diff --git a/CLAUDE.md b/CLAUDE.md
index d039e23..cffc7e2 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -10,7 +10,7 @@ Use `docs/README.md` as the entrypoint for CC:Tweaked, Advanced Peripherals, and
 
 ## Constraints
 
-- Do not run Lua locally or add a test harness unless asked; code executes in-game or CraftOS-PC.
+- Do not add a standalone Lua test harness unless asked. Local execution happens through the CraftOS-PC harness (see `docs/install-craftos-pc.md` and ADR-0005); code otherwise executes in-game.
 - After editing Lua, run `just check` and fix all `luacheck` warnings.
 - Use 2-space indent, semicolons, and `local function`.
 - `require` paths are absolute ComputerCraft paths, for example `require('/apis/net')()`.
diff --git a/DEVELOPMENT.md b/DEVELOPMENT.md
index d84951e..aa0e5ac 100644
--- a/DEVELOPMENT.md
+++ b/DEVELOPMENT.md
@@ -3,6 +3,7 @@
 Requirements:
 - `just`
 - `luacheck`
+- [CraftOS-PC](docs/install-craftos-pc.md) ≥ v2.8.3 — local harness for the CC:Tweaked sandbox.
 
 After cloning the repository, run:
 
@@ -10,4 +11,6 @@ After cloning the repository, run:
 just install
 ```
 
-This installs the local Git hooks, including a pre-push hook that runs `just check`.
+This installs the local Git hooks, including a pre-commit hook that runs `just ci`.
+
+`just ci` is the local verification entry point. Today it verifies that `craftos --version` reports v2.8.3 or newer and runs `just check` for `luacheck`. `just test` is a placeholder for future CraftOS-PC headless integration tests.
diff --git a/Justfile b/Justfile
index feb8b47..fde5021 100644
--- a/Justfile
+++ b/Justfile
@@ -1,19 +1,62 @@
 # Justfile for cc-libs
-# Run `just check` to lint all Lua code with luacheck.
+# Run `just ci` to verify local tooling and lint Lua code.
 
 # List available recipes.
 default:
     @just --list
 
 # Install local development tooling.
-install: install-git-hooks
+install: install-git-hooks check-craftos
 
 # Install Git hooks for this repository.
 install-git-hooks:
     @mkdir -p .git/hooks
-    @printf '%s\n' '#!/bin/sh' '' 'just check' > .git/hooks/pre-push
-    @chmod +x .git/hooks/pre-push
-    @printf '%s\n' 'Installed .git/hooks/pre-push'
+    @printf '%s\n' '#!/bin/sh' '' 'just ci' > .git/hooks/pre-commit
+    @chmod +x .git/hooks/pre-commit
+    @printf '%s\n' 'Installed .git/hooks/pre-commit'
+
+# Verify the CraftOS-PC harness is installed and recent enough.
+check-craftos:
+    @command -v craftos >/dev/null 2>&1 || { \
+        printf '%s\n' 'craftos not found on $PATH. See docs/install-craftos-pc.md.' >&2; \
+        exit 1; \
+    }
+    @version="$(craftos --version)"; \
+    number="${version##* v}"; \
+    case "$number" in \
+        *.*.*) \
+            ;; \
+        *) \
+            printf '%s\n' "$version"; \
+            printf '%s\n' 'Could not parse CraftOS-PC version. See docs/install-craftos-pc.md.' >&2; \
+            exit 1; \
+            ;; \
+    esac; \
+    major="${number%%.*}"; \
+    rest="${number#*.}"; \
+    minor="${rest%%.*}"; \
+    patch="${rest#*.}"; \
+    patch="${patch%%[^0-9]*}"; \
+    printf '%s\n' "$version"; \
+    case "$major.$minor.$patch" in \
+        *[!0-9.]*|.*|*..*|*.) \
+            printf '%s\n' 'Could not parse CraftOS-PC version. See docs/install-craftos-pc.md.' >&2; \
+            exit 1; \
+            ;; \
+    esac; \
+    if ! { [ "${major:-0}" -gt 2 ] || \
+        { [ "${major:-0}" -eq 2 ] && [ "${minor:-0}" -gt 8 ]; } || \
+        { [ "${major:-0}" -eq 2 ] && [ "${minor:-0}" -eq 8 ] && [ "${patch:-0}" -ge 3 ]; }; }; then \
+        printf '%s\n' 'CraftOS-PC v2.8.3 or newer is required. See docs/install-craftos-pc.md.' >&2; \
+        exit 1; \
+    fi
+
+# Local CI entry point used by Git hooks.
+ci: check-craftos check test
+
+# Placeholder for future CraftOS-PC headless integration tests.
+test:
+    @printf '%s\n' 'No CraftOS-PC headless integration tests yet.'
 
 # Lint all Lua source with luacheck.
 check:
diff --git a/docs/adrs/README.md b/docs/adrs/README.md
index cc2819e..93d3000 100644
--- a/docs/adrs/README.md
+++ b/docs/adrs/README.md
@@ -12,3 +12,4 @@ Future ADRs can reuse the shape of the existing files when it is useful.
 - [`adr-0002-use-eventloop-for-async-code.md`](adr-0002-use-eventloop-for-async-code.md) - Use eventloop for async code.
 - [`adr-0003-current-net-api-state.md`](adr-0003-current-net-api-state.md) - Current net API state.
 - [`adr-0004-trapos-branding-and-manifest.md`](adr-0004-trapos-branding-and-manifest.md) - TrapOS branding and manifest-driven installs.
+- [`adr-0005-craftos-pc-harness.md`](adr-0005-craftos-pc-harness.md) - CraftOS-PC as the local harness.
diff --git a/docs/adrs/adr-0005-craftos-pc-harness.md b/docs/adrs/adr-0005-craftos-pc-harness.md
new file mode 100644
index 0000000..07db332
--- /dev/null
+++ b/docs/adrs/adr-0005-craftos-pc-harness.md
@@ -0,0 +1,44 @@
+# ADR 0005: CraftOS-PC As The Local Harness
+
+## Status
+
+Accepted
+
+## Date
+
+2026-06-08
+
+## Context
+
+This repository targets CC:Tweaked on Minecraft 1.21. The Lua we ship runs inside the ComputerCraft sandbox: it depends on `os.pullEventRaw`, `peripheral`, `rednet`, `textutils.serializeJSON`, modem channels, and a handful of CC-specific globals. Standard Lua (or LuaJIT) cannot execute this code as-is, so a normal local test harness was never a serious option.
+
+In practice contributors have been running the code in two places:
+
+- In-game on a real Minecraft server, which is slow to iterate on.
+- In **CraftOS-PC** (https://www.craftos-pc.cc/), a desktop emulator that ships the same ROM/BIOS as CC:Tweaked, supports modem peripherals via `periphemu`, and can run fully headless (`--cli --headless --script <file>`).
+
+CraftOS-PC has been the *de facto* local harness for months, but that fact lived only as a single line in `CLAUDE.md`. There was no install guide, no minimum version, and `just install` did not check that the binary was present. The recent upgrade from v2.6.6 → v2.8.3 (the first one in years) made it obvious that this dependency needed to be made explicit.
+
+## Decision
+
+Treat CraftOS-PC as a first-class local development dependency.
+
+- **Minimum version `v2.8.3`** — recent enough to have the current CC:Tweaked ROM, old enough that contributors already on a 2.8.x build will not be forced to upgrade again immediately.
+- **Documented install** in [`docs/install-craftos-pc.md`](../install-craftos-pc.md), with a SHA-256-verified macOS flow and pointers to the official Windows/Linux artifacts.
+- **Verified by `just install`** via a `check-craftos` recipe that runs `craftos --version` and requires v2.8.3 or newer. Failure prints a one-line pointer to the install guide instead of a long stack trace.
+- **`just ci` is the local verification entry point.** Today it runs `check-craftos` and `check`. The installed pre-commit hook invokes `just ci` directly. `just test` exists as the future home for CraftOS-PC-driven smoke tests.
+
+The existing `CLAUDE.md` constraint ("Do not run Lua locally or add a test harness unless asked") is reframed rather than removed: there is still no standalone Lua harness, and we are not adding a Busted-style test runner. The harness *is* CraftOS-PC, invoked deliberately.
+
+## Consequences
+
+- Contributors must install CraftOS-PC before `just install` succeeds. The install guide makes this a 4-step copy/paste on macOS.
+- Future smoke tests have a clear home: a script under `tests/` (or similar) driven by `craftos --cli --headless --script tests/smoke.lua`, called from `just test`.
+- The harness version becomes a project-level concern. When CC:Tweaked ships breaking changes that require a newer CraftOS-PC build, we bump the minimum version in `docs/install-craftos-pc.md` and `check-craftos` keeps contributors honest.
+- No CI integration yet. Running CraftOS-PC headless in GitHub Actions is feasible (the AppImage works on Ubuntu runners) but is out of scope here; the contract is local-only for now.
+
+## Future Work
+
+- **Headless smoke test.** A `tests/smoke.lua` that boots an emulated computer, `require`s `/apis/eventloop`, `/apis/net`, and the router, and asserts the wiring loads without errors. Hooked into `just test`, then into `just ci` when it is ready to run on every commit.
+- **CI.** Run the smoke test on push using the Linux AppImage.
+- **Pinned ROM.** CraftOS-PC ships its own copy of the CC:Tweaked ROM per release. If we ever need to test against a specific in-game version, point CraftOS-PC at a vendored ROM via `--rom`.
diff --git a/docs/install-craftos-pc.md b/docs/install-craftos-pc.md
new file mode 100644
index 0000000..76ca6e9
--- /dev/null
+++ b/docs/install-craftos-pc.md
@@ -0,0 +1,69 @@
+# Install CraftOS-PC
+
+CraftOS-PC is the local harness used to run this repo's Lua outside of Minecraft. See [ADR-0005](adrs/adr-0005-craftos-pc-harness.md) for why.
+
+Minimum version: **v2.8.3**. `just install` runs `craftos --version` to verify it is on `$PATH` and recent enough.
+
+The upstream installation page is https://www.craftos-pc.cc/docs/installation. The notes below pin the version we test against and add a SHA-256 verification step.
+
+## macOS
+
+There is no Homebrew cask, so the install is a manual drag-to-Applications from the official GitHub release.
+
+```sh
+# 1. Download the dmg and the published hashes.
+curl -L -o ~/Downloads/CraftOS-PC.dmg \
+  https://github.com/MCJack123/craftos2/releases/download/v2.8.3/CraftOS-PC.dmg
+
+# 2. Verify the SHA-256.
+curl -sL https://github.com/MCJack123/craftos2/releases/download/v2.8.3/sha256-hashes.txt \
+  | grep CraftOS-PC.dmg
+shasum -a 256 ~/Downloads/CraftOS-PC.dmg
+# The two hashes must match.
+
+# 3. Mount, install, unmount.
+hdiutil attach ~/Downloads/CraftOS-PC.dmg -nobrowse
+rm -rf /Applications/CraftOS-PC.app
+cp -R "/Volumes/CraftOS-PC/CraftOS-PC.app" /Applications/
+hdiutil detach "/Volumes/CraftOS-PC"
+
+# 4. Clear Gatekeeper quarantine so the first launch is not blocked.
+xattr -dr com.apple.quarantine /Applications/CraftOS-PC.app
+```
+
+The binary is not on `$PATH` by default. Add a symlink so `just check-craftos` can find it:
+
+```sh
+ln -sf /Applications/CraftOS-PC.app/Contents/MacOS/craftos /usr/local/bin/craftos
+```
+
+User data (computer state, settings) lives in `~/Library/Application Support/CraftOS-PC` and survives a reinstall — the steps above only touch the `.app` bundle.
+
+## Windows
+
+Download `CraftOS-PC-Setup.exe` from the [latest release](https://github.com/MCJack123/craftos2/releases/latest) and run it. The installer puts `craftos.exe` on `$PATH`.
+
+User data: `%appdata%\CraftOS-PC`.
+
+## Linux
+
+Download `CraftOS-PC.x86_64.AppImage` from the [latest release](https://github.com/MCJack123/craftos2/releases/latest), make it executable, and symlink it into `$PATH`:
+
+```sh
+chmod +x ~/Downloads/CraftOS-PC.x86_64.AppImage
+sudo ln -sf "$HOME/Downloads/CraftOS-PC.x86_64.AppImage" /usr/local/bin/craftos
+```
+
+User data: `~/.local/share/craftos-pc`.
+
+## Verify
+
+```sh
+craftos --version
+```
+
+Must report `CraftOS-PC v2.8.3` or newer. Once this works, `just install` will succeed.
+
+## Updating
+
+Repeat the steps above against the newer release. The bundle replacement is in-place; the user data directory is preserved.