diff --git a/AGENTS.md b/AGENTS.md
index 8c7fb3c..19666af 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -203,7 +203,19 @@ wait for the 5-min timer. (This mechanism — and the server-side
 `setup-*.sh` scripts as of 2026-07-12; before that it silently matched
 nothing, so nothing auto-ran on any client via this path.)
 
-## 9. When in doubt
+## 9. Versioning
+
+Every commit to `main` **MUST bump the version** in the `VERSION` file at the
+repo root. The format is semver-ish: `major.minor.patch` (e.g. `0.2.3`).
+
+Rules:
+- **patch** (`0.2.2` → `0.2.3`): bugfixes, small tweaks, docs-only changes
+- **minor** (`0.2.3` → `0.3.0`): new features, new tools, visible functionality
+- **major** (`0.3.0` → `1.0.0`): breaking changes (API removal, tool retirement)
+
+The version is shown in the UI sidebar. The `v` prefix is added at build time.
+
+## 10. When in doubt
 
 Use MCP tools: `search_knowledge <query>` for narrative context,
 `get_entity <slug>` for structured data, `get_entity_knowledge <slug>` for
diff --git a/VERSION b/VERSION
new file mode 100644
index 0000000..0d91a54
--- /dev/null
+++ b/VERSION
@@ -0,0 +1 @@
+0.3.0
diff --git a/web/src/App.svelte b/web/src/App.svelte
index 8ee1e5c..bef3c82 100644
--- a/web/src/App.svelte
+++ b/web/src/App.svelte
@@ -19,6 +19,7 @@
   import { Button } from '$lib/components/ui/button'
   import { Badge } from '$lib/components/ui/badge'
   import { Separator } from '$lib/components/ui/separator'
+  import { VERSION } from '$lib/version'
   import { Toaster } from '$lib/components/ui/sonner'
   import PlusIcon from '@lucide/svelte/icons/plus'
   import LayoutDashboardIcon from '@lucide/svelte/icons/layout-dashboard'
@@ -99,7 +100,7 @@
           <Sidebar.MenuButton
             class="data-[slot=sidebar-menu-button]:!p-1.5"
             onclick={() => navigate('overview')}
-            tooltipContent="Oikos"
+            tooltipContent={`Oikos ${VERSION}`}
           >
             {#snippet child({ props })}
               <button {...props}>
@@ -112,6 +113,9 @@
           </Sidebar.MenuButton>
         </Sidebar.MenuItem>
       </Sidebar.Menu>
+      <div class="group-data-[collapsible=icon]:hidden px-2.5 pb-1">
+        <span class="text-[11px] text-muted-foreground select-none">{VERSION}</span>
+      </div>
       <Sidebar.Menu>
         <Sidebar.MenuItem>
           <Sidebar.MenuButton
diff --git a/web/src/lib/version.ts b/web/src/lib/version.ts
new file mode 100644
index 0000000..2773223
--- /dev/null
+++ b/web/src/lib/version.ts
@@ -0,0 +1 @@
+export const VERSION: string = __OIKOS_VERSION__
diff --git a/web/src/vite-env.d.ts b/web/src/vite-env.d.ts
new file mode 100644
index 0000000..060b062
--- /dev/null
+++ b/web/src/vite-env.d.ts
@@ -0,0 +1 @@
+declare const __OIKOS_VERSION__: string
diff --git a/web/vite.config.ts b/web/vite.config.ts
index b0c3e24..b6a7fc2 100644
--- a/web/vite.config.ts
+++ b/web/vite.config.ts
@@ -1,6 +1,9 @@
 import { svelte } from '@sveltejs/vite-plugin-svelte'
 import tailwindcss from '@tailwindcss/vite'
 import { defineConfig } from 'vite'
+import { readFileSync } from 'fs'
+
+const version = readFileSync('../VERSION', 'utf-8').trim()
 
 // Injects OIKOS_API_TOKEN into proxied /api requests in dev — the API no
 // longer has a dev-open bypass (plans/2026-07-12-wails-desktop-app.md 0.4),
@@ -21,6 +24,9 @@ function authProxy(target: string, rewrite?: (path: string) => string) {
 export default defineConfig({
   plugins: [tailwindcss(), svelte()],
   base: '/',
+  define: {
+    __OIKOS_VERSION__: JSON.stringify(`v${version}`),
+  },
   resolve: {
     alias: { $lib: '/src/lib' }
   },