diff --git a/CHANGELOG.md b/CHANGELOG.md
index 03d3236..878fd75 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,8 @@ See [keep a changelog] for information about writing changes to this log.
 
 ## [Unreleased]
 
+- [PR-52](https://github.com/itk-dev/event-database-api/pull/52)
+  Pin the problem+json error contract: 401 served as RFC 7807, resource reads in problem+json yield 406
 - [PR-51](https://github.com/itk-dev/event-database-api/pull/51)
   Show the API-spec diff summary inline in the PR comment (collapsed) alongside the oasdiff review link
 - [PR-49](https://github.com/itk-dev/event-database-api/pull/49)
diff --git a/tests/ApiPlatform/Contract/ErrorContractTest.php b/tests/ApiPlatform/Contract/ErrorContractTest.php
index 037d66e..5c39069 100644
--- a/tests/ApiPlatform/Contract/ErrorContractTest.php
+++ b/tests/ApiPlatform/Contract/ErrorContractTest.php
@@ -52,4 +52,58 @@ public function testNotFoundErrorShape(): void
         $this->assertSame(404, $data['status']);
         $this->assertSame('/errors/404', $data['type']);
     }
+
+    /**
+     * `rfc_7807_compliant_errors: true` (config/packages/api_platform.yaml)
+     * registers problem+json as an error format, so an auth error requested as
+     * `application/problem+json` is served as RFC 7807 (unprefixed
+     * title/detail/status/type) rather than hydra. The 401 also short-circuits
+     * before resource content negotiation, so the format is honoured.
+     *
+     * @see https://api-platform.com/docs/core/content-negotiation/
+     */
+    public function testUnauthenticatedErrorIsAvailableAsProblemJson(): void
+    {
+        $client = self::createClient(defaultOptions: [
+            'headers' => ['accept' => ['application/problem+json']],
+        ]);
+        $response = $client->request('GET', '/api/v2/events');
+
+        $this->assertSame(Response::HTTP_UNAUTHORIZED, $response->getStatusCode());
+        $this->assertStringContainsString('application/problem+json', $response->getHeaders(false)['content-type'][0] ?? '');
+
+        $data = $response->toArray(false);
+        $this->assertSame(401, $data['status']);
+        $this->assertSame('/errors/401', $data['type']);
+        $this->assertArrayHasKey('title', $data);
+        $this->assertArrayHasKey('detail', $data);
+    }
+
+    /**
+     * The only resource format configured under `formats` in
+     * config/packages/api_platform.yaml is `application/ld+json`, so content
+     * negotiation cannot satisfy `Accept: application/problem+json` on a resource
+     * read: it yields 406 Not Acceptable (NOT a 404), and the 406 body is itself
+     * a problem+json error. Pinning this documents that problem+json is not a
+     * resource representation (consumers must read resources as ld+json) and
+     * guards the `formats` allow-list — adding problem+json there would flip this.
+     *
+     * @see https://api-platform.com/docs/core/content-negotiation/
+     */
+    public function testProblemJsonIsNotAcceptableForResourceReads(): void
+    {
+        $client = self::createClient(defaultOptions: [
+            'headers' => [
+                'accept' => ['application/problem+json'],
+                'x-api-key' => 'test_api_key',
+            ],
+        ]);
+        $response = $client->request('GET', '/api/v2/events/99999');
+
+        $this->assertSame(Response::HTTP_NOT_ACCEPTABLE, $response->getStatusCode());
+        $this->assertStringContainsString('application/problem+json', $response->getHeaders(false)['content-type'][0] ?? '');
+
+        $data = $response->toArray(false);
+        $this->assertSame(406, $data['status']);
+    }
 }