feat: serve /llms.txt from the API origin (agent crawler discovery)

Author: S. K. Rotwang MMMMMMMMM

docs/llms.txt

@@ -13,6 +13,7 @@ Seneschal lets a receiver get an HMAC-signed webhook the instant a Monero (XMR)
 - Free UFVK derivation from a seed phrase (rate-limited): POST https://api.seneschal.space/v1/private/derive-viewkey
 
 ## API discovery
+- OpenAPI 3.1 document (all routes, x402 prices per route): https://api.seneschal.space/openapi.json
 - x402 payment manifest: https://api.seneschal.space/.well-known/x402
 - Health + table sizes: https://api.seneschal.space/v1/health
 - MCP server (Streamable HTTP transport): https://mcp.seneschal.space/

src/rest-server.js

@@ -632,6 +632,29 @@ export async function buildApp(options = {}) {
 		return reply.send(faviconCache);
 	});
 
+	// `/llms.txt` — the llmstxt.org convention agent crawlers fetch from
+	// the API origin (the docs site serves its own copy, but indexers
+	// that found us via openapi.json look here, not on the docs host).
+	// Same lazy-load-and-cache pattern as the favicon.
+	let llmsTxtCache;
+	app.get('/llms.txt', async (req, reply) => {
+		if (llmsTxtCache === undefined) {
+			try {
+				llmsTxtCache = await readFile(new URL('../docs/llms.txt', import.meta.url), 'utf8');
+			}
+			catch {
+				llmsTxtCache = null;
+			}
+		}
+		if (!llmsTxtCache) {
+			reply.code(404);
+			return { error: { code: 'not_found', message: 'no llms.txt' } };
+		}
+		reply.header('content-type', 'text/plain; charset=utf-8');
+		reply.header('cache-control', 'public, max-age=3600');
+		return reply.send(llmsTxtCache);
+	});
+
 	// `/.well-known/x402` — opt-in discovery manifest matching the
 	// emerging convention that x402 service-discovery agents (e.g.
 	// Agorion, coinbase/x402 #1379) scrape from a provider's apex

test/rest-server.test.js

@@ -109,6 +109,22 @@ describe('routing & shape', () => {
 		expect(r.statusCode).toBe(404);
 		expect(r.json().error.code).toBe('not_found');
 	});
+
+	// Agent crawlers fetch these from the API origin (not the docs
+	// host); a 404 here loses the listing.
+	test('GET /llms.txt serves the agent-facing service summary', async () => {
+		const r = await call('GET', '/llms.txt');
+		expect(r.statusCode).toBe(200);
+		expect(r.headers['content-type']).toMatch(/text\/plain/);
+		expect(r.body).toMatch(/Seneschal/);
+		expect(r.body).toMatch(/openapi\.json/);
+	});
+
+	test('GET /favicon.ico serves an icon', async () => {
+		const r = await call('GET', '/favicon.ico');
+		expect(r.statusCode).toBe(200);
+		expect(r.headers['content-type']).toMatch(/image\/x-icon/);
+	});
 });
 
 describe('/v1/ops/health (watchdog state surface)', () => {