diff --git a/AGENTS.md b/AGENTS.md
index dd7fa86..3d2a9ce 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -10,3 +10,11 @@
 1. **Compose instance is the app under test** — e2e always runs against the docker-compose build behind Traefik (`https://flight.dev.kkhost.pl`), never a local `next dev` on :3000 (dev builds don't render for non-loopback clients, and a second instance splits state).
 2. **Update before testing** — deploy current code first: `docker compose up -d --build`.
 3. **Remote browsers** — tests run through the shared Playwright server at `192.168.1.4:3000` (Docker image tag must match this repo's @playwright/test version); plain `npx playwright test` handles deploy + readiness + remote connect (see `playwright.config.ts`).
+
+## API
+1. **Live base URL** — `https://flight.dev.kkhost.pl` (the docker-compose build behind Traefik).
+2. **Machine-readable spec** — OpenAPI 3.x at `https://flight.dev.kkhost.pl/api/openapi`; current app version at `/api/version`.
+3. **Spec lives in code** — generated from `lib/openapi.ts`, served by `app/api/openapi/route.ts`; must be updated whenever an endpoint changes.
+
+## Other
+1. For longer terminal tasks you should use pty_* tools.
\ No newline at end of file
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4301c0e..a8fd93f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,12 @@
 # Changelog
 
+## v0.5.0 — 2026-09-17 10:15
+- New `GET /api/version` (`app/api/version/route.ts`): returns `{ "version": "<semver>" }`, version read from package.json via a server-side import (same single-source-of-truth pattern as the header tag)
+- OpenAPI 3.1 spec in code: `lib/openapi.ts` exports an object documenting exactly the two public endpoints — `GET /api/airports` (GeoJSON FeatureCollection, with component schemas for feature collection/feature/point geometry/properties) and `GET /api/version`; `info.version` is imported from package.json so it stays in sync automatically; `servers: [{ url: "" }]` means "current origin"
+- New `GET /api/openapi` (`app/api/openapi/route.ts`) serves that spec as JSON — machine-readable API docs for AI agents and external tooling (no interactive docs page, no new dependencies)
+- AGENTS.md gains a short "## API" section pointing agents at the live base URL and the spec at `https://flight.dev.kkhost.pl/api/openapi`, noting the spec is generated from `lib/openapi.ts` and must be updated whenever endpoints change; this commit also includes the pending local note in AGENTS.md's "## Other" section (use pty_* tools for longer terminal tasks)
+- Tests: unit — new `app/api/version/route.test.ts` (200 + JSON version equals package.json's, exact body shape) and `lib/openapi.test.ts` (both paths present, info.version in sync with package.json, empty server url, airports schema refs the FeatureCollection/Feature/Properties component schemas with name/icao/lat/lon); e2e — new `e2e/api.spec.ts`: fetches `/api/version` from the live origin and asserts 200 + version matching package.json read at test time; fetches `/api/openapi` and checks content-type, title, info.version and both paths
+
 ## v0.4.1 — 2026-09-17 08:50
 - Map-asset pack (~100 MB, 780 files) removed from git and from the Docker image: `git rm -r --cached public/{styles,fonts,sprites}` (files kept on disk), `.gitignore` now excludes exactly the generated paths (`public/styles/`, `public/fonts/`, `public/sprites/`, `public/map-assets-manifest.json`) — not all of `public/`; `.dockerignore` gets the same four entries; the v0.4.0 runtime-stage `COPY --from=build /app/public ./public` line is gone from the Dockerfile
 - The pack now reaches the container via a docker-compose volume mount: `./public:/app/public` on the `flight` service — host dir present with assets → served locally; absent/empty → 404s → runtime fallback (below). Establishes the pattern for the future multi-GB tile file (v0.5.0) that must never go into git
diff --git a/app/api/openapi/route.ts b/app/api/openapi/route.ts
new file mode 100644
index 0000000..4fdbda4
--- /dev/null
+++ b/app/api/openapi/route.ts
@@ -0,0 +1,5 @@
+import { openApiSpec } from '@/lib/openapi';
+
+export async function GET() {
+  return Response.json(openApiSpec);
+}
diff --git a/app/api/version/route.test.ts b/app/api/version/route.test.ts
new file mode 100644
index 0000000..957bc25
--- /dev/null
+++ b/app/api/version/route.test.ts
@@ -0,0 +1,18 @@
+import pkg from '@/package.json';
+import { describe, expect, it } from 'vitest';
+import { GET } from './route';
+
+describe('GET /api/version', () => {
+  it('returns 200 with the version sourced from package.json', async () => {
+    const res = await GET();
+    expect(res.status).toBe(200);
+    expect(res.headers.get('content-type')).toContain('application/json');
+    const body = (await res.json()) as { version: string };
+    expect(body.version).toBe(pkg.version);
+  });
+
+  it('returns exactly the version field', async () => {
+    const res = await GET();
+    expect(await res.json()).toEqual({ version: pkg.version });
+  });
+});
diff --git a/app/api/version/route.ts b/app/api/version/route.ts
new file mode 100644
index 0000000..50bafe8
--- /dev/null
+++ b/app/api/version/route.ts
@@ -0,0 +1,5 @@
+import pkg from '@/package.json';
+
+export async function GET() {
+  return Response.json({ version: pkg.version });
+}
diff --git a/e2e/api.spec.ts b/e2e/api.spec.ts
new file mode 100644
index 0000000..b985a82
--- /dev/null
+++ b/e2e/api.spec.ts
@@ -0,0 +1,24 @@
+import { readFileSync } from 'node:fs';
+import { join } from 'node:path';
+import { test, expect } from '@playwright/test';
+
+// Read at test time so the assertions track package.json automatically.
+const pkg = JSON.parse(readFileSync(join(__dirname, '..', 'package.json'), 'utf8')) as { version: string };
+
+test('GET /api/version returns the app version from package.json', async ({ page }) => {
+  const res = await page.request.get('/api/version');
+  expect(res.status()).toBe(200);
+  expect(res.headers()['content-type']).toContain('application/json');
+  const body = (await res.json()) as { version: string };
+  expect(body.version).toBe(pkg.version);
+});
+
+test('GET /api/openapi serves the spec with both endpoints and the package.json version', async ({ page }) => {
+  const res = await page.request.get('/api/openapi');
+  expect(res.status()).toBe(200);
+  expect(res.headers()['content-type']).toContain('application/json');
+  const spec = (await res.json()) as { info: { title: string; version: string }; paths: Record<string, unknown> };
+  expect(spec.info.title).toBe('Flight Ops API');
+  expect(spec.info.version).toBe(pkg.version);
+  expect(Object.keys(spec.paths).sort()).toEqual(['/api/airports', '/api/version']);
+});
diff --git a/lib/openapi.test.ts b/lib/openapi.test.ts
new file mode 100644
index 0000000..81247cc
--- /dev/null
+++ b/lib/openapi.test.ts
@@ -0,0 +1,49 @@
+import pkg from '@/package.json';
+import { describe, expect, it } from 'vitest';
+import { openApiSpec } from './openapi';
+
+describe('OpenAPI spec', () => {
+  it('documents exactly the two public endpoints', () => {
+    expect(Object.keys(openApiSpec.paths).sort()).toEqual(['/api/airports', '/api/version']);
+  });
+
+  it('keeps info.version in sync with package.json', () => {
+    expect(openApiSpec.info.title).toBe('Flight Ops API');
+    expect(openApiSpec.info.version).toBe(pkg.version);
+  });
+
+  it('uses an empty server url so "try it" targets the current origin', () => {
+    expect(openApiSpec.servers).toEqual([{ url: '' }]);
+  });
+
+  it('describes /api/airports as a GeoJSON FeatureCollection of airport features', () => {
+    const schema = openApiSpec.paths['/api/airports'].get.responses[200].content['application/json'].schema;
+    expect(schema.$ref).toBe('#/components/schemas/AirportFeatureCollection');
+
+    const fc = openApiSpec.components.schemas.AirportFeatureCollection.properties;
+    expect(fc.features.items.$ref).toBe('#/components/schemas/AirportFeature');
+
+    const feature = openApiSpec.components.schemas.AirportFeature.properties;
+    expect(feature.geometry.$ref).toBe('#/components/schemas/PointGeometry');
+    expect(feature.properties.$ref).toBe('#/components/schemas/AirportProperties');
+
+    const props = openApiSpec.components.schemas.AirportProperties.properties;
+    expect(Object.keys(props).sort()).toEqual(['icao', 'lat', 'lon', 'name']);
+    expect(props.name.type).toBe('string');
+    expect(props.icao.type).toBe('string');
+    expect(props.lat.type).toBe('number');
+    expect(props.lon.type).toBe('number');
+
+    const coords = openApiSpec.components.schemas.PointGeometry.properties.coordinates;
+    expect(coords.items.type).toBe('number');
+  });
+
+  it('describes /api/version as { version } with a string version', () => {
+    const schema = openApiSpec.paths['/api/version'].get.responses[200].content['application/json'].schema;
+    expect(schema.$ref).toBe('#/components/schemas/AppVersion');
+
+    const props = openApiSpec.components.schemas.AppVersion.properties;
+    expect(Object.keys(props)).toEqual(['version']);
+    expect(props.version.type).toBe('string');
+  });
+});
diff --git a/lib/openapi.ts b/lib/openapi.ts
new file mode 100644
index 0000000..570850d
--- /dev/null
+++ b/lib/openapi.ts
@@ -0,0 +1,109 @@
+import pkg from '@/package.json';
+
+// OpenAPI 3.1 document for the app's public JSON API, kept in code so it can be
+// served machine-readable at /api/openapi (see AGENTS.md "## API"). Must be updated
+// whenever an endpoint changes.
+export const openApiSpec = {
+  openapi: '3.1.0',
+  info: {
+    title: 'Flight Ops API',
+    description:
+      'JSON API for Flight Ops — EU airport data for the map, plus app metadata.',
+    // Sourced from package.json so it stays in sync with every version bump.
+    version: pkg.version,
+  },
+  // Empty url = current origin (the spec is served from the same host that runs the API).
+  servers: [{ url: '' }],
+  paths: {
+    '/api/airports': {
+      get: {
+        summary: 'List EU airports',
+        description:
+          'All fixed-wing EU airports (large, medium and seaplane bases) as a GeoJSON FeatureCollection.',
+        operationId: 'listAirports',
+        responses: {
+          200: {
+            description: 'GeoJSON FeatureCollection of EU airports',
+            content: {
+              'application/json': {
+                schema: { $ref: '#/components/schemas/AirportFeatureCollection' },
+              },
+            },
+          },
+        },
+      },
+    },
+    '/api/version': {
+      get: {
+        summary: 'App version',
+        description: 'The running app version, read from package.json.',
+        operationId: 'getVersion',
+        responses: {
+          200: {
+            description: 'Current app version',
+            content: {
+              'application/json': {
+                schema: { $ref: '#/components/schemas/AppVersion' },
+              },
+            },
+          },
+        },
+      },
+    },
+  },
+  components: {
+    schemas: {
+      AppVersion: {
+        type: 'object',
+        properties: {
+          version: { type: 'string', description: 'Semver of the running build' },
+        },
+        required: ['version'],
+      },
+      AirportFeatureCollection: {
+        type: 'object',
+        properties: {
+          type: { const: 'FeatureCollection' },
+          features: {
+            type: 'array',
+            items: { $ref: '#/components/schemas/AirportFeature' },
+          },
+        },
+        required: ['type', 'features'],
+      },
+      AirportFeature: {
+        type: 'object',
+        properties: {
+          type: { const: 'Feature' },
+          geometry: { $ref: '#/components/schemas/PointGeometry' },
+          properties: { $ref: '#/components/schemas/AirportProperties' },
+        },
+        required: ['type', 'geometry', 'properties'],
+      },
+      PointGeometry: {
+        type: 'object',
+        properties: {
+          type: { const: 'Point' },
+          coordinates: {
+            type: 'array',
+            description: '[longitude, latitude]',
+            items: { type: 'number' },
+            minItems: 2,
+            maxItems: 2,
+          },
+        },
+        required: ['type', 'coordinates'],
+      },
+      AirportProperties: {
+        type: 'object',
+        properties: {
+          name: { type: 'string' },
+          icao: { type: 'string' },
+          lat: { type: 'number' },
+          lon: { type: 'number' },
+        },
+        required: ['name', 'icao', 'lat', 'lon'],
+      },
+    },
+  },
+};
diff --git a/package.json b/package.json
index b5c9cc9..c0e81aa 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "flight-ops",
-  "version": "0.4.1",
+  "version": "0.5.0",
   "private": true,
   "scripts": {
     "dev": "next dev -p 3000",