diff --git a/.github/workflows/ci-closed-pr-caches.yml b/.github/workflows/ci-closed-pr-caches.yml
index 38cf5803a14..468559ccd4d 100644
--- a/.github/workflows/ci-closed-pr-caches.yml
+++ b/.github/workflows/ci-closed-pr-caches.yml
@@ -1,4 +1,4 @@
-name: Clean closed PR caches
+name: Clean closed PR work
 
 on:
   pull_request_target:
@@ -6,14 +6,81 @@ on:
 
 permissions:
   actions: write
+  pull-requests: read
 
 jobs:
   clean:
     runs-on: ubuntu-latest
     timeout-minutes: 5
     steps:
+      - name: Cancel checks for an unmerged closed PR
+        if: github.event.pull_request.merged == false
+        uses: actions/github-script@v8
+        with:
+          script: |
+            const closed = context.payload.pull_request
+            const targets = [
+              ['pr-checks', 'pr.yml'],
+              ['node-server', 'node-server-tests.yml'],
+              ['ssh-windows-hosts', 'ssh-windows-hosts.yml'],
+              ['ssh-hostile-hosts', 'ssh-hostile-hosts.yml'],
+              ['mobile', 'mobile.yml'],
+              ['computer-e2e', 'computer-e2e.yml']
+            ]
+            const stillClosed = async () => {
+              const { data: current } = await github.rest.pulls.get({
+                ...context.repo, pull_number: closed.number
+              })
+              return current.state === 'closed' && current.merged_at === null &&
+                current.closed_at === closed.closed_at
+            }
+            if (!Number.isSafeInteger(closed.number) || closed.number <= 0 ||
+                !Number.isFinite(Date.parse(closed.closed_at))) {
+              throw new Error('Missing closed PR identity')
+            }
+            if (!await stillClosed()) return
+            for (const [prefix, workflow] of targets) {
+              const group = `${prefix}-${closed.number}`
+              let data
+              try {
+                const response = await github.request(
+                  'GET /repos/{owner}/{repo}/actions/concurrency_groups/{concurrency_group_name}', {
+                    ...context.repo, concurrency_group_name: group,
+                    headers: { 'X-GitHub-Api-Version': '2026-03-10' }
+                  }
+                )
+                data = response.data
+              } catch (error) {
+                if (error.status === 404) continue
+                throw error
+              }
+              if (data.group_name !== group || !Array.isArray(data.group_members)) {
+                throw new Error(`Unexpected concurrency group: ${group}`)
+              }
+              for (const member of data.group_members) {
+                // Only whole PR runs; release/manual jobs never share this identity.
+                if (member.job_id !== undefined || !Number.isSafeInteger(member.run_id)) continue
+                const { data: run } = await github.rest.actions.getWorkflowRun({
+                  ...context.repo, run_id: member.run_id
+                })
+                if (run.event !== 'pull_request' || run.path !== `.github/workflows/${workflow}` ||
+                    run.status === 'completed' ||
+                    !Number.isFinite(Date.parse(run.created_at)) ||
+                    Date.parse(run.created_at) > Date.parse(closed.closed_at)) continue
+                if (!await stillClosed()) return
+                try {
+                  await github.rest.actions.cancelWorkflowRun({
+                    ...context.repo, run_id: member.run_id
+                  })
+                  core.info(`Requested cancellation of ${group}: ${member.run_id}`)
+                } catch (error) {
+                  if (error.status !== 409) throw error
+                }
+              }
+            }
       # No checkout: this runs trusted default-branch code, including for fork PRs.
       - uses: actions/github-script@v8
+        if: '!cancelled()'
         with:
           script: |
             const ref = `refs/pull/${context.payload.pull_request.number}/merge`
diff --git a/config/scripts/ci-closed-pr-caches.test.mjs b/config/scripts/ci-closed-pr-caches.test.mjs
index 44edb5e48df..dbab17e889a 100644
--- a/config/scripts/ci-closed-pr-caches.test.mjs
+++ b/config/scripts/ci-closed-pr-caches.test.mjs
@@ -4,7 +4,7 @@ import { expect, it, vi } from 'vitest'
 import { parse } from 'yaml'
 
 const workflow = parse(readFileSync('.github/workflows/ci-closed-pr-caches.yml', 'utf8'))
-const script = workflow.jobs.clean.steps[0].with.script
+const script = workflow.jobs.clean.steps[1].with.script
 const ref = 'refs/pull/123/merge'
 
 function run(caches, remove = vi.fn().mockResolvedValue(undefined)) {
@@ -23,9 +23,11 @@ function run(caches, remove = vi.fn().mockResolvedValue(undefined)) {
 
 it('uses default-branch code without checking out a closed PR', () => {
   expect(workflow.on).toEqual({ pull_request_target: { types: ['closed'] } })
-  expect(workflow.permissions).toEqual({ actions: 'write' })
-  expect(workflow.jobs.clean.steps).toHaveLength(1)
-  expect(workflow.jobs.clean.steps[0].uses).toBe('actions/github-script@v8')
+  expect(workflow.permissions).toEqual({ actions: 'write', 'pull-requests': 'read' })
+  expect(workflow.jobs.clean.steps).toHaveLength(2)
+  expect(workflow.jobs.clean.steps.every((step) => step.uses === 'actions/github-script@v8')).toBe(
+    true
+  )
 })
 
 it('lists and deletes only caches scoped to the closed merge ref', async () => {
diff --git a/config/scripts/ci-closed-pr-cancellation.test.mjs b/config/scripts/ci-closed-pr-cancellation.test.mjs
new file mode 100644
index 00000000000..13f933932ca
--- /dev/null
+++ b/config/scripts/ci-closed-pr-cancellation.test.mjs
@@ -0,0 +1,152 @@
+import { readFileSync } from 'node:fs'
+import { runInNewContext } from 'node:vm'
+import { expect, it, vi } from 'vitest'
+import { parse } from 'yaml'
+
+const workflow = parse(readFileSync('.github/workflows/ci-closed-pr-caches.yml', 'utf8'))
+const step = workflow.jobs.clean.steps[0]
+const closed = { number: 123, closed_at: '2026-10-06T00:00:00Z' }
+const current = { ...closed, state: 'closed', merged_at: null }
+const run = {
+  id: 42,
+  event: 'pull_request',
+  path: '.github/workflows/pr.yml',
+  status: 'in_progress',
+  created_at: '2026-10-05T23:59:00Z'
+}
+
+function execute(options = {}) {
+  const request = vi.fn(async (_route, args) => {
+    if (options.requestError) {
+      throw options.requestError
+    }
+    if (args.concurrency_group_name !== 'pr-checks-123') {
+      throw { status: 404 }
+    }
+    return {
+      data: {
+        group_name: options.group ?? 'pr-checks-123',
+        group_members: options.members ?? [{ run_id: 42 }]
+      }
+    }
+  })
+  const getPr = vi.fn(async () => ({
+    data: options.current ?? current
+  }))
+  if (options.reopened) {
+    getPr.mockResolvedValueOnce({ data: current })
+  }
+  const getRun = vi.fn(async () => ({ data: { ...run, ...options.run } }))
+  const cancel = vi.fn(async () => {
+    if (options.cancelError) {
+      throw options.cancelError
+    }
+  })
+  const result = runInNewContext(`(async () => { ${step.with.script} })()`, {
+    context: {
+      repo: { owner: 'owner', repo: 'repo' },
+      payload: { pull_request: options.closed ?? closed }
+    },
+    github: {
+      request,
+      rest: {
+        pulls: { get: getPr },
+        actions: { getWorkflowRun: getRun, cancelWorkflowRun: cancel }
+      }
+    },
+    core: { info: vi.fn() }
+  })
+  return { result, request, getPr, getRun, cancel }
+}
+
+it('uses trusted inline code and only enters cancellation on an unmerged close', () => {
+  expect(workflow.on).toEqual({ pull_request_target: { types: ['closed'] } })
+  expect(step.if).toBe('github.event.pull_request.merged == false')
+  expect(step.uses).toBe('actions/github-script@v8')
+  expect(
+    workflow.jobs.clean.steps.some((entry) => entry.uses?.startsWith('actions/checkout'))
+  ).toBe(false)
+})
+
+it('looks up exact PR groups without branch or head-SHA inference', async () => {
+  const { result, request, cancel } = execute()
+  await result
+  expect(request.mock.calls.map(([, args]) => args.concurrency_group_name)).toEqual([
+    'pr-checks-123',
+    'node-server-123',
+    'ssh-windows-hosts-123',
+    'ssh-hostile-hosts-123',
+    'mobile-123',
+    'computer-e2e-123'
+  ])
+  expect(
+    request.mock.calls.every(
+      ([route, args]) =>
+        route === 'GET /repos/{owner}/{repo}/actions/concurrency_groups/{concurrency_group_name}' &&
+        args.headers['X-GitHub-Api-Version'] === '2026-03-10'
+    )
+  ).toBe(true)
+  expect(cancel.mock.calls).toEqual([[{ owner: 'owner', repo: 'repo', run_id: 42 }]])
+})
+
+it.each([
+  { event: 'push' },
+  { event: 'workflow_dispatch' },
+  { path: '.github/workflows/release-cut.yml' },
+  { status: 'completed' },
+  { created_at: '2026-10-06T00:00:01Z' },
+  { created_at: 'invalid' }
+])('retains unrelated, completed and post-close runs: %j', async (otherRun) => {
+  const { result, cancel } = execute({ run: otherRun })
+  await result
+  expect(cancel).not.toHaveBeenCalled()
+})
+
+it.each([
+  { ...current, state: 'open' },
+  { ...current, merged_at: closed.closed_at },
+  { ...current, closed_at: '2026-10-06T00:01:00Z' }
+])('retains checks if the closure is no longer current: %j', async (pr) => {
+  const { result, request, cancel } = execute({ current: pr })
+  await result
+  expect(request).not.toHaveBeenCalled()
+  expect(cancel).not.toHaveBeenCalled()
+})
+
+it('rechecks closure before cancelling when a PR reopens during lookup', async () => {
+  const { result, getRun, cancel } = execute({
+    current: { ...current, state: 'open' },
+    reopened: true
+  })
+  await result
+  expect(getRun).toHaveBeenCalledOnce()
+  expect(cancel).not.toHaveBeenCalled()
+})
+
+it('ignores job-level leases and invalid run identities', async () => {
+  const { result, getRun, cancel } = execute({
+    members: [{ run_id: 42, job_id: 1 }, { run_id: '42' }]
+  })
+  await result
+  expect(getRun).not.toHaveBeenCalled()
+  expect(cancel).not.toHaveBeenCalled()
+})
+
+it('refuses a mismatched concurrency group', async () => {
+  const { result, cancel } = execute({ group: 'pr-checks-124' })
+  await expect(result).rejects.toThrow('Unexpected concurrency group')
+  expect(cancel).not.toHaveBeenCalled()
+})
+
+it('surfaces lookup and cancellation permission failures', async () => {
+  await expect(execute({ requestError: { status: 403 } }).result).rejects.toEqual({ status: 403 })
+  await expect(execute({ cancelError: { status: 403 } }).result).rejects.toEqual({ status: 403 })
+  await expect(execute({ cancelError: { status: 409 } }).result).resolves.toBeUndefined()
+})
+
+it('validates the event identity before looking up any work', async () => {
+  const { result, request, getPr } = execute({ closed: { ...closed, number: '123' } })
+  await expect(result).rejects.toThrow('Missing closed PR identity')
+  expect(request).not.toHaveBeenCalled()
+  expect(getPr).not.toHaveBeenCalled()
+})