diff --git a/docs/Architecture.md b/docs/Architecture.md
index 8a83361..eb92ba0 100644
--- a/docs/Architecture.md
+++ b/docs/Architecture.md
@@ -95,8 +95,9 @@ hard wall; the classifier is a filter, not a guarantee.
 ### P5. Reversibility first
 
 - Work happens on **branches**, never on what others depend on (D4).
-- Work starts from a **fresh, clean workspace** (D3): a broken environment is
-  replaced, not diagnosed.
+- Work starts from a **fresh, clean workspace** (D3). A broken environment is
+  fixed the usual ways, and replaced when that doesn't work: the agent may ask
+  for its build VM to be reimaged rather than diagnose it at length.
 - **What must survive lives on the agent host and is snapshotted** (X4), and
   snapshots are taken before risky changes where the platform allows. Tool VMs,
   build artifacts and caches are disposable and rebuilt rather than repaired (X1).
@@ -703,6 +704,14 @@ reported but not asked about. Changes meant to last go into the repository's fil
 through a toolchain step; recurring ones go into the template (X4). Installing on
 any other host is dangerous.
 
+**A broken build VM blocks its projects.** When the usual fixes don't help, the
+driver asks for the VM to be reimaged from its template. The harness pushes the
+work in progress first, then blocks the VM: nothing runs on any project on it,
+across restarts, until the operator says `vm ready` (which also trusts its new
+host key), and a reimaged VM is set up again by the toolchain step. A VM that
+can't be reached blocks the same way (#137). The operator recovers it for now;
+`nisse-ops` later (O5).
+
 ### D4. Git, branches and PRs
 
 - **The harness owns branches;** plans and models never create or switch them.
diff --git a/docs/Development-Plan.md b/docs/Development-Plan.md
index 5f85132..7080c4e 100644
--- a/docs/Development-Plan.md
+++ b/docs/Development-Plan.md
@@ -415,6 +415,29 @@ files agree, with the repository as the record rather than a harness-kept one.
 - With it, D1's rule made structural: until the go-ahead the fast path refuses
   anything that isn't harmless, so needs go in the restatement.
 
+**M4.3** *(revised with the operator: a broken VM blocks its projects).*
+**Done** (this PR). Cleaning caches and reinstalling dependencies were already
+contained and reported as the actions they are, so what is left is a VM broken
+past the usual fixes (D3).
+- The driver can `request_reimage(reason)`, before the go-ahead too (it asks,
+  it doesn't act). Its prompt says to try the usual fixes first, then ask rather
+  than go on diagnosing.
+- The harness parks the work first (pushed to Gitea, as for a switch; a VM too
+  broken to push loses it, and the operator is told), then **blocks the build
+  VM**: a durable block per host, so every project on it waits and none of its
+  items is taken up, across restarts. Said once at top level with the projects
+  blocked and the command that clears it, and in each blocked item's thread.
+- The operator's `vm ready <project>` (no name needed when only one is blocked)
+  lifts the block and trusts the VM's new host key. A reimaged VM has no marker,
+  so the toolchain step sets it up again before work; one said ready without a
+  reimage goes straight on. The item that found it goes first.
+- `loop.qnt` gained the block (`blocked_vm_runs_nothing`); it found that work in
+  progress was lost on a reimage, hence parking first. The walk requests
+  reimages and recovers them, reimaged or not.
+- Next: an unreachable VM blocks through the same mechanism, unblocked by a slow
+  background retry once it is back (#137). Later, nisse-ops reimages a VM itself
+  (M6.2, M7.6); a changed address means editing `build_host` and restarting.
+
 ### Phase 5 — Memory
 
 **M5.1 The vault.** *Pre-approved.* The folder, per-agent mounts, control tools,
@@ -473,6 +496,7 @@ vision; one local model for every agent; several forges (#26); a room per projec
 | #69 inline review comments | M2.1 |
 | #70 watch stall | M2.1 |
 | #77 conversational agent | M2.3 (questions), M2.7 |
+| #137 block on an unreachable build VM | After M4.3, which built the block |
 
 ## 6. Implementation notes
 
diff --git a/nisse-dev/src/nisse_dev/loop/agent.py b/nisse-dev/src/nisse_dev/loop/agent.py
index 32abf62..63b1a06 100644
--- a/nisse-dev/src/nisse_dev/loop/agent.py
+++ b/nisse-dev/src/nisse_dev/loop/agent.py
@@ -84,6 +84,7 @@ from nisse_dev.loop.tools import (
     PROPOSE_PROJECT,
     PROPOSE_WORK,
     REPLY,
+    REQUEST_REIMAGE,
     RESTATE,
     START_MILESTONE,
     TOOLCHAIN_STEP,
@@ -109,6 +110,8 @@ _COMMITTED = (
     "finish_milestone, which pushes the milestone's branch and opens or "
     "updates its PR. There is no PR until then."
 )
+# 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"})
 # Near-misses for abandon: asked about, never taken for it or for a reply.
 _NEAR_ABANDON = frozenset(
@@ -125,6 +128,8 @@ _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)
 PROGRESS_EVERY_S = 60.0  # progress in prose at a readable rate (L5, #42)
 PROGRESS_CHARS = 400
 DETAILS = 10  # tool calls shown by `details`
@@ -162,6 +167,11 @@ docs and CI configuration in step, as part of the work. Before the go-ahead,
 you only look: say what the work needs in your restatement.
 Never change branches yourself.
 
+If the build VM itself seems broken, fix it the usual ways first: reinstall
+the project's dependencies, or a toolchain_step if the toolchain is wrong. If
+that doesn't help, don't spend the work diagnosing it: request_reimage, and it
+is rebuilt from its template.
+
 Before each group of tool calls, say in a sentence what you're doing and why:
 the operator sees it as your progress.
 
@@ -1273,6 +1283,16 @@ class LoopAgent:
             )
             self._schedule()
             return
+        ready = _VM_READY.fullmatch(command)
+        if ready is not None:
+            self._store.transcript(item.id if item else None, "operator").append(
+                UserMessage(text=text, source=f"transport:{msg.sender}")
+            )
+            self._transport.post(
+                self._vm_ready(ready.group(1), msg.sender), thread=msg.thread
+            )
+            self._schedule()
+            return
         named = _NAMED.fullmatch(command)
         if named is not None:
             # L5: a command that names its item applies to it.
@@ -1814,9 +1834,21 @@ class LoopAgent:
         """Ready items in the order they run (L1): the operator's order
         first, then the oldest."""
         items = self._store.work_items("open")  # oldest first
-        ready = [i for i in items if i.position == READY]
+        blocks = self._store.vm_blocks()
+        ready = [
+            i
+            for i in items
+            if i.position == READY and self._host_of(i.scope) not in blocks
+        ]
         return sorted(ready, key=lambda i: -loop_of(i).first)
 
+    def _host_of(self, project: str | None) -> str | None:
+        """The build VM a project's work runs on, as blocks name it (D3)."""
+        found = self._projects.get(project or "")
+        if found is None or found.workspace is None:
+            return None
+        return found.workspace.host.describe()
+
     def status(self) -> str:
         items = self._store.work_items("open")
         if not items:
@@ -1839,6 +1871,10 @@ class LoopAgent:
                 what += f", going ahead in {left} min unless you reply"
         elif item.position == BLOCKED:
             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})"
         elif item.position == READY:
             what = f"ready, {queue.index(item.id) + 1} of {len(queue)} in line"
         name = (
@@ -2425,6 +2461,7 @@ class LoopAgent:
         tools = [
             *([] if hold else [RESTATE]),
             ASK_HUMAN,
+            *([REQUEST_REIMAGE] if checked else []),
             *(
                 _LOOK
                 if not loop.started
@@ -2656,6 +2693,8 @@ class LoopAgent:
             question = str(args.get("question", "")).strip()
             self._answer_where_asked(item, loop, question)
             self._wait(item, replace(loop, wait=ANSWER, note=question), None)
+        elif outcome.kind == "wait" and outcome.tool == "request_reimage":
+            self._reimage(item, loop, str(args.get("reason", "")).strip())
         elif outcome.kind == "wait" and outcome.tool == "wait_for_review":
             note = str(args.get("note", "")).strip() or f"waiting on PR #{loop.pr}"
             if loop.pr:
@@ -2699,6 +2738,105 @@ class LoopAgent:
             self._wait(item, replace(loop, wait=ANSWER, note=note), note)
         self._store.set_active(None, cause="item waiting")
 
+    def _reimage(self, item: WorkItem, loop: Loop, reason: str) -> None:
+        """P5, D3: the driver found its build VM broken past the usual fixes.
+        The VM is blocked, so nothing runs on it, until the operator says
+        `vm ready`; the item that found it goes first after that."""
+        project = self._at(item)
+        host = self._host_of(project.name)
+        if host is None:
+            note = f"The build VM seems broken: {reason}. Reply once it's fixed."
+            self._wait(item, replace(loop, wait=ANSWER, note=note), note)
+            return
+        self._fact(item, f"build VM reimage requested: {reason}")
+        # Nothing of the work may live only on a VM about to be wiped: park
+        # it, as for a switch (pushed to Gitea). The VM may be too broken.
+        risk = ""
+        if project.milestones is not None:
+            try:
+                project.milestones.park(keep=self._keep(project.name))
+            except Exception as exc:  # noqa: BLE001 -- said; the reimage loses it
+                risk = (
+                    f"I couldn't push the work in progress first ({exc}): it "
+                    "will be lost with everything else on it."
+                )
+                self._fact(item, f"couldn't park the work before a reimage: {exc}")
+        first = max(loop_of(i).first for i in self._store.work_items("open")) + 1
+        save(
+            self._store,
+            item.id,
+            READY,
+            replace(loop, first=first, question=False, back_to="", back_note=""),
+            cause="build VM blocked",
+        )
+        self._block_vm(host, BROKEN, reason, item, risk)
+
+    def _block_vm(
+        self, host: str, cause: str, reason: str, by: WorkItem, 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}`."
+            )
+            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 "
+            "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."
+        )
+
+    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
+        run sets it up again from the repository."""
+        blocks = self._store.vm_blocks()
+        if name is None:
+            names = sorted(p for p in self._projects if self._host_of(p) in blocks)
+            if len(names) != 1:
+                return (
+                    "No build VM is blocked."
+                    if not names
+                    else "Which one? Say "
+                    + " or ".join(f"`vm ready {n}`" for n in names)
+                )
+            [name] = names
+        if name not in self._projects:
+            return f"I don't know a project called {name}."
+        host = self._host_of(name)
+        workspace = self._projects[name].workspace
+        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."
+
     # -- milestones (D4) -----------------------------------------------------
 
     def _start(self, item_id: str, args: dict[str, object]) -> tuple[str, bool]:
diff --git a/nisse-dev/src/nisse_dev/loop/tools.py b/nisse-dev/src/nisse_dev/loop/tools.py
index bcef23a..2936c81 100644
--- a/nisse-dev/src/nisse_dev/loop/tools.py
+++ b/nisse-dev/src/nisse_dev/loop/tools.py
@@ -78,6 +78,16 @@ TOOLCHAIN_STEP = _tool(
     reason="What the work needs, and why.",
 )
 
+REQUEST_REIMAGE = _tool(
+    "request_reimage",
+    "The build VM seems broken beyond the usual fixes (reinstalling "
+    "dependencies, a toolchain_step): ask for it to be reimaged from its "
+    "template rather than diagnosing it further. Anything not pushed on it is "
+    "lost. Ends your turn; the project waits until the operator says the VM "
+    "is ready, and it is set up again from the repository first.",
+    reason="What is broken, and what you tried.",
+)
+
 # A conversation's tools (L1, L5): it talks, and may propose work.
 REPLY = _tool(
     "reply",
@@ -122,6 +132,7 @@ WAITING = frozenset(
         "propose_work",
         "propose_project",
         "toolchain_ready",
+        "request_reimage",
     }
 )
 
diff --git a/nisse-dev/src/nisse_dev/state.py b/nisse-dev/src/nisse_dev/state.py
index 6405558..4a68897 100644
--- a/nisse-dev/src/nisse_dev/state.py
+++ b/nisse-dev/src/nisse_dev/state.py
@@ -29,7 +29,7 @@ from nisse_dev.canonical import (
 from nisse_dev.oplog import OpLog
 from nisse_dev.views import project
 
-CURRENT_SCHEMA_VERSION = 4
+CURRENT_SCHEMA_VERSION = 5
 
 # Stores older than this predate work items (Development Plan M1.1). There was
 # no production data to carry over, so there is no migration from them: start
@@ -38,8 +38,23 @@ OLDEST_SUPPORTED_VERSION = 4
 
 Migration = Callable[[sqlite3.Connection], None]
 
+_VM_BLOCKS = """
+    CREATE TABLE vm_blocks (
+        host TEXT PRIMARY KEY,
+        cause TEXT NOT NULL,
+        reason TEXT NOT NULL,
+        work_item TEXT,
+        since TEXT NOT NULL
+    )
+"""
+
+
+def _add_vm_blocks(conn: sqlite3.Connection) -> None:
+    conn.execute(_VM_BLOCKS)
+
+
 # Maps version N to the migration that takes the schema from N to N+1.
-MIGRATIONS: Mapping[int, Migration] = {}
+MIGRATIONS: Mapping[int, Migration] = {4: _add_vm_blocks}
 
 # Who produced an event (L7): the operator, the orchestrator's own code, or a
 # model role (L9). planner and executor are the old loop's (until M2.8), kept
@@ -77,6 +92,18 @@ class WorkItem:
     closed_at: str | None
 
 
+@dataclass(frozen=True)
+class VMBlock:
+    """A build VM nothing may run on until it is recovered (D3): every
+    project on it waits."""
+
+    host: str
+    cause: str  # broken (the driver said so); unreachable (#137)
+    reason: str
+    work_item: str | None  # the item that found it
+    since: str
+
+
 @dataclass(frozen=True)
 class Intent:
     id: int
@@ -142,6 +169,7 @@ _SCHEMA = [
         marked_at TEXT NOT NULL
     )
     """,
+    _VM_BLOCKS,
 ]
 
 
@@ -369,6 +397,34 @@ class StateStore:
             "work_item.close", cause=cause, work_item=item_id, outcome=outcome
         )
 
+    # --- build VMs (D3) ----------------------------------------------------
+
+    def block_vm(
+        self, host: str, *, cause: str, reason: str, work_item: str | None
+    ) -> None:
+        """Block a build VM; a second block on it keeps the first."""
+        row = self._conn.execute(
+            "INSERT INTO vm_blocks (host, cause, reason, work_item, since) "
+            "VALUES (?, ?, ?, ?, ?) ON CONFLICT(host) DO NOTHING RETURNING host",
+            (host, cause, reason, work_item, _now()),
+        ).fetchone()
+        if row is not None:
+            self.oplog.emit("vm.block", cause=cause, host=host, work_item=work_item)
+
+    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(
+            "DELETE FROM vm_blocks WHERE host = ? RETURNING *", (host,)
+        ).fetchone()
+        if row is None:
+            return None
+        self.oplog.emit("vm.unblock", cause=cause, host=host)
+        return VMBlock(**dict(row))
+
+    def vm_blocks(self) -> dict[str, VMBlock]:
+        rows = self._conn.execute("SELECT * FROM vm_blocks").fetchall()
+        return {row["host"]: VMBlock(**dict(row)) for row in rows}
+
     def carry_idle_messages(self, item_id: str) -> int:
         """Copy what the operator said while the agent was idle into a work
         item that has just opened (D1: messages sent before `use` are kept
diff --git a/nisse-dev/tests/test_loop_properties.py b/nisse-dev/tests/test_loop_properties.py
index b921e22..217fe2f 100644
--- a/nisse-dev/tests/test_loop_properties.py
+++ b/nisse-dev/tests/test_loop_properties.py
@@ -107,6 +107,9 @@ class PolicyModel:
                     toolchain=[{"name": "python", "version_command": command}],  # type: ignore[arg-type]
                 )
             ]
+        if "request_reimage" in offered and self.rng.random() < 0.03:
+            # M4.3: the build VM seems broken past the usual fixes.
+            return [call("request_reimage", reason=f"broken {self.n}")]
         roll = self.rng.random()
         if roll < 0.08:
             return [call("ask_human", question=f"question {self.n}?")]
@@ -196,7 +199,8 @@ class PolicyModel:
 
 class Watched:
     """The driver's model, watched: a run on a project item (not its
-    preflight) must find the project's marker on the VM."""
+    preflight) must find the project's marker on the VM, and no model runs
+    for an item whose build VM is blocked."""
 
     def __init__(self, walk: "Walk", inner: Any) -> None:
         self.walk = walk
@@ -211,6 +215,10 @@ class Watched:
         )
         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:
+            host = self.walk.worlds[item.scope]["host"].describe()
+            if host in self.walk.world["store"].vm_blocks():
+                self.walk.on_blocked.append(item.id)
         return self.inner.complete(events, tools, model, system)
 
 
@@ -291,6 +299,7 @@ class Walk:
         self.markers = Path(self.world["repo"].root).parent / "markers"
         self.unready: list[str] = []
         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":
             for w in worlds.values():
                 self._watch_merges(w["forge"])
@@ -395,6 +404,8 @@ class Walk:
             "issue": 1,
             "chat": 1,
         }
+        if self.world["store"].vm_blocks():
+            weights.update({"vm-ready": 3})
         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.
@@ -423,12 +434,20 @@ class Walk:
         if action == "intake":
             self.agent.intake()
         elif action == "wipe":
-            # The build VM is rebuilt (D3): the clone and its marker gone.
-            w = self.worlds[project]
-            shutil.rmtree(w["repo"].root, ignore_errors=True)
-            events = self.world["store"].all_events()
-            self.wiped_at[project] = events[-1].id if events else 0
-            (self.markers / f"{project}.ready").unlink(missing_ok=True)
+            self._wipe(project)
+        elif action == "vm-ready":
+            # M4.3: the operator recovers a blocked VM, reimaged or not; a
+            # reimage wipes every project on it.
+            blocks = self.world["store"].vm_blocks()
+            on_it = [
+                name
+                for name, w in self.worlds.items()
+                if w["host"].describe() in blocks
+            ]
+            if self.rng.random() < 0.5:
+                for name in on_it:
+                    self._wipe(name)
+            self.transport.receive(DON, f"vm ready {self.rng.choice(on_it)}")
         elif action == "done" and loop:
             # The operator says a new project is ready; no entry was added.
             self.transport.receive(DON, "done", thread=loop.thread)
@@ -557,6 +576,14 @@ class Walk:
             self.agent.watch()
             self.agent._schedule()
 
+    def _wipe(self, project: str) -> None:
+        """The build VM is rebuilt (D3): the clone and its marker gone."""
+        w = self.worlds[project]
+        shutil.rmtree(w["repo"].root, ignore_errors=True)
+        events = self.world["store"].all_events()
+        self.wiped_at[project] = events[-1].id if events else 0
+        (self.markers / f"{project}.ready").unlink(missing_ok=True)
+
     # -- the properties (specs/loop.qnt) -------------------------------------
 
     def _dangerous_only_when_allowed(self, where: str) -> None:
@@ -592,6 +619,10 @@ class Walk:
         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}"
+        # 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}"
+        )
         # D5: the agent asks Gitea to merge only what an operator approved
         # as it is (Gitea would refuse the rest; the harness mustn't ask).
         unapproved = [pr for pr, approved in self.agent_merges if not approved]
diff --git a/nisse-dev/tests/test_loop_toolchain.py b/nisse-dev/tests/test_loop_toolchain.py
index f0d2f1d..d20b907 100644
--- a/nisse-dev/tests/test_loop_toolchain.py
+++ b/nisse-dev/tests/test_loop_toolchain.py
@@ -3,7 +3,9 @@ the build VM is checked (baseline, clone, marker); without a marker the harness
 starts a toolchain step that sets the VM up from the repository and edits
 nothing there; the driver can ask for one once work is agreed, whose edits are
 part of the work; later runs check versions again; a rebuilt VM is re-cloned
-and set up again, and a changed host key waits for the operator."""
+and set up again, and a changed host key waits for the operator. A VM the
+driver finds broken blocks its projects until the operator says it's ready
+(M4.3)."""
 
 import io
 import shutil
@@ -27,6 +29,7 @@ from test_loop_milestones import world as world  # noqa: PLC0414 -- the fixture
 from nisse_dev.cancel import Cancel
 from nisse_dev.canonical import Fact, ToolResult
 from nisse_dev.execution import HostKeyChanged
+from nisse_dev.gitea import Issue
 from nisse_dev.loop import preflight
 from nisse_dev.loop.agent import LoopAgent, Project
 from nisse_dev.loop.driver import Driver
@@ -338,3 +341,130 @@ def test_before_the_go_ahead_the_driver_only_looks(
     assert "Python 3." in results[0]  # looking runs
     assert results[1].startswith("not run: not before the go-ahead")
     assert loop_of(the_item(world)).wait == GO_AHEAD
+
+
+# --- M4.3: a broken VM blocks its projects until it's ready ---------------------------
+
+
+def reimage(reason: str = "pip crashes in every venv, even a new one") -> list[Any]:
+    return [call("request_reimage", reason=reason)]
+
+
+def by_issue(world: dict[str, Any], number: int) -> Any:
+    [found] = [i for i in world["store"].work_items() if loop_of(i).issue == number]
+    return found
+
+
+def facts_of(world: dict[str, Any], item: Any) -> list[str]:
+    events = world["store"].transcript(item.id, "orchestrator").read()
+    return [e.event.text for e in events if isinstance(e.event, Fact)]
+
+
+def test_a_broken_vm_blocks_its_project_until_the_operator_says_ready(
+    world: dict[str, Any], project: str, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    world["forge"].issues.append(Issue(15, "Add count_lines", "count lines", "don", ()))
+    forgotten: list[str] = []
+    monkeypatch.setattr(
+        type(world["host"]), "forget_host_key", lambda self: forgotten.append("yes")
+    )
+    # Asked for before the go-ahead: it's a request, not an action.
+    model = ScriptedModel(ready(), reimage(), ready(), RESTATE, RESTATE)
+    agent, transport = agent_for(world, project, model)
+    agent.intake()
+    first, second = by_issue(world, 14), by_issue(world, 15)
+    assert "build VM reimage requested: pip crashes in every venv, even a new one" in (
+        facts_of(world, first)
+    )
+    [asked] = [
+        m for m in transport.outbound if "reimage it from its template" in m.text
+    ]
+    assert asked.thread is None  # top level: it can cover several projects
+    assert "pip crashes in every venv" in asked.text
+    assert f"`vm ready {project}`" in asked.text
+    assert any(f.startswith("Blocked: the build VM") for f in facts_of(world, second))
+    # 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()
+    # 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))}"')
+    transport.receive(DON, f"vm ready {project}")
+    assert forgotten == ["yes"]  # its new host key is trusted
+    assert world["store"].vm_blocks() == {}
+    assert any("cloned the workspace again" in f for f in facts_of(world, first))
+    # The item that found it went first, and set the VM up again.
+    assert loop_of(by_issue(world, 14)).wait == GO_AHEAD
+    assert loop_of(by_issue(world, 15)).wait == GO_AHEAD
+    setups = [
+        f
+        for item in (first, second)
+        for f in facts_of(world, item)
+        if "setting the build VM up from the repository" in f
+    ]
+    assert len(setups) == 2  # the first, and after the reimage
+    assert setups == [f for f in facts_of(world, first) if "setting the build VM" in f]
+
+
+def test_a_vm_said_ready_without_a_reimage_goes_straight_on(
+    world: dict[str, Any], project: str
+) -> None:
+    model = ScriptedModel(
+        ready(),
+        RESTATE,
+        [call("start_milestone", name="m"), *reimage("the build hangs")],
+        [*WORK[1:], *FINISH],
+    )
+    agent, transport = agent_for(world, project, model)
+    agent.intake()
+    say(transport, world, "go")
+    assert world["store"].vm_blocks()
+    transport.receive(DON, "vm ready")  # the only blocked VM: no name needed
+    # Not reimaged: the marker is still there, so no new setup step.
+    assert (
+        sum("setting the build VM up from the repository" in f for f in facts(world))
+        == 1
+    )
+    assert world["forge"].prs  # the work went on to a PR
+
+
+def test_vm_ready_with_nothing_blocked_says_so(
+    world: dict[str, Any], project: str
+) -> None:
+    _, transport = agent_for(world, project, ScriptedModel())
+    transport.receive(DON, "vm ready")
+    transport.receive(DON, f"vm ready {project}")
+    transport.receive(DON, "vm ready nowhere")
+    assert said(transport) == [
+        "No build VM is blocked.",
+        f"Nothing is blocking {project}'s build VM.",
+        "I don't know a project called nowhere.",
+    ]
+
+
+def test_work_in_progress_is_pushed_before_a_reimage(
+    world: dict[str, Any], project: str
+) -> None:
+    model = ScriptedModel(
+        ready(),
+        RESTATE,
+        [
+            call("start_milestone", name="m"),
+            call("write_file", path="w.py", content="x = 1\n"),
+            *reimage(),
+        ],
+        ready(),  # set up again after the reimage
+        [call("finish_milestone", summary="w.py, tested", final="yes")],
+    )
+    agent, transport = agent_for(world, project, model)
+    agent.intake()
+    say(transport, world, "go")
+    [asked] = [t for t in said(transport) if "reimage it from its template" in t]
+    assert "The work in progress is pushed" in asked
+    shutil.rmtree(world["repo"].root)
+    world["host"].execute(f'rm -f "{preflight.marker_path(project, markers(world))}"')
+    transport.receive(DON, f"vm ready {project}")
+    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)  # it survived the reimage
diff --git a/nisse-dev/tests/test_state.py b/nisse-dev/tests/test_state.py
index b392135..25861b3 100644
--- a/nisse-dev/tests/test_state.py
+++ b/nisse-dev/tests/test_state.py
@@ -121,3 +121,28 @@ def test_newer_schema_is_refused(db: Path) -> None:
         pass
     with pytest.raises(StateSchemaError, match="newer"):
         StateStore(db, schema_version=1)
+
+
+def test_a_vm_block_persists_across_reopen(db: Path) -> None:
+    with StateStore(db) as store:
+        store.block_vm("root@vm1", cause="broken", reason="pip crashes", work_item="x")
+        store.block_vm("root@vm1", cause="broken", reason="later", work_item="y")
+    with StateStore(db) as store:
+        [block] = store.vm_blocks().values()
+        assert (block.host, block.reason, block.work_item) == (
+            "root@vm1",
+            "pip crashes",  # the first block is kept
+            "x",
+        )
+        assert store.unblock_vm("root@vm1", cause="test") == block
+        assert store.unblock_vm("root@vm1", cause="test") is None
+        assert store.vm_blocks() == {}
+
+
+def test_a_version_4_store_gains_vm_blocks(db: Path) -> None:
+    with StateStore(db, schema_version=4) as store:
+        store._conn.execute("DROP TABLE vm_blocks")  # as a real version 4 store
+    with StateStore(db) as store:
+        assert store.schema_version == 5
+        store.block_vm("root@vm1", cause="broken", reason="r", work_item=None)
+        assert list(store.vm_blocks()) == ["root@vm1"]
diff --git a/specs/loop.qnt b/specs/loop.qnt
index 4370a52..825869d 100644
--- a/specs/loop.qnt
+++ b/specs/loop.qnt
@@ -61,6 +61,8 @@ module loop {
     opened: int -> int,       // PR key -> PRs ever opened for it
     // -- durable: Matrix holds messages until the agent handles them --
     inbox: List[Msg],
+    // -- durable: the state store's VM blocks (D3) --
+    vmBlocked: Set[str],      // projects whose build VM waits for recovery
     // -- volatile --
     work: str,                // none / publish
     pubStage: int,
@@ -70,6 +72,7 @@ module loop {
     earlyMs: bool,            // a milestone started before the previous merged
     lostFix: bool,            // a review fix pushed to a PR no longer open
     lostWork: bool,           // a parked item's unpushed work was set aside or wiped
+    ranBlocked: bool,         // an item was taken up on a blocked build VM
   }
 
   var s: Sys
@@ -98,9 +101,10 @@ module loop {
     items: Map(), nextItem: 1, active: 0, intents: Set(),
     issues: ISSUES.mapBy(_ => { open: false, assigned: false, linked: 0 }),
     depOpen: DEP_ISSUES.mapBy(_ => true),
-    prs: Map(), opened: Map(), inbox: List(),
+    prs: Map(), opened: Map(), inbox: List(), vmBlocked: Set(),
     work: "none", pubStage: 0, pending: 0,
     badGo: false, earlyMs: false, lostFix: false, lostWork: false,
+    ranBlocked: false,
   }
 
   // ---- the operator ----------------------------------------------------------
@@ -236,10 +240,12 @@ module loop {
   // sets aside whatever is left there.
   action activate(i: int): bool = all {
     s.active == 0, s.work == "none", live(s).contains(i), itm(s, i).status == "ready",
+    not(s.vmBlocked.contains(itm(s, i).project)),  // D3: nothing runs on a blocked VM
     s' = {
       val leftovers = s.items.keys().exists(j => j != i and itm(s, j).project == itm(s, i).project
                                                   and itm(s, j).unpushed)
-      { ...s, active: i, lostWork: s.lostWork or leftovers }
+      { ...s, active: i, lostWork: s.lostWork or leftovers,
+        ranBlocked: s.ranBlocked or s.vmBlocked.contains(itm(s, i).project) }
     },
   }
 
@@ -392,6 +398,30 @@ module loop {
         j != s.active and itm(s, j).project == p and itm(s, j).unpushed) },
   }
 
+  // The driver finds its build VM broken (request_reimage, D3): its work is
+  // parked (pushed), the VM blocked and the run ends; the item stays ready,
+  // to go on once it's fixed. (A VM too broken to push loses the work; the
+  // operator is told before reimaging it, accepted as for any wipe.)
+  action requestReimage: bool = {
+    val it = itm(s, s.active)
+    all {
+      s.active != 0, s.work == "none", s.intents.size() == 0, it.status == "ready",
+      s' = { ...upd(s, s.active, { ...it, unpushed: false }),
+             vmBlocked: s.vmBlocked.union(Set(it.project)), active: 0 },
+    }
+  }
+
+  // The operator's `vm ready`: the block lifts. A reimage loses what wasn't
+  // pushed on the VM; the operator was told so, and agreed by doing it.
+  action vmReady(p: str, reimaged: bool): bool = all {
+    s.vmBlocked.contains(p),
+    s' = { ...s,
+      vmBlocked: s.vmBlocked.exclude(Set(p)),
+      items: if (reimaged) s.items.keys().mapBy(j =>
+               if (itm(s, j).project == p) { ...itm(s, j), unpushed: false } else itm(s, j))
+             else s.items },
+  }
+
   // ---- crash and boot (L8) --------------------------------------------------
 
   // Volatile state is lost; boot reconciles each pending intent against
@@ -427,7 +457,7 @@ module loop {
       intake(anyIssue, p, b), observe(i), handleMsg,
       park, activate(i), restate(b), graceOver(i), createRecord, linkRecord,
       startMilestone, turn(outcome), finishMilestone, publishStep(b), answerReview,
-      finish, wipe(p), crash,
+      finish, wipe(p), crash, requestReimage, vmReady(p, b),
     }
   }
 
@@ -473,9 +503,13 @@ module loop {
       s.intents.forall(k => k / 100 != i),
     })
 
+  // Nothing is taken up on a build VM that is blocked (D3).
+  val blocked_vm_runs_nothing = not(s.ranBlocked)
+
   val all_properties = and {
     one_item_per_issue, waits_are_explainable, go_binds, milestone_after_merge,
     parked_work_safe, one_pr_per_milestone, review_fixes_land, closed_items_leave_nothing,
+    blocked_vm_runs_nothing,
   }
 
   // ---- reachability witnesses (each should be violated) --------------------------
@@ -485,5 +519,6 @@ module loop {
   val w_blocked = not(s.items.keys().exists(i => itm(s, i).status == "blocked"))
   val w_review = not(s.items.keys().exists(i => itm(s, i).review))
   val w_record = not(s.items.keys().exists(i => itm(s, i).src == "conv" and itm(s, i).issue != 0))
+  val w_vm_blocked = s.vmBlocked.size() == 0
   val w_finished = not(s.items.keys().exists(i => itm(s, i).status == "closed" and itm(s, i).ms > 0))
 }
diff --git a/specs/loop_scenarios.qnt b/specs/loop_scenarios.qnt
index 1d793d7..d031526 100644
--- a/specs/loop_scenarios.qnt
+++ b/specs/loop_scenarios.qnt
@@ -74,4 +74,15 @@ module loop_scenarios {
       .expect(s.issues.get(99).linked == 1)
       .then(linkRecord)
       .expect(one_item_per_issue and itm(s, 1).issue == 99)
+
+  // M4.3: a build VM the driver finds broken blocks its project; nothing is
+  // taken up on it until the operator says `vm ready`, then work goes on.
+  run aBrokenVmBlocksUntilReadyTest =
+    init
+      .then(assignIssue(1)).then(intake(1, "A", false))
+      .then(activate(1)).then(requestReimage)
+      .expect(s.vmBlocked.contains("A") and s.active == 0)
+      .then(vmReady("A", true))
+      .then(activate(1))
+      .expect(blocked_vm_runs_nothing and s.active == 1)
 }