Hold off resuming while the host is short on memory or swapping

README.md

@@ -62,6 +62,20 @@ runs `claude stop <old id>` after a successful resume. Without that the husk
 would be picked up again on a later tick and resumed into a *second* session
 working the same conversation — two agents racing on the same git worktree.
 
+### Host headroom
+
+A resume starts a fresh process that reloads a whole conversation, so before
+listing sessions — and again before each resume — the watchdog reads
+`/proc/meminfo` and holds off while the host is short: less than
+`--min-available-mb` of `MemAvailable`, or swap fuller than
+`--max-swap-used-pct`. Each resume of the same run is charged for its own
+headroom, because a session launched seconds ago has not grown into its memory
+yet.
+
+Holding off costs nothing: the block is still there on the next tick. On a host
+that reports no `MemAvailable` the guard stays inactive rather than blocking
+every resume.
+
 ### Circuit breakers
 
 Three limits stop a broken session, or a still-ongoing outage, from spawning an
@@ -165,12 +179,14 @@ Every option is a CLI flag with an environment-variable fallback — nothing is
 hardcoded. To make one permanent, add it to the crontab line.
 
 | Flag                  | Env var                      | Default      |
-| ----------------- | ------------------------- | -------------------- |
+| --------------------- | ---------------------------- | ------------ |
 | `--claude-bin`        | `CLAUDE_BIN`                 | `claude`     |
 | `--claude-home`       | `CLAUDE_HOME`                | `~/.claude`  |
 | `--max-attempts`      | `WATCHDOG_MAX_ATTEMPTS`      | `1`          |
 | `--max-per-day`       | `WATCHDOG_MAX_PER_DAY`       | `50`         |
 | `--max-per-run`       | `WATCHDOG_MAX_PER_RUN`       | `0` (all)    |
+| `--max-swap-used-pct` | `WATCHDOG_MAX_SWAP_USED_PCT` | `50`         |
+| `--min-available-mb`  | `WATCHDOG_MIN_AVAILABLE_MB`  | `2048`       |
 | `--prompt`            | `WATCHDOG_PROMPT`            | see `--help` |
 | `--dry-run`           | —                            | off          |
 

resume-stalled-sessions.py

@@ -22,6 +22,8 @@ from zoneinfo import ZoneInfo
 # Where `claude` installs itself, and what a scheduler's minimal PATH omits.
 CLAUDE_FALLBACK_DIRS = ("~/.local/bin", "/usr/local/bin", "/opt/homebrew/bin")
 
+MEMINFO = Path("/proc/meminfo")
+
 DEFAULT_PROMPT = (
     "Continue where you left off. Your previous run was interrupted by a "
     "transient failure (usage limit, API outage, or network), which has cleared."
@@ -86,6 +88,37 @@ def log(message):
     print(f"{datetime.now().astimezone().isoformat(timespec='seconds')} {message}", flush=True)
 
 
+def memory_strain(min_available_mb, max_swap_used_pct, meminfo=MEMINFO):
+    """Why the host cannot take another session, or None when it can.
+
+    Also None where the host does not report memory this way: a guard that
+    cannot read the numbers must not be the thing that stops recovery.
+    """
+    try:
+        lines = meminfo.read_text().splitlines()
+    except OSError:
+        return None
+    fields = {}
+    for line in lines:
+        key, _, rest = line.partition(":")
+        amount = rest.split()
+        if amount and amount[0].isdigit():
+            fields[key] = int(amount[0])
+
+    if "MemAvailable" not in fields:
+        return None
+    available_mb = fields["MemAvailable"] // 1024
+    if available_mb < min_available_mb:
+        return f"{available_mb} MB available, need {min_available_mb}"
+
+    swap_total = fields.get("SwapTotal", 0)
+    if swap_total:
+        used_pct = round(100 * (swap_total - fields.get("SwapFree", 0)) / swap_total)
+        if used_pct > max_swap_used_pct:
+            return f"swap {used_pct}% used, max {max_swap_used_pct}%"
+    return None
+
+
 def resolve_claude(name):
     """Absolute path to the Claude Code binary, or None if it cannot be found."""
     found = shutil.which(name)
@@ -318,6 +351,16 @@ def main():
         default=int(os.environ.get("WATCHDOG_MAX_PER_RUN", "0")),
         help="resumes per invocation, 0 for every eligible session (default: 0)",
     )
+    parser.add_argument(
+        "--max-swap-used-pct", type=int,
+        default=int(os.environ.get("WATCHDOG_MAX_SWAP_USED_PCT", "50")),
+        help="swap fill above which the host is already trading, so no resume (default: 50)",
+    )
+    parser.add_argument(
+        "--min-available-mb", type=int,
+        default=int(os.environ.get("WATCHDOG_MIN_AVAILABLE_MB", "2048")),
+        help="memory a resume needs the host to have spare, in MB (default: 2048)",
+    )
     parser.add_argument("--prompt", default=os.environ.get("WATCHDOG_PROMPT", DEFAULT_PROMPT))
     args = parser.parse_args()
 
@@ -347,6 +390,11 @@ def main():
         log(f"circuit breaker: {len(recent)} resumes in the last 24h (max {args.max_per_day})")
         return 0
 
+    strain = memory_strain(args.min_available_mb, args.max_swap_used_pct)
+    if strain:
+        log(f"holding off: {strain}")
+        return 0
+
     sessions = active_sessions(claude_bin)
     if sessions is None:
         return 1
@@ -358,6 +406,12 @@ def main():
         tried = ledger["attempts"].get(job["session_id"], 0)
         if tried >= args.max_attempts:
             continue
+        # A session launched seconds ago has not grown into its memory yet, so
+        # charge every resume of this run for the headroom it is about to take.
+        strain = memory_strain(args.min_available_mb * (resumed + 1), args.max_swap_used_pct)
+        if strain:
+            log(f"holding off: {strain}")
+            break
         if not resume(job, claude_bin, args.prompt, args.dry_run):
             continue
         ledger["attempts"][job["session_id"]] = tried + 1

test-resume-stalled-sessions.py

@@ -109,6 +109,45 @@ class ResetMoment(unittest.TestCase):
         self.assertIsNone(watchdog.reset_moment("API error — see detail", self.blocked_at))
 
 
+class MemoryStrain(unittest.TestCase):
+    """A host with no room left must not be given another session to run."""
+
+    def meminfo(self, available_kb, swap_total_kb=4194300, swap_free_kb=4194300):
+        temp = tempfile.NamedTemporaryFile("w", suffix=".meminfo", delete=False)
+        self.addCleanup(os.unlink, temp.name)
+        temp.write(
+            "MemTotal:       32877496 kB\n"
+            "MemFree:         1048576 kB\n"
+            f"MemAvailable:   {available_kb} kB\n"
+            f"SwapTotal:      {swap_total_kb} kB\n"
+            f"SwapFree:       {swap_free_kb} kB\n"
+        )
+        temp.close()
+        return Path(temp.name)
+
+    def test_healthy_host_takes_another_session(self):
+        self.assertIsNone(watchdog.memory_strain(2048, 50, self.meminfo(30083284)))
+
+    def test_low_memory_holds_off(self):
+        strain = watchdog.memory_strain(2048, 50, self.meminfo(1048576))
+        self.assertIn("1024 MB available", strain)
+
+    def test_swap_in_heavy_use_holds_off(self):
+        strain = watchdog.memory_strain(2048, 50, self.meminfo(30083284, swap_free_kb=1048575))
+        self.assertIn("swap 75% used", strain)
+
+    def test_swapless_host_is_judged_on_memory_alone(self):
+        self.assertIsNone(
+            watchdog.memory_strain(2048, 50, self.meminfo(30083284, swap_total_kb=0, swap_free_kb=0))
+        )
+
+    def test_host_that_does_not_report_memory_is_not_blocked(self):
+        self.assertIsNone(watchdog.memory_strain(2048, 50, Path("/nonexistent/meminfo")))
+        kernel_without_memavailable = self.meminfo(0)
+        kernel_without_memavailable.write_text("MemTotal:       32877496 kB\n")
+        self.assertIsNone(watchdog.memory_strain(2048, 50, kernel_without_memavailable))
+
+
 class Candidates(unittest.TestCase):
     """The whole path: a spend-limited background job with no process left."""