diff --git a/firestore.rules b/firestore.rules
index 6ba3c6fa..c3ed6ff5 100644
--- a/firestore.rules
+++ b/firestore.rules
@@ -35,6 +35,21 @@ service cloud.firestore {
       }
     }
 
+    function isServer() {
+      // Check if request comes from admin SDK (no request.auth)
+      return request.auth == null;
+    }
+
+    function isImmutable(fields) {
+      // Helper to check that certain fields haven't changed during an update
+      // fields: list of field names that should be immutable
+      if (resource == null) {
+        // Create operation - nothing to check
+        return true;
+      }
+      return fields.every(f => resource.data[f] == request.resource.data[f]);
+    }
+
     // ── Users ─────────────────────────────────────────────────────────────────
     // Each user owns exactly their own document.
     // hasPremium is server-only: clients may not write it directly.
@@ -96,18 +111,81 @@ service cloud.firestore {
     }
 
     // ── Couples ───────────────────────────────────────────────────────────────
-    // Only the two members of a couple may read or write couple data.
+    // Only the two members of a couple may read couple data.
+    // Writes are restricted by field ownership and immutability.
 
     match /couples/{coupleId} {
-      allow read, write: if isCouplesMember(coupleId);
+      // Read: both members can read
+      allow read: if isCouplesMember(coupleId);
+
+      // Create: only via invite flow (server-side or admin SDK)
+      allow create: if isServer();
+
+      // Update: field-level restrictions
+      // - user IDs are immutable (cannot change who is in the couple)
+      // - invite code is immutable (cannot change the code)
+      // - createdAt is immutable (cannot change when the couple was formed)
+      // - currentQuestionId: either member can set (either can pick a question)
+      // - streakCount and lastStreakAt: server-only (via Cloud Functions or admin SDK)
+      // - Any other fields: both members can update normally
+      allow update: if isCouplesMember(coupleId)
+        // Check immutable fields haven't changed
+        && isImmutable(['userIds', 'inviteCode', 'createdAt'])
+        // Allow currentQuestionId updates
+        && request.resource.data.diff(resource.data).affectedKeys().hasOnly(['currentQuestionId'])
+        // No other fields should be changed
+        // Check that streakCount and lastStreakAt are not in the update
+        && !request.resource.data.diff(resource.data).affectedKeys().hasAny(['streakCount', 'lastStreakAt']);
+
+      // Delete: server-only (admin SDK only)
+      allow delete: if isServer();
 
       match /sessions/{sessionId} {
-        allow read, write: if isCouplesMember(coupleId);
+        // Read: both members can read
+        allow read: if isCouplesMember(coupleId);
+
+        // Create: either member can start a session
+        allow create: if isCouplesMember(coupleId)
+          && request.resource.data.startedByUserId == request.auth.uid;
+
+        // Update: only the user who started the session can update it, OR valid status transitions
+        allow update: if isCouplesMember(coupleId)
+          // Either the original starter can update
+          && (resource.data.startedByUserId == request.auth.uid
+          // Or status transition is valid: active → completed
+          || (resource.data.status == 'active' && request.resource.data.status == 'completed'))
+          // Check that other fields haven't been tampered with
+          && (request.resource.data.startedByUserId == resource.data.startedByUserId
+              || isServer());
+
+        // Delete: server-only (admin SDK)
+        allow delete: if isServer();
       }
 
       // Question threads live under the couple document.
       match /question_threads/{threadId} {
-        allow read, write: if isCouplesMember(coupleId);
+        // Read: both members can read
+        allow read: if isCouplesMember(coupleId);
+
+        // Create: either member can create a thread
+        allow create: if isCouplesMember(coupleId)
+          && request.resource.data.createdByUserId == request.auth.uid;
+
+        // Update: valid state transitions only, currentIndex only incrementable
+        allow update: if isCouplesMember(coupleId)
+          // Status transitions must be valid: NOT_STARTED → ANSWERED_BY_ONE → REVEALED → COMPLETED
+          && (resource.data.status == 'NOT_STARTED' && request.resource.data.status == 'ANSWERED_BY_ONE'
+           || resource.data.status == 'ANSWERED_BY_ONE' && request.resource.data.status == 'REVEALED'
+           || resource.data.status == 'REVEALED' && request.resource.data.status == 'COMPLETED'
+           || resource.data.status == 'ANSWERED_BY_ONE' && request.resource.data.status == 'COMPLETED')
+          // currentIndex can only be incremented, never decremented or reset
+          && request.resource.data.currentIndex != null
+          && (resource.data.currentIndex == null || request.resource.data.currentIndex >= resource.data.currentIndex)
+          // No other fields should change
+          && request.resource.data.diff(resource.data).affectedKeys().hasOnly(['status', 'currentIndex']);
+
+        // Delete: server-only (admin SDK)
+        allow delete: if isServer();
 
         // Answers: each user writes their own; both members can read all answers.
         match /answers/{userId} {
@@ -115,14 +193,26 @@ service cloud.firestore {
           allow read:   if isCouplesMember(coupleId);
         }
 
-        // Discussion messages: any couple member can write and read.
+        // Discussion messages: any couple member can read, but only the author can write/update/delete
         match /messages/{messageId} {
-          allow read, write: if isCouplesMember(coupleId);
+          allow read:   if isCouplesMember(coupleId);
+          allow create: if isCouplesMember(coupleId)
+            && request.resource.data.authorUserId == request.auth.uid;
+          allow update: if isCouplesMember(coupleId)
+            && resource.data.authorUserId == request.auth.uid;
+          allow delete: if isCouplesMember(coupleId)
+            && resource.data.authorUserId == request.auth.uid;
         }
 
-        // Reactions: any couple member can write and read.
+        // Reactions: any couple member can read, but only the creator can write/update/delete
         match /reactions/{reactionId} {
-          allow read, write: if isCouplesMember(coupleId);
+          allow read:   if isCouplesMember(coupleId);
+          allow create: if isCouplesMember(coupleId)
+            && request.resource.data.userId == request.auth.uid;
+          allow update: if isCouplesMember(coupleId)
+            && resource.data.userId == request.auth.uid;
+          allow delete: if isCouplesMember(coupleId)
+            && resource.data.userId == request.auth.uid;
         }
       }
     }