diff --git a/.github/workflows/elixir-ci.yml b/.github/workflows/elixir-ci.yml
index 2ac0420..53ba7a3 100644
--- a/.github/workflows/elixir-ci.yml
+++ b/.github/workflows/elixir-ci.yml
@@ -3,7 +3,16 @@
 # See standards#174 for the reusable's purpose.
 
 name: Elixir CI
-on: [push, pull_request]
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+
+# Estate guardrail: scope push to default branches so a PR fires once (not
+# push+PR), and cancel superseded runs. Safe — read-only PR-triggered check.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 
 permissions:
   contents: read
diff --git a/.github/workflows/rescript-deno-ci.yml b/.github/workflows/rescript-deno-ci.yml
index a9d8c07..15c6056 100644
--- a/.github/workflows/rescript-deno-ci.yml
+++ b/.github/workflows/rescript-deno-ci.yml
@@ -3,7 +3,16 @@ permissions:
   contents: read
 
 name: Deno CI
-on: [push, pull_request]
+on:
+  push:
+    branches: [main, master]
+  pull_request:
+
+# Estate guardrail: scope push to default branches so a PR fires once (not
+# push+PR), and cancel superseded runs. Safe — read-only PR-triggered check.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 
 jobs:
   build: