diff --git a/apps/backend/src/lib/seed-dummy-data.ts b/apps/backend/src/lib/seed-dummy-data.ts
index 5b5784278..479cd1088 100644
--- a/apps/backend/src/lib/seed-dummy-data.ts
+++ b/apps/backend/src/lib/seed-dummy-data.ts
@@ -1983,7 +1983,7 @@ async function seedSamlConnections(projectId: string): Promise<void> {
   // dot-keys — config normalization with onDotIntoNonObject="ignore"
   // drops dot-keys that try to navigate into a record entry that
   // doesn't yet exist (same convention as auth.oauth.providers).
-  const overlay: Record<string, unknown> = {};
+  const overlay: Parameters<typeof overrideEnvironmentConfigOverride>[0]["environmentConfigOverrideOverride"] = {};
   for (const f of fetched) {
     overlay[`auth.saml.connections.${f.slug}`] = {
       displayName: f.displayName,
@@ -1998,7 +1998,7 @@ async function seedSamlConnections(projectId: string): Promise<void> {
   await overrideEnvironmentConfigOverride({
     projectId,
     branchId: DEFAULT_BRANCH_ID,
-    environmentConfigOverrideOverride: overlay as Parameters<typeof overrideEnvironmentConfigOverride>[0]["environmentConfigOverrideOverride"],
+    environmentConfigOverrideOverride: overlay,
   });
 }
 
@@ -2131,7 +2131,6 @@ export async function seedDummyProject(options: SeedDummyProjectOptions): Promis
         "payments.testMode": true,
       },
     }),
-    ...(getEnvVariable("STACK_SEED_ENABLE_SAML", "false") === "true" ? [seedSamlConnections(projectId)] : []),
     ...options.skipGithubConfigSource ? [] : [setBranchConfigOverrideSource({
       projectId,
       branchId: DEFAULT_BRANCH_ID,
@@ -2154,6 +2153,16 @@ export async function seedDummyProject(options: SeedDummyProjectOptions): Promis
     }),
   ]);
 
+  // Run sequentially after the parallel block. Both this and the
+  // `payments.testMode` write above target the same environment config,
+  // and `overrideEnvironmentConfigOverride` is read-modify-write — running
+  // them in parallel races and one write clobbers the other (TODO at
+  // config.ts:491 already documents this). Sequencing avoids the race
+  // until the underlying override is wrapped in a serializable txn.
+  if (getEnvVariable("STACK_SEED_ENABLE_SAML", "false") === "true") {
+    await seedSamlConnections(projectId);
+  }
+
   await seedDummyTransactions({
     prisma: dummyPrisma,
     tenancyId: dummyTenancy.id,