diff --git a/CLAUDE.md b/CLAUDE.md
index aec94e1..4ddd7fa 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -13,7 +13,9 @@ Whatnot podcast).
 ## Commands
 
 - **Dev server:** `pnpm dev` (runs on localhost:4321)
-- **Build:** `pnpm build` (runs `astro check` then `astro build`)
+- **Build:** `pnpm build` (runs `astro check`, `astro build`, then
+  `scripts/vercel-md-negotiation.mjs`, which injects `Accept: text/markdown`
+  content-negotiation routes into the Vercel build output)
 - **Lint:** `pnpm lint` (ESLint with caching)
 - **Lint fix:** `pnpm lint:fix`
 - **All tests:** `pnpm test` (runs unit + e2e concurrently)
@@ -56,7 +58,11 @@ connection is configured in `db/index.ts`.
 
 - `src/pages/` — Astro pages and API routes. Dynamic episode pages use
   `[episode].astro`. LLM-friendly `.html.md.ts` endpoints generate markdown
-  versions.
+  versions. `openapi.json.ts` publishes an OpenAPI spec for the JSON API.
+  `[...notFound].astro` is an on-demand (prerender=false) catch-all that
+  returns agent-friendly 404s: JSON errors for `/api/*`, a markdown body for
+  `Accept: text/markdown` clients, and the styled 404 page otherwise. API
+  routes return structured JSON errors via `src/lib/api-errors.ts`.
 - `src/components/` — Mix of `.astro` (static) and `.tsx` (Preact interactive)
   components. The audio player (`src/components/player/`) and search dialog are
   Preact.
diff --git a/README.md b/README.md
index ca08401..4c9416d 100644
--- a/README.md
+++ b/README.md
@@ -250,12 +250,40 @@ fine without them, using episode descriptions and metadata from your RSS feed.
 All of the following endpoints are automatically generated at build time from
 your `starpod.config.ts` and RSS feed:
 
-- `/llms.txt` - Main discovery file
+- `/llms.txt` - Main discovery file, including "when to use this site" guidance
+  for agents and a developer resources section
 - `/for-llms` - Human-readable guide page
 - `/for-llms.html.md` - Markdown version of guide
+- `/index.html.md` - Markdown version of the homepage
 - `/about.html.md` - Markdown version of about page
+- `/contact.html.md` - Markdown version of the contact page
 - `/episodes-index.html.md` - Complete episode listing
 - `/{episode-slug}.html.md` - Individual episode with transcript
 - `/{episode-number}.html.md` - Alternative episode URL
+- `/openapi.json` - OpenAPI 3.1 spec describing the JSON API endpoints
+  (episode search, episode pagination, contact form)
 
 No configuration needed - it just works!
+
+#### Markdown Content Negotiation
+
+Agents can also request any page that has a markdown twin with an
+`Accept: text/markdown` header and get the markdown version back from the same
+URL, per [acceptmarkdown.com](https://acceptmarkdown.com). Both variants are
+served with `Vary: Accept` so CDNs cache them separately.
+
+This is implemented by `scripts/vercel-md-negotiation.mjs`, which runs as part
+of `pnpm build` and injects Accept-based rewrite routes into the Vercel build
+output. If you customize the `build` script in `package.json`, keep the
+`node scripts/vercel-md-negotiation.mjs` step after `astro build`. (Deploying
+somewhere other than Vercel? The `.html.md` URLs still work everywhere; only
+the Accept-header negotiation is Vercel-specific.)
+
+#### Agent-Friendly Errors
+
+- Nonexistent paths return a real HTTP 404: browsers get the styled 404 page,
+  `Accept: text/markdown` clients get a short markdown body pointing at the
+  sitemap, `llms.txt`, and the episodes index, and `/api/*` paths get a
+  structured JSON error
+- API errors are structured JSON with a stable `error.code`, a message, and a
+  resolution `hint` - never an HTML error page