diff --git a/nix/checks.nix b/nix/checks.nix
index 046ad8e3e..496dfa496 100644
--- a/nix/checks.nix
+++ b/nix/checks.nix
@@ -83,6 +83,9 @@ in
   # Runs ./host-modules/lib/otlp-gauges.nix's `otlp_gauges` against a stub `curl`: the
   # payload shape, a failed push and a bad value both return 0, no endpoint pushes nothing.
   script-test-otlp-gauges = import ./script-tests/otlp-gauges.nix { inherit pkgs; };
+  # Runs the metric functions of the archive backup and prune scripts: the gauges for a
+  # success, a failure and a partial summary, the newest snapshot, and the output filter.
+  script-test-archive-metrics = import ./script-tests/archive-metrics.nix { inherit pkgs; };
   # Runs ./host-modules/lib/retry-in-job.nix's `retry_in_job`, which the agent
   # trust-bundle fetch retries through inside its start job: success after two
   # failures, the wait schedule, and a return of 1 once the budget is spent.
diff --git a/nix/host-modules/lib/archive-backup.nix b/nix/host-modules/lib/archive-backup.nix
index f5abf1c0a..1b1b24572 100644
--- a/nix/host-modules/lib/archive-backup.nix
+++ b/nix/host-modules/lib/archive-backup.nix
@@ -57,6 +57,14 @@ in
   root=${lib.escapeShellArg agentRoot}
   grants=${lib.escapeShellArg (grants hive)}
 
+  # restic's `--json` output on stdin, reduced to its error and summary lines;
+  # lines that are not JSON objects pass through.
+  restic_output_filter() {
+    jq -R -r '. as $line | ((fromjson? | select(type == "object")) // null) as $j
+      | if $j == null or $j.message_type == "error" or $j.message_type == "summary"
+        then $line else empty end'
+  }
+
   # One agent: a read-only snapshot of its subvolume when it is one, so the
   # databases are copied at one instant, else its directory in place.
   backup_agent() {
@@ -97,9 +105,7 @@ in
       ${restic}/bin/restic backup --json --host "$agent" --tag archive --exclude-caches \
         ${lib.concatMapStringsSep " " (e: "--exclude ${lib.escapeShellArg e}") excludes} \
         -- "''${targets[@]}" |
-        jq -R -r '. as $line | ((fromjson? | select(type == "object")) // null) as $j
-          | if $j == null or $j.message_type == "error" or $j.message_type == "summary"
-            then $line else empty end' || rc=$?
+        restic_output_filter || rc=$?
     fi
     if [ -n "$snap" ]; then
       btrfs subvolume delete "$snap" >/dev/null || rc=1
@@ -120,9 +126,9 @@ in
       metrics+=("hyperhive.archive.backup.last_success_timestamp_seconds=$finished")
       summary="$(printf '%s\n' "$out" | jq -R -r '
         fromjson? | select(type == "object" and .message_type == "summary")
-        | "hyperhive.archive.backup.data_added_bytes=\(.data_added)",
-          "hyperhive.archive.backup.files_new=\(.files_new)",
-          "hyperhive.archive.backup.files_changed=\(.files_changed)"' 2>/dev/null)" || summary=""
+        | {data_added_bytes: .data_added, files_new: .files_new, files_changed: .files_changed}
+        | to_entries[] | select(.value | type == "number")
+        | "hyperhive.archive.backup.\(.key)=\(.value)"' 2>/dev/null)" || summary=""
       while IFS= read -r line; do
         [ -z "$line" ] || metrics+=("$line")
       done <<<"$summary"
diff --git a/nix/script-tests/archive-metrics.nix b/nix/script-tests/archive-metrics.nix
new file mode 100644
index 000000000..d2c6db25c
--- /dev/null
+++ b/nix/script-tests/archive-metrics.nix
@@ -0,0 +1,59 @@
+# `checks.script-test-archive-metrics` — runs the metric functions of the archive's
+# backup and prune scripts (../host-modules/lib/archive-backup.nix and
+# archive-server.nix) against a stub `curl` and a fake `restic`. The cases are in
+# ./archive-metrics.sh, which cuts each function out of the rendered script.
+{
+  pkgs,
+  ...
+}:
+let
+  fakeRestic = pkgs.writeShellScriptBin "restic" ''
+    # Answers from the files named by FAKE_SNAPSHOTS and FAKE_STATS; an unset
+    # variable makes that subcommand fail.
+    case " $* " in
+      *" snapshots "*) [ -n "''${FAKE_SNAPSHOTS:-}" ] && cat "$FAKE_SNAPSHOTS" ;;
+      *" stats "*) [ -n "''${FAKE_STATS:-}" ] && cat "$FAKE_STATS" ;;
+      *) exit 1 ;;
+    esac
+  '';
+  endpoint = "http://10.42.0.1:4318";
+in
+pkgs.runCommand "hyperhive-script-test-archive-metrics"
+  {
+    nativeBuildInputs = [
+      pkgs.coreutils
+      pkgs.jq
+    ];
+    BACKUP_SCRIPT = pkgs.writeText "archive-backup.sh" (
+      import ../host-modules/lib/archive-backup.nix { inherit (pkgs) lib; } {
+        restic = fakeRestic;
+        hive = "h";
+        agentRoot = "/agents";
+        url = "https://archive.test";
+        scope = "paths";
+        paths = [ "state" ];
+        excludes = [ ];
+        caBundle = null;
+        otlpEndpoint = endpoint;
+      }
+    );
+    PRUNE_SCRIPT = pkgs.writeText "archive-prune.sh" (
+      (import ../host-modules/lib/archive-server.nix { inherit (pkgs) lib; } {
+        rclone = pkgs.rclone;
+        restic = fakeRestic;
+        reposDir = "/repos";
+        addr = "127.0.0.1:1";
+        authFile = "/auth";
+        authGroup = "g";
+        retention = {
+          keepWithin = null;
+          keepLast = 1;
+        };
+        otlpEndpoint = endpoint;
+      }).pruneScript
+    );
+  }
+  ''
+    ${pkgs.bash}/bin/bash ${./archive-metrics.sh}
+    touch "$out"
+  ''
diff --git a/nix/script-tests/archive-metrics.sh b/nix/script-tests/archive-metrics.sh
new file mode 100644
index 000000000..8d7aaae67
--- /dev/null
+++ b/nix/script-tests/archive-metrics.sh
@@ -0,0 +1,189 @@
+# Cases for the archive scripts' metric functions. Each function is cut out of
+# the rendered script by name (they start and end at column 0), so the test runs
+# the text the units run. `curl` is a stub that records the payload.
+set -uo pipefail
+
+failures=0
+
+fail() {
+  echo "FAILED: $case: $*" >&2
+  failures=$((failures + 1))
+}
+
+root=$(mktemp -d)
+mkdir -p "$root/bin"
+
+cat >"$root/bin/curl" <<STUB
+#!$BASH
+cat >"$root/curl.payload"
+STUB
+chmod +x "$root/bin/curl"
+
+# Prints function $2 of script $1: from `$2() {` to the first closing brace at column 0.
+cut_function() {
+  awk -v name="$2" '$0 ~ "^"name"\\(\\) \\{$" {on=1} on {print} on && /^}$/ {exit}' "$1"
+}
+
+{
+  cut_function "$BACKUP_SCRIPT" otlp_gauges
+  cut_function "$BACKUP_SCRIPT" push_backup_metrics
+  cut_function "$BACKUP_SCRIPT" restic_output_filter
+} >"$root/backup-fns.sh"
+{
+  cut_function "$PRUNE_SCRIPT" otlp_gauges
+  cut_function "$PRUNE_SCRIPT" push_repo_metrics
+} >"$root/prune-fns.sh"
+for f in backup-fns prune-fns; do
+  grep -q 'otlp_gauges()' "$root/$f.sh" || { echo "FAILED: $f has no otlp_gauges"; exit 1; }
+done
+grep -q 'push_backup_metrics()' "$root/backup-fns.sh" || { echo "FAILED: push_backup_metrics not found"; exit 1; }
+grep -q 'restic_output_filter()' "$root/backup-fns.sh" || { echo "FAILED: restic_output_filter not found"; exit 1; }
+grep -q 'push_repo_metrics()' "$root/prune-fns.sh" || { echo "FAILED: push_repo_metrics not found"; exit 1; }
+
+# Runs "$2 ..." with the functions of file $1 loaded; payload lands in curl.payload.
+run() {
+  local fns="$1"
+  shift
+  rm -f "$root/curl.payload"
+  PATH="$root/bin:$PATH" bash -c 'source "$1"; shift; "$@"' _ "$fns" "$@" >"$root/out" 2>"$root/err"
+}
+
+# The pushed gauges as `name=value` lines, in order.
+gauges() {
+  jq -r '.resourceMetrics[0].scopeMetrics[0].metrics[]
+    | "\(.name)=\(.gauge.dataPoints[0].asDouble)"' "$root/curl.payload"
+}
+
+want_gauges() {
+  local got
+  got="$(gauges 2>&1)"
+  [ "$got" = "$1" ] || fail "gauges were:
+$got
+want:
+$1"
+}
+
+summary='{"message_type":"summary","files_new":3,"files_changed":2,"files_unmodified":9,"data_added":1234,"total_duration":1.5}'
+status='{"message_type":"status","percent_done":0.5,"total_files":14}'
+error='{"message_type":"error","error":{"message":"read denied"}}'
+b=hyperhive.archive.backup
+
+case="a successful run with a summary pushes every gauge"
+if run "$root/backup-fns.sh" push_backup_metrics updo 1 100 130 "$status
+$summary"; then
+  want_gauges "$b.last_run_timestamp_seconds=130
+$b.last_run_success=1
+$b.duration_seconds=30
+$b.last_success_timestamp_seconds=130
+$b.data_added_bytes=1234
+$b.files_new=3
+$b.files_changed=2"
+  [ "$(jq -r '.resourceMetrics[0].resource.attributes[0].value.stringValue' "$root/curl.payload")" = hive-archive-backup ] ||
+    fail "service.name"
+  [ "$(jq -r '.resourceMetrics[0].scopeMetrics[0].metrics[0].gauge.dataPoints[0].attributes[0] | "\(.key)=\(.value.stringValue)"' "$root/curl.payload")" = agent=updo ] ||
+    fail "agent attribute"
+else
+  fail "failed: $(cat "$root/err")"
+fi
+
+case="a failed run pushes the result and no success time or counts"
+if run "$root/backup-fns.sh" push_backup_metrics updo 0 100 190 "$error"; then
+  want_gauges "$b.last_run_timestamp_seconds=190
+$b.last_run_success=0
+$b.duration_seconds=90"
+else
+  fail "failed: $(cat "$root/err")"
+fi
+
+case="a failed run ignores a summary in its output"
+if run "$root/backup-fns.sh" push_backup_metrics updo 0 100 190 "$summary"; then
+  want_gauges "$b.last_run_timestamp_seconds=190
+$b.last_run_success=0
+$b.duration_seconds=90"
+else
+  fail "failed: $(cat "$root/err")"
+fi
+
+case="a success without a summary (nothing to back up) pushes no counts"
+if run "$root/backup-fns.sh" push_backup_metrics updo 1 100 105 "updo has none of its backup paths yet; nothing to back up."; then
+  want_gauges "$b.last_run_timestamp_seconds=105
+$b.last_run_success=1
+$b.duration_seconds=5
+$b.last_success_timestamp_seconds=105"
+else
+  fail "failed: $(cat "$root/err")"
+fi
+
+case="a summary missing fields pushes the fields it has"
+if run "$root/backup-fns.sh" push_backup_metrics updo 1 100 130 '{"message_type":"summary","data_added":77}'; then
+  want_gauges "$b.last_run_timestamp_seconds=130
+$b.last_run_success=1
+$b.duration_seconds=30
+$b.last_success_timestamp_seconds=130
+$b.data_added_bytes=77"
+else
+  fail "failed: $(cat "$root/err")"
+fi
+
+case="the output filter keeps summary and error lines and drops progress"
+got="$(printf '%s\n' "$status" "$error" "$status" "$summary" "open repository" 5 |
+  PATH="$root/bin:$PATH" bash -c "source $root/backup-fns.sh; restic_output_filter")"
+want="$error
+$summary
+open repository
+5"
+[ "$got" = "$want" ] || fail "kept:
+$got
+want:
+$want"
+
+# --- the prune side ---
+r=hyperhive.archive
+printf '%s' '[{"id":"a","time":"2026-11-13T23:02:28.123456789+01:00"},{"id":"b","time":"2026-11-14T08:00:00.5Z"},{"id":"c","time":"2026-11-10T00:00:00Z"}]' >"$root/snapshots.json"
+printf '%s' '{"total_size":98765,"total_blob_count":3}' >"$root/stats.json"
+newest=$(date -d 2026-11-14T08:00:00.5Z +%s)
+
+case="a prune that worked pushes the result, snapshot count, newest snapshot and size"
+if FAKE_SNAPSHOTS="$root/snapshots.json" FAKE_STATS="$root/stats.json" \
+  run "$root/prune-fns.sh" push_repo_metrics /repos/updo updo 1; then
+  got="$(gauges)"
+  [ "$(printf '%s\n' "$got" | sed -n 1p)" = "$r.prune.last_run_success=1" ] || fail "first gauge: $got"
+  printf '%s\n' "$got" | grep -q "^$r.prune.last_run_timestamp_seconds=[0-9]" || fail "no prune timestamp: $got"
+  printf '%s\n' "$got" | grep -qx "$r.repo.snapshots=3" || fail "snapshot count: $got"
+  printf '%s\n' "$got" | grep -qx "$r.repo.latest_snapshot_timestamp_seconds=$newest" || fail "newest snapshot, want $newest: $got"
+  printf '%s\n' "$got" | grep -qx "$r.repo.size_bytes=98765" || fail "size: $got"
+  [ "$(jq -r '.resourceMetrics[0].resource.attributes[0].value.stringValue' "$root/curl.payload")" = swarm-archive-prune ] ||
+    fail "service.name"
+else
+  fail "failed: $(cat "$root/err")"
+fi
+
+case="a failed prune reports 0 and still reports the repository it can read"
+if FAKE_SNAPSHOTS="$root/snapshots.json" FAKE_STATS="$root/stats.json" \
+  run "$root/prune-fns.sh" push_repo_metrics /repos/updo updo 0; then
+  [ "$(gauges | sed -n 1p)" = "$r.prune.last_run_success=0" ] || fail "result: $(gauges)"
+  gauges | grep -qx "$r.repo.snapshots=3" || fail "count: $(gauges)"
+else
+  fail "failed: $(cat "$root/err")"
+fi
+
+case="an unreadable repository pushes only the prune result"
+if run "$root/prune-fns.sh" push_repo_metrics /repos/updo updo 0; then
+  got="$(gauges)"
+  [ "$(printf '%s\n' "$got" | wc -l)" = 2 ] || fail "want two gauges: $got"
+  printf '%s\n' "$got" | grep -q 'repo\.' && fail "repo gauge pushed: $got"
+else
+  fail "failed: $(cat "$root/err")"
+fi
+
+case="an empty repository has no newest snapshot"
+printf '%s' '[]' >"$root/empty.json"
+if FAKE_SNAPSHOTS="$root/empty.json" run "$root/prune-fns.sh" push_repo_metrics /repos/updo updo 1; then
+  gauges | grep -qx "$r.repo.snapshots=0" || fail "count: $(gauges)"
+  gauges | grep -q latest_snapshot && fail "newest pushed: $(gauges)"
+else
+  fail "failed: $(cat "$root/err")"
+fi
+
+[ "$failures" -eq 0 ] || exit 1
+echo "all cases passed"