diff --git a/src/domains/chapter-assignments/presence/chapter-assignments-presence.repository.ts b/src/domains/chapter-assignments/presence/chapter-assignments-presence.repository.ts
index ea1049cb..a502a751 100644
--- a/src/domains/chapter-assignments/presence/chapter-assignments-presence.repository.ts
+++ b/src/domains/chapter-assignments/presence/chapter-assignments-presence.repository.ts
@@ -1,4 +1,4 @@
-import { and, asc, eq, lt } from 'drizzle-orm';
+import { and, asc, eq, lt, sql } from 'drizzle-orm';
 
 import type { Result } from '@/lib/types';
 
@@ -16,6 +16,12 @@ export async function upsertAndQueryFirstEditor(
 ): Promise<Result<PresenceResponse>> {
   try {
     return await db.transaction(async (tx) => {
+      // Serialize concurrent presence registrations for the same chapter so the
+      // first-editor check can't race: a near-simultaneous second registration
+      // must observe the first's committed row instead of both deciding they are
+      // first. The advisory lock is transaction-scoped and auto-released on commit.
+      await tx.execute(sql`SELECT pg_advisory_xact_lock(${chapterAssignmentId}::bigint)`);
+
       const now = new Date();
       const staleTime = new Date(now.getTime() - STALE_THRESHOLD_MINUTES * 60 * 1000);
 
@@ -48,7 +54,7 @@ export async function upsertAndQueryFirstEditor(
         .from(active_chapter_editors)
         .innerJoin(users, eq(active_chapter_editors.userId, users.id))
         .where(eq(active_chapter_editors.chapterAssignmentId, chapterAssignmentId))
-        .orderBy(asc(active_chapter_editors.startedAt))
+        .orderBy(asc(active_chapter_editors.startedAt), asc(active_chapter_editors.userId))
         .limit(1);
 
       if (!firstEditor) {
diff --git a/src/domains/chapter-assignments/presence/chapter-assignments-presence.route.ts b/src/domains/chapter-assignments/presence/chapter-assignments-presence.route.ts
index ac2bc7f2..b0c0768a 100644
--- a/src/domains/chapter-assignments/presence/chapter-assignments-presence.route.ts
+++ b/src/domains/chapter-assignments/presence/chapter-assignments-presence.route.ts
@@ -4,6 +4,8 @@ import * as HttpStatusPhrases from 'stoker/http-status-phrases';
 import { jsonContent } from 'stoker/openapi/helpers';
 import { createMessageObjectSchema } from 'stoker/openapi/schemas';
 
+import { requireChapterAssignmentAccess } from '@/domains/chapter-assignments/chapter-assignment-auth.middleware';
+import { CHAPTER_ASSIGNMENT_ACTIONS } from '@/domains/chapter-assignments/chapter-assignments.types';
 import { PERMISSIONS } from '@/lib/permissions';
 import { getHttpStatus } from '@/lib/types';
 import { authenticateUser, requirePermission } from '@/middlewares/role-auth';
@@ -28,14 +30,30 @@ const registerPresenceRoute = createRoute({
   tags: ['Chapter Assignments - Presence'],
   method: 'post',
   path: '/chapter-assignments/{chapterAssignmentId}/presence',
-  middleware: [authenticateUser, requirePermission(PERMISSIONS.CONTENT_UPDATE)] as const,
+  middleware: [
+    authenticateUser,
+    requirePermission(PERMISSIONS.CONTENT_UPDATE),
+    requireChapterAssignmentAccess(CHAPTER_ASSIGNMENT_ACTIONS.IS_PARTICIPANT),
+  ] as const,
   request: { params: chapterAssignmentIdParam },
   responses: {
     [HttpStatusCodes.OK]: jsonContent(presenceResponseSchema, 'Presence registered successfully'),
+    [HttpStatusCodes.BAD_REQUEST]: jsonContent(
+      createMessageObjectSchema('Bad Request'),
+      'Invalid chapter assignment ID'
+    ),
     [HttpStatusCodes.UNAUTHORIZED]: jsonContent(
       createMessageObjectSchema('Unauthorized'),
       'Authentication required'
     ),
+    [HttpStatusCodes.FORBIDDEN]: jsonContent(
+      createMessageObjectSchema('Forbidden'),
+      'Access denied'
+    ),
+    [HttpStatusCodes.NOT_FOUND]: jsonContent(
+      createMessageObjectSchema(HttpStatusPhrases.NOT_FOUND),
+      'Chapter assignment not found'
+    ),
     [HttpStatusCodes.INTERNAL_SERVER_ERROR]: jsonContent(
       createMessageObjectSchema(HttpStatusPhrases.INTERNAL_SERVER_ERROR),
       'Internal server error'
@@ -50,14 +68,30 @@ const removePresenceRoute = createRoute({
   tags: ['Chapter Assignments - Presence'],
   method: 'delete',
   path: '/chapter-assignments/{chapterAssignmentId}/presence',
-  middleware: [authenticateUser, requirePermission(PERMISSIONS.CONTENT_UPDATE)] as const,
+  middleware: [
+    authenticateUser,
+    requirePermission(PERMISSIONS.CONTENT_UPDATE),
+    requireChapterAssignmentAccess(CHAPTER_ASSIGNMENT_ACTIONS.IS_PARTICIPANT),
+  ] as const,
   request: { params: chapterAssignmentIdParam },
   responses: {
     [HttpStatusCodes.NO_CONTENT]: { description: 'Presence removed successfully' },
+    [HttpStatusCodes.BAD_REQUEST]: jsonContent(
+      createMessageObjectSchema('Bad Request'),
+      'Invalid chapter assignment ID'
+    ),
     [HttpStatusCodes.UNAUTHORIZED]: jsonContent(
       createMessageObjectSchema('Unauthorized'),
       'Authentication required'
     ),
+    [HttpStatusCodes.FORBIDDEN]: jsonContent(
+      createMessageObjectSchema('Forbidden'),
+      'Access denied'
+    ),
+    [HttpStatusCodes.NOT_FOUND]: jsonContent(
+      createMessageObjectSchema(HttpStatusPhrases.NOT_FOUND),
+      'Chapter assignment not found'
+    ),
     [HttpStatusCodes.INTERNAL_SERVER_ERROR]: jsonContent(
       createMessageObjectSchema(HttpStatusPhrases.INTERNAL_SERVER_ERROR),
       'Internal server error'