diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c13edc21..aabecbb7 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -14,7 +14,7 @@ permissions:
 
 jobs:
   ci:
-    if: (github.event_name != 'pull_request' && github.event_name != 'pull_request_target') || github.head_ref == 'trunk'
+    if: (github.event_name != 'pull_request' && github.event_name != 'pull_request_target') || (github.head_ref == 'trunk' && github.event.pull_request.head.repo.full_name == github.repository && github.base_ref == 'main')
     name: Lint, Build & Test
     runs-on: ubuntu-latest
     steps:
@@ -33,7 +33,7 @@ jobs:
       - run: npm run test:container
 
   container:
-    if: (github.event_name != 'pull_request' && github.event_name != 'pull_request_target') || github.head_ref == 'trunk'
+    if: (github.event_name != 'pull_request' && github.event_name != 'pull_request_target') || (github.head_ref == 'trunk' && github.event.pull_request.head.repo.full_name == github.repository && github.base_ref == 'main')
     name: Container (${{ matrix.platform }})
     runs-on: ubuntu-latest
     timeout-minutes: 25
@@ -134,7 +134,7 @@ jobs:
         run: docker rm --force --volumes relaycast-ci-${{ matrix.arch }} 2>/dev/null || true
 
   rust-sdk:
-    if: (github.event_name != 'pull_request' && github.event_name != 'pull_request_target') || github.head_ref == 'trunk'
+    if: (github.event_name != 'pull_request' && github.event_name != 'pull_request_target') || (github.head_ref == 'trunk' && github.event.pull_request.head.repo.full_name == github.repository && github.base_ref == 'main')
     name: Rust SDK
     runs-on: ubuntu-latest
     steps:
diff --git a/.github/workflows/container-integration.yml b/.github/workflows/container-integration.yml
index 1303e4d8..21d46e28 100644
--- a/.github/workflows/container-integration.yml
+++ b/.github/workflows/container-integration.yml
@@ -47,7 +47,7 @@ permissions:
 
 jobs:
   container-integration:
-    if: (github.event_name != 'pull_request' && github.event_name != 'pull_request_target') || github.head_ref == 'trunk'
+    if: (github.event_name != 'pull_request' && github.event_name != 'pull_request_target') || (github.head_ref == 'trunk' && github.event.pull_request.head.repo.full_name == github.repository && github.base_ref == 'main')
     name: Build and test the real image
     runs-on: ubuntu-latest
     timeout-minutes: 15
diff --git a/.github/workflows/preview.yml b/.github/workflows/preview.yml
index 63d2a521..f0568d21 100644
--- a/.github/workflows/preview.yml
+++ b/.github/workflows/preview.yml
@@ -17,7 +17,7 @@ jobs:
   cleanup-preview:
     name: Cleanup Preview
     if: >-
-      ((github.event_name != 'pull_request' && github.event_name != 'pull_request_target') || github.head_ref == 'trunk') &&
+      ((github.event_name != 'pull_request' && github.event_name != 'pull_request_target') || (github.head_ref == 'trunk' && github.event.pull_request.head.repo.full_name == github.repository && github.base_ref == 'main')) &&
       (github.event.action == 'closed' && (github.event.pull_request.merged == true || github.event.pull_request.state == 'closed') && github.event.pull_request.head.repo.full_name == github.repository)
     runs-on: ubuntu-latest
     env:
diff --git a/.github/workflows/trunk-guard.yml b/.github/workflows/trunk-guard.yml
index 2ddb4975..fbe379e4 100644
--- a/.github/workflows/trunk-guard.yml
+++ b/.github/workflows/trunk-guard.yml
@@ -1,17 +1,22 @@
 name: Trunk guard
 
-# Only the `trunk` branch may merge into main. Feature branches merge into
-# `trunk` (via the merge train); CI runs only on the trunk -> main PR.
+# Only this repository's `trunk` branch may merge into main. Feature branches
+# merge into `trunk`; CI runs only on the trunk -> main PR.
+# pull_request_target loads this workflow from the base branch, so a PR cannot
+# edit the guard to pass itself. It never checks out PR code.
 on:
-  pull_request:
+  pull_request_target:
     branches: [main]
+    types: [opened, synchronize, reopened, edited]
+
+permissions: {}
 
 jobs:
   only-trunk-merges-to-main:
-    if: github.head_ref != 'trunk'
+    if: github.head_ref != 'trunk' || github.event.pull_request.head.repo.full_name != github.repository
     runs-on: ubuntu-latest
     timeout-minutes: 1
     steps:
       - run: |
-          echo "::error::Only the 'trunk' branch may merge into main. Retarget this PR to 'trunk'."
+          echo "::error::Only this repository's 'trunk' branch may merge into main. Retarget this PR to 'trunk'."
           exit 1