diff --git a/src/components/project/handlers/set-department-id.handler.ts b/src/components/project/handlers/set-department-id.handler.ts
index b11664f077..2fd2e7d697 100644
--- a/src/components/project/handlers/set-department-id.handler.ts
+++ b/src/components/project/handlers/set-department-id.handler.ts
@@ -97,15 +97,21 @@ export class SetDepartmentId implements IEventHandler<SubscribedEvent> {
       // Get used IDs
       .subQuery((sub) =>
         sub
-          .match([
-            node('', 'Project'),
-            relation('out', '', 'departmentId', ACTIVE),
-            node('deptIdNode', 'Property'),
-          ])
-          .where({ 'deptIdNode.value': not(isNull()) })
-          .return(collect('deptIdNode.value').as('used')),
+          .subQuery((sub2) =>
+            sub2
+              .match([
+                node('', 'Project'),
+                relation('out', '', 'departmentId', ACTIVE),
+                node('deptIdNode', 'Property'),
+              ])
+              .where({ 'deptIdNode.value': not(isNull()) })
+              .return('deptIdNode.value as id')
+              .union()
+              .match(node('external', 'ExternalDepartmentId'))
+              .return('external.departmentId as id'),
+          )
+          .return(collect('id').as('used')),
       )
-      // Distill to available
       .with('[id in enumerated where not id in used][0] as next')
       // collapse cardinality to zero if none available
       .raw('unwind next as nextId')
diff --git a/src/components/project/migrations/create-used-dept-id-list.migration.ts b/src/components/project/migrations/create-used-dept-id-list.migration.ts
new file mode 100644
index 0000000000..2780316e34
--- /dev/null
+++ b/src/components/project/migrations/create-used-dept-id-list.migration.ts
@@ -0,0 +1,51 @@
+import { node } from 'cypher-query-builder';
+import { readFile } from 'node:fs/promises';
+import { BaseMigration, Migration } from '~/core/database';
+import { apoc, variable } from '~/core/database/query';
+
+interface ExternalDepartmentId {
+  id: string;
+  name?: string;
+}
+
+@Migration('2025-09-17T09:00:00')
+export class CreateUsedDeptIdListMigration extends BaseMigration {
+  async up() {
+    const intaactFilePath = new URL(
+      '../../../../../.vscode/localFiles/All-Department-Ids-From-Intaact.csv',
+      import.meta.url,
+    );
+    const cordFilePath = new URL(
+      '../../../../../.vscode/localFiles/prod_deptIds_ids_only.csv',
+      import.meta.url,
+    );
+
+    const intaactFileContent = await readFile(intaactFilePath, 'utf-8');
+    const cordFileContent = await readFile(cordFilePath, 'utf-8');
+
+    const intaactRows = intaactFileContent.trim().split(/\r?\n/).slice(1); // Skip header
+    const cordRows = cordFileContent.trim().split(/\r?\n/).slice(1); // Skip header
+
+    const intaactList: ExternalDepartmentId[] = intaactRows.flatMap((row) => {
+      const [id, name] = row.split(',');
+      return id ? { id, name } : [];
+    });
+
+    const prunedIntaactList = intaactList.flatMap((row) =>
+      !cordRows.includes(row.id) ? row : [],
+    );
+
+    await this.db
+      .query()
+      .unwind(prunedIntaactList, 'dept')
+      .create(node('external', 'ExternalDepartmentId'))
+      .setValues({
+        'external.id': variable(apoc.create.uuid()),
+        'external.departmentId': variable('dept.id'),
+        'external.name': variable('dept.name'),
+        'external.createdAt': variable('datetime()'),
+      })
+      .return('count(external) as created')
+      .executeAndLogStats();
+  }
+}
diff --git a/src/components/project/project.module.ts b/src/components/project/project.module.ts
index a798a66a78..1cc0f7d400 100644
--- a/src/components/project/project.module.ts
+++ b/src/components/project/project.module.ts
@@ -14,6 +14,7 @@ import { ProjectEngagementConnectionResolver } from './engagement-connection.res
 import { FinancialApproverModule } from './financial-approver/financial-approver.module';
 import * as handlers from './handlers';
 import { InternshipProjectResolver } from './internship-project.resolver';
+import { CreateUsedDeptIdListMigration } from './migrations/create-used-dept-id-list.migration';
 import { FixDeptIdLabelMigration } from './migrations/fix-dept-id-label.migration';
 import { RenameTranslationToMomentumMigration } from './migrations/rename-translation-to-momentum.migration';
 import { ProjectEngagementIdResolvers } from './project-engagement-id.resolver';
@@ -55,6 +56,7 @@ import { ProjectWorkflowModule } from './workflow/project-workflow.module';
     ...Object.values(ConcreteRepos),
     ProjectLoader,
     ...Object.values(handlers),
+    CreateUsedDeptIdListMigration,
     RenameTranslationToMomentumMigration,
     FixDeptIdLabelMigration,
   ],