diff --git a/server/src/router/v1/player/teamForum.router.js b/server/src/router/v1/player/teamForum.router.js
index ffc1f40..5038333 100644
--- a/server/src/router/v1/player/teamForum.router.js
+++ b/server/src/router/v1/player/teamForum.router.js
@@ -176,7 +176,15 @@ forumRouter.post(
   /* #swagger.responses[400] = { description: 'An action that applies to a thread, not a post', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
   /* #swagger.responses[403] = { description: 'Not a leader of this Team', content: { "application/json": { schema: { $ref: "#/components/schemas/Error" } } } } */
   param('id').isInt({ min: 1 }).toInt(),
-  body('action').isIn(['hide', 'unhide', 'delete', 'restore']),
+  // **Deliberately the FULL action list, not the four a post accepts.** The model
+  // answers `pin` with "that applies to a thread, not to a post" and an invented
+  // action with "unknown", and a validator that allowed only the four would turn
+  // the first of those into a generic "Validation failed" — leaving the precise
+  // message reachable only from a unit test. Found on the live rig, where `pin`
+  // came back as a validation error rather than as the sentence written for it.
+  // Both are 400 and neither is a security boundary; the difference is entirely
+  // whether the caller is told which mistake they made.
+  body('action').isIn(['pin', 'unpin', 'lock', 'unlock', 'hide', 'unhide', 'delete', 'restore']),
   body('reason').optional().isString().trim().isLength({ max: 255 }),
   validate,
   ctrl.moderatePost,
diff --git a/server/test/teamRoutes.test.js b/server/test/teamRoutes.test.js
index 293fc83..f020e22 100644
--- a/server/test/teamRoutes.test.js
+++ b/server/test/teamRoutes.test.js
@@ -495,3 +495,28 @@ test('the grant routes answer even while the forum is switched off', async () =>
     assert.equal((await get(app, '/api/v1/player/teams/a/grants')).status, 200)
   })
 })
+
+test('pin on a POST reaches the model, so the caller is told which mistake they made', async () => {
+  // The route's validator deliberately accepts all eight actions. Narrowing it to
+  // the four a post takes would turn "that applies to a thread, not to a post"
+  // into a generic "Validation failed" — the precise message would exist, be
+  // unit-tested, and be unreachable through the API. Found on the live rig.
+  signInAs(admin)
+  patch(forumSettings, 'forumsEnabled', async () => true)
+  patch(teamsDbModule, 'findBySlug', async () => ({ id: 1, name: 'A' }))
+  patch(access, 'forumAccess', async () => ({ allowed: true, viaMembership: true, viaGrant: false, isLeader: false }))
+  patch(forum, 'moderatePost', async ({ action }) => ({
+    ok: false, status: 400, error: `"${action}" applies to a thread, not to a post`,
+  }))
+
+  await withApp('/api/v1/player', playerRouter, async (app) => {
+    const res = await post(app, '/api/v1/player/teams/a/forum/posts/1/moderate', { action: 'pin' })
+    assert.equal(res.status, 400)
+    assert.match((await res.json()).message, /applies to a thread/)
+
+    // An action that is not in the enum at all still stops at the validator —
+    // widening the list is not the same as removing it.
+    const nonsense = await post(app, '/api/v1/player/teams/a/forum/posts/1/moderate', { action: 'incinerate' })
+    assert.equal(nonsense.status, 400)
+  })
+})