Model Delegation — The Main Session Thinks, Subagents Implement

Token budgets are the constraint: the main session's context is where design quality lives and is expensive to rebuild. Spend it on thinking; delegate execution.

The split is by role, not by model. Whatever model the session happens to run (~/.claude/settings.json sets it), the main session is the thinker and subagents are the implementers — the rule holds when the config changes, and the same model may well sit on both sides.

Never name a model version anywhere in this repo — not in skills, docs, workflow scripts, or delegation prompts. Model families ship new versions every few weeks and this project always wants the latest, so write the unversioned family alias only. A version-pinned id (claude-<family>-<version>) or a prose family-plus-number is stale the moment it's written and silently keeps work on an old model.

Division of labor

• Main session: specs, proposals, architecture decisions, triage, naming, docs conventions, reviewing agent output. Anything where judgment compounds.
• Background subagent: executing an already-written spec — renames, sweeps, migrations, mechanical refactors, well-scoped feature implementation. Launch via the Agent tool with subagent_type: "general-purpose", run in background so the main session keeps working.

The docs skill already encodes the handoff: proposals must be self-contained enough for a cold implementation session. The delegation prompt is that cold session's entire world.

A reading pass is not delegable work

The division above splits on judgment vs. execution. There is a second axis that overrides it: how much of the agent's context is spent reading versus writing. A spec execution reads a handful of files and writes most of them. A convention sweep reads a whole tree to change a tenth of it, and the agent pays full context cost for every file it opens and discards.

That inverts the economics. Cost tracks files read, but value tracks files changed, and on a sweep those differ by an order of magnitude — so the price of one delivered edit is roughly ten times the price of reading one file, landing in the tens of thousands of tokens per changed file. That is a large multiple of doing the same pass in the main session, where the tree is read once and the rule is already in context. Four parallel sweep agents can burn a session's remaining budget and stop mid-unit, leaving partially-swept trees that cannot be ticked.

So: delegate by edit ratio, not by tedium. Mechanical does not mean delegable. If the task is "read everything under X and change what matches", run it in the main session and chunk it by unit. Delegate when the files to change are known up front.

Three costs compound and are easy to miss when the work looks parallel:

• Cold start per agent. Each one re-reads the same skill files, references and conventions the main session already holds. Fan-out multiplies that fixed cost by the number of agents.
• Reading dominates. The change is a few lines; the judgment needs the whole file. Tokens track files read.
• No shared learning. A carve-out one agent discovers (an exception the rule failed to state) is re-derived by every sibling, or missed. In the main session it is found once and applied to everything after it.

Writing the delegation prompt

The agent starts with zero conversation context. The prompt must carry:

1. The spec — point at the proposal file (or inline it) and pre-resolve every judgment call you can: exact rename maps, negative lists (what NOT to touch), edge cases already decided. Ambiguity left in the prompt becomes a judgment call made without you.
2. Repo conventions the agent can't infer — always pnpm, never npx; verify with pnpm format + typecheck (and relevant tests); lint with pnpm lint:fix from the repo root, which is the only pass that oxlints the app (a package's own lint:fix is ESLint over that package alone, so a change that passes it still fails CI); try/catch banned (getResult/getResultAsync + .match); no relative imports (@/, #shared, @esposter/*); never run db:gen/db:up and never hand-craft migration folders (cloning snapshot.json forks the migration chain — db:gen is the only sanctioned producer); when the spec needs a migration, edit the Drizzle schema only (the TS types alone keep typecheck green) and report that the user must run pnpm db:gen and apply it.
3. A verifiable done-definition — grep audits that must return zero hits, test files that must pass. "Done" the agent can prove beats "done" it can claim.
4. Git discipline — commit style from the git skill; push when green. Never git add -A: other sessions' WIP may be dirty (historically pnpm-lock.yaml, pnpm-workspace.yaml, scripts/src/refreshLockfile/index.ps1, but check git status fresh) — stage explicit paths only.
5. Report-back contract — files changed, judgment calls made, verification results, and anything only the user can do (e.g. running pnpm db:gen for a pending schema change).

While the agent runs

• The main session may only edit files the agent will not stage — agree the file boundary in the prompt (e.g. agent excludes proposals/resource/blueprint-*), and queue everything else until its commit lands.
• Never spawn a duplicate agent for the same task; wait for the completion notification, then verify its commit yourself (git log, spot-check the grep audits) before building on it.

Running several agents at once

One agent per PR, each in its own git worktree (isolation: "worktree" on the Agent tool), is the way to open a batch of PRs in parallel. The shared-working-tree boundary rule above only holds for a single agent; two agents in one tree trample each other. Isolation is what makes concurrency safe, so it is not optional for a batch.

Fan-out is earned by the prompt and paid for by the budget. The historical failure mode was agents burning their budget re-reading context and never producing work — that happens when the prompt is a topic instead of a spec. Two conditions gate a parallel batch, and both must hold: every prompt is a self-contained spec-execution task per the section above, and there are excess tokens to burn. Under a tight budget, or for exploratory, ideation, or docs-authoring work, stay sequential in the main session where judgment compounds.

Plan the batch around what the agents touch:

• Size each PR to fill the review budget, not just stay under it — the number itself is the coderabbit skill's ("PR File Budget"), and a single proposal comes nowhere near it. One-proposal-per-agent therefore wastes most of a review slot and multiplies review rounds; batch several related proposals from one area into a single agent's spec.
• Give each agent its own branch cut from develop and a stated merge order; a PR that depends on another's output is a stacked branch, not a parallel one — fold it into its parent's PR instead.
• Overlap must be additive only (separate rows on a shared component, separate procedures in a shared router). Shared schema sections or a shared write path mean one PR, not two agents.
• Each agent commits, pushes, and opens its own PR from its worktree. Verify each landed commit yourself before the next PR merges on top.

Cleaning up worktrees

Agent worktrees and their branches outlive the agent. Sweep them once their PR merges — git worktree remove <path> (it refuses while dirty, which is the signal to look before deleting), then git worktree prune, then git branch -d per branch. Use -d, never -D: the refusal to delete an unmerged branch is the only thing standing between a stale worktree and lost work. Orphaned worktree-agent-* branches with zero commits beyond develop are debris from already-cleaned worktrees and delete cleanly. Only branches you created for an agent are yours to sweep. A branch with a name someone chose deliberately (not the worktree-agent-* pattern) is presumed long-lived: leave it and ask, even when asked to "clean up old branches" — staleness or a merged PR is not authorization to delete it.

Code reviews

Reviews are execution roles, not the thinking role. The full convention — single entry point, opus-pinned workflow script, scriptPath invocation, findings handling — lives in the code-review skill; load it on any review request. Never review inline in the session and never use the review skill.

A delegated fix round carries that skill's closing checklist verbatim in its prompt (code-review, fixing-findings.md). An agent handed only a findings list optimises for the finding: it makes each one's own test pass and stops, which is precisely how a round ships a worse defect than it closed — a guard exempted, a sibling site left behind, a mitigation asserted in a comment and never written. The checklist is what the prompt's done-definition is built from, alongside the usual grep audits.

Design for agents

Every feature is designed agentic-first: resource creation (and eventually most authoring) may be done by AI, so specs must keep that path open — content is schema-validated JSON, writes go through ordinary validated procedures, no hidden client-side state, validation before side effects. apps/web/content/docs/resource/blueprint-resource.md is the canonical statement: whatever creates resources — human, form, or model — goes through the same front door.