diff --git a/scripts/mosaic-task.mjs b/scripts/mosaic-task.mjs
index 2a43f8db..86b71dcb 100755
--- a/scripts/mosaic-task.mjs
+++ b/scripts/mosaic-task.mjs
@@ -529,6 +529,52 @@ function retryRun(runId) {
   runTask(tempTaskFile, { retriedFrom: runId });
 }
 
+function pruneRuns(args) {
+  const resolved = loadConfig();
+  const root = runsRoot(resolved);
+  let keep = 50;
+  let apply = false;
+  for (const arg of args) {
+    if (arg === "--yes") apply = true;
+    else if (arg === "--keep") fail(4, "prune: --keep requires a value");
+    else if (arg.startsWith("--keep=")) {
+      keep = Number(arg.slice("--keep=".length));
+      if (!Number.isInteger(keep) || keep < 1) fail(4, `--keep must be a positive integer (got ${arg.slice(7)})`);
+    } else fail(4, `unknown prune option: ${arg}`);
+  }
+
+  let entries = [];
+  try {
+    entries = fs.readdirSync(root, { withFileTypes: true })
+      .filter((e) => e.name.startsWith("r-") && e.isDirectory() && !e.isSymbolicLink())
+      .map((e) => e.name)
+      .sort();
+  } catch {
+    // No runs yet.
+  }
+
+  if (entries.length <= keep) {
+    process.stdout.write(`prune: ${entries.length} run(s) present, keep=${keep} -> nothing to prune\n`);
+    process.exit(0);
+  }
+
+  const doomed = entries.slice(0, entries.length - keep); // oldest first
+  if (!apply) {
+    process.stdout.write(`prune (dry-run): would remove ${doomed.length} oldest run(s), keep ${entries.length - doomed.length}:\n`);
+    for (const id of doomed) process.stdout.write(`  would remove: ${id}\n`);
+    process.stdout.write("prune: re-run with --yes to apply\n");
+    process.exit(0);
+  }
+
+  const receipt = path.join(root, ".pruned.log");
+  for (const id of doomed) {
+    fs.rmSync(path.join(root, id), { recursive: true, force: true });
+    fs.appendFileSync(receipt, `${JSON.stringify({ at: new Date().toISOString(), event: "pruned", runId: id })}\n`);
+  }
+  process.stdout.write(`prune: removed ${doomed.length} run(s), kept ${keep}; receipt: ${receipt}\n`);
+  process.exit(0);
+}
+
 const operation = process.argv[2];
 const target = process.argv[3];
 
@@ -552,10 +598,13 @@ switch (operation) {
   case "list":
     listRuns();
     process.exit(0);
+  case "prune":
+    pruneRuns(process.argv.slice(3));
+    break;
   case "retry":
     if (!target) fail(4, "usage: mosaic-task.mjs retry <runId>");
     retryRun(target);
     break;
   default:
-    fail(4, `unknown operation: ${JSON.stringify(operation ?? "")} (expected validate | run | show | list | retry)`);
+    fail(4, `unknown operation: ${JSON.stringify(operation ?? "")} (expected validate | run | show | list | retry | prune)`);
 }
diff --git a/scripts/test-task.sh b/scripts/test-task.sh
index 53fce5f7..a7bb72a2 100755
--- a/scripts/test-task.sh
+++ b/scripts/test-task.sh
@@ -42,10 +42,6 @@ latest_reason() {
     [ -n "$latest" ] && node -e 'try{const r=JSON.parse(require("fs").readFileSync(process.argv[1],"utf8"));console.log(r.reason??"")}catch{console.log("")}' "$latest/result.json" 2>/dev/null
 }
 
-check() {
-    if [ "$2" = "0" ]; then PASS=$((PASS+1)); echo "${C_OK}OK${C_RESET}    $1"; else FAIL=$((FAIL+1)); echo "${C_FAIL}FAIL${C_RESET} $1"; fi
-}
-
 CONFIG="$SANDBOX/config.json"
 DATA_ROOT="$SANDBOX/data"
 mkdir -p "$DATA_ROOT"
@@ -108,6 +104,37 @@ $TASK validate "$SANDBOX/ok.json" >/dev/null 2>&1
 M2=$(stat -c %Y "$SANDBOX/ok.json")
 [ "$M1" = "$M2" ] && check "validation does not modify the task file" 0 || check "validation does not modify the task file" 1
 
+# ---------- retention: prune (deterministic, no Docker) ----------
+mkdir -p "$SANDBOX/data"
+cat > "$SANDBOX/prune-config.json" <<EOF
+{"configVersion":1,"environment":"development","dataRoot":"$SANDBOX/data","execution":{"backend":"docker","provider":"zai","model":"m"}}
+EOF
+for i in 1 2 3 4 5; do
+    D="$SANDBOX/data/runs/r-20260903T0100_0${i}Z-suite00$i"
+    mkdir -p "$D"
+    printf '{"runVersion":1,"runId":"r-20260903T0100_0%sZ-suite00%s","taskId":"t","status":"succeeded"}' "$i" "$i" > "$D/result.json"
+done
+mkdir -p "$SANDBOX/data/sessions/sentinel" "$SANDBOX/data/workspaces/sentinel"
+
+expect_exit "prune dry-run exits 0" 0 -- env MOSAIC_CONFIG="$SANDBOX/prune-config.json" node scripts/mosaic-task.mjs prune
+[ "$(ls "$SANDBOX/data/runs" | grep -c '^r-')" -eq 5 ] \
+    && check "dry-run deleted nothing" 0 || check "dry-run deleted nothing" 1
+
+expect_exit "prune --keep=2 --yes removes oldest" 0 -- env MOSAIC_CONFIG="$SANDBOX/prune-config.json" node scripts/mosaic-task.mjs prune --keep=2 --yes
+[ "$(ls "$SANDBOX/data/runs" | grep -c '^r-')" -eq 2 ] \
+    && check "kept exactly 2 newest runs" 0 || check "kept exactly 2 newest runs" 1
+NEWEST="r-20260903T0100_05Z-suite005"
+[ -d "$SANDBOX/data/runs/$NEWEST" ] \
+    && check "newest run kept, oldest pruned" 0 || check "newest run kept, oldest pruned" 1
+[ -f "$SANDBOX/data/runs/.pruned.log" ] \
+    && [ "$(grep -c 'pruned' "$SANDBOX/data/runs/.pruned.log")" -eq 3 ] \
+    && check "append-only receipt written (3 entries)" 0 \
+    || check "append-only receipt written (3 entries)" 1
+[ -d "$SANDBOX/data/sessions/sentinel" ] && [ -d "$SANDBOX/data/workspaces/sentinel" ] \
+    && check "sessions/workspaces untouched by prune" 0 \
+    || check "sessions/workspaces untouched by prune" 1
+expect_exit "prune with invalid keep exits 4" 4 -- env MOSAIC_CONFIG="$SANDBOX/prune-config.json" node scripts/mosaic-task.mjs prune --keep=0 --yes
+
 # ---------- adapter seam: deterministic mock cases (Docker, no provider) ----------
 if docker info >/dev/null 2>&1; then
     good_task "$SANDBOX/ok.json"
@@ -244,12 +271,6 @@ dump_latest_run() {
     fi
 }
 
-latest_reason() {
-    local latest
-    latest="$(ls -dt "$SANDBOX/data/runs"/r-* 2>/dev/null | head -1)"
-    [ -n "$latest" ] && node -e 'try{const r=JSON.parse(require("fs").readFileSync(process.argv[1],"utf8"));console.log(r.reason??"")}catch{console.log("")}' "$latest/result.json" 2>/dev/null
-}
-
 if docker info >/dev/null 2>&1; then
     RUNS1=$(ls "$DATA_ROOT/runs" 2>/dev/null | wc -l)
     if scripts/run-task.sh run "$SANDBOX/ok.json" >/dev/null 2>&1; then