diff --git a/src/components/sharing/DiagramSharingModal.tsx b/src/components/sharing/DiagramSharingModal.tsx
index 0af60aa..fca58be 100644
--- a/src/components/sharing/DiagramSharingModal.tsx
+++ b/src/components/sharing/DiagramSharingModal.tsx
@@ -1,6 +1,6 @@
 import { useEffect, useState } from 'react'
 import type { CollaboratorPermission } from '../../data/DiagramCollaboratorRepository'
-import { UnknownUsernameError } from '../../data/DiagramCollaboratorRepository'
+import { SelfCollaboratorError, UnknownUsernameError } from '../../data/DiagramCollaboratorRepository'
 import { useDiagramCollaboratorStore } from '../../state/diagramCollaboratorStore'
 import { useProjectStore } from '../../state/projectStore'
 import Modal from '../common/Modal'
@@ -36,7 +36,11 @@ export default function DiagramSharingModal({ onClose }: { onClose: () => void }
       await add(diagramId, trimmed, permission)
       setUsername('')
     } catch (err) {
-      setError(err instanceof UnknownUsernameError ? err.message : 'Could not add that person. Try again.')
+      setError(
+        err instanceof UnknownUsernameError || err instanceof SelfCollaboratorError
+          ? err.message
+          : 'Could not add that person. Try again.',
+      )
     } finally {
       setBusy(false)
     }
diff --git a/src/data/DiagramCollaboratorRepository.ts b/src/data/DiagramCollaboratorRepository.ts
index 9df2267..18f470b 100644
--- a/src/data/DiagramCollaboratorRepository.ts
+++ b/src/data/DiagramCollaboratorRepository.ts
@@ -11,6 +11,11 @@ export interface DiagramCollaborator {
  *  not a system failure. */
 export class UnknownUsernameError extends Error {}
 
+/** Thrown by `add` when the resolved username is the diagram owner's own
+ *  account — RLS blocks this at the database level too (the real
+ *  enforcement), this is just a friendlier message than the raw 42501. */
+export class SelfCollaboratorError extends Error {}
+
 /** Storage abstraction for a diagram's collaborator list, per
  *  organized-ideas.md §8: the owner shares with specific people by
  *  username, choosing view or edit access per person. RLS restricts
diff --git a/src/data/SupabaseDiagramCollaboratorRepository.ts b/src/data/SupabaseDiagramCollaboratorRepository.ts
index 1925fe7..9d33c55 100644
--- a/src/data/SupabaseDiagramCollaboratorRepository.ts
+++ b/src/data/SupabaseDiagramCollaboratorRepository.ts
@@ -3,7 +3,7 @@ import type {
   DiagramCollaborator,
   DiagramCollaboratorRepository,
 } from './DiagramCollaboratorRepository'
-import { UnknownUsernameError } from './DiagramCollaboratorRepository'
+import { SelfCollaboratorError, UnknownUsernameError } from './DiagramCollaboratorRepository'
 import { supabase } from './supabaseClient'
 
 interface CollaboratorRow {
@@ -43,6 +43,12 @@ export class SupabaseDiagramCollaboratorRepository implements DiagramCollaborato
     if (!userId) {
       throw new UnknownUsernameError(`No account found for username "${username}".`)
     }
+    const {
+      data: { user: currentUser },
+    } = await supabase.auth.getUser()
+    if (currentUser && userId === currentUser.id) {
+      throw new SelfCollaboratorError("You already have full access to your own diagram — you can't share it with yourself.")
+    }
     const { error } = await supabase.from('diagram_collaborators').insert({ diagram_id: diagramId, user_id: userId, permission })
     if (error) {
       console.error('Failed to add collaborator in Supabase', error)
diff --git a/supabase/migrations/20260914000000_prevent_self_collaborator.sql b/supabase/migrations/20260914000000_prevent_self_collaborator.sql
new file mode 100644
index 0000000..5c04544
--- /dev/null
+++ b/supabase/migrations/20260914000000_prevent_self_collaborator.sql
@@ -0,0 +1,29 @@
+-- Prevents a diagram's owner from ending up as their own collaborator row
+-- (found via manual testing: the UI let you "share" a diagram with
+-- yourself). Ownership already implies full access, so a self-collaborator
+-- row is never meaningful — enforced here, not just in the client, since
+-- the client-side check alone wouldn't stop a Super Admin's "add on behalf
+-- of the owner" path or any other direct API access from creating one.
+
+drop policy "diagram_collaborators_insert" on public.diagram_collaborators;
+
+create policy "diagram_collaborators_insert" on public.diagram_collaborators for insert
+  with check (
+    (public.is_super_admin() or public.diagram_owner_id(diagram_id) = auth.uid())
+    and user_id != public.diagram_owner_id(diagram_id)
+  );
+
+-- The existing update policy had no WITH CHECK at all (only USING) — added
+-- here too, defensively, in case user_id (part of the primary key) is ever
+-- changed via UPDATE rather than delete+insert.
+drop policy "diagram_collaborators_update" on public.diagram_collaborators;
+
+create policy "diagram_collaborators_update" on public.diagram_collaborators for update
+  using (
+    public.is_super_admin()
+    or public.diagram_owner_id(diagram_id) = auth.uid()
+  )
+  with check (
+    (public.is_super_admin() or public.diagram_owner_id(diagram_id) = auth.uid())
+    and user_id != public.diagram_owner_id(diagram_id)
+  );
diff --git a/supabase/tests/rls.sql b/supabase/tests/rls.sql
index d80b804..9ed0466 100644
--- a/supabase/tests/rls.sql
+++ b/supabase/tests/rls.sql
@@ -25,7 +25,7 @@ begin;
 
 create extension if not exists pgtap with schema extensions;
 
-select plan(52);
+select plan(53);
 
 -- ----------------------------------------------------------------------
 -- Fixtures (as postgres — RLS does not apply)
@@ -138,6 +138,13 @@ select throws_ok(
 
 select set_config('request.jwt.claim.sub', '11111111-1111-1111-1111-111111111111', true);
 
+select throws_ok(
+  $$ insert into public.diagram_collaborators (diagram_id, user_id, permission)
+     values ('b0000000-0000-0000-0000-000000000001', '11111111-1111-1111-1111-111111111111', 'edit') $$,
+  '42501'::char(5), null,
+  'alice (owner) cannot add herself as a collaborator on her own diagram'
+);
+
 select lives_ok(
   $$ insert into public.diagram_collaborators (diagram_id, user_id, permission)
      values ('b0000000-0000-0000-0000-000000000001', '22222222-2222-2222-2222-222222222222', 'view') $$,