRYPOINT (or similar env var) for Agent-spawned sub-agents

Summary

When Claude Code spawns a sub-agent via the Agent tool, the sub-agent's process inherits CLAUDE_CODE_ENTRYPOINT='cli' from its parent — making it indistinguishable from the main-thread session via env-var inspection alone.

This breaks any custom hook (PreToolUse, PostToolUse, etc.) that wants to apply different policy based on whether a tool call originates from the main thread vs a sub-agent.

Empirical observation (Claude Code 2.1.122 on macOS)

Two general-purpose sub-agents spawned in parallel via the Agent tool both reported identical environment values:

CLAUDE_CODE_ENTRYPOINT='cli'             ← same as main thread
CLAUDE_PROJECT_DIR='UNSET'                ← differs from main, but unset is inconsistent across spawn types
CLAUDECODE='1'                            ← same as main thread
AI_AGENT='claude-code/2.1.122/agent'      ← same as main thread
parent_pid=<intermediate-claude-PID>      ← differs but only via process tree walking

The Team Lead's own env on the same session is identical on every key except parent_pid. There is no env-var-only discriminator in current Claude Code.

Use case

Hook-based enforcement of a 'main thread only orchestrates; sub-agents do the work' invariant. The natural implementation is:

• PreToolUse hook checks whether this tool call is from main thread or sub-agent.
• Allow all tools for sub-agents.
• Restrict main thread to a permitted set (Agent, SendMessage, TaskCreate, etc.).

The hook's is_sub_agent() function naturally tries to read CLAUDE_CODE_ENTRYPOINT. Per the existing comment at the top of one such hook implementation:

# Sub-agent detection:
# - CLAUDE_CODE_ENTRYPOINT env var: \"cli\" = main thread, \"local-agent\" = sub-agent.
# - If we can't reliably detect sub-agent context, we LOG ONLY (exit 0) instead of
#   blocking (exit 2), to avoid bricking legitimate agent threads.

The detection cannot be made reliable today because the env var simply doesn't differ.

Proposed change

When Claude Code spawns a sub-agent via the Agent tool, set one of:

• CLAUDE_CODE_ENTRYPOINT=local-agent (or another value distinct from cli); OR
• CLAUDE_AGENT_ID=<id> populated only for sub-agents; OR
• CLAUDE_AGENT_DEPTH=<n> (1 for main thread, 2+ for sub-agents).

Any of the three would solve the problem. The first option matches an existing pattern that hook authors have already started building against.

Workaround (in current versions)

Process-tree marker file: a PreToolUse hook on Agent writes a marker file when a sub-agent dispatch fires; the main hook walks parent PIDs to detect ancestor markers. Works but adds complexity to userspace hook code that this internal change would eliminate.

Why this matters

Custom hook authors want to enforce per-actor policy without resorting to ancestor PID walking. A single distinguishing env var would make such hooks robust and version-stable.