diff --git a/README.md b/README.md
index 7ca0299..a773fae 100644
--- a/README.md
+++ b/README.md
@@ -115,6 +115,18 @@ The pages are React components using [FiestaUI](https://github.com/Fiestaboard/F
   loads the UI when a person is needed, so the common case does not wait for React.
 - `src/prerender.tsx` and `scripts/prerender.mjs` render each page into the built HTML.
 
+## OAuth client metadata documents
+
+Some providers (Todoist, Hugging Face) accept a URL as the `client_id` and fetch the app's
+details from it, so nobody has to register an app
+([draft-ietf-oauth-client-id-metadata-document](https://datatracker.ietf.org/doc/draft-ietf-oauth-client-id-metadata-document/)).
+`public/clients/<name>.json` is copied into the site unchanged and served at
+`https://fiestaboard.app/auth/clients/<name>.json`, which is also its `client_id`.
+
+Like the redirect URL, **these URLs must never change**: FiestaBoard plugins ship them as
+their client ID. Each lists both redirect URIs and `"token_endpoint_auth_method": "none"`
+(a public client using PKCE); none may ever carry a secret.
+
 ## Development
 
 ```sh
diff --git a/package.json b/package.json
index aef0e56..3d368ab 100644
--- a/package.json
+++ b/package.json
@@ -9,7 +9,7 @@
     "build": "tsc --noEmit && vite build && vite build --ssr src/prerender.tsx --outDir dist-ssr && node scripts/prerender.mjs",
     "preview": "vite preview --port 8787",
     "typecheck": "tsc --noEmit",
-    "test": "node --test tests/board-address.test.js",
+    "test": "node --test tests/board-address.test.js tests/clients.test.js",
     "test:dist": "node --test tests/dist.test.js"
   },
   "dependencies": {
diff --git a/public/clients/huggingface.json b/public/clients/huggingface.json
new file mode 100644
index 0000000..9559f08
--- /dev/null
+++ b/public/clients/huggingface.json
@@ -0,0 +1,10 @@
+{
+  "client_id": "https://fiestaboard.app/auth/clients/huggingface.json",
+  "client_name": "FiestaBoard",
+  "client_uri": "https://fiestaboard.app/",
+  "redirect_uris": [
+    "https://fiestaboard.app/auth/oauth/redirect",
+    "https://fiestaboard.app/auth/oauth/redirect.html"
+  ],
+  "token_endpoint_auth_method": "none"
+}
diff --git a/public/clients/todoist.json b/public/clients/todoist.json
new file mode 100644
index 0000000..4a01e84
--- /dev/null
+++ b/public/clients/todoist.json
@@ -0,0 +1,13 @@
+{
+  "client_id": "https://fiestaboard.app/auth/clients/todoist.json",
+  "client_name": "FiestaBoard",
+  "client_uri": "https://fiestaboard.app/",
+  "redirect_uris": [
+    "https://fiestaboard.app/auth/oauth/redirect",
+    "https://fiestaboard.app/auth/oauth/redirect.html"
+  ],
+  "grant_types": ["authorization_code", "refresh_token"],
+  "response_types": ["code"],
+  "token_endpoint_auth_method": "none",
+  "scope": "data:read"
+}
diff --git a/tests/clients.test.js b/tests/clients.test.js
new file mode 100644
index 0000000..c21214d
--- /dev/null
+++ b/tests/clients.test.js
@@ -0,0 +1,35 @@
+// OAuth client metadata documents (draft-ietf-oauth-client-id-metadata-document).
+//
+// A provider that supports these fetches the client_id itself: the client_id
+// is the URL of the document. Vite copies public/ into dist/ unchanged, so
+// public/clients/<name>.json is served at https://fiestaboard.app/auth/clients/<name>.json.
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { resolve } from "node:path";
+import { test } from "node:test";
+
+const SITE = "https://fiestaboard.app/auth/";
+// Both forms are registered: FiestaBoard 9.5.0 through 9.7.x sends the .html one.
+const REDIRECT_URIS = [`${SITE}oauth/redirect`, `${SITE}oauth/redirect.html`];
+const CLIENTS = ["todoist", "huggingface"];
+
+const load = (name) => JSON.parse(readFileSync(resolve(import.meta.dirname, `../public/clients/${name}.json`), "utf8"));
+
+for (const name of CLIENTS) {
+  test(`${name}: client_id is the URL the document is served at`, () => {
+    assert.equal(load(name).client_id, `${SITE}clients/${name}.json`);
+  });
+
+  test(`${name}: a public client that returns through the relay`, () => {
+    const doc = load(name);
+    assert.equal(doc.token_endpoint_auth_method, "none");
+    assert.deepEqual(doc.redirect_uris, REDIRECT_URIS);
+    assert.equal(typeof doc.client_name, "string");
+    assert.ok(!("client_secret" in doc));
+  });
+
+  test(`${name}: small enough for providers' fetch limits`, () => {
+    // Todoist refuses documents over 5 KB.
+    assert.ok(readFileSync(resolve(import.meta.dirname, `../public/clients/${name}.json`)).length < 5000);
+  });
+}
diff --git a/tests/dist.test.js b/tests/dist.test.js
index d162cbb..3187fc9 100644
--- a/tests/dist.test.js
+++ b/tests/dist.test.js
@@ -94,3 +94,11 @@ test("no script or stylesheet references another host", () => {
     assert.doesNotMatch(css, /@import\s+(url\()?["']?https?:/);
   }
 });
+
+test("OAuth client metadata documents are published at their client_id URLs", () => {
+  // The client_id is this URL; moving a document breaks every sign-in that uses it.
+  for (const name of ["todoist", "huggingface"]) {
+    const doc = JSON.parse(read(`clients/${name}.json`));
+    assert.equal(doc.client_id, `https://fiestaboard.app/auth/clients/${name}.json`);
+  }
+});