diff --git a/.github/workflows/delete-branch.yml b/.github/workflows/delete-branch.yml
new file mode 100644
index 0000000..a074ffc
--- /dev/null
+++ b/.github/workflows/delete-branch.yml
@@ -0,0 +1,86 @@
+# Deletes one named branch in one repository this account owns, by hand.
+#
+# branch-prune only deletes branches whose pull request merged in the same
+# repository at the branch's tip. Some branches can never qualify: a fork's
+# branch whose pull request was merged upstream, or a branch whose pull request
+# was closed without merging (superseded by a re-cut). This deletes exactly one
+# of those, after the checks in scripts/delete-branch.sh:
+#   - the repository belongs to this account;
+#   - the branch's tip is exactly the SHA you give. The REST delete has no
+#     precondition, so this narrows, but cannot close, the seconds-long window
+#     in which a new push could land and be lost;
+#   - not the default branch, not a long-lived name (the prune's keep pattern),
+#     not protected, and no open pull request uses it as its base here or as its
+#     head anywhere in the fork network the token can read (parent, source or a
+#     public sibling fork; someone else's private fork is invisible to it).
+#
+# DRY RUN BY DEFAULT: `mode` defaults to dry-run, which makes every check and
+# reports. Only `mode: apply` deletes. The job summary prints the command that
+# restores the branch at the same commit.
+#
+# TOKEN: the same fine-grained PAT as branch-prune, the repository secret
+# PRUNE_TOKEN (Contents: Read and write, Pull requests: Read-only). GITHUB_TOKEN
+# only reaches this repository. Every input reaches the shell through env. The
+# job only runs from main, so a dispatch on another branch can never run an
+# unreviewed copy of the script with this token.
+name: delete-branch
+
+on:
+  workflow_dispatch:
+    inputs:
+      repository:
+        description: Repository as owner/name. It must belong to this account.
+        required: true
+        type: string
+      branch:
+        description: Branch to delete. Never the default branch or a long-lived name.
+        required: true
+        type: string
+      sha:
+        description: Full 40-character commit SHA the branch must point at.
+        required: true
+        type: string
+      mode:
+        description: dry-run makes every check and reports; apply deletes.
+        type: choice
+        options:
+          - dry-run
+          - apply
+        default: dry-run
+
+permissions: {}
+
+concurrency:
+  group: delete-branch-${{ github.repository }}
+  cancel-in-progress: false
+
+jobs:
+  delete:
+    name: Delete one branch
+    if: github.ref == 'refs/heads/main'
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    # Checkout only. Every API call uses PRUNE_TOKEN, never GITHUB_TOKEN.
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+
+      - name: Delete the branch
+        env:
+          GH_TOKEN: ${{ secrets.PRUNE_TOKEN }}
+          OWNER: ${{ github.repository_owner }}
+          REPOSITORY: ${{ inputs.repository }}
+          BRANCH: ${{ inputs.branch }}
+          SHA: ${{ inputs.sha }}
+          MODE: ${{ inputs.mode }}
+        run: |
+          set -euo pipefail
+          if [ -z "${GH_TOKEN}" ]; then
+            echo "::error title=delete-branch::PRUNE_TOKEN is not set. See branch-prune.yml for how to create it."
+            exit 1
+          fi
+          bash scripts/delete-branch.sh --owner "${OWNER}" --repo "${REPOSITORY}" \
+            --branch "${BRANCH}" --sha "${SHA}" --mode "${MODE}"
diff --git a/scripts/delete-branch.sh b/scripts/delete-branch.sh
new file mode 100644
index 0000000..dc87006
--- /dev/null
+++ b/scripts/delete-branch.sh
@@ -0,0 +1,186 @@
+#!/usr/bin/env bash
+# Deletes ONE named branch in ONE repository owned by this account, on demand,
+# after the same kind of checks the nightly prune makes. It exists for the
+# branches the prune never touches: a fork's branch whose pull request was
+# merged upstream, or a branch whose pull request was closed without merging.
+#
+#   delete-branch.sh --owner OWNER --repo OWNER/NAME --branch BRANCH \
+#                    --sha FULL_SHA [--mode dry-run|apply]
+#
+# Nothing is deleted, and the run fails, unless every check holds:
+#   1. the repository belongs to --owner;
+#   2. --sha is a full 40-character commit SHA and the branch's tip is exactly
+#      that commit. This narrows, but cannot close, the window in which a push
+#      lands between the check and the DELETE: the REST delete has no
+#      precondition on the old SHA;
+#   3. the branch is not the repository's default branch and does not match
+#      KEEP_RE, the long-lived pattern prune-merged-branches.sh also keeps;
+#   4. GitHub does not report the branch as protected;
+#   5. no open pull request uses it as its head or as its base. The head check
+#      asks GraphQL for the open pull requests whose head is this ref, in any
+#      repository of the fork network the token can read (the parent, the
+#      source or a public sibling fork; someone else's private fork is
+#      invisible to it), then also lists them over REST in this repository and
+#      in its parent and source. Pull requests into a branch can only live in
+#      the branch's own repository, so the base check asks only there. Any
+#      query that fails refuses the deletion.
+# A branch that is already gone is reported and is not an error. The default
+# mode is dry-run: it makes every check and reports what it would delete.
+#
+# Token (GH_TOKEN, used by gh): read the repository, its branches and pull
+# requests, and delete refs (Contents: Read and write, Pull requests:
+# Read-only). The workflow passes PRUNE_TOKEN.
+set -euo pipefail
+
+KEEP_RE='^(main|master|develop|staging|test|gh-pages|v[0-9][0-9.]*|deploy-.*|release/.*)$'
+
+# fail MESSAGE: report a refused check in the log and the job summary, exit 1.
+fail() {
+  echo "::error title=delete-branch::$1"
+  summary "Refused" "$1"
+  exit 1
+}
+
+# usage_error MESSAGE: report a malformed command line and exit 2, like the prune.
+usage_error() {
+  echo "::error title=delete-branch::$1"
+  exit 2
+}
+
+# clean VALUE: print VALUE without CR, LF or backticks. Inputs are echoed into
+# logs and the job summary before they are validated: a newline would let a
+# value start its own workflow command line.
+clean() { local v="${1//[$'\n\r']/ }"; printf '%s' "${v//\`/\'}"; }
+
+# summary TITLE TEXT: append the inputs and TEXT to the job summary, if any.
+summary() {
+  if [ -n "${GITHUB_STEP_SUMMARY:-}" ]; then
+    {
+      echo "### delete-branch: $1"
+      echo
+      echo "- Repository: \`$(clean "${repo:-?}")\`"
+      echo "- Branch: \`$(clean "${branch:-?}")\`"
+      echo "- Expected tip: \`$(clean "${sha:-?}")\`"
+      echo "- Mode: \`$(clean "${mode:-?}")\`"
+      echo
+      echo "$2"
+    } >> "${GITHUB_STEP_SUMMARY}"
+  fi
+}
+
+# urlencode VALUE: percent-encode VALUE for one URL path or query component.
+urlencode() { jq -rn --arg v "$1" '$v | @uri'; }
+
+owner="" repo="" branch="" sha="" mode="dry-run"
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --owner|--repo|--branch|--sha|--mode)
+      [ $# -ge 2 ] || usage_error "$1 needs a value"
+      case "$1" in
+        --owner) owner="$2" ;;
+        --repo) repo="$2" ;;
+        --branch) branch="$2" ;;
+        --sha) sha="$2" ;;
+        --mode) mode="$2" ;;
+      esac
+      shift 2 ;;
+    -h|--help) sed -n '2,/^set -euo/p' "$0" | sed '$d' | sed 's/^# \{0,1\}//'; exit 0 ;;
+    *) usage_error "unknown argument: $(clean "$1")" ;;
+  esac
+done
+
+case "${mode}" in
+  dry-run|apply) ;;
+  *) fail "mode must be dry-run or apply, not '$(clean "${mode}")'" ;;
+esac
+[[ "${owner}" =~ ^[A-Za-z0-9-]+$ ]] || fail "--owner is missing or not a GitHub account name"
+[[ "${repo}" =~ ^[A-Za-z0-9-]+/[A-Za-z0-9._-]+$ ]] || fail "repository must be owner/name, got '$(clean "${repo}")'"
+# GitHub account names are case-insensitive.
+repo_owner="${repo%%/*}"
+[ "${repo_owner,,}" = "${owner,,}" ] || fail "repository ${repo} does not belong to ${owner}"
+[[ "${sha}" =~ ^[0-9a-f]{40}$ ]] || fail "sha must be a full 40-character lowercase commit SHA"
+if [[ ! "${branch}" =~ ^[A-Za-z0-9._/@+-]+$ ]] || [[ "${branch}" == -* ]] || [[ "${branch}" == /* ]] \
+  || [[ "${branch}" == */ ]] || [[ "${branch}" == *..* ]] || [[ "${branch}" == *//* ]] \
+  || [[ "${branch}" == *.lock ]] || [[ "${branch}" == *@\{* ]]; then
+  fail "'$(clean "${branch}")' is not a branch name this workflow accepts"
+fi
+if [[ "${branch}" =~ ${KEEP_RE} ]]; then
+  fail "${branch} matches the long-lived pattern and is never deleted here"
+fi
+
+meta="$(gh api "repos/${repo}")" || fail "cannot read repos/${repo}"
+# gh follows redirects: a renamed or transferred repository must not pass as this one.
+full_name="$(jq -r '.full_name // empty' <<<"${meta}")"
+[ "${full_name,,}" = "${repo,,}" ] || fail "repos/${repo} resolved to '${full_name}' (renamed or transferred?)"
+default_branch="$(jq -r '.default_branch // empty' <<<"${meta}")"
+[ -n "${default_branch}" ] || fail "repos/${repo} returned no default branch"
+# A fork branch's pull requests usually live in its parent (or source) repository.
+upstreams="$(jq -r '[.parent.full_name, .source.full_name] | map(select(. != null)) | unique | .[]' <<<"${meta}")"
+[ "${branch}" != "${default_branch}" ] || fail "${branch} is the default branch of ${repo}"
+
+enc="$(urlencode "${branch}")"
+if ! info="$(gh api "repos/${repo}/branches/${enc}" 2>&1)"; then
+  if grep -q 'HTTP 404' <<<"${info}"; then
+    echo "${repo}: ${branch} does not exist (already deleted)."
+    summary "Already gone" "The branch does not exist, so there was nothing to delete."
+    exit 0
+  fi
+  fail "cannot read ${repo} branch ${branch}: ${info}"
+fi
+# gh follows the redirect for a renamed branch: the name must match exactly.
+name="$(jq -r '.name // empty' <<<"${info}")"
+[ "${name}" = "${branch}" ] || fail "branch ${branch} in ${repo} resolved to '${name}' (renamed?)"
+tip="$(jq -r '.commit.sha // empty' <<<"${info}")"
+# Not `.protected // empty`: jq's // treats false as missing.
+protected="$(jq -r 'if .protected == null then "" else (.protected | tostring) end' <<<"${info}")"
+[ -n "${tip}" ] || fail "branch ${branch} in ${repo} returned no tip commit"
+[ "${tip}" = "${sha}" ] || fail "${branch} in ${repo} points at ${tip}, not ${sha}: it has moved since you looked"
+[ "${protected}" = "false" ] || fail "${branch} in ${repo} is protected (protected=${protected:-unknown})"
+
+# Every open pull request whose head is this ref, whichever repository of the
+# fork network it was opened in, as far as the token can see (someone else's
+# private fork is invisible). A sibling fork's /pulls is not listable here.
+# shellcheck disable=SC2016 # $owner, $name and $ref are GraphQL variables.
+gql='query($owner: String!, $name: String!, $ref: String!) {
+  repository(owner: $owner, name: $name) {
+    ref(qualifiedName: $ref) { associatedPullRequests(states: OPEN, first: 1) { totalCount } }
+  }
+}'
+open_any="$(gh api graphql -f query="${gql}" -f owner="${repo%%/*}" -f name="${repo#*/}" \
+  -f ref="refs/heads/${branch}" --jq '.data.repository.ref.associatedPullRequests.totalCount' 2>&1)" \
+  || fail "cannot ask GraphQL for open pull requests from ${branch}: $(clean "${open_any}")"
+[[ "${open_any}" =~ ^[0-9]+$ ]] || fail "GraphQL returned no pull request count for ${branch} (got '$(clean "${open_any}")')"
+[ "${open_any}" = "0" ] || fail "${open_any} open pull request(s) in the fork network use ${repo}:${branch} as their head (GraphQL)"
+open_head="$(gh api "repos/${repo}/pulls?state=open&per_page=1&head=$(urlencode "${owner}:${branch}")" --jq 'length')" \
+  || fail "cannot list open pull requests from ${branch}"
+[ "${open_head}" = "0" ] || fail "an open pull request in ${repo} uses ${branch} as its head"
+open_base="$(gh api "repos/${repo}/pulls?state=open&per_page=1&base=${enc}" --jq 'length')" \
+  || fail "cannot list open pull requests into ${branch}"
+[ "${open_base}" = "0" ] || fail "an open pull request in ${repo} targets ${branch}"
+for up in ${upstreams}; do
+  open_up="$(gh api "repos/${up}/pulls?state=open&per_page=1&head=$(urlencode "${owner}:${branch}")" --jq 'length')" \
+    || fail "cannot list open pull requests in ${up}"
+  [ "${open_up}" = "0" ] || fail "an open pull request in ${up} uses ${owner}:${branch} as its head"
+done
+
+if [ "${mode}" = "dry-run" ]; then
+  echo "dry-run: would delete ${repo} ${branch} at ${tip}."
+  summary "Would delete (dry run)" "Every check passed. Run again with \`mode: apply\` to delete it."
+  exit 0
+fi
+
+gh api -X DELETE "repos/${repo}/git/refs/heads/${enc}" >/dev/null \
+  || fail "DELETE refs/heads/${branch} in ${repo} failed"
+# Only a 404 proves it is gone; any other failure is unknown, not success. A
+# short retry covers a read that briefly still sees the branch.
+for attempt in 1 2 3; do
+  if gone="$(gh api "repos/${repo}/branches/${enc}" 2>&1)"; then
+    [ "${attempt}" -lt 3 ] || fail "${branch} still exists in ${repo} after the DELETE"
+    sleep "${READBACK_SLEEP:-2}"
+    continue
+  fi
+  grep -q 'HTTP 404' <<<"${gone}" && break
+  fail "DELETE sent, but the read-back of ${branch} in ${repo} failed: ${gone}"
+done
+echo "Deleted ${repo} ${branch} (was ${tip})."
+summary "Deleted" "Deleted. To restore it: \`gh api repos/${repo}/git/refs -f ref=refs/heads/${branch} -f sha=${tip}\`."