api: /api/version + OpenAPI 3.x spec generated in code

lib/openapi.ts is the machine-readable spec (served at /api/openapi) per AGENTS.md;
info.version sourced from package.json. Unit tests pin the spec shape and version sync.

app/api/openapi/route.ts

@@ -0,0 +1,5 @@
+import { openApiSpec } from '@/lib/openapi';
+
+export async function GET() {
+  return Response.json(openApiSpec);
+}

app/api/version/route.ts

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+import pkg from '@/package.json';
+
+export async function GET() {
+  return Response.json({ version: pkg.version });
+}

lib/openapi.test.ts

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+import pkg from '@/package.json';
+import { describe, expect, it } from 'vitest';
+import { openApiSpec } from './openapi';
+
+describe('OpenAPI spec', () => {
+  it('documents exactly the one public endpoint (scaffold stage)', () => {
+    expect(Object.keys(openApiSpec.paths).sort()).toEqual(['/api/version']);
+  });
+
+  it('keeps info.version in sync with package.json', () => {
+    expect(openApiSpec.info.title).toBe('SpaceWanderer 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/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 appVersion = openApiSpec.components.schemas.AppVersion.properties;
+    expect(Object.keys(appVersion)).toEqual(['version']);
+    expect(appVersion.version.type).toBe('string');
+  });
+});

lib/openapi.ts

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+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: 'SpaceWanderer API',
+    description:
+      'JSON API for SpaceWanderer — a single-player turn-based space adventure. Scaffold stage: app metadata only; game endpoints (turn actions, state snapshots) land with the first game systems.',
+    // 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/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'],
+      },
+    },
+  },
+};