diff --git a/examples/verification-workers/src/html.ts b/examples/verification-workers/src/html.ts
index 7d2e850..c56d38c 100644
--- a/examples/verification-workers/src/html.ts
+++ b/examples/verification-workers/src/html.ts
@@ -189,6 +189,20 @@ button:focus {
 button:active {
   box-shadow: inset 0 1px 1px 1px rgba(0,0,0,.4);
 }
+input[type="url"] {
+  border: 1px solid #999;
+  border-radius: 6px;
+  box-sizing: border-box;
+  display: block;
+  font: inherit;
+  margin: 0.5rem 0;
+  max-width: 720px;
+  padding: 8px 10px;
+  width: 100%;
+}
+.validation-result {
+  margin-top: 0.75rem;
+}
 
 .question-list {
   margin-bottom: 2rem;
@@ -273,6 +287,17 @@ footer {
       </ul>
     </p>
 
+    <h2>Validate your key directory</h2>
+    <p>
+      Paste the full HTTPS URL for a <code>/.well-known/http-message-signatures-directory</code> endpoint to check whether it returns a usable directory.
+    </p>
+    <form id="directory-validator">
+      <label for="directory-url">Directory URL</label>
+      <input id="directory-url" name="url" type="url" placeholder="https://example.com/.well-known/http-message-signatures-directory" required />
+      <button type="submit">Validate directory</button>
+      <p id="directory-validation-result" class="validation-result" aria-live="polite"></p>
+    </form>
+
     <h2>It's hard to debug. How can this website help?</h2>
     <p>
     This website expose an endpoint dropping incoming request headers on <a>/debug</a>
@@ -289,6 +314,26 @@ footer {
     To contribute to the standard discussion, the current draft is hosted on <a href="https://github.com/thibmeu/http-message-signatures-directory">thibmeu/http-message-signatures-directory</a>, and is being discussed on <a href="https://mailarchive.ietf.org/arch/browse/web-bot-auth/">web-bot-auth</a> IETF mailing list.
     </p>
   </section>
+  <script>
+    const form = document.getElementById("directory-validator");
+    const result = document.getElementById("directory-validation-result");
+
+    form.addEventListener("submit", async (event) => {
+      event.preventDefault();
+      const data = new FormData(form);
+      result.textContent = "Checking directory...";
+
+      try {
+        const response = await fetch("/v0/api/validate-directory?url=" + encodeURIComponent(data.get("url")));
+        const body = await response.json();
+        result.textContent = body.ok
+          ? "Directory looks valid."
+          : "Directory check failed: " + body.errors.join("; ");
+      } catch (_e) {
+        result.textContent = "Directory check failed.";
+      }
+    });
+  </script>
 </body>
 </html>`;
 
diff --git a/examples/verification-workers/src/index.ts b/examples/verification-workers/src/index.ts
index 1e683b0..02e3104 100644
--- a/examples/verification-workers/src/index.ts
+++ b/examples/verification-workers/src/index.ts
@@ -87,6 +87,103 @@ async function fetchDirectory(signatureAgent: string): Promise<Directory> {
 	return response.json();
 }
 
+function validateDirectory(directory: unknown): string[] {
+	const errors: string[] = [];
+
+	if (directory === null || typeof directory !== "object") {
+		return ["Directory must be a JSON object"];
+	}
+
+	const value = directory as Partial<Directory>;
+
+	if (!Array.isArray(value.keys)) {
+		errors.push("Directory must include a keys array");
+	} else if (value.keys.length === 0) {
+		errors.push("Directory keys array must not be empty");
+	} else {
+		for (const [index, key] of value.keys.entries()) {
+			if (key === null || typeof key !== "object") {
+				errors.push(`keys[${index}] must be a JSON object`);
+				continue;
+			}
+			if (typeof key.kty !== "string") {
+				errors.push(`keys[${index}].kty must be a string`);
+			}
+		}
+	}
+
+	if (typeof value.purpose !== "string" || value.purpose.length === 0) {
+		errors.push("Directory must include a non-empty purpose string");
+	}
+
+	return errors;
+}
+
+async function validateDirectoryURL(request: Request): Promise<Response> {
+	const url = new URL(request.url);
+	const directoryURL = url.searchParams.get("url");
+
+	if (directoryURL === null || directoryURL.length === 0) {
+		return Response.json(
+			{ ok: false, errors: ["Missing url query parameter"] },
+			{ status: 400 }
+		);
+	}
+
+	let parsed: URL;
+	try {
+		parsed = new URL(directoryURL);
+	} catch (_e) {
+		return Response.json(
+			{ ok: false, errors: ["URL must be valid"] },
+			{ status: 400 }
+		);
+	}
+
+	if (parsed.protocol !== "https:") {
+		return Response.json(
+			{ ok: false, errors: ['Directory URL must use "https:"'] },
+			{ status: 400 }
+		);
+	}
+
+	if (parsed.pathname !== HTTP_MESSAGE_SIGNATURES_DIRECTORY) {
+		return Response.json(
+			{
+				ok: false,
+				errors: [
+					`Directory URL path must be ${HTTP_MESSAGE_SIGNATURES_DIRECTORY}`,
+				],
+			},
+			{ status: 400 }
+		);
+	}
+
+	const response = await fetch(parsed);
+	if (!response.ok) {
+		return Response.json(
+			{
+				ok: false,
+				errors: [`Directory returned HTTP ${response.status}`],
+			},
+			{ status: 502 }
+		);
+	}
+
+	let directory: unknown;
+	try {
+		directory = await response.json();
+	} catch (_e) {
+		return Response.json(
+			{ ok: false, errors: ["Directory response must be valid JSON"] },
+			{ status: 502 }
+		);
+	}
+
+	const errors = validateDirectory(directory);
+	return Response.json({ ok: errors.length === 0, errors });
+}
+
 async function getSigner(): Promise<Signer> {
 	return Ed25519Signer.fromJWK(jwk);
 }
@@ -180,6 +277,10 @@ export default {
 			return new Response(status);
 		}
 
+		if (url.pathname.startsWith("/v0/api/validate-directory")) {
+			return validateDirectoryURL(request);
+		}
+
 		if (url.pathname.startsWith(HTTP_MESSAGE_SIGNATURES_DIRECTORY)) {
 			const directory = await getExampleDirectory();
 
diff --git a/examples/verification-workers/test/index.spec.ts b/examples/verification-workers/test/index.spec.ts
index 2e1950b..650a3eb 100644
--- a/examples/verification-workers/test/index.spec.ts
+++ b/examples/verification-workers/test/index.spec.ts
@@ -49,3 +49,51 @@ describe("/debug endpoint", () => {
 		expect(await response.text()).toMatch(headersString);
 	});
 });
+
+describe("/v0/api/validate-directory endpoint", () => {
+	it("requires a url query parameter", async () => {
+		const response = await SELF.fetch(
+			new Request(`${sampleURL}/v0/api/validate-directory`)
+		);
+
+		expect(response.status).toEqual(400);
+		expect(await response.json()).toEqual({
+			ok: false,
+			errors: ["Missing url query parameter"],
+		});
+	});
+
+	it("requires an https well-known directory URL", async () => {
+		const response = await SELF.fetch(
+			new Request(
+				`${sampleURL}/v0/api/validate-directory?url=${encodeURIComponent(
+					"http://example.com/"
+				)}`
+			)
+		);
+
+		expect(response.status).toEqual(400);
+		expect(await response.json()).toEqual({
+			ok: false,
+			errors: ['Directory URL must use "https:"'],
+		});
+	});
+
+	it("requires the directory well-known path", async () => {
+		const response = await SELF.fetch(
+			new Request(
+				`${sampleURL}/v0/api/validate-directory?url=${encodeURIComponent(
+					"https://example.com/"
+				)}`
+			)
+		);
+
+		expect(response.status).toEqual(400);
+		expect(await response.json()).toEqual({
+			ok: false,
+			errors: [
+				"Directory URL path must be /.well-known/http-message-signatures-directory",
+			],
+		});
+	});
+});