diff --git a/.fallowrc.json b/.fallowrc.json
index 57f486d..f587e92 100644
--- a/.fallowrc.json
+++ b/.fallowrc.json
@@ -22,6 +22,23 @@
     "zod"
   ],
   "rules": {
+    "boundary-violation": "error",
     "unused-dependencies": "warn"
+  },
+  "boundaries": {
+    "zones": [
+      { "name": "domain", "patterns": ["src/domain/**"] },
+      { "name": "infra", "patterns": ["src/infra/**"] },
+      { "name": "api", "patterns": ["src/api/**"] },
+      { "name": "webapp", "patterns": ["src/webapp/**"] },
+      { "name": "server", "patterns": ["src/server.ts"] }
+    ],
+    "rules": [
+      { "from": "domain", "allow": [] },
+      { "from": "infra", "allow": ["domain"] },
+      { "from": "api", "allow": ["domain", "infra"] },
+      { "from": "webapp", "allow": ["domain", "api"] },
+      { "from": "server", "allow": ["domain", "infra", "api", "webapp"] }
+    ]
   }
 }
diff --git a/AGENTS.md b/AGENTS.md
index aaf261f..cf25f58 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -30,7 +30,7 @@ This project uses **Vite 8**, **Vitest 4**, **Stryker**, **oxlint**, **oxfmt**,
 
 - `pnpm lint` - Lint and auto-fix with oxlint (includes local `starter/*` JS plugin rules in `tools/oxlint-plugins/`)
 - `pnpm format` - Format code with oxfmt
-- `pnpm run ci` - Full CI check: lint, format, and typecheck
+- `pnpm run ci` - Full CI check: lint, format, typecheck, and Fallow audit
 
 ### Dependencies
 
@@ -46,6 +46,7 @@ This project uses **Vite 8**, **Vitest 4**, **Stryker**, **oxlint**, **oxfmt**,
 - **Stryker config**: `stryker.config.mjs` (mutation scope, reporters, 80% break threshold)
 - **oxlint config**: `oxlint.config.ts` (type-aware linting, ignore patterns)
 - **oxfmt config**: `oxfmt.config.ts` (formatting: no semi, single quotes)
+- **Fallow config**: `.fallowrc.json` (dead code analysis, dependency checks, custom architecture boundary zones)
 - **TypeScript**: `tsconfig.json` and layer-specific configs
 
 ## Imports
@@ -60,7 +61,7 @@ This project uses **Vite 8**, **Vitest 4**, **Stryker**, **oxlint**, **oxfmt**,
 
 Git hooks are managed by [lefthook](https://lefthook.dev), configured in `lefthook.yml`.
 
-- **pre-commit** runs `oxlint --fix` and `oxfmt --write` on staged files (auto-restaged), then runs `vitest related` over the staged files — only unit tests that import a staged file execute.
+- **pre-commit** runs `oxlint --fix` and `oxfmt --write` on staged files (auto-restaged), then runs `vitest related` over the staged files — only unit tests that import a staged file execute. It also runs `pnpm fallow:ci` to enforce Fallow checks before each commit.
 - **pre-push** runs `vitest run --changed origin/master --project unit`, executing only the unit tests affected by files changed against `origin/master`.
 
 Mutation testing is intentionally not a git hook because it is slower than the commit/push feedback loop. Run `pnpm test:mutate` before merging changes to `src/domain/**`, `src/api/**`, or unit-test behaviour; CI enforces the same command for those paths.
@@ -71,4 +72,5 @@ Hooks install automatically via the `prepare` script (`lefthook install`). To sk
 
 - [ ] Run `pnpm install` after pulling remote changes and before getting started.
 - [ ] Run `pnpm run ci` and `pnpm test` to validate changes.
+- [ ] Run `pnpm fallow list --boundaries` and `pnpm fallow dead-code --boundary-violations` after changing architecture boundary config.
 - [ ] Run `pnpm test:mutate` when changing mutation-scoped logic or unit tests.
diff --git a/README.md b/README.md
index 202d4d8..00e80dc 100644
--- a/README.md
+++ b/README.md
@@ -53,13 +53,14 @@ Business logic lives in pure TypeScript. Frameworks, databases, and external ser
 - `src/api/` — HTTP composition root (Hono). Wires concrete adapters into use cases and exposes them as routes.
 - `src/webapp/` — React + TanStack Start UI. Calls into the API; composes its own adapters where needed.
 
-**The boundary is enforced three times, on purpose:**
+**The boundary is enforced four times, on purpose:**
 
 1. **Path aliases** — `@domain/*` and `@infra/*` in `tsconfig.json` make the layer of every import readable at a glance.
 2. **Layered typechecks** — `tsconfig.domain.json` compiles the domain alone; `tsconfig.infra.json` compiles domain + infra. If domain code reaches into infra, the domain typecheck fails before anything else runs. `pnpm typecheck:layers` runs both.
 3. **Custom oxlint rule** — `starter/domain-no-infra-imports` (in `tools/oxlint-plugins/rules/`) blocks both static and dynamic imports from `src/infra/**` inside `src/domain/**` files.
+4. **Fallow custom boundaries** — `.fallowrc.json` defines explicit zones for `domain`, `infra`, `api`, `webapp`, and `server`, then fails `boundary-violation` issues in CI.
 
-Any one of the three would catch most mistakes; all three together make the violation loud and local.
+Any one of the four would catch most mistakes; all four together make the violation loud and local.
 
 ## Directory layout
 
@@ -147,7 +148,7 @@ Rule of thumb: if a module contains only declarations, use `*.schema.ts` or `*.t
 
 Every stage has a specific job. Understanding the _why_ matters as much as the commands.
 
-- **Pre-commit (lefthook)** — runs `oxlint --fix` and `oxfmt --write` on staged files (auto-restaged), then `vitest related --run --project unit` over the staged files. _Why_: keeps git history clean and readable (no "fix lint" commits), and ensures every commit is **independently releasable** — no commit silently breaks the behaviour of code near the change.
+- **Pre-commit (lefthook)** — runs `oxlint --fix` and `oxfmt --write` on staged files (auto-restaged), then `vitest related --run --project unit` over the staged files and `pnpm fallow:ci` for Fallow checks. _Why_: keeps git history clean and readable (no "fix lint" commits), and ensures every commit is **independently releasable** — no commit silently breaks the behaviour or architecture of code near the change.
 - **Pre-push (lefthook)** — `vitest run --changed origin/master --project unit`. Catches regressions across the whole change set before they leave the machine.
 - **`pnpm run ci`** (local + CI) — `oxlint && oxfmt --check . && pnpm typecheck:layers && pnpm fallow:ci`. Read-only quality gate; no fixes, no writes. The source of truth for "is this branch green?"
 - **GitHub Actions** — runs the same gate plus the test matrix (unit, browser, integration).
@@ -178,10 +179,12 @@ pnpm fallow               # full analysis (dead code + dupes + health)
 pnpm fallow dead-code     # unused code + circular deps only
 pnpm fallow dupes         # duplication scan
 pnpm fallow health        # complexity + maintainability
+pnpm fallow list --boundaries
+pnpm fallow dead-code --boundary-violations
 pnpm fallow fix --dry-run # preview auto-fixes for unused exports/deps
 ```
 
-`pnpm run ci` runs `pnpm fallow:ci` (`fallow audit`) as a quality gate — it scopes analysis to files changed against the base branch and returns a pass/warn/fail verdict. Configuration lives in `.fallowrc.json`. For the full feature set, see the [fallow docs](https://docs.fallow.tools).
+`pnpm run ci` runs `pnpm fallow:ci` (`fallow audit`) as a quality gate — it scopes analysis to files changed against the base branch and returns a pass/warn/fail verdict. Configuration lives in `.fallowrc.json`, including custom boundary zones and rules for the hexagonal architecture. For the full feature set, see the [fallow docs](https://docs.fallow.tools).
 
 ## Reproducing this setup in your own repo
 
@@ -191,7 +194,7 @@ A short checklist for applying these conventions to a greenfield project:
 2. Create `src/domain`, `src/infra`, `src/utils` on day one, even if empty. Directory shape is a commitment.
 3. Copy the `tsconfig.json` + `tsconfig.domain.json` + `tsconfig.infra.json` trio; adjust the includes/aliases.
 4. Set up `oxlint.config.ts` and `oxfmt.config.ts`. Copy `tools/oxlint-plugins/` and rename the `starter` namespace.
-5. Add `lefthook.yml` with pre-commit (lint + format + `vitest related --project unit`) and pre-push (`vitest run --changed origin/master --project unit`).
+5. Add `lefthook.yml` with pre-commit (lint + format + `vitest related --project unit` + `pnpm fallow:ci`) and pre-push (`vitest run --changed origin/master --project unit`).
 6. Add the `pnpm ci` script: lint → format check → layered typecheck → fallow audit.
 7. Add Stryker (`@stryker-mutator/core`, `@stryker-mutator/vitest-runner`), `stryker.config.mjs`, `vitest.mutation.config.ts`, and the `pnpm test:mutate` script with an 80% `thresholds.break` floor.
 8. Adopt the four test-type filename suffixes (`.unit`, `.browser`, `.integration`, `.e2e`) before writing any tests.
diff --git a/lefthook.yml b/lefthook.yml
index 740d903..67bcfa8 100644
--- a/lefthook.yml
+++ b/lefthook.yml
@@ -10,6 +10,8 @@ pre-commit:
     - name: vitest
       glob: '*.{js,ts,tsx,jsx,mjs,cjs}'
       run: pnpm vitest related --run --project unit --passWithNoTests {staged_files}
+    - name: fallow
+      run: pnpm fallow:ci
 
 pre-push:
   jobs: