diff --git a/.github/workflows/runtime-dev-cleanup.yml b/.github/workflows/runtime-dev-cleanup.yml
new file mode 100644
index 0000000000..b045b25003
--- /dev/null
+++ b/.github/workflows/runtime-dev-cleanup.yml
@@ -0,0 +1,76 @@
+name: runtime-dev-cleanup
+
+# Prune immutable runtime-dev-YYYYMMDD-<sha> prereleases. The floating
+# runtime-dev-latest release is ignored, and the tag currently pinned by
+# config-dev is passed as protected so fresh dev image builds do not lose
+# their baked runtime overlay.
+on:
+  schedule:
+    - cron: '17 4 * * *'
+  push:
+    branches: [dev]
+    paths:
+      - 'runtime-overlay/**'
+      - 'config-dev'
+      - '.github/workflows/runtime-dev-cleanup.yml'
+  workflow_dispatch:
+    inputs:
+      keep_count:
+        description: 'Newest immutable runtime-dev releases to retain'
+        type: string
+        default: '5'
+      keep_days:
+        description: 'Also retain immutable runtime-dev releases this many days old or newer'
+        type: string
+        default: '14'
+      dry_run:
+        description: 'Log what would be deleted without deleting'
+        type: choice
+        options: ['true', 'false']
+        default: 'true'
+
+permissions:
+  contents: write
+
+# Runtime dev publish on dev uses runtime-release-refs/heads/dev. Reusing that
+# group keeps cleanup from deleting while a dev runtime release is publishing.
+concurrency:
+  group: runtime-release-refs/heads/dev
+  cancel-in-progress: false
+
+jobs:
+  prune:
+    runs-on: ubuntu-24.04
+    timeout-minutes: 10
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          # The runtime dev stream is anchored on dev. Scheduled workflows
+          # are loaded from the default branch, so check out dev explicitly
+          # before reading config-dev or running the helper script.
+          ref: dev
+          persist-credentials: false
+
+      - name: Resolve protected config-dev runtime tag
+        id: config-dev
+        run: |
+          set -euo pipefail
+          set -a
+          # shellcheck disable=SC1091
+          . ./config-dev
+          set +a
+          if [[ -z "${AIRPLANES_RUNTIME_OVERLAY_TAG:-}" ]]; then
+              echo "::error::config-dev did not set AIRPLANES_RUNTIME_OVERLAY_TAG"
+              exit 1
+          fi
+          echo "tag=$AIRPLANES_RUNTIME_OVERLAY_TAG" >> "$GITHUB_OUTPUT"
+
+      - name: Prune stale immutable dev runtime releases
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          RUNTIME_DEV_RELEASE_KEEP_COUNT: ${{ github.event.inputs.keep_count || '5' }}
+          RUNTIME_DEV_RELEASE_KEEP_DAYS: ${{ github.event.inputs.keep_days || '14' }}
+          RUNTIME_DEV_RELEASE_DRY_RUN: ${{ github.event.inputs.dry_run || 'false' }}
+        run: |
+          bash runtime-overlay/scripts/release-workflow/prune-dev-runtime-releases.sh \
+              --protected-tag "${{ steps.config-dev.outputs.tag }}"
diff --git a/runtime-overlay/scripts/release-workflow/prune-dev-runtime-releases.sh b/runtime-overlay/scripts/release-workflow/prune-dev-runtime-releases.sh
new file mode 100755
index 0000000000..0948748a2e
--- /dev/null
+++ b/runtime-overlay/scripts/release-workflow/prune-dev-runtime-releases.sh
@@ -0,0 +1,182 @@
+#!/usr/bin/env bash
+# prune-dev-runtime-releases.sh - delete stale immutable runtime-dev releases.
+#
+# Retention is additive:
+#   - keep the newest RUNTIME_DEV_RELEASE_KEEP_COUNT immutable dev releases;
+#   - keep releases created within RUNTIME_DEV_RELEASE_KEEP_DAYS days;
+#   - keep any --protected-tag values, even if old.
+#
+# Only tags matching runtime-dev-YYYYMMDD-<sha> are eligible. The floating
+# runtime-dev-latest release and stable runtime-v* releases are ignored.
+
+set -euo pipefail
+
+die() {
+    echo "prune-dev-runtime-releases: $*" >&2
+    exit 1
+}
+
+usage() {
+    cat >&2 <<'USAGE'
+Usage: prune-dev-runtime-releases.sh [--repo owner/name] [--protected-tag tag]
+
+Environment:
+  GITHUB_REPOSITORY                 default repo when --repo is omitted
+  RUNTIME_DEV_RELEASE_KEEP_COUNT    newest immutable dev releases to retain (default: 5)
+  RUNTIME_DEV_RELEASE_KEEP_DAYS     age window to retain, in days (default: 14)
+  RUNTIME_DEV_RELEASE_DRY_RUN       true|false (default: true)
+  RUNTIME_DEV_RELEASE_LIST_LIMIT    gh release list limit (default: 1000)
+
+Test-only environment:
+  RUNTIME_DEV_RELEASES_JSON         JSON array with tagName and createdAt
+  RUNTIME_DEV_RELEASE_NOW_EPOCH     fixed "now" epoch for age calculation
+USAGE
+}
+
+is_uint() {
+    [[ "$1" =~ ^[0-9]+$ ]]
+}
+
+repo="${GITHUB_REPOSITORY:-}"
+protected_tags=()
+
+while [[ $# -gt 0 ]]; do
+    case "$1" in
+        --repo)
+            [[ $# -ge 2 ]] || die "--repo requires a value"
+            repo="$2"
+            shift 2
+            ;;
+        --protected-tag)
+            [[ $# -ge 2 ]] || die "--protected-tag requires a value"
+            protected_tags+=("$2")
+            shift 2
+            ;;
+        -h|--help)
+            usage
+            exit 0
+            ;;
+        *)
+            usage
+            die "unknown argument: $1"
+            ;;
+    esac
+done
+
+[[ -n "$repo" ]] || die "missing --repo or GITHUB_REPOSITORY"
+
+keep_count="${RUNTIME_DEV_RELEASE_KEEP_COUNT:-5}"
+keep_days="${RUNTIME_DEV_RELEASE_KEEP_DAYS:-14}"
+dry_run="${RUNTIME_DEV_RELEASE_DRY_RUN:-true}"
+list_limit="${RUNTIME_DEV_RELEASE_LIST_LIMIT:-1000}"
+now_epoch="${RUNTIME_DEV_RELEASE_NOW_EPOCH:-$(date -u +%s)}"
+
+is_uint "$keep_count" || die "RUNTIME_DEV_RELEASE_KEEP_COUNT must be an unsigned integer"
+is_uint "$keep_days" || die "RUNTIME_DEV_RELEASE_KEEP_DAYS must be an unsigned integer"
+is_uint "$list_limit" || die "RUNTIME_DEV_RELEASE_LIST_LIMIT must be an unsigned integer"
+is_uint "$now_epoch" || die "RUNTIME_DEV_RELEASE_NOW_EPOCH must be an unsigned integer epoch"
+
+case "$dry_run" in
+    true|false) ;;
+    *) die "RUNTIME_DEV_RELEASE_DRY_RUN must be true or false" ;;
+esac
+
+fetch_releases_json() {
+    if [[ -n "${RUNTIME_DEV_RELEASES_JSON:-}" ]]; then
+        printf '%s\n' "$RUNTIME_DEV_RELEASES_JSON"
+        return
+    fi
+
+    command -v gh >/dev/null 2>&1 || die "gh is required"
+    gh release list \
+        -R "$repo" \
+        --limit "$list_limit" \
+        --json tagName,createdAt
+}
+
+command -v jq >/dev/null 2>&1 || die "jq is required"
+
+declare -A protected=()
+for tag in "${protected_tags[@]}"; do
+    [[ -n "$tag" ]] || continue
+    protected["$tag"]=1
+done
+
+release_json="$(fetch_releases_json)"
+candidate_tsv="$(
+    jq -r '
+        map(select(.tagName | test("^runtime-dev-[0-9]{8}-[0-9a-f]{7,40}$")))
+        | sort_by(.createdAt)
+        | reverse
+        | .[]
+        | [.createdAt, .tagName]
+        | @tsv
+    ' <<<"$release_json"
+)"
+
+if [[ -z "$candidate_tsv" ]]; then
+    echo "No immutable runtime-dev releases found in $repo."
+    exit 0
+fi
+
+cutoff_epoch=0
+if (( keep_days > 0 )); then
+    cutoff_epoch=$((now_epoch - keep_days * 86400))
+fi
+
+index=0
+to_delete=()
+
+echo "Runtime dev release cleanup policy:"
+echo "  repo: $repo"
+echo "  keep_count: $keep_count"
+echo "  keep_days: $keep_days"
+echo "  dry_run: $dry_run"
+if ((${#protected_tags[@]} > 0)); then
+    printf '  protected_tags: %s\n' "${protected_tags[*]}"
+else
+    echo "  protected_tags: none"
+fi
+
+while IFS=$'\t' read -r created_at tag; do
+    [[ -n "$tag" ]] || continue
+
+    reason=""
+    if [[ -n "${protected[$tag]:-}" ]]; then
+        reason="protected"
+    elif (( index < keep_count )); then
+        reason="newest"
+    else
+        if ! created_epoch="$(date -u -d "$created_at" +%s 2>/dev/null)"; then
+            die "could not parse createdAt for $tag: $created_at"
+        fi
+        if (( keep_days > 0 && created_epoch >= cutoff_epoch )); then
+            reason="age"
+        fi
+    fi
+
+    if [[ -n "$reason" ]]; then
+        echo "keep   $tag ($created_at; $reason)"
+    else
+        echo "delete $tag ($created_at)"
+        to_delete+=("$tag")
+    fi
+
+    index=$((index + 1))
+done <<<"$candidate_tsv"
+
+if ((${#to_delete[@]} == 0)); then
+    echo "No stale immutable runtime-dev releases to delete."
+    exit 0
+fi
+
+if [[ "$dry_run" == "true" ]]; then
+    echo "Dry run only; would delete ${#to_delete[@]} release(s)."
+    exit 0
+fi
+
+for tag in "${to_delete[@]}"; do
+    gh release delete "$tag" -R "$repo" --yes --cleanup-tag
+done
+
+echo "Deleted ${#to_delete[@]} stale immutable runtime-dev release(s)."
diff --git a/test/runtime-overlay/test_prune_dev_runtime_releases.bats b/test/runtime-overlay/test_prune_dev_runtime_releases.bats
new file mode 100644
index 0000000000..d76f9a312e
--- /dev/null
+++ b/test/runtime-overlay/test_prune_dev_runtime_releases.bats
@@ -0,0 +1,74 @@
+#!/usr/bin/env bats
+
+# Tests for runtime-overlay/scripts/release-workflow/prune-dev-runtime-releases.sh.
+
+bats_require_minimum_version 1.5.0
+
+setup() {
+    REPO_ROOT="$(cd "$BATS_TEST_DIRNAME/../.." && pwd)"
+    SCRIPT="$REPO_ROOT/runtime-overlay/scripts/release-workflow/prune-dev-runtime-releases.sh"
+    [ -x "$SCRIPT" ] || skip "prune-dev-runtime-releases.sh not executable"
+    command -v jq >/dev/null 2>&1 || skip "jq not installed"
+}
+
+fixture_json() {
+    cat <<'JSON'
+[
+  {"tagName":"runtime-dev-latest","createdAt":"2026-04-01T00:00:00Z"},
+  {"tagName":"runtime-v1.0.0","createdAt":"2026-04-01T00:00:00Z"},
+  {"tagName":"runtime-dev-20260522-aaaaaaa","createdAt":"2026-05-22T00:00:00Z"},
+  {"tagName":"runtime-dev-20260521-bbbbbbb","createdAt":"2026-05-21T00:00:00Z"},
+  {"tagName":"runtime-dev-20260520-ccccccc","createdAt":"2026-05-20T00:00:00Z"},
+  {"tagName":"runtime-dev-20260501-ddddddd","createdAt":"2026-05-01T00:00:00Z"},
+  {"tagName":"runtime-dev-20260430-eeeeeee","createdAt":"2026-04-30T00:00:00Z"}
+]
+JSON
+}
+
+@test "dry run keeps newest, recent, and protected immutable dev releases" {
+    now_epoch="$(date -u -d '2026-05-22T00:00:00Z' +%s)"
+
+    RUNTIME_DEV_RELEASES_JSON="$(fixture_json)" \
+    RUNTIME_DEV_RELEASE_NOW_EPOCH="$now_epoch" \
+    RUNTIME_DEV_RELEASE_KEEP_COUNT=2 \
+    RUNTIME_DEV_RELEASE_KEEP_DAYS=7 \
+    RUNTIME_DEV_RELEASE_DRY_RUN=true \
+        run "$SCRIPT" --repo airplanes-live/image \
+            --protected-tag runtime-dev-20260501-ddddddd
+
+    [ "$status" -eq 0 ]
+    [[ "$output" == *"keep   runtime-dev-20260522-aaaaaaa"* ]]
+    [[ "$output" == *"keep   runtime-dev-20260521-bbbbbbb"* ]]
+    [[ "$output" == *"keep   runtime-dev-20260520-ccccccc"* ]]
+    [[ "$output" == *"keep   runtime-dev-20260501-ddddddd"* ]]
+    [[ "$output" == *"delete runtime-dev-20260430-eeeeeee"* ]]
+    [[ "$output" == *"Dry run only; would delete 1 release(s)."* ]]
+    [[ "$output" != *"runtime-dev-latest ("* ]]
+    [[ "$output" != *"runtime-v1.0.0 ("* ]]
+}
+
+@test "non-dry run deletes selected releases with cleanup-tag" {
+    mkdir -p "$BATS_TEST_TMPDIR/bin"
+    cat > "$BATS_TEST_TMPDIR/bin/gh" <<'STUB'
+#!/usr/bin/env bash
+printf '%s\n' "$*" >> "$GH_LOG"
+STUB
+    chmod 0755 "$BATS_TEST_TMPDIR/bin/gh"
+
+    export PATH="$BATS_TEST_TMPDIR/bin:$PATH"
+    export GH_LOG="$BATS_TEST_TMPDIR/gh.log"
+    now_epoch="$(date -u -d '2026-05-22T00:00:00Z' +%s)"
+
+    RUNTIME_DEV_RELEASES_JSON='[
+      {"tagName":"runtime-dev-20260430-eeeeeee","createdAt":"2026-04-30T00:00:00Z"}
+    ]' \
+    RUNTIME_DEV_RELEASE_NOW_EPOCH="$now_epoch" \
+    RUNTIME_DEV_RELEASE_KEEP_COUNT=0 \
+    RUNTIME_DEV_RELEASE_KEEP_DAYS=0 \
+    RUNTIME_DEV_RELEASE_DRY_RUN=false \
+        run "$SCRIPT" --repo airplanes-live/image
+
+    [ "$status" -eq 0 ]
+    run grep -F "release delete runtime-dev-20260430-eeeeeee -R airplanes-live/image --yes --cleanup-tag" "$GH_LOG"
+    [ "$status" -eq 0 ]
+}