# ADR 0010: Self-host the harness — install discern into its own repo > **Retired — superseded by [ADR 0019](../0019-single-binary-ts-engine.md).** Self-host inverted; the committed shell engine and `selfcheck`/`selfsync` are gone. Kept for history; not current architecture. **Status**: accepted; **superseded by [ADR 0019](../0019-single-binary-ts-engine.md)** — see _Update (single-binary cutover)_ below. ## Update (single-binary cutover) The single-binary cutover ([ADR 0019](../0019-single-binary-ts-engine.md)) inverts self-host. There is no committed shell harness to install into the repo — the engine is TypeScript compiled into the `discern` binary — so the repo self-hosts by running its **own** engine (`deno task dev finish`), with no second copy to keep in sync. The drift-gate this ADR established (`selfcheck`/`selfsync`) is removed: the regression class it guarded is made structurally impossible, because there is nothing that can drift. ## Context discern is an agentic-development harness in two halves: a Deno/TypeScript installer (`src/`) and the POSIX-shell harness it installs (`templates/`, the source of truth). Until now the repo was **not** self-installed — there was no root `discern.toml`, `bin/agent`, or `.discern/`. The README's self-hosting section was therefore aspirational: the repo's real gate was the local convention `deno fmt && deno lint && deno check src/main.ts && deno task test`, and CI only built release binaries. That left the product's two most distinctive pieces unexercised by real use: the `agent finish` gate as a daily driver, and the author-once→compile guidelines pipeline (the repo had no generated `CLAUDE.md`/`AGENTS.md` at all). The test suite drives the engine hermetically (`tests/engine_*` scaffold the real `templates/` and shell out to `bin/agent`), but a test harness is not the same as living with the tool. Installing the harness into its own repo is the truest validation we can run, and the repo is pre-1.0 and pre-adoption, so there is no backward-compatibility cost to doing it now. ## Decision Install the harness into the repo and make `./bin/agent finish` the repo's gate. - **Committed managed copy ("Option C").** `discern setup` runs at the root; the managed set (`bin/agent`, `.discern/engine/**`, `.ai/skills/**`) is committed as a copy of `templates/`, alongside the seed files `setup` writes (`discern.toml`, `docs/**`, `.ai/guidelines/discern.md`, `TODO.md`). - **Drift is a gate-enforced invariant.** A new read-only `discern upgrade --check` (exposed as `deno task selfcheck`) exits non-zero if any managed file differs from `templates/`. It is wired as `[slots.selfcheck]` in the `check` phase, so `agent finish` fails on drift. Healing is one command — `deno task selfsync` (≡ `discern upgrade`) — which propagates `templates/` → the install. - **The golden rule.** To change a managed file you edit `templates/` and run `deno task selfsync`; never edit the root copy (a direct edit is reported as drift and written back as `.new`). Seed files are edited directly at the root and are never distributed to other projects. - **Wired slots.** `deno fmt` (fix); `deno lint`, `deno check src/main.ts`, and `selfcheck` (check); `deno task test` (test). Build stays a no-op (`deno task build` is release-only). - **Guidance via the pipeline.** Agent guidance is authored in `.ai/guidelines/discern.md` (a seed) and compiled by `agent guidelines` into `AGENTS.md` (tracked) and `CLAUDE.md` (generated, gitignored). - **CI runs the repo's own gate** (`agent finish`) on push and PR, with a trailing `git diff --exit-code` so the auto-fixing `fix` phase becomes a hard check. - **fmt/lint exclude the managed artifacts** (`.discern/`, `.ai/`, `AGENTS.md`, `CLAUDE.md`, plus `.idea/`) so the formatter never rewrites a managed file into drift or fights the guidelines compiler. ## Consequences - The repo now has **two copies of the engine**: `templates/.discern/engine/**` (the source the test suite runs) and `.discern/engine/**` (the installed copy that gates this repo). The `selfcheck` gate plus the golden rule keep them identical; the cost is the discipline of editing `templates/` and syncing. - The gate runs on itself: `agent finish` runs `deno task test`, whose `engine_*` tests shell out to `agent finish`. This nesting immediately surfaced a real bug — `finish` exported the `fail_fast`/`stream` env vars only when on and never cleared them, so a nested gate inherited the parent's value and ignored its own `[gate]` config. Fixed (set both ways) with a regression test. Exactly the kind of defect only living with the tool reveals. - The test suite remains the **correctness arbiter** for the engine: a `templates/` change is validated by `deno task test` whether or not the install is synced yet. If a bad engine change ever breaks `agent finish` itself, fall back to `deno task test` or `git checkout .discern/engine`. - `discern upgrade --check` is a genuinely useful new feature for any user (CI can now assert harness sync), not just an internal device. - The isolated-worktree workflow is now installed and exercisable for real, but adopting it for this repo's own development is deferred (see `TODO.md`); the SessionStart/WorktreeCreate hooks are scaffolded and dormant. - Backward compatibility is not a concern (pre-1.0, pre-adoption), so this was done without migration shims. ## Alternatives considered - **Symlink the install to `templates/`.** Simplest to keep in sync, but it bypasses the real copy/`upgrade` machinery — the part most worth validating in real use — and no installed project works that way. Rejected. - **Generate the install on demand, don't commit it.** Avoids the second engine copy, but adds a cold-start build step before the gate can run and makes the install invisible in the tree and in review. Rejected in favour of a committed copy kept honest by `selfcheck`. - **Auto-sync via a git hook.** A pre-commit hook that runs `selfsync` would heal drift silently — and could silently commit a broken engine change. Making drift a loud gate failure (the kind of invariant the harness exists to provide) is the point. Rejected.