diff --git a/.circleci/config.yml b/.circleci/config.yml
index 4c56948a5c..6c525fcd11 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -4,6 +4,14 @@ version: 2.1
 orbs:
   win: circleci/windows@5.0
 
+parameters:
+  # Set only by .github/workflows/fork-ci-mirror.yml through the CircleCI
+  # trigger API, after a maintainer approved a fork PR head with the ci:run
+  # label. GitHub App pipelines never fire for fork PRs on their own.
+  fork_pr:
+    type: string
+    default: ""
+
 jobs:
   pr-check:
     executor:
@@ -22,6 +30,7 @@ jobs:
       # from the environment with [AllowEmptyString] defaults.
       CBX_PR_URL: << pipeline.event.context.github.pr_url >>
       CBX_PR_BASE_SHA: << pipeline.event.github.pull_request.base.sha >>
+      CBX_FORK_PR: << pipeline.parameters.fork_pr >>
     steps:
       - checkout
       - run:
@@ -81,7 +90,8 @@ workflows:
     # Semantic CI boundary: release-integration micro PRs target port/upstream-*.
     # A micro-named head aimed directly at main still receives full hosted CI.
     # Guard the PR-only value behind event.name so push pipelines compile safely.
-    when: pipeline.event.name == "push" or (pipeline.event.name == "pull_request" and not (pipeline.event.github.pull_request.base.ref starts-with "port/upstream-"))
+    # API pipelines run only with fork_pr set (the fork-ci-mirror workflow).
+    when: pipeline.event.name == "push" or (pipeline.event.name == "pull_request" and not (pipeline.event.github.pull_request.base.ref starts-with "port/upstream-")) or pipeline.parameters.fork_pr != ""
     jobs:
       - pr-check:
           filters:
diff --git a/.github/CI.md b/.github/CI.md
index 2b05fa0687..a42dd22a12 100644
--- a/.github/CI.md
+++ b/.github/CI.md
@@ -45,10 +45,23 @@ build one:
    with this project's environment variables.
 2. Add the `ci:run` label. .github/workflows/fork-ci-mirror.yml pushes
    exactly that head commit to ci/pr-<number> in this repository.
-3. CircleCI builds the ci/pr-<number> push. The trigger gate in
-   scripts/circleci-pr-common.ps1 lets those branches through, and the
-   docs-only gate finds the PR base from the number. The check lands on the
-   same commit, so it shows on the fork PR.
+3. The same workflow starts a CircleCI pipeline through the trigger API,
+   with the config from main and the code from ci/pr-<number>. A plain push
+   is not enough: CircleCI ignores a branch push that brings no new commits,
+   and the fork head is already in this repository through its pull ref.
+   The pipeline's fork_pr parameter lets it through the trigger gate in
+   scripts/circleci-pr-common.ps1, and the docs-only gate finds the PR base
+   from the number. The check lands on the same commit, so it shows on the
+   fork PR.
+
+One-time setup for step 3:
+
+- Actions secret CIRCLECI_TOKEN: a CircleCI personal API token (User
+  Settings -> Personal API Tokens) from a user with access to this project.
+- Actions variable CIRCLECI_PROJECT_SLUG: Project Settings -> Overview, for
+  example circleci/<org-id>/<project-id>.
+- Actions variable CIRCLECI_PIPELINE_DEFINITION_ID: Project Settings ->
+  Pipelines, the id of the pipeline that runs .circleci/config.yml.
 
 Any new push to the fork PR removes the label; re-add it after reviewing the
 new commits. Closing the PR removes the mirror branch. GITHUB_TOKEN cannot
diff --git a/.github/workflows/fork-ci-mirror.yml b/.github/workflows/fork-ci-mirror.yml
index 0a2d705159..52b9a3ead4 100644
--- a/.github/workflows/fork-ci-mirror.yml
+++ b/.github/workflows/fork-ci-mirror.yml
@@ -3,15 +3,17 @@ name: Fork PR CI mirror
 # CircleCI GitHub App pipelines never fire for pull requests from forks, so
 # fork PRs used to merge with no hosted Windows check. A maintainer approves a
 # fork PR's current head by adding the `ci:run` label. This workflow then
-# copies exactly that commit to ci/pr-<number> in this repository. CircleCI
-# builds the push (scripts/circleci-pr-common.ps1 lets ci/pr-<number> through),
-# and its check lands on the same commit, so it shows on the fork PR.
+# copies exactly that commit to ci/pr-<number> in this repository and starts a
+# CircleCI pipeline for it through the trigger API (secret CIRCLECI_TOKEN,
+# variables CIRCLECI_PROJECT_SLUG and CIRCLECI_PIPELINE_DEFINITION_ID). The
+# check lands on the same commit, so it shows on the fork PR.
 #
 # Safety:
 # - pull_request_target runs this file from the base branch, never the fork's
 #   copy. No step checks out, installs, or executes fork code here; the job
-#   only moves a git ref. The fork code runs later on CircleCI, which is the
-#   reason for the label gate.
+#   only moves a git ref and calls the CircleCI API. The fork code runs later
+#   on CircleCI, which is the reason for the label gate. CIRCLECI_TOKEN is
+#   exposed only to that API step, never to fork code.
 # - Only users with triage access or higher can add labels.
 # - Any new push to the fork PR removes the label, so every new head needs a
 #   fresh maintainer approval before CircleCI builds it.
@@ -60,7 +62,34 @@ jobs:
           test "$(git rev-parse FETCH_HEAD)" = "$HEAD_SHA"
           # The mirror branch belongs to this workflow; a re-approval replaces it.
           git -c http.extraheader="$auth" push --quiet --force origin "${HEAD_SHA}:refs/heads/ci/pr-${PR_NUMBER}"
-          echo "Mirrored ${HEAD_SHA} to ci/pr-${PR_NUMBER}; CircleCI pr-check will report on this commit." >> "$GITHUB_STEP_SUMMARY"
+          echo "Mirrored ${HEAD_SHA} to ci/pr-${PR_NUMBER}." >> "$GITHUB_STEP_SUMMARY"
+
+      # CircleCI ignores a branch push that brings no new commits, and the
+      # fork head is already in this repository through its pull ref. So the
+      # pipeline is started through the trigger API: config from main (the
+      # fork cannot change the CI definition), code from ci/pr-<number>.
+      - name: Start the CircleCI pipeline
+        env:
+          CIRCLECI_TOKEN: ${{ secrets.CIRCLECI_TOKEN }}
+          PROJECT_SLUG: ${{ vars.CIRCLECI_PROJECT_SLUG }}
+          DEFINITION_ID: ${{ vars.CIRCLECI_PIPELINE_DEFINITION_ID }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+        run: |
+          set -euo pipefail
+          if [ -z "$CIRCLECI_TOKEN" ] || [ -z "$PROJECT_SLUG" ] || [ -z "$DEFINITION_ID" ]; then
+            echo "::error::Set secret CIRCLECI_TOKEN and variables CIRCLECI_PROJECT_SLUG and CIRCLECI_PIPELINE_DEFINITION_ID (see .github/CI.md)."
+            exit 1
+          fi
+          body=$(jq -n --arg def "$DEFINITION_ID" --arg pr "$PR_NUMBER" \
+            '{definition_id: $def, config: {branch: "main"}, checkout: {branch: ("ci/pr-" + $pr)}, parameters: {fork_pr: $pr}}')
+          status=$(curl -sS -o response.json -w '%{http_code}' -X POST \
+            -H "Circle-Token: ${CIRCLECI_TOKEN}" -H 'Content-Type: application/json' \
+            --data "$body" "https://circleci.com/api/v2/project/${PROJECT_SLUG}/pipeline/run")
+          if [ "$status" != "201" ]; then
+            echo "::error::CircleCI trigger returned HTTP ${status}: $(jq -r '.message // empty' response.json)"
+            exit 1
+          fi
+          echo "CircleCI pipeline #$(jq -r .number response.json) started for ci/pr-${PR_NUMBER}; pr-check will report on the fork PR's head commit." >> "$GITHUB_STEP_SUMMARY"
 
   revoke:
     name: Require re-approval after a new push
diff --git a/scripts/circleci-pr-common.ps1 b/scripts/circleci-pr-common.ps1
index 53427742da..eb1e401adc 100644
--- a/scripts/circleci-pr-common.ps1
+++ b/scripts/circleci-pr-common.ps1
@@ -34,7 +34,8 @@ function Get-TriggerGateDecision {
     param(
         [AllowEmptyString()][string]$BudgetMode,
         [AllowEmptyString()][string]$Branch,
-        [AllowEmptyString()][string]$PrUrl
+        [AllowEmptyString()][string]$PrUrl,
+        [AllowEmptyString()][string]$ForkPr = ''
     )
 
     # Gate 1 - budget: CI_BUDGET_MODE=off is the emergency stop
@@ -55,7 +56,7 @@ function Get-TriggerGateDecision {
     # those pushes run the checks too.
     $isPr = -not [string]::IsNullOrWhiteSpace($PrUrl)
     $isMainPush = $Branch -in @('main', 'master')
-    $isForkMirror = -not [string]::IsNullOrWhiteSpace((Get-ForkMirrorPrNumber -Branch $Branch))
+    $isForkMirror = -not [string]::IsNullOrWhiteSpace((Get-ForkMirrorPrNumber -Branch $Branch -ForkPr $ForkPr))
     if (-not $isPr -and -not $isMainPush -and -not $isForkMirror) {
         return [pscustomobject]@{
             Skip = $true
@@ -71,12 +72,18 @@ function Get-TriggerGateDecision {
 
 <#
 .SYNOPSIS
-    PR number of a ci/pr-<number> fork-mirror branch, or '' for any other
-    branch. The fork-ci-mirror workflow is the only writer of these branches.
+    PR number of an approved fork mirror, or '' when the pipeline is not one.
+    The fork-ci-mirror workflow is the only writer of ci/pr-<number>
+    branches, and the only caller that sets the fork_pr pipeline parameter
+    (passed in as ForkPr) through the CircleCI trigger API.
 #>
 function Get-ForkMirrorPrNumber {
-    param([AllowEmptyString()][string]$Branch)
+    param(
+        [AllowEmptyString()][string]$Branch,
+        [AllowEmptyString()][string]$ForkPr = ''
+    )
 
+    if ($ForkPr -match '^[1-9]\d*$') { return $ForkPr }
     if ($Branch -match '^ci/pr-([1-9]\d*)$') { return $Matches[1] }
     return ''
 }
diff --git a/scripts/circleci-pr-gates.ps1 b/scripts/circleci-pr-gates.ps1
index 37db10d499..795a35cd37 100644
--- a/scripts/circleci-pr-gates.ps1
+++ b/scripts/circleci-pr-gates.ps1
@@ -42,6 +42,10 @@ param(
     # push pipelines).
     [AllowEmptyString()][string]$PrBaseSha = $env:CBX_PR_BASE_SHA,
 
+    # Pipeline parameter fork_pr, exported as CBX_FORK_PR. Set only on API
+    # pipelines started by .github/workflows/fork-ci-mirror.yml.
+    [AllowEmptyString()][string]$ForkPr = $env:CBX_FORK_PR,
+
     # CIRCLE_SHA1.
     [AllowEmptyString()][string]$Sha = $env:CIRCLE_SHA1,
 
@@ -74,7 +78,7 @@ function Invoke-GateHalt {
 }
 
 # Gates 1 and 2: budget emergency stop, then PR/main-master scope.
-$trigger = Get-TriggerGateDecision -BudgetMode $BudgetMode -Branch $Branch -PrUrl $PrUrl
+$trigger = Get-TriggerGateDecision -BudgetMode $BudgetMode -Branch $Branch -PrUrl $PrUrl -ForkPr $ForkPr
 if ($trigger.Skip) {
     Invoke-GateHalt -Reason $trigger.Reason
     exit 0
@@ -94,7 +98,7 @@ if ($isMainPush) {
     # PR association without a populated event value (e.g. api trigger):
     # resolve the base from the public GitHub pulls API.
     try {
-        $prNumber = Get-ForkMirrorPrNumber -Branch $Branch
+        $prNumber = Get-ForkMirrorPrNumber -Branch $Branch -ForkPr $ForkPr
         if ($PrUrl -match '/pull/(\d+)') { $prNumber = $Matches[1] }
         if ([string]::IsNullOrWhiteSpace($prNumber)) { throw 'PR number not available from pipeline values.' }
         if ([string]::IsNullOrWhiteSpace($env:CIRCLE_PROJECT_USERNAME) -or [string]::IsNullOrWhiteSpace($env:CIRCLE_PROJECT_REPONAME)) {
diff --git a/scripts/circleci-pr.tests.ps1 b/scripts/circleci-pr.tests.ps1
index 20338cd0e0..a079051153 100644
--- a/scripts/circleci-pr.tests.ps1
+++ b/scripts/circleci-pr.tests.ps1
@@ -98,6 +98,15 @@ foreach ($branch in @('ci/pr-', 'ci/pr-0', 'ci/pr-12x', 'ci/pr-7/extra', 'x/ci/p
 }
 Assert-Equal (Get-ForkMirrorPrNumber -Branch 'ci/pr-750') '750' 'mirror PR number parsed'
 Assert-Equal (Get-ForkMirrorPrNumber -Branch 'main') '' 'non-mirror branch has no PR number'
+$decision = Get-TriggerGateDecision -BudgetMode 'normal' -Branch 'pull/750/head' -PrUrl '' -ForkPr '750'
+Assert-True (-not $decision.Skip) 'API fork pipeline with fork_pr runs whatever the branch name'
+$decision = Get-TriggerGateDecision -BudgetMode 'off' -Branch '' -PrUrl '' -ForkPr '750'
+Assert-True $decision.Skip 'budget off wins over fork_pr'
+foreach ($forkPr in @('0', 'abc', '7 ', '-1', '12x')) {
+    $decision = Get-TriggerGateDecision -BudgetMode 'normal' -Branch 'feature/x' -PrUrl '' -ForkPr $forkPr
+    Assert-True $decision.Skip "malformed fork_pr '$forkPr' skips"
+}
+Assert-Equal (Get-ForkMirrorPrNumber -Branch 'ci/pr-12' -ForkPr '750') '750' 'fork_pr wins over the branch name'
 
 Write-Host '==> Hosted Node provisioning guard'
 $prRunnerText = Get-Content -Raw -LiteralPath (Join-Path $scriptRoot 'run-circleci-pr-check.ps1')