diff --git a/.github/workflows/cancel-closed-pr-ci.yml b/.github/workflows/cancel-closed-pr-ci.yml
new file mode 100644
index 000000000..e52227fcc
--- /dev/null
+++ b/.github/workflows/cancel-closed-pr-ci.yml
@@ -0,0 +1,69 @@
+name: Cancel CI for closed PRs
+
+# A PR's concurrency group only cancels its in-flight runs when a newer run of
+# the same PR starts. Merging or closing starts none, so without this the runs
+# keep holding the GPU runners until they finish.
+#
+# Why a sweep rather than reacting to the close event:
+# - pull_request: closed sets GITHUB_REF to refs/heads/main on merge, so the
+#   workflows' cancel-in-progress is false in exactly the case that matters,
+#   and a fork contributor's run of it waits for maintainer approval.
+# - pull_request_target is never held for approval, but it is a HIGH
+#   dangerous-triggers finding in the security scan.
+# A scheduled run comes from main, is never held for approval, and needs no
+# exception. The cost is up to one interval before a closed PR's runs stop.
+on:
+  schedule:
+    - cron: "7-59/10 * * * *"
+  workflow_dispatch:
+
+permissions:
+  actions: write
+  pull-requests: read
+
+concurrency:
+  group: ${{ github.workflow }}
+  cancel-in-progress: false
+
+jobs:
+  cancel:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - name: Cancel in-flight runs of closed PRs
+        env:
+          GH_TOKEN: ${{ github.token }}
+          REPO: ${{ github.repository }}
+        run: |
+          set -euo pipefail
+          inflight=$(for status in queued in_progress waiting pending; do
+            gh api --paginate "repos/$REPO/actions/runs?event=pull_request&status=$status&per_page=100" \
+              --jq '.workflow_runs[] | [.id, .name, .head_repository.full_name, .head_branch, .head_sha] | @tsv'
+          done)
+          if [ -z "$inflight" ]; then
+            echo "No in-flight PR runs"
+            exit 0
+          fi
+
+          # Branch names come from PR authors: they are only ever data here,
+          # URL-encoded for the query and compared, never evaluated.
+          declare -A pr_state
+          while IFS=$'\t' read -r id name head_repo branch sha; do
+            key="$head_repo:$branch:$sha"
+            if [ -z "${pr_state[$key]+set}" ]; then
+              head=$(printf '%s:%s' "${head_repo%%/*}" "$branch" | jq -sRr @uri)
+              # Fork runs carry no PR number, so find the PR by its head branch.
+              # Only a PR whose head is this commit counts, and any open one wins.
+              pr_state[$key]=$(SHA="$sha" gh api "repos/$REPO/pulls?state=all&head=$head&per_page=100" \
+                --jq '[.[] | select(.head.sha == env.SHA) | .state] | if index("open") then "open" elif length > 0 then "closed" else "none" end')
+            fi
+            if [ "${pr_state[$key]}" != "closed" ]; then
+              continue
+            fi
+            # A run can finish between listing and cancelling; that's fine.
+            if gh api -X POST "repos/$REPO/actions/runs/$id/cancel" > /dev/null 2>&1; then
+              echo "Cancelled $name ($id) from closed PR $head_repo:$branch"
+            else
+              echo "Could not cancel $name ($id); it has probably just finished"
+            fi
+          done <<< "$inflight"
diff --git a/.github/workflows/iris-external-validation-test.yml b/.github/workflows/iris-external-validation-test.yml
index aa12811e1..81a9b22ac 100644
--- a/.github/workflows/iris-external-validation-test.yml
+++ b/.github/workflows/iris-external-validation-test.yml
@@ -8,7 +8,8 @@ on:
   workflow_dispatch:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  # PR number, not head branch: two forks' "patch-1" branches must not cancel each other.
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
 
 env:
diff --git a/.github/workflows/iris-performance-regression-test.yml b/.github/workflows/iris-performance-regression-test.yml
index 96e694493..d760065dd 100644
--- a/.github/workflows/iris-performance-regression-test.yml
+++ b/.github/workflows/iris-performance-regression-test.yml
@@ -8,7 +8,8 @@ on:
   workflow_dispatch:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  # PR number, not head branch: two forks' "patch-1" branches must not cancel each other.
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
 
 env:
diff --git a/.github/workflows/iris-tests.yml b/.github/workflows/iris-tests.yml
index c635b986a..7ea57ef58 100644
--- a/.github/workflows/iris-tests.yml
+++ b/.github/workflows/iris-tests.yml
@@ -8,7 +8,8 @@ on:
   workflow_dispatch:
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  # PR number, not head branch: two forks' "patch-1" branches must not cancel each other.
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
 
 env: