diff --git a/.github/workflows/deploy-on-merge.yml b/.github/workflows/deploy-on-merge.yml
index 15db35f..953f952 100644
--- a/.github/workflows/deploy-on-merge.yml
+++ b/.github/workflows/deploy-on-merge.yml
@@ -7,6 +7,12 @@ on:
     paths-ignore:
       - "terraform/**"
 
+# Serialize fleet deploys so two quick merges can't run Ansible against the
+# same hosts at once. Don't cancel an in-flight deploy mid-run — queue it.
+concurrency:
+  group: deploy-on-merge
+  cancel-in-progress: false
+
 jobs:
   discover:
     name: Discover hosts
diff --git a/.github/workflows/terraform.yml b/.github/workflows/terraform.yml
index 7d97dda..1e3071f 100644
--- a/.github/workflows/terraform.yml
+++ b/.github/workflows/terraform.yml
@@ -10,6 +10,13 @@ on:
 # Requires these repository secrets:
 #   AGE_SECRET_KEY     — age private key for SOPS decryption
 
+# Serialize Terraform runs so two merges can't apply against the state
+# concurrently. Never cancel an in-flight run (an interrupted apply can
+# corrupt state) — queue instead.
+concurrency:
+  group: terraform-state
+  cancel-in-progress: false
+
 jobs:
   plan:
     name: Plan
diff --git a/terraform/providers.tf b/terraform/providers.tf
index 69f73a5..3260ffe 100644
--- a/terraform/providers.tf
+++ b/terraform/providers.tf
@@ -24,6 +24,10 @@ terraform {
     region                      = "eu-central-003"
     skip_credentials_validation = true
     skip_region_validation      = true
+    # NOTE: no state locking — Backblaze B2's S3 API doesn't implement the
+    # conditional PutObject that OpenTofu's use_lockfile needs (returns 501
+    # NotImplemented). Concurrent applies are instead prevented by the
+    # `concurrency` guard in .github/workflows/terraform.yml.
     # Credentials read from AWS_ACCESS_KEY_ID / AWS_SECRET_ACCESS_KEY env vars
   }
 }