diff --git a/README.md b/README.md
index a75ab7a..4413118 100644
--- a/README.md
+++ b/README.md
@@ -33,6 +33,7 @@ No modules.
 | [cloudflare_dns_record.xnoto_dev_root](https://registry.terraform.io/providers/cloudflare/cloudflare/latest/docs/resources/dns_record) | resource |
 | [cloudflare_dns_record.xnoto_dev_www](https://registry.terraform.io/providers/cloudflare/cloudflare/latest/docs/resources/dns_record) | resource |
 | [cloudflare_ruleset.cache_rules](https://registry.terraform.io/providers/cloudflare/cloudflare/latest/docs/resources/ruleset) | resource |
+| [cloudflare_ruleset.response_headers](https://registry.terraform.io/providers/cloudflare/cloudflare/latest/docs/resources/ruleset) | resource |
 | [cloudflare_zero_trust_access_application.alertmanager](https://registry.terraform.io/providers/cloudflare/cloudflare/latest/docs/resources/zero_trust_access_application) | resource |
 | [cloudflare_zero_trust_access_application.grafana_alerts](https://registry.terraform.io/providers/cloudflare/cloudflare/latest/docs/resources/zero_trust_access_application) | resource |
 | [cloudflare_zero_trust_access_application.k3s](https://registry.terraform.io/providers/cloudflare/cloudflare/latest/docs/resources/zero_trust_access_application) | resource |
@@ -101,3 +102,27 @@ This root owns only the bootstrap tunnel DNS required to reach the Kubernetes
 API before cluster workloads are available. `TunnelBinding` owns workload
 tunnel DNS, including the MCP gateway endpoints. Do not add a workload hostname
 to this OpenTofu root.
+
+## OpenCode HSTS
+
+`cf-opencode-hsts.tf` configures the zone response-header ruleset with a rule
+matching only HTTPS requests to `opencode.makeitwork.cloud`. It sets
+`Strict-Transport-Security: max-age=86400` (one day), without
+`includeSubDomains` or `preload`. Native OpenCode authentication and existing
+HTTPS redirection are unchanged. No Cloudflare Access, workload DNS/tunnel,
+or AWX cleanup is included.
+
+This root owns the response-header entrypoint. Review and preserve its full
+rule list before adding other header rules; do not create a competing
+entrypoint or adopt an existing one without reviewing its rules. PR CI plans
+validate the proposed changes; `main` runs a fresh environment-associated
+apply, not the saved PR plan. Merge and apply authorization are separate.
+
+After approved apply, verify the HTTPS UI and unauthenticated API responses
+carry the header, native phone login and session continuity still work, and
+sibling hostnames are unaffected. A successful plan is not live verification.
+
+For rollback, first serve `max-age=0` over HTTPS through this rule, then remove
+it after the rollback has been reviewed and applied. Simply removing the
+header does not clear browser policies; they otherwise expire up to one day
+after their last receipt. Preserve HTTPS throughout.
diff --git a/cf-opencode-hsts.tf b/cf-opencode-hsts.tf
new file mode 100644
index 0000000..b142597
--- /dev/null
+++ b/cf-opencode-hsts.tf
@@ -0,0 +1,25 @@
+resource "cloudflare_ruleset" "response_headers" {
+  zone_id     = local.zone_id
+  name        = "Response header transforms"
+  description = "Hostname-scoped response security headers"
+  kind        = "zone"
+  phase       = "http_response_headers_transform"
+
+  rules = [
+    {
+      ref    = "opencode_hsts"
+      action = "rewrite"
+      action_parameters = {
+        headers = {
+          "strict-transport-security" = {
+            operation = "set"
+            value     = "max-age=86400"
+          }
+        }
+      }
+      expression  = "(http.host eq \"opencode.makeitwork.cloud\" and http.request.scheme eq \"https\")"
+      description = "One-day HSTS for the OpenCode HTTPS hostname only"
+      enabled     = true
+    }
+  ]
+}