diff --git a/.devtools/prune-merged-branches.sh b/.devtools/prune-merged-branches.sh
new file mode 100755
index 0000000..b57967e
--- /dev/null
+++ b/.devtools/prune-merged-branches.sh
@@ -0,0 +1,77 @@
+#!/usr/bin/env bash
+# Deletes local branches whose pull request has been merged.
+#
+# Usage: prune-merged-branches.sh [--dry-run]
+#
+# Runs from the shared post-merge hook, so it fires on the `git pull` that brings a
+# merged pull request into the default branch, and does nothing on any other branch.
+# A branch goes only when all of these hold:
+#   - its remote branch is gone (GitHub deletes it when the pull request merges)
+#   - it is not checked out here or in another worktree
+#   - its work is in the default branch: either its tip is an ancestor of it, or,
+#     for a squash merge that leaves no ancestry, gh reports a merged pull request
+#     whose head is exactly the local tip
+#
+# Anything else is kept and named, so commits made after the merge, or on a branch
+# whose pull request was closed unmerged, are never lost. Without gh, a squash-merged
+# branch is kept rather than guessed at. The exit status is 0 either way: a cleanup
+# must not make a pull look like it failed.
+set -euo pipefail
+
+dry_run=false
+case "${1:-}" in
+    '') ;;
+    --dry-run) dry_run=true ;;
+    -h | --help)
+        sed -n '2,16p' "${BASH_SOURCE[0]}" | sed 's/^# \{0,1\}//'
+        exit 0
+        ;;
+    *)
+        echo "unknown argument: $1" >&2
+        exit 2
+        ;;
+esac
+
+default="$(git symbolic-ref --quiet --short refs/remotes/origin/HEAD 2>/dev/null || true)"
+default="${default#origin/}"
+default="${default:-main}"
+current="$(git symbolic-ref --quiet --short HEAD 2>/dev/null || true)"
+[[ "${current}" == "${default}" ]] || exit 0
+
+# A pull does not prune on its own unless fetch.prune is set, and offline is fine.
+git fetch --prune --quiet origin 2>/dev/null || true
+
+checked_out="$(git worktree list --porcelain | sed -n 's#^branch refs/heads/##p')"
+
+git for-each-ref --format='%(refname:short)%09%(upstream:track)%09%(objectname)' refs/heads |
+    while IFS=$'\t' read -r branch track tip; do
+        [[ "${track}" == '[gone]' ]] || continue
+
+        if grep -Fxq "${branch}" <<<"${checked_out}"; then
+            echo "– kept ${branch}: checked out in a worktree"
+            continue
+        fi
+
+        merged=false
+        if git merge-base --is-ancestor "${tip}" "${default}" 2>/dev/null; then
+            merged=true
+        elif command -v gh >/dev/null 2>&1; then
+            # Captured first: grep -q closing the pipe early would fail it under pipefail.
+            heads="$(gh pr list --head "${branch}" --state merged \
+                --json headRefOid --jq '.[].headRefOid' 2>/dev/null || true)"
+            if grep -Fxq "${tip}" <<<"${heads}"; then
+                merged=true
+            fi
+        fi
+
+        if [[ "${merged}" != true ]]; then
+            echo "– kept ${branch}: not shown to be merged (closed unmerged, or commits after the merge)"
+        elif [[ "${dry_run}" == true ]]; then
+            echo "would delete ${branch}"
+        else
+            git branch --quiet -D "${branch}"
+            echo "✔ deleted ${branch}"
+        fi
+    done
+
+exit 0
diff --git a/.devtools/test-hooks.sh b/.devtools/test-hooks.sh
index a278b9d..0ba78ef 100755
--- a/.devtools/test-hooks.sh
+++ b/.devtools/test-hooks.sh
@@ -131,6 +131,76 @@ expect fail "fails when a named base is missing" "${size}" --base no-such-branch
 git branch --quiet -m main trunk
 expect pass "skips when a guessed base is missing" "${size}"
 
+echo "prune-merged-branches"
+prune="${here}/prune-merged-branches.sh"
+
+# gh stands in for GitHub: it reports GH_MERGED_OID as the head of a merged pull request.
+mkdir "${work}/bin"
+# shellcheck disable=SC2016  # the stub expands it, not this script
+printf '#!/bin/sh\n[ -z "${GH_MERGED_OID:-}" ] || echo "${GH_MERGED_OID}"\n' >"${work}/bin/gh"
+chmod +x "${work}/bin/gh"
+mkdir "${work}/bin-fail"
+printf '#!/bin/sh\nexit 1\n' >"${work}/bin-fail/gh"
+chmod +x "${work}/bin-fail/gh"
+no_gh=(env PATH="${work}/bin-fail:${PATH}")
+
+git init --quiet --bare --initial-branch=main "${work}/origin.git"
+git clone --quiet "${work}/origin.git" "${work}/prune" 2>/dev/null
+cd "${work}/prune"
+git commit --quiet --allow-empty -m 'chore: initial commit'
+git push --quiet origin main
+git remote set-head origin main >/dev/null
+
+# branch_off <name> <file>: a pushed branch with one commit, back on main afterwards.
+branch_off() {
+    git switch --quiet -c "$1" main
+    echo "$1" >"$2"
+    git add "$2"
+    git commit --quiet -m "feat: add $2"
+    git push --quiet -u origin "$1" 2>/dev/null
+    git switch --quiet main
+}
+# remote_gone <name>: what GitHub does to a head branch when its pull request merges.
+remote_gone() { git push --quiet origin --delete "$1" 2>/dev/null; }
+has_branch() { git show-ref --verify --quiet "refs/heads/$1"; }
+
+branch_off feat/ancestry a.txt
+git merge --quiet --no-ff -m 'feat: merge ancestry' feat/ancestry
+git push --quiet origin main
+remote_gone feat/ancestry
+
+branch_off feat/squashed b.txt
+squashed_tip="$(git rev-parse feat/squashed)"
+git merge --quiet --squash feat/squashed >/dev/null
+git commit --quiet -m 'feat: squash b'
+git push --quiet origin main
+remote_gone feat/squashed
+
+branch_off feat/closed c.txt
+remote_gone feat/closed
+branch_off feat/live d.txt
+branch_off feat/worktree e.txt
+remote_gone feat/worktree
+git worktree add --quiet "${work}/wt" feat/worktree 2>/dev/null
+
+expect pass "dry run exits 0" "${prune}" --dry-run
+expect pass "dry run deletes nothing" has_branch feat/ancestry
+expect pass "ignores a branch that is not the default" bash -c "git switch --quiet -c feat/elsewhere && '${prune}' && git switch --quiet main"
+expect pass "and deletes nothing there" has_branch feat/ancestry
+expect pass "keeps a squash merge it cannot confirm without gh" "${no_gh[@]}" "${prune}"
+expect pass "keeps it" has_branch feat/squashed
+expect fail "deletes a branch merged by ancestry" has_branch feat/ancestry
+expect pass "keeps a branch with a live remote" has_branch feat/live
+expect pass "keeps a branch whose pull request closed unmerged" has_branch feat/closed
+expect pass "keeps a branch checked out in a worktree" has_branch feat/worktree
+expect pass "keeps a squash merge whose head moved on" env PATH="${work}/bin:${PATH}" GH_MERGED_OID=0000000000000000000000000000000000000000 "${prune}"
+expect pass "and the branch with it" has_branch feat/squashed
+expect pass "runs with a squash merge gh confirms" env PATH="${work}/bin:${PATH}" GH_MERGED_OID="${squashed_tip}" "${prune}"
+expect fail "and deletes it" has_branch feat/squashed
+expect pass "still keeps the unmerged and the live" has_branch feat/closed
+expect pass "and the checked out one" has_branch feat/worktree
+cd "${work}/repo"
+
 if ((failures > 0)); then
     echo "${failures} failed" >&2
     exit 1
diff --git a/.lefthook/post-merge/prune-branches.sh b/.lefthook/post-merge/prune-branches.sh
new file mode 100755
index 0000000..42ce8ff
--- /dev/null
+++ b/.lefthook/post-merge/prune-branches.sh
@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+# lefthook finds a shared hook's script here, in .lefthook/<hook>/ at the root of the
+# repository the config came from. The cleanup itself stays in .devtools/.
+#
+# git passes a post-merge hook a squash flag. The script takes none, so it is not handed on.
+exec "$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)/.devtools/prune-merged-branches.sh"
diff --git a/README.md b/README.md
index bee13d4..5cb74d8 100644
--- a/README.md
+++ b/README.md
@@ -79,6 +79,13 @@ PR_BASE=main lefthook run pre-push --job pr-size
 `PR_MAX_FILES` changes the limit. Without `PR_BASE` the base is the branch below this one in a
 `gh stack`, else the remote's default branch.
 
+`org.yml` also tidies up after a merge. On the default branch, `post-merge` runs
+[`prune-merged-branches.sh`](.devtools/prune-merged-branches.sh), which deletes the local branches
+whose remote is gone and whose work is in: the tip is an ancestor of the default branch, or, for a
+squash merge, `gh` reports a merged pull request with exactly that head. A branch checked out in
+any worktree, or one it cannot show to be merged, is kept and named. Run it with `--dry-run` to
+see what it would do. A repository gets the hook once it bumps `ref` and runs `lefthook install`.
+
 Commit messages are not part of `org.yml`: each repository runs commitlint against its own
 `.commitlintrc.yaml` in a `commit-msg` job, and `ci-pr.yml` reads the same file.
 
diff --git a/lefthook/org.yml b/lefthook/org.yml
index 7c35316..9b2f056 100644
--- a/lefthook/org.yml
+++ b/lefthook/org.yml
@@ -11,6 +11,9 @@
 #
 # The scripts need bash and git; pr-size also uses gh and jq when they are present,
 # to find a stack's parent. Set BRANCH_NAME to check a name that is not checked out.
+#
+# post-merge tidies up after a merged pull request: on the default branch it deletes the
+# local branches whose remote is gone and whose work is in, and keeps the rest.
 
 pre-push:
   jobs:
@@ -21,3 +24,9 @@ pre-push:
     - name: pr-size
       script: pr-size.sh
       runner: bash
+
+post-merge:
+  jobs:
+    - name: prune-branches
+      script: prune-branches.sh
+      runner: bash