Feature request: context_profile parameter on Agent (Task) tool to reduce subagent dispatch cost

Title

Feature request: context_profile parameter on Agent (Task) tool to reduce subagent dispatch cost

Body

Problem

Every subagent dispatched via the Agent (Task) tool currently inherits the parent session's full context as a cache-creation cost on the subagent's first message. Measured baseline on claude-opus-4-7 (1M context): ~54,000 tokens per dispatch, very tight standard deviation across dispatches (54,271 max vs 53,630 min across an 8-dispatch session).

For workflows that dispatch verification subagents, lens-frame divergent-converge agents, or parallel implementation subagents (8-12 dispatches per session is not unusual), cumulative subagent-startup cost can exceed 400,000 tokens before any subagent has done useful work. That cost is constant whether the subagent is doing a one-shot grep or a multi-hour implementation.

What gets inherited

Per the current behavior, the subagent's first turn includes:

• Claude Code system prompt
• Global ~/.claude/CLAUDE.md (operator-personal posture)
• Repo CLAUDE.md at the parent's cwd (project-specific posture)
• Transitively @-referenced files from either CLAUDE.md
• Memory MEMORY.md index plus on-demand memory entries
• Operator standing-instructions block (from a UserPromptSubmit hook in our setup)
• Standard tool listings + tool schemas
• The parent's task prompt envelope

For the simplest possible verification subagent (one shell command, three-sentence reply), most of the above context is unused yet billed at dispatch time.

Proposed shape

Add an optional context_profile parameter to the Agent tool with at least two named values:

| Value | Behavior |
|---|---|
| full (default) | Current behavior; preserves backward compatibility. |
| lean | Suppresses non-load-bearing context: skip repo CLAUDE.md, skip memory MEMORY.md index, skip @-resolved files. Keep: system prompt, global CLAUDE.md (or even skip; see open questions), tool schemas, agent task prompt. |

A bare value could also be useful for purely mechanical lookups (skip everything except system prompt + agent prompt + tool schemas).

Parent caller chooses the profile at dispatch time. Default stays full so existing parent code is unaffected.

Open design questions

1. Where does the profile decision live? Parent-prompt-time, or a per-subagent-type default in some registry?
2. Should CLAUDE.md content be marker-gateable? For example, <subagent-context-min-section> markers so the file's author can declare which sections survive lean profile inheritance. This lets operators tune the profile per-file rather than per-call.
3. How should lean mode interact with the divergent-converge pattern, where parallel subagents do need shared canonical-discipline content?
4. Should lean mode include a "load on demand" path so the subagent can pull in skipped context via Read if the task warrants it, without re-paying for cache creation on the first turn?

Witnessed pattern that surfaced this

Operator-side workflow (Nickcom4) routinely dispatches:

• 3 divergent-thinking subagents on substantive design or evaluation work (the 2-or-3-witness pattern)
• 1-2 verification subagents per PR before merge
• N parallel implementation subagents for independent tier-1 PRs in an umbrella issue

A typical session that ships 4-8 PRs incurs 10-20 subagent dispatches. At ~54k tokens each, the startup-only cost dominates the meaningful-work tokens for short subagent tasks. The asymmetry encourages skipping verification or batching work in ways that hurt PR quality.

What I've already tried locally

• Trimming the project CLAUDE.md content (saves ~6k per dispatch on identical content; bounded by the operator-personal frame size).
• Removing duplicated content (the same file was loading as both global and project; ~6k saved).
• Measurement script that tracks per-dispatch cache_creation_input_tokens so the trim work is data-driven.

Combined, these recover roughly 24% of the per-dispatch cost. The remaining 76% is Anthropic-side (system prompt, tool listings, the dispatch envelope) and not reachable from the operator side.

Refs

• (link to operator-side measurement issue / closed issue in their repo, if filed)
• (link to umbrella cost-burn diagnostic, if filed)