M4.4: an unreachable build VM blocks its projects until it's back (#137)

M4.3's block, with new triggers and a way out. A VM out of reach at the
check before a run, in a run (which ends there: the driver, a toolchain
step and a requested step), or while preparing the workspace blocks every
project on it; the item stays ready, waiting on no one. On the intake round
it is tried again every tuning.build_vm.retry_s (300), and once it answers
it must still answer settle_s (180) later; then the work a lost run left
there is parked (pushed), the VM unblocked, and that said once.

A changed host key, at the check or on a retry, blocks the VM too, until
the operator's `vm ready`, which trusts the new key: `trust new host key`
is retired, and the message says it's a key problem that may be an
impersonation. Questions are answered while a VM is blocked, without it.
Starting the agent with a VM down no longer fails.

The walk breaks the VM's SSH, now or partway through a run, and checks that
no model works on a VM that is down; loop.qnt models the VM going and
coming back (unblocked_only_when_up). Together they found: a lost run's
work set aside by the next item there; a question left behind a block; a
lost VM swallowed while the toolchain step checked or recorded it; and a
remark waiting to be answered that outlived its PR's merge. All fixed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J5Utyfzw7gF7UhjRDEtjTk

nisse-dev/src/nisse_dev/config.py

@@ -137,6 +137,15 @@ class LoopTuning:
     general_idle_s: float = 86400.0
 
 
+@dataclass(frozen=True)
+class BuildVMTuning:
+    # An unreachable build VM is tried again this often (#137).
+    retry_s: float = 300.0
+    # Once it answers, it must still answer this much later before work
+    # goes on, so it has finished booting.
+    settle_s: float = 180.0
+
+
 @dataclass(frozen=True)
 class Tuning:
     driver: DriverTuning = field(default_factory=DriverTuning)
@@ -146,6 +155,7 @@ class Tuning:
     tools: ToolsTuning = field(default_factory=ToolsTuning)
     context: ContextTuning = field(default_factory=ContextTuning)
     loop: LoopTuning = field(default_factory=LoopTuning)
+    build_vm: BuildVMTuning = field(default_factory=BuildVMTuning)
 
 
 _TUNING_SECTIONS: dict[str, type] = {

nisse-dev/src/nisse_dev/loop/agent.py

@@ -56,7 +56,13 @@ from nisse_dev.llm import LLMError, RoleMap
 from nisse_dev.loop import preflight
 from nisse_dev.loop.actions import classify, commits
 from nisse_dev.loop.classifier import IN_SCOPE, ActionClassifier, Scope
-from nisse_dev.loop.driver import ASK_ALLOW, Driver, RunOutcome, Screening
+from nisse_dev.loop.driver import (
+    ASK_ALLOW,
+    Driver,
+    RunOutcome,
+    Screening,
+    VMUnreachable,
+)
 from nisse_dev.loop.help import ASK_EXPERT, DELEGATE, Helpers
 from nisse_dev.loop.items import (
     ANSWER,
@@ -112,7 +118,7 @@ _COMMITTED = (
 )
 # The operator's word that a blocked build VM is ready again (D3).
 _VM_READY = re.compile(r"vm ready(?: ([a-z0-9][a-z0-9-]*))?")
-_NEEDS_ITEM = frozenset({"go", "approve", "abandon", "allow", "trust new host key"})
+_NEEDS_ITEM = frozenset({"go", "approve", "abandon", "allow"})
 # Near-misses for abandon: asked about, never taken for it or for a reply.
 _NEAR_ABANDON = frozenset(
     {
@@ -129,7 +135,19 @@ _NEAR_ABANDON = frozenset(
 BUG_GRACE_S = 600.0
 GENERAL_IDLE_S = 86400.0
 _SAVED = "The work in progress is pushed; anything else on it will be lost."
-BROKEN = "broken"  # a VM block's cause: the driver found it broken (D3)
+# A build VM block's causes (D3): the driver found it broken (M4.3); it
+# can't be reached, and is tried again; it answered with a new host key.
+BROKEN = "broken"
+UNREACHABLE = "unreachable"
+HOST_KEY = "host key"
+# How each reads in a sentence about the VM.
+_BLOCKED_BY = {
+    BROKEN: "needs reimaging",
+    UNREACHABLE: "is out of reach",
+    HOST_KEY: "has a new host key",
+}
+VM_RETRY_S = 300.0
+VM_SETTLE_S = 180.0
 PROGRESS_EVERY_S = 60.0  # progress in prose at a readable rate (L5, #42)
 PROGRESS_CHARS = 400
 DETAILS = 10  # tool calls shown by `details`
@@ -213,6 +231,9 @@ The operator has asked a question (the last message). Answer it with reply,
 from what you know and what you can look at; don't change anything. The item
 then goes back to waiting exactly as it was, so a restatement up stays up."""
 
+OFFLINE = """ The build VM is out of reach just now, so you can't look at
+anything: answer from what you know, and say what you'd check."""
+
 CONVERSATION = """You are nisse-dev, a software agent, in a conversation with
 the operator in a Matrix thread. It isn't about a project yet: it may be a
 question, thoughts on feasibility or hardware, or the start of some work.
@@ -263,6 +284,11 @@ class Project:
     merge: str = "operator"  # the trust dial (L3): who merges its PRs
 
 
+def _minutes(seconds: float) -> str:
+    m = max(1, round(seconds / 60))
+    return f"{m} minute{'s' if m != 1 else ''}"
+
+
 def _restatement_problem(args: dict[str, object]) -> str | None:
     """L5, D1: a restatement is agreed to as it stands, so it asks nothing.
     One that ends in a question is sent back: questions go by ask_human."""
@@ -313,6 +339,8 @@ class LoopAgent:
         markers: str = preflight.MARKERS,
         clock: Callable[[], float] = time.time,
         intake_interval_s: float = INTAKE_INTERVAL_S,
+        vm_retry_s: float = VM_RETRY_S,
+        vm_settle_s: float = VM_SETTLE_S,
     ) -> None:
         self._store = store
         self._transport = transport
@@ -348,6 +376,11 @@ class LoopAgent:
         self._judged: dict[tuple[str, int, str], tuple[Verdict, tuple[str, str]]] = {}
         self._clock = clock
         self._interval = intake_interval_s
+        # Unreachable build VMs (#137): when each is next tried, and since
+        # when it has answered. Volatile: after a boot, tried at once.
+        self._vm_retry = vm_retry_s
+        self._vm_settle = vm_settle_s
+        self._probes: dict[str, tuple[float, float | None]] = {}
         # thread root -> revision on screen there, read on the transport's
         # thread to stamp replies on arrival (L3); (project, issue) -> thread
         # root, for a command that names its item.
@@ -471,21 +504,12 @@ class LoopAgent:
         root = str(project.workspace.root)
         try:
             ready = preflight.check(host, project.name, root, self._markers)
-        except HostKeyChanged:
-            note = (
-                f"the build VM for {project.name} ({host.describe()}) answered "
-                "with a different host key than the one I pinned. If you "
-                "rebuilt it, say `trust new host key` and I'll set it up "
-                "again; if you didn't, something is impersonating it."
-            )
-            self._wait(
-                item, replace(loop, wait=ANSWER, note=note, host_key_changed=True), note
-            )
+        except HostKeyChanged as exc:
+            self._block_vm(host.describe(), HOST_KEY, str(exc), item)
             self._store.set_active(None, cause="host key changed")
             return None
         except HostUnavailable as exc:
-            note = f"I can't reach the build VM: {exc}. Reply to retry."
-            self._wait(item, replace(loop, wait=ANSWER, note=note), note)
+            self._block_vm(host.describe(), UNREACHABLE, str(exc), item)
             self._store.set_active(None, cause="build VM unreachable")
             return None
         if ready.missing:
@@ -540,8 +564,9 @@ class LoopAgent:
         )
         try:
             now = preflight.versions(host, before)
-        except HostUnavailable:
-            return loop  # the run will find out, and say so
+        except HostUnavailable as exc:
+            self._settle(item.id, RunOutcome("unreachable", note=str(exc)))
+            return None
         changed = preflight.drift(before, now)
         if changed:
             said = "the toolchain changed since it was checked: " + "; ".join(changed)
@@ -590,9 +615,13 @@ class LoopAgent:
             if requested
             else "toolchain step: setting the build VM up from the repository",
         )
-        before = self._changed_files(project)
-        roles = self._roles_of(item)
         outer = self._running  # a requested step runs inside the driver's run
+        try:
+            before = self._changed_files(project)
+        except HostUnavailable as exc:
+            self._lost_in_step(item, outer, exc)
+            return None
+        roles = self._roles_of(item)
         if not outer:
             self._active_thread = loop.thread
             self._active_name = self._label(item)
@@ -620,6 +649,11 @@ class LoopAgent:
             self._cancel.clear()
             self._settle(item.id, outcome)
             return None
+        if outcome.kind == "unreachable":
+            if outer:
+                raise VMUnreachable(outcome.note)  # ends the driver's run too
+            self._settle(item.id, outcome)
+            return None
         host = workspace.host
         now = loop_of(self._store.work_item(item.id) or item)
         if outcome.kind == "wait" and outcome.tool == "toolchain_ready":
@@ -646,8 +680,10 @@ class LoopAgent:
                     "at": datetime.now(UTC).isoformat(timespec="seconds"),
                 },
             )
-        except HostUnavailable:
-            tools = report.tools
+        except HostUnavailable as exc:
+            # #137: no marker, so no work: the VM is blocked instead.
+            self._lost_in_step(item, outer, exc)
+            return None
         missing = [f"{name} ({why})" for name, why in report.missing]
         parts = [f"toolchain: {preflight.said(tools)}"]
         if installed:
@@ -658,7 +694,11 @@ class LoopAgent:
             parts.append(report.summary)
         if not requested:
             # D3: the harness's step edits nothing in the repository.
-            edited = sorted(set(self._changed_files(project)) - set(before))
+            try:
+                edited = sorted(set(self._changed_files(project)) - set(before))
+            except HostUnavailable as exc:
+                self._lost_in_step(item, outer, exc)
+                return None
             if edited:
                 parts.append(
                     "it changed files it shouldn't have, left for the work: "
@@ -671,16 +711,20 @@ class LoopAgent:
         loop = replace(now, toolchain=tuple(tuple(t.to_data()) for t in tools))
         return loop, said
 
+    def _lost_in_step(self, item: WorkItem, outer: bool, exc: Exception) -> None:
+        """A toolchain step lost its VM (#137): a requested one ends the
+        driver's run with it; the harness's own blocks the VM, settled."""
+        if outer:
+            raise VMUnreachable(str(exc)) from exc
+        self._settle(item.id, RunOutcome("unreachable", note=str(exc)))
+
     def _changed_files(self, project: Project) -> list[str]:
         """The workspace's changes, as git sees them (for the setup step's
         promise to edit nothing)."""
         assert project.workspace is not None
-        try:
-            result = project.workspace.host.execute(
-                "git status --porcelain", cwd=str(project.workspace.root), timeout_s=30
-            )
-        except HostUnavailable:
-            return []
+        result = project.workspace.host.execute(
+            "git status --porcelain", cwd=str(project.workspace.root), timeout_s=30
+        )  # HostUnavailable: the step's caller handles a lost VM (#137)
         return [line[3:] for line in result.stdout.splitlines() if line.strip()]
 
     def _requested_step(self, item_id: str, reason: str) -> tuple[str, bool]:
@@ -943,6 +987,7 @@ class LoopAgent:
         for project in self._projects.values():
             self._intake(project)
         self.watch()
+        self.check_vms()
         self.check_blocked()
         self.check_grace()
         self.check_idle()
@@ -1042,6 +1087,10 @@ class LoopAgent:
                 and self._at(item).merge == "approved"
             ):
                 pr, loop = self._merge_if_approved(item, loop, repo, pr)
+            if pr.state != "open" and loop.back_to == MERGE:
+                # A remark on it waited to be answered (a later run): the
+                # merge it would go back to waiting for is over.
+                loop = replace(loop, back_to="", back_note="")
             if pr.merged:
                 self._fact(item, f"PR #{loop.pr} merged")
                 loop, position = replace(loop, pr=0, pr_url="", branch=""), READY
@@ -1392,27 +1441,6 @@ class LoopAgent:
                 ),
                 cause="the operator asked a question",
             )
-        elif command == "trust new host key":
-            project = self._at(item)
-            if loop.host_key_changed and project.workspace is not None:
-                project.workspace.host.forget_host_key()
-                self._fact(
-                    item,
-                    f"the operator ({msg.sender}) trusted the build VM's new "
-                    "host key: it will be set up again",
-                )
-                save(
-                    self._store,
-                    item.id,
-                    READY,
-                    replace(loop, wait="", note="", host_key_changed=False),
-                    cause=f"new host key trusted by {msg.sender}",
-                )
-            else:
-                self._transport.post(
-                    f"Nothing on {self._label(item)} is waiting for that.",
-                    thread=loop.thread,
-                )
         elif command == "allow":
             if item.position == WAITING and loop.pending_action:
                 action = json.loads(loop.pending_action)
@@ -1838,7 +1866,9 @@ class LoopAgent:
         ready = [
             i
             for i in items
-            if i.position == READY and self._host_of(i.scope) not in blocks
+            if i.position == READY
+            # L5: a question is answered even while its VM is blocked.
+            and (self._host_of(i.scope) not in blocks or loop_of(i).question)
         ]
         return sorted(ready, key=lambda i: -loop_of(i).first)
 
@@ -1873,8 +1903,11 @@ class LoopAgent:
             what = f"blocked by {', '.join(loop.blocked_by)}"
         elif item.position == READY and item.id not in queue:
             block = self._store.vm_blocks().get(self._host_of(item.scope) or "")
-            why = block.reason if block else "unknown"
-            what = f"blocked: its build VM needs recovery ({why})"
+            what = (
+                f"blocked: its build VM {_BLOCKED_BY[block.cause]} ({block.reason})"
+                if block
+                else "blocked: its build VM is blocked"
+            )
         elif item.position == READY:
             what = f"ready, {queue.index(item.id) + 1} of {len(queue)} in line"
         name = (
@@ -2415,13 +2448,20 @@ class LoopAgent:
         checked = self._preflight and project.workspace is not None
         marker: dict[str, Any] | None = None
         starting = not loop.prepared
-        if checked and milestones is not None:
+        # L5: a question doesn't wait for a blocked build VM: it is answered
+        # without one (#137).
+        offline = loop.question and self._vm_blocked(project.name)
+        if checked and milestones is not None and not offline:
             ready = self._readiness(item, project, loop)
-            if ready is None:
-                return  # waiting on the operator
-            loop, marker = ready
-            starting = not loop.prepared
-        if milestones is not None:
+            if ready is None and loop.question and self._vm_blocked(project.name):
+                offline = True
+                self._store.set_active(item.id, cause="a question, without the VM")
+            elif ready is None:
+                return  # waiting on the operator, or on the VM
+            else:
+                loop, marker = ready
+                starting = not loop.prepared
+        if milestones is not None and not offline:
             keep = self._keep(project.name)
             try:
                 if not loop.prepared:
@@ -2434,6 +2474,12 @@ class LoopAgent:
                         loop.branch or f"{item.id}-work", keep=keep, mine=f"{item.id}-"
                     )
             except Exception as exc:  # noqa: BLE001 -- the item waits, not the loop
+                host = self._host_of(project.name)
+                if isinstance(exc.__cause__, HostUnavailable) and host is not None:
+                    # #137: the VM went between the check and this.
+                    self._block_vm(host, UNREACHABLE, str(exc.__cause__), item)
+                    self._store.set_active(None, cause="build VM unreachable")
+                    return
                 note = f"I couldn't prepare the workspace: {exc}. Reply to retry."
                 self._wait(item, replace(loop, wait=ANSWER, note=note), note)
                 self._store.set_active(None, cause="workspace failed")
@@ -2446,10 +2492,10 @@ class LoopAgent:
                 self._transport.post(said, thread=loop.thread)
             loop = replace(loop, prepared=True)
         save(self._store, item.id, RUNNING, loop, cause="driver run")
-        if checked and milestones is not None:
+        if checked and milestones is not None and not offline:
             toolchain = self._toolchain(item, project, loop, marker, starting)
             if toolchain is None:
-                return  # preflight was stopped: settled
+                return  # stopped, or the VM went: settled
             loop = toolchain
             save(self._store, item.id, RUNNING, loop, cause="toolchain checked")
         # D1: a `discuss` item talks before it restates.
@@ -2478,23 +2524,27 @@ class LoopAgent:
             ),
         ]
         if loop.question:
-            tools = [REPLY, *_LOOK]
+            tools = [REPLY] if offline else [REPLY, *_LOOK]
         elif loop.back_to and not loop.started:
             tools = [*tools, REPLY]  # before the go-ahead, a remark answered
         roles = self._roles_of(item)
-        workspace = project.workspace
-        system = SYSTEM.format(
-            repo=project.repo,
-            issue=loop.issue,
-            title=item.title,
-            labels="".join(f"{LABELS[x]}\n\n" for x in loop.labels if x in LABELS),
-            helpers=(
-                f"{self._helpers.briefing(roles)}\n\n"
-                if self._helpers and loop.started
-                else ""
-            ),
-            workspace=workspace.describe() if workspace else "",
-        ) + (QUESTION if loop.question else "")
+        workspace = None if offline else project.workspace
+        system = (
+            SYSTEM.format(
+                repo=project.repo,
+                issue=loop.issue,
+                title=item.title,
+                labels="".join(f"{LABELS[x]}\n\n" for x in loop.labels if x in LABELS),
+                helpers=(
+                    f"{self._helpers.briefing(roles)}\n\n"
+                    if self._helpers and loop.started
+                    else ""
+                ),
+                workspace=workspace.describe() if workspace else "",
+            )
+            + (QUESTION if loop.question else "")
+            + (OFFLINE if offline else "")
+        )
         self._active_thread = loop.thread
         self._active_name = self._label(item)
         self._running = True
@@ -2728,6 +2778,13 @@ class LoopAgent:
             summary = str(args.get("summary", "")).strip()
             self._transport.post(f"Done: {summary}", thread=loop.thread)
             self._close(item, loop, "completed", summary)
+        elif outcome.kind == "unreachable":
+            # #137: not the work's fault. The item goes on when the VM is back.
+            save(self._store, item.id, READY, loop, cause="build VM unreachable")
+            project = self._at(item)
+            host = self._host_of(project.name)
+            if host is not None:
+                self._block_vm(host, UNREACHABLE, outcome.note, item)
         elif outcome.kind == "stopped":
             note = f"stopped ({outcome.note}). Why did you stop it? Reply to continue."
             self._wait(item, replace(loop, wait=ANSWER, note=note), note)
@@ -2772,36 +2829,138 @@ class LoopAgent:
         self._block_vm(host, BROKEN, reason, item, risk)
 
     def _block_vm(
-        self, host: str, cause: str, reason: str, by: WorkItem, risk: str = ""
+        self,
+        host: str,
+        cause: str,
+        reason: str,
+        by: WorkItem | None,
+        risk: str = "",
     ) -> None:
         """Block a build VM (D3): every project on it waits until it is
         recovered. Said once at top level, and in each waiting item's thread.
-        The recovery is the operator's for now; nisse-ops' later (M7.6)."""
-        self._store.block_vm(host, cause=cause, reason=reason, work_item=by.id)
-        names = sorted(p for p in self._projects if self._host_of(p) == host)
-        blocked = [
-            i
-            for i in self._store.work_items("open")
-            if i.scope in names and i.scope != GENERAL
-        ]
-        for other in blocked:
-            line = (
-                f"Blocked: the build VM ({host}) needs reimaging. "
-                f"Waiting for `vm ready {other.scope}`."
+        The recovery is the operator's for now, or the VM's coming back;
+        nisse-ops' later (M7.6)."""
+        if host in self._store.vm_blocks():
+            return  # already said
+        self._store.block_vm(
+            host, cause=cause, reason=reason, work_item=by.id if by else None
+        )
+        self._probes.pop(host, None)
+        names = self._on_host(host)
+        for other in self._store.work_items("open"):
+            if other.scope not in names:
+                continue
+            line = f"Blocked: the build VM ({host}) {_BLOCKED_BY[cause]}. " + (
+                "I'll go on once it's back."
+                if cause == UNREACHABLE
+                else f"Waiting for `vm ready {other.scope}`."
             )
             self._fact(other, line)
             self._transport.post(line, thread=loop_of(other).thread)
-        self._transport.post(
-            f"The build VM {host} seems broken, found working on "
-            f"{self._label(by)}: {reason}\n\nThe projects on it are blocked "
-            f"until it's fixed: {', '.join(names)}. Could you reimage it from "
-            f"its template? {risk or _SAVED} If its "
+        self._transport.post(self._blocked_message(host, cause, reason, by, risk))
+
+    def _blocked_message(
+        self, host: str, cause: str, reason: str, by: WorkItem | None, risk: str
+    ) -> str:
+        names = self._on_host(host)
+        ready = " or ".join(f"`vm ready {n}`" for n in names)
+        found = f", found working on {self._label(by)}" if by else ""
+        blocked = f"The projects on it are blocked: {', '.join(names)}."
+        if cause == UNREACHABLE:
+            return (
+                f"I can't reach the build VM {host}{found}: {reason}\n\n"
+                f"{blocked} I'll try again every {_minutes(self._vm_retry)} and "
+                f"go on once it has answered for {_minutes(self._vm_settle)}. If "
+                f"it's gone for good, reimage it and say {ready}; anything not "
+                "pushed on it will be lost."
+            )
+        if cause == HOST_KEY:
+            return (
+                f"The build VM {host} answered with a different host key than "
+                f"the one I pinned{found}. {blocked}\n\nIf you rebuilt it, say "
+                f"{ready}: that trusts its new key, and I'll set it up again. "
+                "If you didn't, something may be impersonating it, so don't."
+            )
+        return (
+            f"The build VM {host} seems broken{found}: {reason}\n\n{blocked} "
+            f"Could you reimage it from its template? {risk or _SAVED} If its "
             "address changes, update `build_host` in my configuration and "
-            "restart me first.\n\nOnce it's ready, say "
-            + " or ".join(f"`vm ready {n}`" for n in names)
-            + "; that also trusts its new host key."
+            f"restart me first.\n\nOnce it's ready, say {ready}; that also "
+            "trusts its new host key."
         )
 
+    def _vm_blocked(self, project: str) -> bool:
+        return self._host_of(project) in self._store.vm_blocks()
+
+    def _on_host(self, host: str) -> list[str]:
+        return sorted(p for p in self._projects if self._host_of(p) == host)
+
+    def _park_on(self, host: str) -> None:
+        """A run cut off by its VM going away couldn't park its work: park
+        each project's workspace on the VM before anything else is taken up
+        there, which would set that work aside (#137). Raises when the VM
+        can't be reached."""
+        for name in self._on_host(host):
+            milestones = self._projects[name].milestones
+            if milestones is not None:
+                milestones.park(keep=self._keep(name))
+
+    def _unblock_vm(self, host: str, cause: str, said: str) -> None:
+        self._store.unblock_vm(host, cause=cause)
+        self._probes.pop(host, None)
+        for other in self._store.work_items("open"):
+            if other.scope in self._on_host(host):
+                self._fact(other, said)
+
+    def check_vms(self) -> None:
+        """#137: each unreachable build VM, tried again when due. Once it
+        answers, it must still answer a settle period later before work goes
+        on (SSH is up before the VM has finished booting). A new host key
+        is the operator's to trust; a broken VM, theirs to reimage."""
+        now = self._clock()
+        for host, block in self._store.vm_blocks().items():
+            if block.cause != UNREACHABLE:
+                continue
+            due, answered = self._probes.get(host, (now, None))
+            if now < due:
+                continue
+            names = self._on_host(host)
+            if not names:
+                continue  # no project on it any more
+            workspace = self._projects[names[0]].workspace
+            assert workspace is not None
+            try:
+                workspace.host.execute("true", timeout_s=30)
+            except HostKeyChanged as exc:
+                self._store.reblock_vm(host, cause=HOST_KEY, reason=str(exc))
+                self._probes.pop(host, None)
+                self._transport.post(
+                    self._blocked_message(host, HOST_KEY, str(exc), None, "")
+                )
+                continue
+            except HostUnavailable as exc:
+                self._oplog.emit("vm.unreachable", cause=str(exc), host=host)
+                self._probes[host] = (now + self._vm_retry, None)
+                continue
+            if answered is None or now - answered < self._vm_settle:
+                since = now if answered is None else answered
+                self._probes[host] = (since + self._vm_settle, since)
+                continue
+            try:
+                self._park_on(host)
+            except Exception as exc:  # noqa: BLE001 -- not back after all
+                self._oplog.emit("vm.unreachable", cause=repr(exc), host=host)
+                self._probes[host] = (now + self._vm_retry, None)
+                continue
+            self._unblock_vm(
+                host,
+                "it answered again",
+                f"the build VM ({host}) is back: work goes on",
+            )
+            self._transport.post(
+                f"The build VM {host} is back: {', '.join(names)} can go on."
+            )
+
     def _vm_ready(self, name: str | None, sender: str) -> str:
         """The operator's `vm ready`: the build VM's block lifts and its new
         host key is trusted (D3). A reimaged VM has no marker, so its first
@@ -2824,18 +2983,17 @@ class LoopAgent:
         if host is None or workspace is None or host not in blocks:
             return f"Nothing is blocking {name}'s build VM."
         workspace.host.forget_host_key()
-        block = self._store.unblock_vm(host, cause=f"vm ready from {sender}")
-        names = sorted(p for p in self._projects if self._host_of(p) == host)
-        for other in self._store.work_items("open"):
-            if other.scope in names:
-                self._fact(
-                    other,
-                    f"the operator ({sender}) said the build VM ({host}) is "
-                    "ready: its new host key is trusted, and it is set up again "
-                    "if it was reimaged",
-                )
-        assert block is not None  # checked above, on the worker
-        return f"{host} is unblocked: {', '.join(names)} can go on."
+        try:
+            self._park_on(host)
+        except Exception as exc:  # noqa: BLE001 -- reimaged: nothing left there
+            self._oplog.emit("vm.park_failed", cause=repr(exc), host=host)
+        self._unblock_vm(
+            host,
+            f"vm ready from {sender}",
+            f"the operator ({sender}) said the build VM ({host}) is ready: its "
+            "new host key is trusted, and it is set up again if it was reimaged",
+        )
+        return f"{host} is unblocked: {', '.join(self._on_host(host))} can go on."
 
     # -- milestones (D4) -----------------------------------------------------
 

nisse-dev/src/nisse_dev/loop/build.py

@@ -331,6 +331,8 @@ def build_loop_agent(
         preflight_timeout_s=config.tuning.preflight.command_timeout_s,
         bug_grace_s=config.tuning.loop.bug_grace_s,
         general_idle_s=config.tuning.loop.general_idle_s,
+        vm_retry_s=config.tuning.build_vm.retry_s,
+        vm_settle_s=config.tuning.build_vm.settle_s,
     )
     return RunningLoop(
         config=config,

nisse-dev/src/nisse_dev/loop/driver.py

@@ -29,13 +29,19 @@ _PROSE_NUDGE = (
 
 @dataclass(frozen=True)
 class RunOutcome:
-    # stuck: failed commands in a row hit the failure limit (L9's trigger)
-    kind: Literal["wait", "capped", "stuck", "stopped", "failed"]
+    # stuck: failed commands in a row hit the failure limit (L9's trigger);
+    # unreachable: the build VM went out of reach (#137), noted in ``note``
+    kind: Literal["wait", "capped", "stuck", "stopped", "failed", "unreachable"]
     tool: str = ""  # the waiting tool called
     arguments: dict[str, Any] | None = None
     note: str = ""
 
 
+class VMUnreachable(Exception):
+    """Raised by a handler whose work found the build VM out of reach: the
+    run ends there, as for an execution call (#137)."""
+
+
 # The waiting outcome a dangerous action ends a run with (P3): the harness
 # puts it to the operator, rendered from the action itself.
 ASK_ALLOW = "ask_allow"
@@ -157,7 +163,10 @@ class Driver:
                     )
                     return RunOutcome("wait", call.tool, dict(call.arguments))
                 if call.tool in handlers and call.tool in offered:
-                    result, ok = handlers[call.tool](dict(call.arguments))
+                    try:
+                        result, ok = handlers[call.tool](dict(call.arguments))
+                    except VMUnreachable as exc:
+                        return self._unreachable(transcript, calls, index, str(exc))
                 elif call.tool not in offered or workspace is None:
                     result, ok = (
                         f"no such tool: {call.tool}; the tools are "
@@ -200,6 +209,12 @@ class Driver:
                         )
                         break
                     outcome = workspace.dispatch(call.tool, call.arguments)
+                    if outcome.unreachable:
+                        # Not the work failing: no model call works on a VM
+                        # that isn't there (P6).
+                        return self._unreachable(
+                            transcript, calls, index, outcome.result
+                        )
                     result, ok = outcome.result, outcome.success
                     if screening.note:
                         result = f"{result}\n\n[harness] {screening.note}"
@@ -219,6 +234,20 @@ class Driver:
                         )
         return RunOutcome("capped", note=f"hit the {cap}-turn cap")
 
+    @staticmethod
+    def _unreachable(
+        transcript: Transcript, calls: list[ToolCall], index: int, said: str
+    ) -> RunOutcome:
+        transcript.append(
+            ToolResult(
+                call_id=calls[index].call_id,
+                result=f"not finished: {said}; your turn has ended",
+                success=False,
+            )
+        )
+        skip_calls(transcript, calls[index + 1 :], "not run: the turn had ended")
+        return RunOutcome("unreachable", note=said)
+
 
 def _brief(call: ToolCall, result: str) -> str:
     """A failed command and the end of what it said, for a helper's brief."""

nisse-dev/src/nisse_dev/loop/items.py

@@ -78,10 +78,8 @@ class Loop:
     # it's said once per commit and reason.
     merge_held: str = ""
     # The toolchain as the harness last checked it (M4.1): name, version
-    # command, version; and a build VM whose host key changed, waiting on
-    # the operator's word.
+    # command, version.
     toolchain: tuple[tuple[str, str, str], ...] = ()
-    host_key_changed: bool = False
 
     def to_data(self) -> dict[str, Any]:
         return asdict(self)
@@ -89,6 +87,8 @@ class Loop:
 
 def loop_of(item: WorkItem) -> Loop:
     data = dict(item.data.get("loop", {}))
+    # Retired: a changed host key now blocks the build VM (#137).
+    data.pop("host_key_changed", None)
     data["notes"] = tuple(data.get("notes", ()))
     data["blocked_by"] = tuple(data.get("blocked_by", ()))
     data["labels"] = tuple(data.get("labels", ()))

nisse-dev/src/nisse_dev/state.py

@@ -98,7 +98,7 @@ class VMBlock:
     project on it waits."""
 
     host: str
-    cause: str  # broken (the driver said so); unreachable (#137)
+    cause: str  # broken (the driver said so), unreachable, host key (#137)
     reason: str
     work_item: str | None  # the item that found it
     since: str
@@ -411,6 +411,15 @@ class StateStore:
         if row is not None:
             self.oplog.emit("vm.block", cause=cause, host=host, work_item=work_item)
 
+    def reblock_vm(self, host: str, *, cause: str, reason: str) -> None:
+        """A blocked VM's block, for another cause (an unreachable VM that
+        answers with a new host key)."""
+        self._conn.execute(
+            "UPDATE vm_blocks SET cause = ?, reason = ? WHERE host = ?",
+            (cause, reason, host),
+        )
+        self.oplog.emit("vm.block", cause=cause, host=host)
+
     def unblock_vm(self, host: str, *, cause: str) -> VMBlock | None:
         """Lift a build VM's block; returns it, or None if there was none."""
         row = self._conn.execute(

nisse-dev/src/nisse_dev/tools.py

@@ -93,6 +93,7 @@ EXECUTION_TOOLS: list[dict[str, Any]] = [
 class ToolOutcome:
     result: str
     success: bool
+    unreachable: bool = False  # the build VM couldn't be reached (#137)
 
 
 class Workspace:
@@ -118,7 +119,12 @@ class Workspace:
         # The project secrets the model's commands may see (D7); redacted from
         # whatever comes back.
         self._secrets = dict(secrets or {})
-        self._run(f"mkdir -p -- {shlex.quote(str(self.root))}")
+        try:
+            self._run(f"mkdir -p -- {shlex.quote(str(self.root))}")
+        except HostUnavailable:
+            # A VM down at startup mustn't stop the agent: the check before
+            # each run finds it, and blocks it (#137); cloning makes the root.
+            pass
 
     def with_timeout(self, timeout_s: int) -> "Workspace":
         """The same workspace with a longer limit per command (installs)."""
@@ -173,7 +179,9 @@ class Workspace:
             return ToolOutcome(f"missing argument {exc} for {tool}", success=False)
         except HostUnavailable as exc:
             # Not the work failing: the build VM is out of reach.
-            return ToolOutcome(f"the build VM is unreachable: {exc}", success=False)
+            return ToolOutcome(
+                f"the build VM is unreachable: {exc}", success=False, unreachable=True
+            )
 
     def read_file(self, path: str) -> ToolOutcome:
         target = self._resolve(path)
@@ -244,7 +252,9 @@ class Workspace:
                 timeout_s=self._timeout_s,
             )
         except HostUnavailable as exc:
-            return ToolOutcome(f"the build VM is unreachable: {exc}", success=False)
+            return ToolOutcome(
+                f"the build VM is unreachable: {exc}", success=False, unreachable=True
+            )
         if result.timed_out:
             return ToolOutcome(
                 f"command timed out after {self._timeout_s}s: {command}",

nisse-dev/tests/test_config.py

@@ -167,6 +167,7 @@ def test_tuning_is_read() -> None:
             "delegation": {"max_turns": 20, "max_tokens": 100000, "wall_clock_s": 600},
             "tools": {"command_timeout_s": 300, "max_output_chars": 2000},
             "loop": {"bug_grace_s": 120},
+            "build_vm": {"retry_s": 60},
         }
     )
     tuning = config.tuning
@@ -176,6 +177,7 @@ def test_tuning_is_read() -> None:
     assert tuning.delegation.wall_clock_s == 600.0
     assert tuning.tools.max_output_chars == 2000
     assert tuning.loop.bug_grace_s == 120.0
+    assert (tuning.build_vm.retry_s, tuning.build_vm.settle_s) == (60.0, 180.0)
 
 
 def test_a_partly_given_tuning_section_keeps_the_other_defaults() -> None:

nisse-dev/tests/test_loop.py

@@ -24,6 +24,7 @@ from nisse_dev.loop.items import CLOSED, GO_AHEAD, READY, RUNNING, WAITING, loop
 from nisse_dev.loop.reader import command_form
 from nisse_dev.oplog import OpLog
 from nisse_dev.state import StateStore
+from nisse_dev.tools import ToolOutcome
 from nisse_dev.transport import InMemoryTransport
 from nisse_dev.worker import InlineWorker
 
@@ -430,7 +431,7 @@ def test_a_command_that_works_resets_the_failure_count(tmp_path: Path) -> None:
     class Commands:
         def dispatch(self, tool: str, arguments: dict[str, Any]) -> Any:
             ok = arguments["command"] == "good"
-            return type("Outcome", (), {"result": "out", "success": ok})()
+            return ToolOutcome("out", ok)
 
     def run(command: str) -> list[Any]:
         return [call("run_command", command=command)]

nisse-dev/tests/test_loop_live_fixes.py

@@ -156,6 +156,34 @@ def test_a_review_comment_can_be_answered_without_a_fix(
     assert "It's a constant on purpose" in forge.posted[-1][1]
 
 
+def test_a_remark_waiting_to_be_answered_doesnt_outlive_the_merge(
+    world: dict[str, Any],
+) -> None:
+    """#137's walk: a review comment readies the item to answer it and go back
+    to waiting for the merge; if the merge comes before that run (a blocked
+    VM, a queue), there is no merge left to go back to."""
+    forge: Forge = world["forge"]
+    model = ScriptedModel(
+        RESTATE,
+        [
+            call("start_milestone", name="words"),
+            call("write_file", path="w.py", content="W = 1\n"),
+            call("finish_milestone", summary="words", final="yes"),
+        ],
+    )
+    agent, transport = loop_agent(world, model)
+    agent.intake()
+    say(transport, world, "go")
+    [pr] = forge.prs
+    forge.comments[pr] = [Comment(7, "don", "why a constant", "t")]
+    agent.watch()  # not run yet
+    assert loop_of(the_item(world)).back_to == MERGE
+    forge.merge(pr)
+    agent.watch()
+    loop = loop_of(the_item(world))
+    assert (loop.pr, loop.back_to, loop.back_note) == (0, "", "")
+
+
 # --- the session's later findings ----------------------------------------------
 
 

nisse-dev/tests/test_loop_properties.py

@@ -35,7 +35,7 @@ from nisse_dev.canonical import (
     UserMessage,
 )
 from nisse_dev.delegation import DELEGATION_INTENT, Caps
-from nisse_dev.execution import SSHHost
+from nisse_dev.execution import HostUnavailable, SSHHost
 from nisse_dev.gitea import Comment, Issue
 from nisse_dev.llm import LLMError, RoleMap
 from nisse_dev.loop.agent import LoopAgent, Project
@@ -209,13 +209,30 @@ class Watched:
     def complete(self, events, tools, model, system=None) -> list[Any]:
         offered = {t["function"]["name"] for t in tools}
         item = self.walk.world["store"].active_item()
-        working = "toolchain_ready" not in offered and "propose_work" not in offered
+        # Working on the VM: offered its tools (an answer given while the VM
+        # is blocked isn't, #137), and not setting it up.
+        working = (
+            "read_file" in offered
+            and "toolchain_ready" not in offered
+            and "propose_work" not in offered
+        )
         marked = (
             item is not None and (self.walk.markers / f"{item.scope}.ready").exists()
         )
         if item is not None and item.scope != "general" and working and not marked:
             self.walk.unready.append(item.id)
-        if item is not None and item.scope in self.walk.worlds:
+        if (
+            item is not None
+            and item.scope != "general"
+            and "read_file" in offered
+            and self.walk.down
+        ):
+            self.walk.while_down.append(item.id)
+        if (
+            item is not None
+            and "read_file" in offered
+            and item.scope in self.walk.worlds
+        ):
             host = self.walk.worlds[item.scope]["host"].describe()
             if host in self.walk.world["store"].vm_blocks():
                 self.walk.on_blocked.append(item.id)
@@ -282,7 +299,12 @@ class Walk:
     """``worlds``: project name -> its world (forge, bare repo, clone,
     milestones), all over one store."""
 
-    def __init__(self, worlds: dict[str, dict[str, Any]], seed: int) -> None:
+    def __init__(
+        self,
+        worlds: dict[str, dict[str, Any]],
+        seed: int,
+        monkeypatch: pytest.MonkeyPatch,
+    ) -> None:
         self.worlds = worlds
         self.world = next(iter(worlds.values()))  # the store's home
         self.rng = random.Random(seed)
@@ -298,6 +320,26 @@ class Walk:
         # without one.
         self.markers = Path(self.world["repo"].root).parent / "markers"
         self.unready: list[str] = []
+        # #137: the build VM out of reach, now or after a few more commands
+        # (so mid-run); model calls made while it was.
+        self.down = False
+        self.drop_in = 0
+        self.while_down: list[str] = []
+        self.checking = False  # the walk's own checks see the real VM
+        real = SSHHost.execute
+        walk = self
+
+        def execute(host: SSHHost, command: str, **kw: Any) -> Any:
+            if walk.checking:
+                return real(host, command, **kw)
+            if walk.drop_in:
+                walk.drop_in -= 1
+                walk.down = walk.down or walk.drop_in == 0
+            if walk.down:
+                raise HostUnavailable("ssh: connect to host: Connection timed out")
+            return real(host, command, **kw)
+
+        monkeypatch.setattr(SSHHost, "execute", execute)
         self.wiped_at: dict[str, int] = {}  # project -> last event before a wipe
         self.on_blocked: list[str] = []  # items run while their VM was blocked
         if self.merge == "approved":
@@ -406,6 +448,7 @@ class Walk:
         }
         if self.world["store"].vm_blocks():
             weights.update({"vm-ready": 3})
+        weights.update({"vm-up": 3} if self.down else {"vm-down": 1})
         item = self.rng.choice(open_items) if open_items else None
         loop = loop_of(item) if item else None
         # The forge an item's project lives on; a new issue picks one.
@@ -435,6 +478,14 @@ class Walk:
             self.agent.intake()
         elif action == "wipe":
             self._wipe(project)
+        elif action == "vm-down":
+            # #137: out of reach now, or partway through what comes next.
+            if self.rng.random() < 0.5:
+                self.down = True
+            else:
+                self.drop_in = self.rng.randint(1, 30)
+        elif action == "vm-up":
+            self.down, self.drop_in = False, 0
         elif action == "vm-ready":
             # M4.3: the operator recovers a blocked VM, reimaged or not; a
             # reimage wipes every project on it.
@@ -616,9 +667,18 @@ class Walk:
                     )
 
     def check(self) -> None:
+        self.checking = True
+        try:
+            self._check()
+        finally:
+            self.checking = False
+
+    def _check(self) -> None:
         where = f"seed {self.seed}, steps {self.steps}"
         # M4.1: the driver never works on a project whose VM has no marker.
         assert not self.unready, f"ready_before_work: {self.unready}: {where}"
+        # #137: no model works on a project whose build VM is out of reach.
+        assert not self.while_down, f"no_model_on_a_lost_vm: {self.while_down}: {where}"
         # M4.3: nothing runs for a project whose build VM is blocked.
         assert not self.on_blocked, (
             f"blocked_vm_runs_nothing: {self.on_blocked}: {where}"
@@ -859,13 +919,21 @@ class Walk:
         for item in self.items():
             if item.status != "open" or item.scope != name:
                 continue
+            events = self.world["store"].transcript(item.id, "driver").read()
+            # Only writes that happened: one cut off by a lost VM (#137) didn't.
+            done = {
+                e.event.call_id
+                for e in events
+                if isinstance(e.event, ToolResult) and e.event.success
+            }
             written = {
                 str(e.event.arguments.get("path"))
-                for e in self.world["store"].transcript(item.id, "driver").read()
+                for e in events
                 # The driver's own: a failed helper's are discarded on purpose.
                 if isinstance(e.event, ToolCall)
                 and e.event.tool == "write_file"
                 and e.actor == "driver"
+                and e.event.call_id in done
                 # D3: a wiped VM loses what wasn't pushed, accepted.
                 and e.id > self.wiped_at.get(name, 0)
             }
@@ -932,7 +1000,7 @@ def test_the_loop_keeps_its_properties(
     world: dict[str, Any], seed: int, monkeypatch: pytest.MonkeyPatch
 ) -> None:
     _seeded_ids(monkeypatch, seed)
-    walk = Walk({"sandbox": world}, seed)
+    walk = Walk({"sandbox": world}, seed, monkeypatch)
     for _ in range(STEPS):
         walk.step()
         walk.check()
@@ -948,7 +1016,7 @@ def test_two_projects_keep_the_properties(
     other = make_world(
         ssh_host, tmp_path / "other", store=sandbox["store"], repo_name="don/other"
     )
-    walk = Walk({"sandbox": sandbox, "other": other}, seed)
+    walk = Walk({"sandbox": sandbox, "other": other}, seed, monkeypatch)
     for _ in range(STEPS):
         walk.step()
         walk.check()

nisse-dev/tests/test_loop_toolchain.py

@@ -66,7 +66,7 @@ def markers(world: dict[str, Any]) -> str:
 
 
 def agent_for(
-    world: dict[str, Any], project: str, model: ScriptedModel
+    world: dict[str, Any], project: str, model: ScriptedModel, **more: Any
 ) -> tuple[LoopAgent, InMemoryTransport]:
     transport = InMemoryTransport()
     cancel = Cancel()
@@ -92,6 +92,7 @@ def agent_for(
         preflight=True,
         preflight_turns=5,
         markers=markers(world),
+        **more,
     )
     transport.on_message(loop.on_message)
     return loop, transport
@@ -236,7 +237,7 @@ def test_a_missing_baseline_waits_on_the_operator(
     assert loop.wait == ANSWER and "lacks no-such-tool-xyz" in loop.note
 
 
-def test_a_changed_host_key_waits_for_the_operators_word(
+def test_a_changed_host_key_blocks_the_vm_until_the_operators_word(
     world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
 ) -> None:
     real = preflight.check
@@ -254,13 +255,15 @@ def test_a_changed_host_key_waits_for_the_operators_word(
     )
     agent, transport = agent_for(world, project, ScriptedModel(ready(), RESTATE))
     agent.intake()
-    loop = loop_of(the_item(world))
-    assert loop.host_key_changed and "trust new host key" in loop.note
+    [block] = world["store"].vm_blocks().values()
+    assert block.cause == "host key"
+    [asked] = [t for t in said(transport) if "different host key" in t]
+    assert f"`vm ready {project}`" in asked and "impersonating" in asked
+    assert "has a new host key" in agent.status()
     say(transport, world, "it's fine")  # an answer isn't the operator's word
-    still = loop_of(the_item(world))
-    assert (still.wait, still.host_key_changed, forgotten) == (ANSWER, True, [])
+    assert world["store"].vm_blocks() and forgotten == []
     changed["on"] = False
-    say(transport, world, "trust new host key")
+    transport.receive(DON, f"vm ready {project}")
     assert forgotten == ["yes"]
     assert loop_of(the_item(world)).wait == GO_AHEAD  # set up, then planned
 
@@ -386,7 +389,7 @@ def test_a_broken_vm_blocks_its_project_until_the_operator_says_ready(
     # Neither runs while the VM is blocked, even after a restart.
     again, _ = agent_for(world, project, ScriptedModel())
     assert again._queue() == []
-    assert "its build VM needs recovery" in again.status()
+    assert "its build VM needs reimaging" in again.status()
     # The operator reimages it: the clone and the marker are gone.
     shutil.rmtree(world["repo"].root)
     world["host"].execute(f'rm -f "{preflight.marker_path(project, markers(world))}"')

nisse-dev/tests/test_loop_vm.py

@@ -0,0 +1,340 @@
+"""A build VM out of reach (D3, #137): it blocks every project on it, the run
+that finds it ends there, and it is tried again in the background; once it
+answers, and still answers a settle period later, work goes on by itself. A
+new host key is the operator's to trust (`vm ready`), and a VM the driver
+found broken is never unblocked by a probe."""
+
+from typing import Any
+
+import pytest
+from test_loop_milestones import DON, RESTATE, ScriptedModel, call, git, say
+from test_loop_milestones import item as the_item
+from test_loop_milestones import world as world  # noqa: PLC0414 -- the fixture
+from test_loop_toolchain import (
+    agent_for,
+    by_issue,
+    ready,
+    reimage,
+    said,
+)
+from test_loop_toolchain import project as project  # noqa: PLC0414 -- the fixture
+
+from nisse_dev.canonical import ToolResult
+from nisse_dev.execution import HostKeyChanged, HostUnavailable
+from nisse_dev.gitea import Issue
+from nisse_dev.loop.items import GO_AHEAD, READY, loop_of
+
+RETRY, SETTLE = 300.0, 180.0
+
+
+class Faults:
+    """The build VM's SSH, broken on demand: ``down`` makes it unreachable,
+    ``rekey`` makes it answer with a new host key, and ``drop_on`` takes it
+    down when a command containing that text runs. Probes are counted."""
+
+    def __init__(self, monkeypatch: pytest.MonkeyPatch, host: Any) -> None:
+        self.down = False
+        self.rekey = False
+        self.drop_on = ""
+        self.probes = 0
+        real = type(host).execute
+        faults = self
+
+        def execute(self: Any, command: str, **kw: Any) -> Any:
+            if faults.drop_on and faults.drop_on in command:
+                faults.down = True
+            if command == "true":
+                faults.probes += 1
+            if faults.rekey:
+                raise HostKeyChanged("the host key changed")
+            if faults.down:
+                raise HostUnavailable("ssh: connect to host: Connection timed out")
+            return real(self, command, **kw)
+
+        monkeypatch.setattr(type(host), "execute", execute)
+
+
+class Clock:
+    def __init__(self) -> None:
+        self.now = 1000.0
+
+    def __call__(self) -> float:
+        return self.now
+
+
+def blocks(world: dict[str, Any]) -> dict[str, str]:
+    return {h: b.cause for h, b in world["store"].vm_blocks().items()}
+
+
+def test_an_unreachable_vm_blocks_and_goes_on_once_settled(
+    world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    world["forge"].issues.append(Issue(15, "Add count_lines", "count lines", "don", ()))
+    faults = Faults(monkeypatch, world["host"])
+    clock = Clock()
+    faults.down = True
+    agent, transport = agent_for(
+        world,
+        project,
+        ScriptedModel(ready(), RESTATE, RESTATE),  # only once it's back
+        clock=clock,
+        vm_retry_s=RETRY,
+        vm_settle_s=SETTLE,
+    )
+    agent.intake()
+    assert list(blocks(world).values()) == ["unreachable"]
+    [told] = [m for m in transport.outbound if "I can't reach the build VM" in m.text]
+    assert told.thread is None
+    assert "try again every 5 minutes" in told.text
+    assert "answered for 3 minutes" in told.text
+    assert f"`vm ready {project}`" in told.text  # if it's gone for good
+    assert "is out of reach" in agent.status()
+    assert loop_of(by_issue(world, 14)).wait == ""  # not waiting on anyone
+    assert by_issue(world, 14).position == READY
+    # Tried again only when due, and quietly.
+    before = len(transport.outbound)
+    agent.intake()  # the first round after it went: tried at once
+    clock.now += 60
+    agent.intake()  # not due yet
+    assert faults.probes == 1
+    clock.now += RETRY
+    agent.intake()
+    assert faults.probes == 2
+    assert len(transport.outbound) == before
+    # Back: it answers, but work waits for it to settle.
+    faults.down = False
+    clock.now += RETRY
+    agent.intake()
+    assert blocks(world)
+    # Gone again while settling: the wait starts over.
+    faults.down = True
+    clock.now += SETTLE
+    agent.intake()
+    assert blocks(world)
+    faults.down = False
+    clock.now += RETRY
+    agent.intake()  # answers
+    clock.now += SETTLE - 10
+    agent.intake()  # not settled yet
+    assert blocks(world)
+    clock.now += 10
+    agent.intake()  # settled: unblocked, and the work runs
+    assert blocks(world) == {}
+    assert [t for t in said(transport) if "is back" in t] == [
+        f"The build VM {world['host'].describe()} is back: {project} can go on."
+    ]
+    assert loop_of(by_issue(world, 14)).wait == GO_AHEAD
+    assert loop_of(by_issue(world, 15)).wait == GO_AHEAD
+
+
+def test_a_vm_lost_mid_run_ends_the_run_and_blocks(
+    world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    faults = Faults(monkeypatch, world["host"])
+    faults.drop_on = "GO-DOWN"
+    model = ScriptedModel(
+        ready(),
+        RESTATE,
+        [
+            call("start_milestone", name="m"),
+            call("run_command", command="echo GO-DOWN"),
+            call("run_command", command="echo never"),
+        ],
+        # Nothing more: the model is never shown the failure.
+    )
+    agent, transport = agent_for(world, project, model, clock=Clock())
+    agent.intake()
+    say(transport, world, "go")
+    assert list(blocks(world).values()) == ["unreachable"]
+    item = the_item(world)
+    assert item.position == READY
+    results = [
+        e.event.result
+        for e in world["store"].transcript(item.id, "driver").read()
+        if isinstance(e.event, ToolResult)
+    ]
+    assert results[-2].startswith("not finished: the build VM is unreachable")
+    assert results[-1] == "not run: the turn had ended"
+    assert not model.turns
+
+
+def test_a_vm_lost_in_a_requested_step_ends_the_drivers_run(
+    world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    faults = Faults(monkeypatch, world["host"])
+    faults.drop_on = "GO-DOWN"
+    model = ScriptedModel(
+        ready(),
+        RESTATE,
+        [
+            call("start_milestone", name="m"),
+            call("toolchain_step", reason="it needs httpx"),
+        ],
+        [call("run_command", command="pip install httpx # GO-DOWN")],  # the step
+    )
+    agent, transport = agent_for(world, project, model, clock=Clock())
+    agent.intake()
+    say(transport, world, "go")
+    assert list(blocks(world).values()) == ["unreachable"]
+    assert the_item(world).position == READY
+    assert not model.turns  # neither the step nor the driver went on
+
+
+def test_a_vm_back_with_a_new_host_key_waits_for_the_operator(
+    world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    faults = Faults(monkeypatch, world["host"])
+    forgotten: list[str] = []
+    monkeypatch.setattr(
+        type(world["host"]), "forget_host_key", lambda self: forgotten.append("yes")
+    )
+    clock = Clock()
+    faults.down = True
+    agent, transport = agent_for(
+        world, project, ScriptedModel(ready(), RESTATE), clock=clock
+    )
+    agent.intake()
+    faults.down, faults.rekey = False, True
+    clock.now += RETRY
+    agent.intake()
+    assert list(blocks(world).values()) == ["host key"]
+    [asked] = [t for t in said(transport) if "different host key" in t]
+    assert f"`vm ready {project}`" in asked
+    clock.now += RETRY + SETTLE
+    agent.intake()
+    agent.intake()
+    assert blocks(world)  # never cleared by a probe
+    faults.rekey = False
+    transport.receive(DON, f"vm ready {project}")
+    assert forgotten == ["yes"]
+    assert blocks(world) == {}
+    assert loop_of(the_item(world)).wait == GO_AHEAD
+
+
+def test_the_operator_can_clear_an_unreachable_block(
+    world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    faults = Faults(monkeypatch, world["host"])
+    faults.down = True
+    agent, transport = agent_for(
+        world, project, ScriptedModel(ready(), RESTATE), clock=Clock()
+    )
+    agent.intake()
+    assert blocks(world)
+    faults.down = False
+    transport.receive(DON, "vm ready")
+    assert blocks(world) == {}
+    assert loop_of(the_item(world)).wait == GO_AHEAD
+
+
+def test_a_broken_vm_is_never_unblocked_by_a_probe(
+    world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    faults = Faults(monkeypatch, world["host"])
+    clock = Clock()
+    agent, _ = agent_for(world, project, ScriptedModel(ready(), reimage()), clock=clock)
+    agent.intake()
+    for _ in range(3):
+        clock.now += RETRY + SETTLE
+        agent.intake()
+    assert list(blocks(world).values()) == ["broken"]
+    assert faults.probes == 0
+
+
+def test_an_unreachable_block_survives_a_restart_and_is_tried_at_once(
+    world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    faults = Faults(monkeypatch, world["host"])
+    faults.down = True
+    clock = Clock()
+    agent, _ = agent_for(world, project, ScriptedModel(), clock=clock)
+    agent.intake()
+    agent.intake()
+    assert faults.probes == 1
+    again, _ = agent_for(world, project, ScriptedModel(), clock=clock)
+    again.boot()
+    again.intake()  # a new process knows the block, not when it last tried
+    assert faults.probes == 2
+    assert list(blocks(world).values()) == ["unreachable"]
+
+
+def test_a_question_is_answered_while_the_vm_is_blocked(
+    world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    faults = Faults(monkeypatch, world["host"])
+    model = ScriptedModel(
+        ready(), RESTATE, [call("reply", text="Because counting words is asked.")]
+    )
+    agent, transport = agent_for(world, project, model, clock=Clock())
+    agent.intake()
+    faults.down = True
+    say(transport, world, "why count words?")  # L5: a question
+    assert list(blocks(world).values()) == ["unreachable"]
+    assert "Because counting words is asked." in said(transport)
+    loop = loop_of(the_item(world))
+    assert loop.wait == GO_AHEAD  # back to its wait, restatement still up
+
+
+def test_a_vm_lost_while_the_step_records_it_blocks_before_any_work(
+    world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    faults = Faults(monkeypatch, world["host"])
+    faults.drop_on = "base64 -d >"  # writing the marker
+    model = ScriptedModel(ready())  # no driver run: there is no marker
+    agent, _ = agent_for(world, project, model, clock=Clock())
+    agent.intake()
+    assert list(blocks(world).values()) == ["unreachable"]
+    assert not model.turns
+
+
+def test_a_lost_runs_work_is_parked_before_the_vm_is_used_again(
+    world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    faults = Faults(monkeypatch, world["host"])
+    clock = Clock()
+    model = ScriptedModel(
+        ready(),
+        RESTATE,
+        [
+            call("start_milestone", name="m"),
+            call("write_file", path="w.py", content="x = 1\n"),
+            call("run_command", command="echo GO-DOWN"),
+        ],
+        [call("finish_milestone", summary="w.py, tested", final="yes")],
+    )
+    agent, transport = agent_for(world, project, model, clock=clock)
+    agent.intake()
+    faults.drop_on = "GO-DOWN"
+    say(transport, world, "go")  # writes w.py, then loses its VM
+    assert blocks(world)
+    faults.drop_on, faults.down = "", False
+    parked: list[str] = []
+    real = agent._schedule
+
+    def schedule() -> None:
+        # What Gitea holds when the VM is first used again.
+        refs = git("--git-dir", str(world["forge"].bare), "branch", "--list")
+        parked.append(refs)
+        real()
+
+    monkeypatch.setattr(agent, "_schedule", schedule)
+    clock.now += RETRY
+    agent.intake()  # answers
+    clock.now += SETTLE
+    agent.intake()  # settled: parked, unblocked, and the item goes on
+    assert blocks(world) == {}
+    assert f"{the_item(world).id}-parked" in parked[-1]
+    forge = world["forge"]
+    [pr] = [n for n, p in forge.prs.items() if not p["head"].startswith("nisse-check")]
+    assert "w.py" in forge.files(pr)
+
+
+def test_a_vm_lost_just_before_the_step_blocks_before_any_model_call(
+    world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    faults = Faults(monkeypatch, world["host"])
+    faults.drop_on = "git status --porcelain"  # what the step checks first
+    model = ScriptedModel()  # no model call at all
+    agent, _ = agent_for(world, project, model, clock=Clock())
+    agent.intake()
+    assert list(blocks(world).values()) == ["unreachable"]