diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 0000000..759f3c5
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,39 @@
+# Working on Sausage
+
+Sausage is meant to be read and changed by agents, including its own agent.
+Keep it that way.
+
+## Priorities
+
+1. **Correct.** Every behavior in `PROTOCOL.md` has a test in `cli/tests/`.
+2. **Simple and short.** Prefer the obvious code. No abstraction for one
+   caller. No configuration that nobody asked for.
+3. **Readable.** A new reader must understand a file without the others.
+   Each module starts with a doc comment that says what it owns.
+4. **Efficient** on the paths that run per message.
+5. **Raw.** Sausage has no permissions, sandboxing beyond WASI, limits, or
+   authentication. Do not add them to the core. People who want them can
+   build them as actors or as a fork.
+
+## Layout
+
+- `cli/`: the host (`sausage` binary and library). `broker.rs` routes and
+  supervises, `fs.rs` is the filesystem layer, `runtime.rs` runs Wasmtime,
+  `candidate.rs` tests new versions, `cargo_actor.rs` builds Rust actors.
+- `sdk/`: `sausage-actor`, the SDK for Rust actors. Never published: the
+  `cargo` actor points `sausage-actor = "0.1"` at its local copy.
+- `agent/`: the agent actor. `sausage init` embeds and builds it.
+- `examples/`: small actors. They are not workspace members.
+- `PROTOCOL.md`: the contract between the host and the actors. Change it in
+  the same commit as the behavior.
+
+## Checks
+
+```sh
+nix develop -c cargo test -p sausage -p sausage-actor
+nix develop -c cargo build --release -p agent --target wasm32-wasip2
+nix develop -c cargo clippy --workspace
+```
+
+`nix run -- init` and `nix run -- run` must keep working without other
+setup.
diff --git a/Cargo.lock b/Cargo.lock
index 8fced98..58e4cee 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1593,6 +1593,7 @@ dependencies = [
  "serde",
  "serde_json",
  "tokio",
+ "toml_edit",
  "wasmtime",
  "wasmtime-wasi",
  "wasmtime-wasi-http",
@@ -1939,6 +1940,19 @@ dependencies = [
  "serde_core",
 ]
 
+[[package]]
+name = "toml_edit"
+version = "0.25.17+spec-1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e3641d5bbb5349a79e1020a242d251efbc546ad8048d133958323ce9c40a9c9c"
+dependencies = [
+ "indexmap",
+ "toml_datetime",
+ "toml_parser",
+ "toml_writer",
+ "winnow",
+]
+
 [[package]]
 name = "toml_parser"
 version = "1.1.5+spec-1.1.0"
@@ -2801,6 +2815,9 @@ name = "winnow"
 version = "1.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "23b97319f7b8343df12cc98938e5c3eb436064524c8d2b4e30a1d3a36eecdf81"
+dependencies = [
+ "memchr",
+]
 
 [[package]]
 name = "winx"
diff --git a/PROTOCOL.md b/PROTOCOL.md
new file mode 100644
index 0000000..312c692
--- /dev/null
+++ b/PROTOCOL.md
@@ -0,0 +1,138 @@
+# The Sausage protocol
+
+This document is the contract between the host and the actors. An actor in
+any language that follows it works in Sausage.
+
+## Actors
+
+An actor is a WASI 0.2 command (`wasm32-wasip2`): a program with `main`. The
+host runs one instance per actor. An instance can run forever.
+
+The instance gets:
+
+- **stdin:** one message per line, from the broker
+- **stdout:** one message per line, to the broker
+- **stderr:** free text, appended to `/actor/<id>/stderr.log`
+- **environment:** `SAUSAGE_ACTOR` (its ID), `SAUSAGE_VERSION` (its version),
+  and `SAUSAGE_CANDIDATE=1` during a test run
+- **outgoing HTTP and HTTPS** through `wasi:http` (no raw sockets)
+- **the filesystem** described below
+
+## Messages
+
+A line on **stdin**:
+
+```json
+{"id": "m-17", "from": "console", "reply_to": "agent/a5", "body": {"type": "text", "text": "hi"}}
+```
+
+A line on **stdout**:
+
+```json
+{"to": "console", "id": "a6", "reply_to": "m-17", "body": {"type": "text", "text": "hello"}}
+```
+
+- `body` is required. By convention it is an object with a `"type"`.
+- `to` is the recipient. Without `to`, the broker publishes the message to
+  every subscriber of the sender.
+- `id` is optional. If the actor sets it, the message ID is `"<sender>/<id>"`.
+  Otherwise, the broker assigns `m-<n>`. The broker always sets `from`.
+- `reply_to` marks a reply. There is no request/response: a reply is a new
+  message whose `reply_to` is the ID of the message it answers.
+- Delivery is at most once. Each actor has a mailbox of 1,000 messages. The
+  mailbox keeps messages while the actor restarts or is upgraded.
+- If the broker cannot deliver a message, the sender gets
+  `{"type": "undeliverable", "reason": "...", "to": "..."}` from `broker`.
+
+Before the broker stops an instance, it sends
+`{"id": "broker/shutdown", "from": "broker", "body": {"type": "shutdown"}}`
+and then closes stdin. After 5 seconds, it kills the instance.
+
+## Filesystem
+
+Each instance sees three folders:
+
+| Path | Access | Contents |
+| --- | --- | --- |
+| `/data` | read-write | private state of the actor |
+| `/public` | read-write | files that the actor publishes |
+| `/actor` | read-only | the `actor/` folder of the world |
+
+`/actor/<id>/` contains `actor.wasm`, `actor.json`, `source/`, `public/`,
+`status.json`, `stderr.log`, and `versions/<n>/` of every actor. The `data/`
+folders (and the data snapshots in `versions/`) are hidden.
+
+A new or truncated file becomes visible when the writer closes it. Before
+that, readers see the old file or no file. If the instance crashes, the write
+is lost. A write to an existing file without truncate (append, edit) goes in
+place.
+
+Send large or binary data as a path under `/actor/<id>/public/`, not inside a
+message.
+
+## The broker
+
+Requests to `broker` (the reply has the same `reply_to`):
+
+| `type` | Fields | Effect |
+| --- | --- | --- |
+| `spawn` | `actor`, `wasm`, `source`?, `built_by`? | test, then start a new actor |
+| `upgrade` | `actor`, `wasm`, `source`?, `built_by`? | test, then swap the version |
+| `rollback` | `actor`, `version`? | go back to an older version and its data |
+| `stop`, `start` | `actor` | stop or start an actor |
+| `remove` | `actor` | move the actor's folder to `archive/` |
+| `subscribe`, `unsubscribe` | `actor` | receive (or not) what `actor` publishes |
+| `scan` | | boot new or changed `actor.wasm` files |
+| `list` | | all actors and their state |
+
+Paths: `/actor/...`, `/public/...` (the sender's), or `/data/...` (the
+sender's).
+
+**Versions.** `versions/<n>/` keeps every version that the broker saw:
+`actor.wasm`, `source/`, and `meta.json`. A version that the broker refused
+has `"rejected"` in its `meta.json`.
+
+**Tests.** Before a spawn or an upgrade, the broker runs the candidate on a
+copy of the actor's `/data` and `/public`, without HTTP. The tests are JSON
+files in `source/tests/*.json`:
+
+```json
+{
+  "description": "add, then list",
+  "steps": [
+    {"send": {"type": "add", "text": "milk"}, "expect": {"type": "added"}},
+    {"send": {"type": "list"}, "expect": [{"type": "notes"}]}
+  ]
+}
+```
+
+An `expect` entry matches when it is a subset of the reply body. A step can
+also have `from` (the sender, default `test`) and `to` (the expected
+recipient). Each file runs in a fresh instance. Without test files, the
+candidate must stay up for 2 seconds.
+
+**Swap.** The broker holds the mailbox, stops the old instance, saves a
+snapshot of `/data` and `/public` in the old version's folder, installs the new
+files, and starts the new instance.
+
+**Crashes.** The broker restarts a crashed actor (0.5 s, then doubling, at
+most 30 s). It keeps the message that the actor got just before the crash,
+and delivers it again after the next upgrade. A new version that crashes 3
+times within 30 seconds after an upgrade goes back to the previous version,
+with its data. The actor that asked for the upgrade gets `rolled_back`.
+
+**The folder is the truth.** At start, and on `scan`, the broker boots every
+`actor/<id>/actor.wasm`. If the file differs from the installed version, the
+broker copies it to `versions/<n>/` and upgrades to it. If the upgrade fails,
+the installed files come back.
+
+## The `cargo` actor
+
+```json
+{"type": "build", "source": "/actor/agent/public/drafts/notes"}
+```
+
+The reply is `{"type": "built", "wasm": "/actor/cargo/public/builds/<n>/<name>.wasm", ...}`
+or `{"type": "build_failed", "errors": "..."}`. A Rust actor depends on the SDK
+with `sausage-actor = "0.1"`. `cargo` points that dependency at the SDK of the
+host.
diff --git a/cli/Cargo.toml b/cli/Cargo.toml
index 6b24ed7..29943cf 100644
--- a/cli/Cargo.toml
+++ b/cli/Cargo.toml
@@ -18,6 +18,7 @@ include_dir = "0.7"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 tokio = { version = "1", features = ["full"] }
+toml_edit = "0.25"
 wasmtime = "49"
 wasmtime-wasi = "49"
 wasmtime-wasi-http = "49"
diff --git a/cli/src/broker.rs b/cli/src/broker.rs
index f185e68..be929d1 100644
--- a/cli/src/broker.rs
+++ b/cli/src/broker.rs
@@ -1,10 +1,12 @@
 //! The broker: message routing, mailboxes, subscriptions, and the lifecycle of
-//! WASM actors (spawn, upgrade, rollback, stop, start, remove).
+//! WASM actors (spawn, upgrade, rollback, stop, start, remove, scan).
 //!
 //! The broker accepts only `.wasm` files. It does not know any source language.
+//! The world folder is the source of truth: `scan` boots every
+//! `actor/<id>/actor.wasm` and treats a changed file as an upgrade.
 
-use std::collections::{BTreeSet, HashMap};
-use std::path::{Path, PathBuf};
+use std::collections::{BTreeMap, HashMap};
+use std::path::Path;
 use std::sync::atomic::{AtomicU64, Ordering};
 use std::sync::{Arc, Mutex};
 use std::time::{Duration, Instant};
@@ -17,7 +19,7 @@ use wasmtime::component::Component;
 use crate::candidate;
 use crate::fs::Mount;
 use crate::runtime::{Exit, Instance, InstanceConfig, Runtime};
-use crate::world::{self, ActorRecord, NATIVE, Registry, VersionMeta, World};
+use crate::world::{self, ActorMeta, NATIVE, VersionMeta, World};
 
 pub const MAILBOX_SIZE: usize = 1000;
 pub const SHUTDOWN_DEADLINE: Duration = Duration::from_secs(5);
@@ -51,13 +53,15 @@ pub fn shutdown_line() -> String {
     json!({"id": "broker/shutdown", "from": "broker", "body": {"type": "shutdown"}}).to_string()
 }
 
+type Reply = oneshot::Sender<Result<Value>>;
+
 enum Control {
-    Upgrade { version: u64, requester: String, reply: oneshot::Sender<Result<Value>> },
-    Rollback { target: Option<u64>, reply: oneshot::Sender<Result<Value>> },
-    Stop { reply: oneshot::Sender<Result<Value>> },
-    Start { reply: oneshot::Sender<Result<Value>> },
-    Remove { reply: oneshot::Sender<Result<Value>> },
-    Shutdown { reply: oneshot::Sender<Result<Value>> },
+    Upgrade { version: u64, requester: String, reply: Reply },
+    Rollback { target: Option<u64>, reply: Reply },
+    Stop { reply: Reply },
+    Start { reply: Reply },
+    Remove { reply: Reply },
+    Shutdown { reply: Reply },
 }
 
 pub struct Broker {
@@ -65,33 +69,36 @@ pub struct Broker {
     pub runtime: Arc<Runtime>,
     mailboxes: Mutex<HashMap<String, mpsc::Sender<Envelope>>>,
     controls: Mutex<HashMap<String, mpsc::Sender<Control>>>,
-    registry: Mutex<Registry>,
+    /// `actor.json` of every actor, native and WASM.
+    metas: Mutex<BTreeMap<String, ActorMeta>>,
     op_locks: Mutex<HashMap<String, Arc<tokio::sync::Mutex<()>>>>,
     seq: AtomicU64,
 }
 
 impl Broker {
     pub fn new(world: World, runtime: Arc<Runtime>) -> Result<Arc<Self>> {
-        let registry = world.load_registry()?;
+        let metas = world.actor_ids()?.into_iter().map(|id| (id.clone(), world.read_meta(&id))).collect();
         Ok(Arc::new(Self {
             world,
             runtime,
             mailboxes: Mutex::new(HashMap::new()),
             controls: Mutex::new(HashMap::new()),
-            registry: Mutex::new(registry),
+            metas: Mutex::new(metas),
             op_locks: Mutex::new(HashMap::new()),
             seq: AtomicU64::new(1),
         }))
     }
 
-    pub fn registry(&self) -> Registry {
-        self.registry.lock().unwrap().clone()
+    pub fn meta(&self, id: &str) -> Option<ActorMeta> {
+        self.metas.lock().unwrap().get(id).cloned()
     }
 
-    fn update_registry(&self, f: impl FnOnce(&mut Registry)) -> Result<()> {
-        let mut reg = self.registry.lock().unwrap();
-        f(&mut reg);
-        self.world.save_registry(&reg)
+    /// Change the `actor.json` of `id` (creating it if needed) and save it.
+    fn update_meta(&self, id: &str, f: impl FnOnce(&mut ActorMeta)) -> Result<()> {
+        let mut metas = self.metas.lock().unwrap();
+        let meta = metas.entry(id.to_string()).or_insert_with(|| ActorMeta { running: true, ..Default::default() });
+        f(meta);
+        self.world.write_meta(id, meta)
     }
 
     /// Create the mailbox of an actor. Messages wait here while the actor is
@@ -102,27 +109,23 @@ impl Broker {
         rx
     }
 
-    fn next_id(&self) -> String {
-        format!("m-{}", self.seq.fetch_add(1, Ordering::Relaxed))
-    }
-
-    /// Send a message from a native actor. `to: None` publishes to the
-    /// subscribers of `from`. Returns the message ID.
-    pub fn send(&self, from: &str, to: Option<&str>, id: Option<&str>, reply_to: Option<&str>, body: Value) -> String {
-        let id = match id {
+    /// Send a message. `to: None` publishes to the subscribers of `from`.
+    /// With `own_id`, the message ID is `"<from>/<own_id>"`. Returns the ID.
+    pub fn send(&self, from: &str, to: Option<&str>, own_id: Option<&str>, reply_to: Option<&str>, body: Value) -> String {
+        let id = match own_id {
             Some(own) => format!("{from}/{own}"),
-            None => self.next_id(),
+            None => format!("m-{}", self.seq.fetch_add(1, Ordering::Relaxed)),
         };
         let recipients: Vec<String> = match to {
             Some(to) => vec![to.to_string()],
             None => self
-                .registry
+                .metas
                 .lock()
                 .unwrap()
-                .subscriptions
-                .get(from)
-                .map(|s| s.iter().cloned().collect())
-                .unwrap_or_default(),
+                .iter()
+                .filter(|(_, m)| m.subscriptions.contains(from))
+                .map(|(id, _)| id.clone())
+                .collect(),
         };
         for to in recipients {
             self.deliver(Envelope {
@@ -142,8 +145,7 @@ impl Broker {
         if line.is_empty() {
             return;
         }
-        let parsed: Option<Value> = serde_json::from_str(line).ok();
-        let Some(Value::Object(obj)) = parsed else {
+        let Ok(Value::Object(obj)) = serde_json::from_str::<Value>(line) else {
             self.log(from, &format!("stdout line is not a JSON object, ignored: {}", truncate(line, 500)));
             return;
         };
@@ -151,12 +153,12 @@ impl Broker {
             self.log(from, &format!("message without \"body\", ignored: {}", truncate(line, 500)));
             return;
         };
-        let to = obj.get("to").and_then(Value::as_str);
-        let own_id = obj.get("id").and_then(|v| match v {
-            Value::String(s) => Some(s.clone()),
-            Value::Number(n) => Some(n.to_string()),
+        let own_id = match obj.get("id") {
+            Some(Value::String(s)) => Some(s.clone()),
+            Some(Value::Number(n)) => Some(n.to_string()),
             _ => None,
-        });
+        };
+        let to = obj.get("to").and_then(Value::as_str);
         let reply_to = obj.get("reply_to").and_then(Value::as_str);
         self.send(from, to, own_id.as_deref(), reply_to, body);
     }
@@ -164,24 +166,16 @@ impl Broker {
     fn deliver(&self, env: Envelope) {
         let tx = self.mailboxes.lock().unwrap().get(&env.to).cloned();
         let error = match tx {
-            None => Some(format!("no actor with ID {}", env.to)),
+            None => format!("no actor with ID {}", env.to),
             Some(tx) => match tx.try_send(env.clone()) {
-                Ok(()) => None,
-                Err(mpsc::error::TrySendError::Full(_)) => Some(format!("mailbox of {} is full", env.to)),
-                Err(mpsc::error::TrySendError::Closed(_)) => Some(format!("actor {} is gone", env.to)),
+                Ok(()) => return,
+                Err(mpsc::error::TrySendError::Full(_)) => format!("mailbox of {} is full", env.to),
+                Err(mpsc::error::TrySendError::Closed(_)) => format!("actor {} is gone", env.to),
             },
         };
-        if let Some(reason) = error
-            && env.from != "broker"
-            && env.from != "test"
-        {
-            self.send(
-                "broker",
-                Some(&env.from),
-                None,
-                Some(&env.id),
-                json!({"type": "undeliverable", "reason": reason, "to": env.to}),
-            );
+        if env.from != "broker" {
+            let body = json!({"type": "undeliverable", "reason": error, "to": env.to});
+            self.send("broker", Some(&env.from), None, Some(&env.id), body);
         }
     }
 
@@ -198,7 +192,7 @@ impl Broker {
         self.op_locks.lock().unwrap().entry(id.to_string()).or_default().clone()
     }
 
-    /// Mounts of a running actor.
+    /// Mounts of a running actor. `/actor` hides every `data/` folder.
     pub fn mounts(&self, id: &str) -> Vec<Mount> {
         vec![
             Mount { guest: "/actor".into(), host: self.world.actors_root(), writable: false },
@@ -211,185 +205,280 @@ impl Broker {
     // Lifecycle
     // ---------------------------------------------------------------------
 
-    /// Start the supervisors of all registered WASM actors.
-    pub fn start_all(self: &Arc<Self>) {
-        let reg = self.registry();
-        for (id, rec) in reg.actors {
-            self.start_supervisor(&id, rec.version, rec.running);
-        }
-    }
-
     fn start_supervisor(self: &Arc<Self>, id: &str, version: u64, running: bool) {
         let mailbox = self.open_mailbox(id);
         let (ctl_tx, ctl_rx) = mpsc::channel(16);
         self.controls.lock().unwrap().insert(id.to_string(), ctl_tx);
-        let sv = Supervisor::new(self.clone(), id.to_string(), version, running, mailbox, ctl_rx);
-        tokio::spawn(sv.run());
+        tokio::spawn(Supervisor::new(self.clone(), id.to_string(), version, running, mailbox, ctl_rx).run());
     }
 
-    async fn control(&self, id: &str, make: impl FnOnce(oneshot::Sender<Result<Value>>) -> Control) -> Result<Value> {
+    fn is_supervised(&self, id: &str) -> bool {
+        self.controls.lock().unwrap().contains_key(id)
+    }
+
+    async fn control(&self, id: &str, make: impl FnOnce(Reply) -> Control) -> Result<Value> {
         let tx = self.controls.lock().unwrap().get(id).cloned().ok_or_else(|| anyhow!("no WASM actor with ID {id}"))?;
         let (reply, rx) = oneshot::channel();
         tx.send(make(reply)).await.map_err(|_| anyhow!("supervisor of {id} is gone"))?;
         rx.await.map_err(|_| anyhow!("supervisor of {id} did not answer"))?
     }
 
-    /// Stop every actor gracefully (host shutdown). The registry keeps their
-    /// `running` flags, so the next `sausage run` starts them again.
-    pub async fn shutdown_all(&self) {
+    /// Stop every actor gracefully, in parallel (host shutdown). The
+    /// `running` flags stay, so the next `sausage run` starts them again.
+    pub async fn shutdown_all(self: &Arc<Self>) {
         let ids: Vec<String> = self.controls.lock().unwrap().keys().cloned().collect();
-        let mut waits = Vec::new();
+        let mut tasks = tokio::task::JoinSet::new();
         for id in ids {
-            waits.push(async move {
-                let _ = self.control(&id, |reply| Control::Shutdown { reply }).await;
-            });
+            let this = self.clone();
+            tasks.spawn(async move { this.control(&id, |reply| Control::Shutdown { reply }).await });
         }
-        futures_join_all(waits).await;
+        tasks.join_all().await;
     }
 
-    /// Store a new version: copy the `.wasm` file and the source folder into
-    /// the version store. Returns the version directory and the component.
-    fn store_version(&self, id: &str, version: u64, wasm: &Path, source: Option<&Path>, meta: &VersionMeta) -> Result<(PathBuf, Component)> {
-        let dir = self.world.version_dir(id, version);
-        let _ = std::fs::remove_dir_all(&dir);
+    /// Copy a `.wasm` file and its source folder into `versions/<n>/`.
+    /// Every version that the broker sees stays there, also rejected ones.
+    fn store_version(&self, id: &str, meta: &VersionMeta, wasm: &Path, source: Option<&Path>) -> Result<Component> {
+        let dir = self.world.version_dir(id, meta.version);
         std::fs::create_dir_all(&dir)?;
         std::fs::copy(wasm, dir.join("actor.wasm")).with_context(|| format!("copy {}", wasm.display()))?;
-        match source {
-            Some(src) => world::copy_tree(src, &dir.join("source"))?,
-            None => std::fs::create_dir_all(dir.join("source"))?,
-        }
+        world::replace_tree(source.unwrap_or(&dir.join("no-source")), &dir.join("source"))?;
         world::write_atomic(&dir.join("meta.json"), serde_json::to_string_pretty(meta)?.as_bytes())?;
-        let component = self.runtime.load(&dir.join("actor.wasm"))?;
-        Ok((dir, component))
+        self.runtime.load(&dir.join("actor.wasm"))
+    }
+
+    /// Record in `versions/<n>/meta.json` why the broker refused a version.
+    fn reject(&self, id: &str, version: u64, reason: &anyhow::Error) {
+        let path = self.world.version_dir(id, version).join("meta.json");
+        let mut meta: VersionMeta = std::fs::read_to_string(&path)
+            .ok()
+            .and_then(|s| serde_json::from_str(&s).ok())
+            .unwrap_or_default();
+        meta.rejected = Some(format!("{reason:#}"));
+        let _ = world::write_atomic(&path, serde_json::to_string_pretty(&meta).unwrap_or_default().as_bytes());
+    }
+
+    fn version_rejected(&self, id: &str, version: u64) -> bool {
+        std::fs::read_to_string(self.world.version_dir(id, version).join("meta.json"))
+            .ok()
+            .and_then(|s| serde_json::from_str::<VersionMeta>(&s).ok())
+            .is_some_and(|m| m.rejected.is_some())
+    }
+
+    /// Install `version` as the current files of a new actor.
+    fn install_files(&self, id: &str, version: u64) -> Result<()> {
+        let dir = self.world.version_dir(id, version);
+        self.world.ensure_actor_dirs(id)?;
+        world::write_atomic(&self.world.wasm_path(id), &std::fs::read(dir.join("actor.wasm"))?)?;
+        world::replace_tree(&dir.join("source"), &self.world.source_dir(id))?;
+        self.update_meta(id, |m| {
+            m.version = Some(version);
+            m.running = true;
+        })
     }
 
     /// Create a new actor from a `.wasm` file. With `start`, also start its
-    /// supervisor (in a running host). Without `start` (during `init`), only
-    /// register it.
-    pub async fn create_actor(
-        self: &Arc<Self>,
-        id: &str,
-        wasm: &Path,
-        source: Option<&Path>,
-        meta: VersionMeta,
-        start: bool,
-    ) -> Result<Value> {
+    /// supervisor (in a running host). Without `start` (`init`), only install it.
+    pub async fn create_actor(self: &Arc<Self>, id: &str, wasm: &Path, source: Option<&Path>, meta: VersionMeta, start: bool) -> Result<Value> {
         if !world::valid_id(id) {
             bail!("invalid actor ID {id:?}: use a-z, 0-9, '-' and '_'");
         }
-        if NATIVE.contains(&id) || self.registry().actors.contains_key(id) {
+        if NATIVE.contains(&id) || self.is_supervised(id) || self.world.wasm_path(id).exists() {
             bail!("an actor with ID {id} already exists");
         }
         let lock = self.op_lock(id);
         let _guard = lock.lock().await;
-        let (dir, component) = self.store_version(id, 1, wasm, source, &meta)?;
-        let report = candidate::test(self, id, &component, &dir.join("source"), None, None).await;
-        let report = match report {
+        let version = self.world.next_version(id);
+        let meta = VersionMeta { version, ..meta };
+        self.create_version(id, version, &meta, wasm, source, start).await
+    }
+
+    async fn create_version(self: &Arc<Self>, id: &str, version: u64, meta: &VersionMeta, wasm: &Path, source: Option<&Path>, start: bool) -> Result<Value> {
+        let tested = async {
+            let component = self.store_version(id, meta, wasm, source)?;
+            candidate::test(self, id, &component, &self.world.version_dir(id, version).join("source"), None, None).await
+        }
+        .await;
+        let report = match tested {
             Ok(r) => r,
             Err(e) => {
-                let _ = std::fs::remove_dir_all(self.world.versions_dir(id));
+                self.reject(id, version, &e);
                 return Err(e);
             }
         };
-        self.world.ensure_actor_dirs(id)?;
-        world::replace_tree(&dir.join("source"), &self.world.source_dir(id))?;
-        self.update_registry(|r| {
-            r.actors.insert(id.to_string(), ActorRecord { version: 1, next_version: 2, running: true });
-        })?;
-        write_status(&self.world, id, &json!({"id": id, "version": 1, "state": "created"}));
+        self.install_files(id, version)?;
+        write_status(&self.world, id, &json!({"id": id, "version": version, "state": "created"}));
         if start {
-            self.start_supervisor(id, 1, true);
+            self.start_supervisor(id, version, true);
         }
-        Ok(json!({"type": "spawned", "actor": id, "version": 1, "tests": report}))
+        Ok(json!({"type": "spawned", "actor": id, "version": version, "tests": report}))
     }
 
-    pub async fn upgrade_actor(self: &Arc<Self>, id: &str, wasm: &Path, source: Option<&Path>, mut meta: VersionMeta) -> Result<Value> {
+    pub async fn upgrade_actor(self: &Arc<Self>, id: &str, wasm: &Path, source: Option<&Path>, meta: VersionMeta) -> Result<Value> {
         let lock = self.op_lock(id);
         let _guard = lock.lock().await;
-        let rec = self.registry().actors.get(id).cloned().ok_or_else(|| anyhow!("no WASM actor with ID {id}"))?;
-        let version = rec.next_version;
-        self.update_registry(|r| {
-            if let Some(a) = r.actors.get_mut(id) {
-                a.next_version = version + 1;
-            }
-        })?;
-        meta.version = version;
-        let (dir, component) = self.store_version(id, version, wasm, source, &meta)?;
-        let report = candidate::test(
-            self,
-            id,
-            &component,
-            &dir.join("source"),
-            Some(&self.world.data_dir(id)),
-            Some(&self.world.public_dir(id)),
-        )
-        .await;
-        if let Err(e) = report {
-            let _ = std::fs::remove_dir_all(&dir);
-            return Err(e);
+        if !self.is_supervised(id) {
+            bail!("no WASM actor with ID {id}");
         }
-        let mut result = self
-            .control(id, |reply| Control::Upgrade { version, requester: meta.requested_by.clone(), reply })
-            .await?;
-        result["tests"] = report.unwrap();
+        let version = self.world.next_version(id);
+        self.upgrade_version(id, &VersionMeta { version, ..meta }, wasm, source).await
+    }
+
+    async fn upgrade_version(self: &Arc<Self>, id: &str, meta: &VersionMeta, wasm: &Path, source: Option<&Path>) -> Result<Value> {
+        let version = meta.version;
+        let tested = async {
+            let component = self.store_version(id, meta, wasm, source)?;
+            let source = self.world.version_dir(id, version).join("source");
+            candidate::test(self, id, &component, &source, Some(&self.world.data_dir(id)), Some(&self.world.public_dir(id))).await
+        }
+        .await;
+        let report = match tested {
+            Ok(r) => r,
+            Err(e) => {
+                self.reject(id, version, &e);
+                return Err(e);
+            }
+        };
+        let requester = meta.requested_by.clone();
+        let mut result = self.control(id, |reply| Control::Upgrade { version, requester, reply }).await?;
+        result["tests"] = report;
         Ok(result)
     }
 
+    /// Boot the actors in `actor/`. Called at start and on `scan`.
+    ///
+    /// - A known `actor.wasm` (equal to the installed version) starts as is.
+    /// - A new or changed `actor.wasm` is first copied to `versions/<n>/`.
+    ///   A new actor is tested and installed. A changed one is an upgrade:
+    ///   if it fails, the installed files come back.
+    /// - A file that equals an already rejected version is not tried again.
+    pub async fn scan(self: &Arc<Self>) -> Vec<Value> {
+        let mut report = Vec::new();
+        for id in self.world.actor_ids().unwrap_or_default() {
+            if NATIVE.contains(&id.as_str()) || !self.world.wasm_path(&id).is_file() {
+                continue;
+            }
+            let lock = self.op_lock(&id);
+            let _guard = lock.lock().await;
+            if let Some(r) = self.scan_actor(&id).await {
+                report.push(r);
+            }
+        }
+        report
+    }
+
+    async fn scan_actor(self: &Arc<Self>, id: &str) -> Option<Value> {
+        let meta = self.world.read_meta(id);
+        let wasm = self.world.wasm_path(id);
+        let same = |v: u64| same_file(&wasm, &self.world.version_dir(id, v).join("actor.wasm"));
+        let installed = meta.version.filter(|v| self.world.version_dir(id, *v).join("actor.wasm").is_file());
+        self.metas.lock().unwrap().insert(id.to_string(), meta.clone());
+        if let Some(v) = installed
+            && !self.is_supervised(id)
+        {
+            self.start_supervisor(id, v, meta.running);
+        }
+        if installed.is_some_and(same) {
+            return None;
+        }
+        let latest = self.world.versions(id).last().copied();
+        if let Some(v) = latest
+            && same(v)
+            && self.version_rejected(id, v)
+        {
+            if let Some(current) = installed {
+                let _ = self.restore_files(id, current);
+            }
+            self.log(id, &format!("actor.wasm equals the rejected version {v}; not installed"));
+            return Some(json!({"actor": id, "skipped": "rejected version", "version": v}));
+        }
+        let version = self.world.next_version(id);
+        let source = self.world.source_dir(id);
+        let vmeta = VersionMeta {
+            version,
+            wasm: format!("/actor/{id}/actor.wasm"),
+            source: Some(format!("/actor/{id}/source")),
+            built_by: None,
+            requested_by: "scan".into(),
+            created_at: world::now(),
+            rejected: None,
+        };
+        let source = source.is_dir().then_some(source.as_path());
+        let result = match installed {
+            None => self.create_version(id, version, &vmeta, &wasm, source, true).await,
+            Some(current) => {
+                let r = self.upgrade_version(id, &vmeta, &wasm, source).await;
+                if r.is_err() {
+                    // Put the installed version back in place.
+                    let _ = self.restore_files(id, current);
+                }
+                r
+            }
+        };
+        Some(match result {
+            Ok(v) => v,
+            Err(e) => {
+                self.log(id, &format!("version {version} from disk rejected: {e:#}"));
+                json!({"type": "error", "actor": id, "version": version, "reason": format!("{e:#}")})
+            }
+        })
+    }
+
+    fn restore_files(&self, id: &str, version: u64) -> Result<()> {
+        let dir = self.world.version_dir(id, version);
+        world::write_atomic(&self.world.wasm_path(id), &std::fs::read(dir.join("actor.wasm"))?)?;
+        world::replace_tree(&dir.join("source"), &self.world.source_dir(id))
+    }
+
     /// Handle one message to `broker`.
     async fn handle(self: Arc<Self>, env: Envelope) {
-        let result = self.clone().command(&env).await;
-        let body = match result {
-            Ok(v) => v,
-            Err(e) => json!({"type": "error", "request": env.body.get("type"), "reason": format!("{e:#}")}),
-        };
+        let body = self.clone().command(&env).await.unwrap_or_else(|e| {
+            json!({"type": "error", "request": env.body.get("type"), "reason": format!("{e:#}")})
+        });
         self.send("broker", Some(&env.from), None, Some(&env.id), body);
     }
 
     async fn command(self: Arc<Self>, env: &Envelope) -> Result<Value> {
         let b = &env.body;
-        let ty = b.get("type").and_then(Value::as_str).unwrap_or("");
-        let actor = || -> Result<String> {
-            b.get("actor").and_then(Value::as_str).map(str::to_string).ok_or_else(|| anyhow!("missing \"actor\""))
-        };
-        let meta = |version: u64| -> Result<(PathBuf, Option<PathBuf>, VersionMeta)> {
-            let wasm_s = b.get("wasm").and_then(Value::as_str).ok_or_else(|| anyhow!("missing \"wasm\""))?;
+        let ty = b["type"].as_str().unwrap_or("");
+        let actor = || b["actor"].as_str().map(str::to_string).ok_or_else(|| anyhow!("missing \"actor\""));
+        let version_meta = || -> Result<(std::path::PathBuf, Option<std::path::PathBuf>, VersionMeta)> {
+            let wasm_s = b["wasm"].as_str().ok_or_else(|| anyhow!("missing \"wasm\""))?;
             let wasm = self.world.resolve(&env.from, wasm_s)?;
             if !wasm.is_file() {
                 bail!("{wasm_s} is not a file");
             }
-            let source_s = b.get("source").and_then(Value::as_str);
+            let source_s = b["source"].as_str();
             let source = source_s.map(|s| self.world.resolve(&env.from, s)).transpose()?;
-            if let Some(s) = &source
-                && !s.is_dir()
+            if let (Some(s), Some(p)) = (source_s, &source)
+                && !p.is_dir()
             {
-                bail!("{} is not a folder", source_s.unwrap());
+                bail!("{s} is not a folder");
             }
-            let m = VersionMeta {
-                version,
+            let meta = VersionMeta {
+                version: 0,
                 wasm: wasm_s.to_string(),
                 source: source_s.map(str::to_string),
-                built_by: b.get("built_by").and_then(Value::as_str).map(str::to_string),
+                built_by: b["built_by"].as_str().map(str::to_string),
                 requested_by: env.from.clone(),
                 created_at: world::now(),
+                rejected: None,
             };
-            Ok((wasm, source, m))
+            Ok((wasm, source, meta))
         };
         match ty {
             "spawn" => {
-                let id = actor()?;
-                let (wasm, source, m) = meta(1)?;
-                self.create_actor(&id, &wasm, source.as_deref(), m, true).await
+                let (wasm, source, meta) = version_meta()?;
+                self.create_actor(&actor()?, &wasm, source.as_deref(), meta, true).await
             }
             "upgrade" => {
-                let id = actor()?;
-                let (wasm, source, m) = meta(0)?;
-                self.upgrade_actor(&id, &wasm, source.as_deref(), m).await
+                let (wasm, source, meta) = version_meta()?;
+                self.upgrade_actor(&actor()?, &wasm, source.as_deref(), meta).await
             }
             "rollback" => {
                 let id = actor()?;
-                let target = b.get("version").and_then(Value::as_u64);
+                let target = b["version"].as_u64();
                 let lock = self.op_lock(&id);
-                let _g = lock.lock().await;
+                let _guard = lock.lock().await;
                 self.control(&id, |reply| Control::Rollback { target, reply }).await
             }
             "stop" => self.control(&actor()?, |reply| Control::Stop { reply }).await,
@@ -397,49 +486,44 @@ impl Broker {
             "remove" => {
                 let id = actor()?;
                 let lock = self.op_lock(&id);
-                let _g = lock.lock().await;
+                let _guard = lock.lock().await;
                 let r = self.control(&id, |reply| Control::Remove { reply }).await?;
                 self.mailboxes.lock().unwrap().remove(&id);
                 self.controls.lock().unwrap().remove(&id);
-                self.update_registry(|reg| {
-                    reg.actors.remove(&id);
-                    reg.subscriptions.remove(&id);
-                    for subs in reg.subscriptions.values_mut() {
-                        subs.remove(&id);
-                    }
-                })?;
+                self.metas.lock().unwrap().remove(&id);
                 Ok(r)
             }
             "subscribe" | "unsubscribe" => {
                 let publisher = actor()?;
-                let sub = env.from.clone();
                 let on = ty == "subscribe";
-                self.update_registry(|reg| {
-                    let set = reg.subscriptions.entry(publisher.clone()).or_insert_with(BTreeSet::new);
+                self.update_meta(&env.from, |m| {
                     if on {
-                        set.insert(sub.clone());
+                        m.subscriptions.insert(publisher.clone());
                     } else {
-                        set.remove(&sub);
+                        m.subscriptions.remove(&publisher);
                     }
                 })?;
                 Ok(json!({"type": format!("{ty}d"), "actor": publisher}))
             }
+            "scan" => Ok(json!({"type": "scanned", "changes": self.scan().await})),
             "list" => {
-                let reg = self.registry();
-                let mut actors: Vec<Value> = NATIVE.iter().map(|id| json!({"id": id, "kind": "native"})).collect();
-                for (id, rec) in reg.actors {
-                    let status = std::fs::read_to_string(self.world.actor_dir(&id).join("status.json"))
-                        .ok()
-                        .and_then(|s| serde_json::from_str::<Value>(&s).ok());
-                    actors.push(json!({
-                        "id": id, "kind": "wasm", "version": rec.version, "running": rec.running,
-                        "state": status.as_ref().and_then(|s| s.get("state")).cloned(),
-                    }));
-                }
-                Ok(json!({"type": "actors", "actors": actors, "subscriptions": reg.subscriptions}))
+                let metas = self.metas.lock().unwrap().clone();
+                let actors: Vec<Value> = metas
+                    .iter()
+                    .map(|(id, m)| {
+                        let status = std::fs::read_to_string(self.world.actor_dir(id).join("status.json"))
+                            .ok()
+                            .and_then(|s| serde_json::from_str::<Value>(&s).ok())
+                            .unwrap_or_default();
+                        json!({"id": id, "kind": if NATIVE.contains(&id.as_str()) { "native" } else { "wasm" },
+                               "version": m.version, "running": m.running, "state": status["state"],
+                               "subscriptions": m.subscriptions})
+                    })
+                    .collect();
+                Ok(json!({"type": "actors", "actors": actors}))
             }
             "shutdown" => Ok(json!({"type": "ignored"})),
-            other => bail!("unknown broker request {other:?}. Known: spawn, upgrade, rollback, stop, start, remove, subscribe, unsubscribe, list"),
+            other => bail!("unknown broker request {other:?}. Known: spawn, upgrade, rollback, stop, start, remove, subscribe, unsubscribe, scan, list"),
         }
     }
 
@@ -451,14 +535,8 @@ impl Broker {
     }
 }
 
-async fn futures_join_all<F: Future<Output = ()>>(futs: Vec<F>) {
-    let mut set = Vec::new();
-    for f in futs {
-        set.push(Box::pin(f));
-    }
-    for f in set {
-        f.await;
-    }
+fn same_file(a: &Path, b: &Path) -> bool {
+    matches!((std::fs::read(a), std::fs::read(b)), (Ok(x), Ok(y)) if x == y)
 }
 
 pub fn truncate(s: &str, max: usize) -> String {
@@ -723,20 +801,23 @@ impl Supervisor {
         Ok(())
     }
 
+    /// Make `version` the current files: `actor.wasm`, `source/`, `actor.json`.
     fn install(&mut self, version: u64) -> Result<()> {
-        let src = self.world().version_dir(&self.id, version).join("source");
-        world::replace_tree(&src, &self.world().source_dir(&self.id))?;
+        let dir = self.world().version_dir(&self.id, version);
+        world::write_atomic(&self.world().wasm_path(&self.id), &std::fs::read(dir.join("actor.wasm"))?)?;
+        world::replace_tree(&dir.join("source"), &self.world().source_dir(&self.id))?;
         self.version = version;
         self.component = None;
-        let id = self.id.clone();
-        self.broker.update_registry(|r| {
-            if let Some(a) = r.actors.get_mut(&id) {
-                a.version = version;
-                a.running = true;
-            }
+        self.broker.update_meta(&self.id, |m| {
+            m.version = Some(version);
+            m.running = true;
         })
     }
 
+    fn set_running(&self, running: bool) -> Result<()> {
+        self.broker.update_meta(&self.id, |m| m.running = running)
+    }
+
     async fn upgrade(&mut self, version: u64, requester: String) -> Result<Value> {
         self.stop_instance().await;
         self.snapshot_current()?;
@@ -759,7 +840,7 @@ impl Supervisor {
             Some(t) => t,
             None => (1..self.version)
                 .rev()
-                .find(|v| self.world().version_dir(&self.id, *v).join("actor.wasm").is_file())
+                .find(|v| self.world().version_dir(&self.id, *v).join("actor.wasm").is_file() && !self.broker.version_rejected(&self.id, *v))
                 .ok_or_else(|| anyhow!("{} has no version before {}", self.id, self.version))?,
         };
         let dir = self.world().version_dir(&self.id, target);
@@ -800,23 +881,13 @@ impl Supervisor {
                 self.restart_at = None;
                 self.stop_instance().await;
                 self.state = "stopped";
-                let id = self.id.clone();
-                let r = self.broker.update_registry(|r| {
-                    if let Some(a) = r.actors.get_mut(&id) {
-                        a.running = false;
-                    }
-                });
+                let r = self.set_running(false);
                 let _ = reply.send(r.map(|_| json!({"type": "stopped", "actor": self.id})));
             }
             Control::Start { reply } => {
                 self.desired_running = true;
                 self.restart_at = None;
-                let id = self.id.clone();
-                let r = self.broker.update_registry(|r| {
-                    if let Some(a) = r.actors.get_mut(&id) {
-                        a.running = true;
-                    }
-                });
+                let r = self.set_running(true);
                 let _ = reply.send(r.map(|_| json!({"type": "started", "actor": self.id})));
             }
             Control::Remove { reply } => {
@@ -838,19 +909,12 @@ impl Supervisor {
         false
     }
 
-    /// Move every folder of the actor into `archive/`. Nothing is deleted.
+    /// Move the folder of the actor into `archive/`. Nothing is deleted.
     fn archive(&self) -> Result<Value> {
-        let dest = self.world().root.join("archive").join(format!("{}-{}", self.id, world::now()));
-        std::fs::create_dir_all(&dest)?;
-        for (from, name) in [
-            (self.world().actor_dir(&self.id), "actor"),
-            (self.world().root.join("private").join(&self.id), "private"),
-            (self.world().versions_dir(&self.id), "versions"),
-        ] {
-            if from.exists() {
-                std::fs::rename(&from, dest.join(name))?;
-            }
-        }
+        let archive = self.world().root.join("archive");
+        std::fs::create_dir_all(&archive)?;
+        let dest = archive.join(format!("{}-{}", self.id, world::now()));
+        std::fs::rename(self.world().actor_dir(&self.id), &dest)?;
         Ok(json!({"type": "removed", "actor": self.id, "archive": dest.display().to_string()}))
     }
 }
diff --git a/cli/src/candidate.rs b/cli/src/candidate.rs
index 00a2e79..e6a4eda 100644
--- a/cli/src/candidate.rs
+++ b/cli/src/candidate.rs
@@ -110,7 +110,9 @@ pub async fn test(
 }
 
 fn start(broker: &Broker, id: &str, component: &Component, data: Option<&Path>, public: Option<&Path>) -> Result<(Instance, Scratch)> {
-    let dir = broker.world.scratch_dir().join(format!("{id}-{}-{}", world::now(), rand_suffix()));
+    static N: std::sync::atomic::AtomicU64 = std::sync::atomic::AtomicU64::new(0);
+    let n = N.fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+    let dir = broker.world.scratch_dir().join(format!("{id}-{}-{n}", std::process::id()));
     let scratch = Scratch { dir: dir.clone() };
     std::fs::create_dir_all(dir.join("data"))?;
     std::fs::create_dir_all(dir.join("public"))?;
@@ -133,13 +135,6 @@ fn start(broker: &Broker, id: &str, component: &Component, data: Option<&Path>,
     Ok((broker.runtime.start(component, cfg)?, scratch))
 }
 
-fn rand_suffix() -> u32 {
-    use std::hash::{BuildHasher, Hasher};
-    let mut h = std::collections::hash_map::RandomState::new().build_hasher();
-    h.write_u64(std::process::id() as u64);
-    h.finish() as u32
-}
-
 async fn run_file(
     broker: &Broker,
     id: &str,
diff --git a/cli/src/cargo_actor.rs b/cli/src/cargo_actor.rs
index c05075f..1e423c5 100644
--- a/cli/src/cargo_actor.rs
+++ b/cli/src/cargo_actor.rs
@@ -6,22 +6,30 @@
 //!
 //! The broker knows nothing about Rust. This actor is one way to get a
 //! `.wasm` file. Other actors can add other languages.
+//!
+//! Actors depend on the SDK with `sausage-actor = "0.1"`. The SDK is not
+//! published: this actor copies the SDK of this host to `data/sdk/`, and
+//! points the dependency there in the build copy of `Cargo.toml`.
 
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
-use std::time::{Duration, Instant};
+use std::time::Instant;
 
 use anyhow::{Context, Result, anyhow, bail};
 use serde_json::{Value, json};
 use tokio::process::Command;
 use tokio::sync::{Mutex, mpsc};
 
-use crate::broker::{Broker, Envelope, truncate};
+use include_dir::{Dir, include_dir};
+
+use crate::broker::{Broker, Envelope};
 use crate::world::{self, World};
 
 pub const ID: &str = "cargo";
 pub const TARGET: &str = "wasm32-wasip2";
-const BUILD_TIMEOUT: Duration = Duration::from_secs(900);
+
+/// The source of the `sausage-actor` SDK.
+static SDK: Dir<'_> = include_dir!("$CARGO_MANIFEST_DIR/../sdk");
 
 pub struct BuildOutput {
     /// Host path of the published `.wasm` file.
@@ -37,8 +45,10 @@ pub struct Cargo {
 }
 
 impl Cargo {
-    pub fn new(world: World) -> Self {
-        Self { world, lock: Mutex::new(()) }
+    /// Create the actor and install the SDK of this host into `data/sdk/`.
+    pub fn new(world: World) -> Result<Self> {
+        write_dir(&SDK, &world.data_dir(ID).join("sdk"))?;
+        Ok(Self { world, lock: Mutex::new(()) })
     }
 
     /// Build the crate in `source` (a host path) and publish the `.wasm` file
@@ -53,12 +63,8 @@ impl Cargo {
         let key = format!("{:016x}", fxhash(source.to_string_lossy().as_bytes()));
         let build_dir = data.join("work").join(&key);
         world::replace_tree(source, &build_dir)?;
-        // The crate must not join a parent workspace by accident.
         let manifest = build_dir.join("Cargo.toml");
-        let text = std::fs::read_to_string(&manifest)?;
-        if !text.contains("[workspace]") {
-            std::fs::write(&manifest, format!("{text}\n[workspace]\n"))?;
-        }
+        std::fs::write(&manifest, prepare_manifest(&std::fs::read_to_string(&manifest)?, Some(&data.join("sdk")))?)?;
 
         let mut cmd = Command::new("cargo");
         cmd.arg("build")
@@ -70,13 +76,10 @@ impl Cargo {
             .arg(data.join("target"))
             .current_dir(&build_dir)
             .kill_on_drop(true);
-        let out = tokio::time::timeout(BUILD_TIMEOUT, cmd.output())
-            .await
-            .map_err(|_| anyhow!("cargo build took longer than {BUILD_TIMEOUT:?}"))?
-            .context("cannot run cargo (is it on PATH, with the wasm32-wasip2 target?)")?;
+        let out = cmd.output().await.context("cannot run cargo (is it on PATH, with the wasm32-wasip2 target?)")?;
         let stderr = String::from_utf8_lossy(&out.stderr).to_string();
         if !out.status.success() {
-            bail!("{}", truncate(&strip_progress(&stderr), 20_000));
+            bail!("{}", strip_progress(&stderr));
         }
         let mut artifact = None;
         for line in String::from_utf8_lossy(&out.stdout).lines() {
@@ -123,7 +126,7 @@ impl Cargo {
                     "source": source_s,
                     "built_by": ID,
                     "seconds": (out.seconds * 10.0).round() / 10.0,
-                    "warnings": truncate(&out.warnings, 4000),
+                    "warnings": out.warnings,
                 }))
             }
             Some("shutdown") => Ok(json!({"type": "ignored"})),
@@ -132,6 +135,47 @@ impl Cargo {
     }
 }
 
+/// Point a `sausage-actor` dependency at `sdk` (keeping its features), and
+/// add an empty `[workspace]`, so the crate never joins a parent workspace.
+fn prepare_manifest(text: &str, sdk: Option<&Path>) -> Result<String> {
+    let mut doc: toml_edit::DocumentMut = text.parse()?;
+    if let Some(sdk) = sdk {
+        for section in ["dependencies", "dev-dependencies", "build-dependencies"] {
+            let Some(dep) = doc.get_mut(section).and_then(|t| t.get_mut("sausage-actor")) else { continue };
+            let mut table = dep.as_inline_table().cloned().unwrap_or_default();
+            for key in ["version", "path", "git", "branch", "tag", "rev", "registry"] {
+                table.remove(key);
+            }
+            table.insert("path", sdk.display().to_string().into());
+            *dep = toml_edit::value(table);
+        }
+    }
+    if doc.get("workspace").is_none() {
+        doc["workspace"] = toml_edit::table();
+    }
+    Ok(doc.to_string())
+}
+
+/// Write the files of `dir` below `to`. Skip files that did not change, so
+/// that cargo does not rebuild the SDK at every start.
+fn write_dir(dir: &Dir<'_>, to: &Path) -> Result<()> {
+    for file in dir.files() {
+        let dest = to.join(file.path());
+        let contents = match file.path().file_name() {
+            Some(n) if n == "Cargo.toml" => prepare_manifest(file.contents_utf8().context("Cargo.toml")?, None)?.into_bytes(),
+            _ => file.contents().to_vec(),
+        };
+        if std::fs::read(&dest).ok() != Some(contents.clone()) {
+            std::fs::create_dir_all(dest.parent().context("no parent")?)?;
+            std::fs::write(&dest, contents)?;
+        }
+    }
+    for sub in dir.dirs() {
+        write_dir(sub, to)?;
+    }
+    Ok(())
+}
+
 fn next_build_number(dir: &Path) -> Result<u64> {
     std::fs::create_dir_all(dir)?;
     let max = std::fs::read_dir(dir)?
diff --git a/cli/src/console.rs b/cli/src/console.rs
index c739c56..6a015ec 100644
--- a/cli/src/console.rs
+++ b/cli/src/console.rs
@@ -115,7 +115,7 @@ impl Console {
         if built.body["type"] != "built" {
             bail!("build failed");
         }
-        let exists = self.broker.registry().actors.contains_key(actor);
+        let exists = self.broker.meta(actor).is_some_and(|m| m.version.is_some());
         let ty = if exists { "upgrade" } else { "spawn" };
         println!("· {ty} {actor} …");
         let body = json!({
diff --git a/cli/src/fs.rs b/cli/src/fs.rs
index 2afe1de..2f0d4bf 100644
--- a/cli/src/fs.rs
+++ b/cli/src/fs.rs
@@ -9,6 +9,9 @@
 //!
 //! An open of an existing file without truncate (append, in-place edit)
 //! writes in place, so that appending to a log does not copy the log.
+//!
+//! A read-only mount of the world's `actor/` folder hides the private
+//! `data/` folders of all actors (see `is_private`).
 
 use std::collections::HashMap;
 use std::path::{Component, Path, PathBuf};
@@ -37,7 +40,19 @@ pub struct Mount {
 struct DirInfo {
     host: PathBuf,
     root: PathBuf,
+    /// Writable mount: new and truncated files commit on close.
     commit: bool,
+    /// Read-only mount: private folders are hidden.
+    hide: bool,
+}
+
+/// `<id>/data` and `<id>/versions/<n>/snapshot/data`, relative to `actor/`.
+fn is_private(rel: &Path) -> bool {
+    let c: Vec<_> = rel.components().map(|c| c.as_os_str()).collect();
+    c.get(1).is_some_and(|n| *n == "data")
+        || (c.get(1).is_some_and(|n| *n == "versions")
+            && c.get(3).is_some_and(|n| *n == "snapshot")
+            && c.get(4).is_some_and(|n| *n == "data"))
 }
 
 struct Pending {
@@ -49,12 +64,14 @@ struct Pending {
 pub struct FsExt {
     mounts: Vec<Mount>,
     dirs: HashMap<u32, DirInfo>,
+    /// Directory streams of hiding mounts, and the directory they list.
+    streams: HashMap<u32, DirInfo>,
     pending: HashMap<u32, Pending>,
 }
 
 impl FsExt {
     pub fn new(mounts: Vec<Mount>) -> Self {
-        Self { mounts, dirs: HashMap::new(), pending: HashMap::new() }
+        Self { mounts, dirs: HashMap::new(), streams: HashMap::new(), pending: HashMap::new() }
     }
 
     /// Commit every open write. Use this when the guest exits normally,
@@ -110,6 +127,17 @@ pub struct ActorFs<'a> {
     pub ext: &'a mut FsExt,
 }
 
+impl ActorFs<'_> {
+    /// Whether `path` (relative to the directory `fd`) must look absent:
+    /// an uncommitted file, or a private folder in a hiding mount.
+    fn blocked(&self, fd: &Resource<types::Descriptor>, path: &str) -> bool {
+        is_tmp_name(path)
+            || self.ext.dirs.get(&fd.rep()).is_some_and(|d| {
+                d.hide && resolve(d, path).is_some_and(|p| p.strip_prefix(&d.root).is_ok_and(is_private))
+            })
+    }
+}
+
 pub struct ActorFsData;
 
 impl HasData for ActorFsData {
@@ -123,7 +151,7 @@ impl preopens::Host for ActorFs<'_> {
             if let Some(m) = self.ext.mounts.iter().find(|m| &m.guest == name) {
                 self.ext.dirs.insert(
                     r.rep(),
-                    DirInfo { host: m.host.clone(), root: m.host.clone(), commit: m.writable },
+                    DirInfo { host: m.host.clone(), root: m.host.clone(), commit: m.writable, hide: !m.writable },
                 );
             }
         }
@@ -202,7 +230,12 @@ impl HostDescriptor for ActorFs<'_> {
         &mut self,
         fd: Resource<types::Descriptor>,
     ) -> FsResult<Resource<types::DirectoryEntryStream>> {
-        HostDescriptor::read_directory(&mut self.inner, fd).await
+        let dir = self.ext.dirs.get(&fd.rep()).filter(|d| d.hide).cloned();
+        let stream = HostDescriptor::read_directory(&mut self.inner, fd).await?;
+        if let Some(dir) = dir {
+            self.ext.streams.insert(stream.rep(), dir);
+        }
+        Ok(stream)
     }
 
     async fn sync(&mut self, fd: Resource<types::Descriptor>) -> FsResult<()> {
@@ -223,7 +256,7 @@ impl HostDescriptor for ActorFs<'_> {
         path_flags: types::PathFlags,
         path: String,
     ) -> FsResult<types::DescriptorStat> {
-        if is_tmp_name(&path) {
+        if self.blocked(&fd, &path) {
             return Err(ErrorCode::NoEntry.into());
         }
         HostDescriptor::stat_at(&mut self.inner, fd, path_flags, path).await
@@ -259,7 +292,7 @@ impl HostDescriptor for ActorFs<'_> {
         oflags: types::OpenFlags,
         flags: types::DescriptorFlags,
     ) -> FsResult<Resource<types::Descriptor>> {
-        if is_tmp_name(&path) {
+        if self.blocked(&fd, &path) {
             return Err(ErrorCode::NoEntry.into());
         }
         let parent = self.ext.dirs.get(&fd.rep()).cloned();
@@ -294,7 +327,7 @@ impl HostDescriptor for ActorFs<'_> {
             && matches!(self.inner.table.get(&r)?, Descriptor::Dir(_))
             && let Some(host) = resolve(&dir, &path)
         {
-            self.ext.dirs.insert(r.rep(), DirInfo { host, root: dir.root, commit: dir.commit });
+            self.ext.dirs.insert(r.rep(), DirInfo { host, ..dir });
         }
         Ok(r)
     }
@@ -310,6 +343,9 @@ impl HostDescriptor for ActorFs<'_> {
     }
 
     async fn readlink_at(&mut self, fd: Resource<types::Descriptor>, path: String) -> FsResult<String> {
+        if self.blocked(&fd, &path) {
+            return Err(ErrorCode::NoEntry.into());
+        }
         HostDescriptor::readlink_at(&mut self.inner, fd, path).await
     }
 
@@ -385,6 +421,9 @@ impl HostDescriptor for ActorFs<'_> {
         path_flags: types::PathFlags,
         path: String,
     ) -> FsResult<types::MetadataHashValue> {
+        if self.blocked(&fd, &path) {
+            return Err(ErrorCode::NoEntry.into());
+        }
         HostDescriptor::metadata_hash_at(&mut self.inner, fd, path_flags, path).await
     }
 }
@@ -401,14 +440,21 @@ impl HostDirectoryEntryStream for ActorFs<'_> {
                 Resource::new_borrow(rep),
             )
             .await?;
+            let hidden = |name: &str| {
+                name.starts_with(TMP_PREFIX)
+                    || self.ext.streams.get(&rep).is_some_and(|d| {
+                        d.host.join(name).strip_prefix(&d.root).is_ok_and(is_private)
+                    })
+            };
             match entry {
-                Some(e) if e.name.starts_with(TMP_PREFIX) => continue,
+                Some(e) if hidden(&e.name) => continue,
                 other => return Ok(other),
             }
         }
     }
 
     fn drop(&mut self, stream: Resource<types::DirectoryEntryStream>) -> wasmtime::Result<()> {
+        self.ext.streams.remove(&stream.rep());
         HostDirectoryEntryStream::drop(&mut self.inner, stream)
     }
 }
diff --git a/cli/src/lib.rs b/cli/src/lib.rs
index d176c1a..2cfa971 100644
--- a/cli/src/lib.rs
+++ b/cli/src/lib.rs
@@ -44,9 +44,11 @@ impl Host {
         let rx = broker.open_mailbox("broker");
         tokio::spawn(broker.clone().run_actor(rx));
         let rx = broker.open_mailbox(cargo_actor::ID);
-        tokio::spawn(Arc::new(cargo_actor::Cargo::new(world.clone())).run_actor(broker.clone(), rx));
+        tokio::spawn(Arc::new(cargo_actor::Cargo::new(world.clone())?).run_actor(broker.clone(), rx));
 
-        broker.start_all();
+        for change in broker.scan().await {
+            eprintln!("sausage: {change}");
+        }
         Ok(Host { world, broker, _lock: lock })
     }
 
diff --git a/cli/src/main.rs b/cli/src/main.rs
index 8a84d7b..a7994b5 100644
--- a/cli/src/main.rs
+++ b/cli/src/main.rs
@@ -7,7 +7,7 @@ use clap::{Parser, Subcommand};
 use include_dir::{Dir, include_dir};
 use serde_json::json;
 
-use sausage::world::{self, VersionMeta, World};
+use sausage::world::{self, ActorMeta, VersionMeta, World};
 use sausage::{Broker, Host, cargo_actor, console, runtime};
 
 /// The source of the agent actor. `init` builds it like any other actor.
@@ -69,9 +69,8 @@ async fn init(dir: PathBuf) -> Result<()> {
     let token = setting("SAUSAGE_API_TOKEN", "API token", None)?;
 
     let world = World::create(&dir)?;
-    let mut reg = world.load_registry()?;
-    reg.subscriptions.entry("agent".into()).or_default().insert("console".into());
-    world.save_registry(&reg)?;
+    let console = ActorMeta { subscriptions: ["agent".to_string()].into(), ..world.read_meta(console::ID) };
+    world.write_meta(console::ID, &console)?;
 
     // The agent's private configuration. Only the agent can read /data.
     world.ensure_actor_dirs("agent")?;
@@ -84,11 +83,11 @@ async fn init(dir: PathBuf) -> Result<()> {
         std::fs::set_permissions(&config_path, std::fs::Permissions::from_mode(0o600))?;
     }
 
-    let source = world.root.join("broker").join("seed").join("agent");
+    let source = world.scratch_dir().join("seed").join("agent");
     std::fs::create_dir_all(&source)?;
     AGENT_SOURCE.extract(&source).context("extract agent source")?;
     println!("Building the agent actor (the first build takes a few minutes) …");
-    let out = cargo_actor::Cargo::new(world.clone()).build(&source).await.context("build agent")?;
+    let out = cargo_actor::Cargo::new(world.clone())?.build(&source).await.context("build agent")?;
     println!("Built in {:.1} s.", out.seconds);
 
     let broker = Broker::new(world.clone(), runtime::Runtime::new()?)?;
@@ -99,6 +98,7 @@ async fn init(dir: PathBuf) -> Result<()> {
         built_by: Some(cargo_actor::ID.into()),
         requested_by: "init".into(),
         created_at: world::now(),
+        rejected: None,
     };
     broker.create_actor("agent", &out.wasm, Some(&source), meta, false).await?;
     println!("World ready. Start it with: sausage run {}", dir.display());
diff --git a/cli/src/world.rs b/cli/src/world.rs
index 13e9deb..1c78d1a 100644
--- a/cli/src/world.rs
+++ b/cli/src/world.rs
@@ -1,19 +1,22 @@
-//! On-disk layout of a Sausage workspace ("world").
+//! On-disk layout of a Sausage world. The folder is the source of truth:
+//! `sausage run` boots every actor that it finds in `actor/`.
 //!
 //! ```text
 //! <world>/
-//!   sausage.json                 registry: actors, versions, subscriptions
-//!   actor/<id>/public/           written by the actor, read-only for others
-//!   actor/<id>/source/           source of the running version (broker)
-//!   actor/<id>/status.json       state of the actor (broker)
-//!   actor/<id>/stderr.log        stderr of the actor (broker)
-//!   private/<id>/data/           private state of the actor
-//!   broker/versions/<id>/<n>/    actor.wasm, source/, meta.json, snapshot/
-//!   archive/                     removed actors
+//!   actor/<id>/
+//!     actor.wasm        the current version
+//!     actor.json        version, running, subscriptions (broker)
+//!     source/           source of the current version
+//!     public/           written by the actor, read-only for others
+//!     data/             private state; hidden from other actors
+//!     status.json       runtime state (broker)
+//!     stderr.log        stderr of the actor and notes of the broker
+//!     versions/<n>/     actor.wasm, source/, meta.json, snapshot/{data,public}
+//!   archive/            removed actors
 //! ```
 
-use std::collections::{BTreeMap, BTreeSet};
-use std::path::{Path, PathBuf};
+use std::collections::BTreeSet;
+use std::path::{Component, Path, PathBuf};
 
 use anyhow::{Context, Result, bail};
 use serde::{Deserialize, Serialize};
@@ -28,20 +31,25 @@ pub struct World {
     pub root: PathBuf,
 }
 
-#[derive(Default, Serialize, Deserialize, Clone)]
-pub struct Registry {
-    pub actors: BTreeMap<String, ActorRecord>,
-    /// Publisher ID → subscriber IDs.
-    pub subscriptions: BTreeMap<String, BTreeSet<String>>,
-}
-
-#[derive(Serialize, Deserialize, Clone, Debug)]
-pub struct ActorRecord {
-    pub version: u64,
-    pub next_version: u64,
+/// `actor/<id>/actor.json`.
+#[derive(Serialize, Deserialize, Clone, Debug, Default)]
+pub struct ActorMeta {
+    /// The installed version. `None` for native actors and for a new
+    /// `actor.wasm` that the broker has not installed yet.
+    #[serde(default)]
+    pub version: Option<u64>,
+    #[serde(default = "yes")]
     pub running: bool,
+    /// The actors whose published messages this actor receives.
+    #[serde(default)]
+    pub subscriptions: BTreeSet<String>,
 }
 
+fn yes() -> bool {
+    true
+}
+
+/// `versions/<n>/meta.json`.
 #[derive(Serialize, Deserialize, Clone, Debug, Default)]
 pub struct VersionMeta {
     pub version: u64,
@@ -51,9 +59,12 @@ pub struct VersionMeta {
     pub source: Option<String>,
     /// The actor that built the `.wasm` file.
     pub built_by: Option<String>,
-    /// The actor that requested the spawn or upgrade.
+    /// The actor that requested the version ("scan" for a file found on disk).
     pub requested_by: String,
     pub created_at: u64,
+    /// Why the broker did not install this version, if it did not.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub rejected: Option<String>,
 }
 
 pub fn valid_id(id: &str) -> bool {
@@ -74,15 +85,15 @@ impl World {
         Self { root: root.into() }
     }
 
-    pub fn registry_path(&self) -> PathBuf {
-        self.root.join("sausage.json")
-    }
-    pub fn actor_dir(&self, id: &str) -> PathBuf {
-        self.root.join("actor").join(id)
-    }
     pub fn actors_root(&self) -> PathBuf {
         self.root.join("actor")
     }
+    pub fn actor_dir(&self, id: &str) -> PathBuf {
+        self.actors_root().join(id)
+    }
+    pub fn wasm_path(&self, id: &str) -> PathBuf {
+        self.actor_dir(id).join("actor.wasm")
+    }
     pub fn public_dir(&self, id: &str) -> PathBuf {
         self.actor_dir(id).join("public")
     }
@@ -90,20 +101,21 @@ impl World {
         self.actor_dir(id).join("source")
     }
     pub fn data_dir(&self, id: &str) -> PathBuf {
-        self.root.join("private").join(id).join("data")
+        self.actor_dir(id).join("data")
     }
     pub fn versions_dir(&self, id: &str) -> PathBuf {
-        self.root.join("broker").join("versions").join(id)
+        self.actor_dir(id).join("versions")
     }
     pub fn version_dir(&self, id: &str, v: u64) -> PathBuf {
         self.versions_dir(id).join(v.to_string())
     }
+    /// Private scratch space of the broker (candidate copies, seeds).
     pub fn scratch_dir(&self) -> PathBuf {
-        self.root.join("broker").join("scratch")
+        self.data_dir("broker").join("scratch")
     }
 
     pub fn exists(&self) -> bool {
-        self.registry_path().is_file()
+        self.actors_root().is_dir()
     }
 
     /// Create an empty world with the folders of the native actors.
@@ -112,8 +124,6 @@ impl World {
         if world.exists() {
             bail!("{} is already a Sausage world", root.display());
         }
-        std::fs::create_dir_all(&world.root)?;
-        world.save_registry(&Registry::default())?;
         for id in NATIVE {
             world.ensure_actor_dirs(id)?;
             let status = serde_json::json!({"id": id, "kind": "native", "state": "built-in"});
@@ -122,21 +132,46 @@ impl World {
         Ok(world)
     }
 
-    /// Create the directories of one actor.
     pub fn ensure_actor_dirs(&self, id: &str) -> Result<()> {
         std::fs::create_dir_all(self.public_dir(id))?;
         std::fs::create_dir_all(self.data_dir(id))?;
         Ok(())
     }
 
-    pub fn load_registry(&self) -> Result<Registry> {
-        let text = std::fs::read_to_string(self.registry_path())
-            .with_context(|| format!("no Sausage world at {}", self.root.display()))?;
-        Ok(serde_json::from_str(&text)?)
+    /// IDs of every folder in `actor/`, sorted.
+    pub fn actor_ids(&self) -> Result<Vec<String>> {
+        let mut ids: Vec<String> = std::fs::read_dir(self.actors_root())?
+            .filter_map(|e| e.ok())
+            .filter(|e| e.path().is_dir())
+            .filter_map(|e| e.file_name().into_string().ok())
+            .filter(|id| valid_id(id))
+            .collect();
+        ids.sort();
+        Ok(ids)
     }
 
-    pub fn save_registry(&self, reg: &Registry) -> Result<()> {
-        write_atomic(&self.registry_path(), serde_json::to_string_pretty(reg)?.as_bytes())
+    pub fn read_meta(&self, id: &str) -> ActorMeta {
+        std::fs::read_to_string(self.actor_dir(id).join("actor.json"))
+            .ok()
+            .and_then(|s| serde_json::from_str(&s).ok())
+            .unwrap_or(ActorMeta { running: true, ..Default::default() })
+    }
+
+    pub fn write_meta(&self, id: &str, meta: &ActorMeta) -> Result<()> {
+        write_atomic(&self.actor_dir(id).join("actor.json"), serde_json::to_string_pretty(meta)?.as_bytes())
+    }
+
+    /// Version numbers in `versions/`, ascending.
+    pub fn versions(&self, id: &str) -> Vec<u64> {
+        let mut v: Vec<u64> = std::fs::read_dir(self.versions_dir(id))
+            .map(|rd| rd.filter_map(|e| e.ok()?.file_name().to_str()?.parse().ok()).collect())
+            .unwrap_or_default();
+        v.sort();
+        v
+    }
+
+    pub fn next_version(&self, id: &str) -> u64 {
+        self.versions(id).last().map_or(1, |v| v + 1)
     }
 
     /// Map a path in an actor's namespace to a host path.
@@ -145,25 +180,28 @@ impl World {
     /// - `/public/...` and `/data/...` belong to `sender`.
     /// - Native senders (the console) can also give absolute host paths.
     pub fn resolve(&self, sender: &str, path: &str) -> Result<PathBuf> {
-        let clean = |base: PathBuf, rest: &str| -> Result<PathBuf> {
-            let mut out = base.clone();
+        let under = |base: PathBuf, rest: &str| -> Result<PathBuf> {
+            let mut out = base;
             for c in Path::new(rest).components() {
                 match c {
-                    std::path::Component::Normal(n) => out.push(n),
-                    std::path::Component::CurDir | std::path::Component::RootDir => {}
+                    Component::Normal(n) => out.push(n),
+                    Component::CurDir | Component::RootDir => {}
                     _ => bail!("path must not contain '..': {path}"),
                 }
             }
             Ok(out)
         };
-        if let Some(rest) = path.strip_prefix("/actor/") {
-            return clean(self.actors_root(), rest);
+        let prefixed = |prefix: &str| {
+            path.strip_prefix(prefix).filter(|rest| rest.is_empty() || rest.starts_with('/'))
+        };
+        if let Some(rest) = prefixed("/actor") {
+            return under(self.actors_root(), rest);
         }
-        if let Some(rest) = path.strip_prefix("/public/").or(path.strip_prefix("/public").filter(|r| r.is_empty())) {
-            return clean(self.public_dir(sender), rest);
+        if let Some(rest) = prefixed("/public") {
+            return under(self.public_dir(sender), rest);
         }
-        if let Some(rest) = path.strip_prefix("/data/").or(path.strip_prefix("/data").filter(|r| r.is_empty())) {
-            return clean(self.data_dir(sender), rest);
+        if let Some(rest) = prefixed("/data") {
+            return under(self.data_dir(sender), rest);
         }
         if NATIVE.contains(&sender) && Path::new(path).is_absolute() {
             return Ok(PathBuf::from(path));
@@ -182,11 +220,8 @@ impl World {
 pub fn write_atomic(path: &Path, bytes: &[u8]) -> Result<()> {
     let dir = path.parent().context("path has no parent")?;
     std::fs::create_dir_all(dir)?;
-    let tmp = dir.join(format!(
-        "{TMP_PREFIX}host-{}-{}",
-        std::process::id(),
-        path.file_name().and_then(|n| n.to_str()).unwrap_or("file")
-    ));
+    let name = path.file_name().and_then(|n| n.to_str()).unwrap_or("file");
+    let tmp = dir.join(format!("{TMP_PREFIX}host-{}-{name}", std::process::id()));
     std::fs::write(&tmp, bytes)?;
     std::fs::rename(&tmp, path)?;
     Ok(())
@@ -198,16 +233,13 @@ pub fn copy_tree(from: &Path, to: &Path) -> Result<()> {
     for entry in std::fs::read_dir(from).with_context(|| format!("read {}", from.display()))? {
         let entry = entry?;
         let name = entry.file_name();
-        let name_str = name.to_string_lossy();
-        if name_str.starts_with(TMP_PREFIX) {
+        let name = name.to_string_lossy();
+        if name.starts_with(TMP_PREFIX) {
             continue;
         }
+        let dst = to.join(&*name);
         let ty = entry.file_type()?;
-        let dst = to.join(&name);
-        if ty.is_dir() {
-            if name_str == "target" || name_str == ".git" {
-                continue;
-            }
+        if ty.is_dir() && name != "target" && name != ".git" {
             copy_tree(&entry.path(), &dst)?;
         } else if ty.is_file() {
             std::fs::copy(entry.path(), &dst)?;
@@ -216,8 +248,8 @@ pub fn copy_tree(from: &Path, to: &Path) -> Result<()> {
     Ok(())
 }
 
-/// Replace the directory `dst` with a copy of `src`, as close to atomic as a
-/// directory can be: copy to a sibling, then swap with two renames.
+/// Replace the directory `dst` with a copy of `src` (an empty folder if `src`
+/// does not exist): copy to a sibling, then swap with two renames.
 pub fn replace_tree(src: &Path, dst: &Path) -> Result<()> {
     let parent = dst.parent().context("path has no parent")?;
     let name = dst.file_name().and_then(|n| n.to_str()).unwrap_or("dir");
diff --git a/examples/crasher/Cargo.lock b/examples/crasher/Cargo.lock
new file mode 100644
index 0000000..9ab03ef
--- /dev/null
+++ b/examples/crasher/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "crasher"
+version = "0.1.0"
diff --git a/examples/notes-v2/Cargo.lock b/examples/notes-v2/Cargo.lock
new file mode 100644
index 0000000..7c1167a
--- /dev/null
+++ b/examples/notes-v2/Cargo.lock
@@ -0,0 +1,105 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "itoa"
+version = "1.0.18"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8f42a60cbdf9a97f5d2305f08a87dc4e09308d1276d28c869c684d7777685682"
+
+[[package]]
+name = "memchr"
+version = "2.8.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cf8baf1c55e62ffcace7a9f06f4bd9cd3f0c4beb022d3b367256b91b87513d98"
+
+[[package]]
+name = "notes"
+version = "0.1.0"
+dependencies = [
+ "serde_json",
+]
+
+[[package]]
+name = "proc-macro2"
+version = "1.0.107"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "985e7ec9bb745e6ce6535b544d84d6cd6f7ad8bd711c398938ae983b91a766d9"
+dependencies = [
+ "unicode-ident",
+]
+
+[[package]]
+name = "quote"
+version = "1.0.47"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1fbf4db142a473a8d80c26bbf18454ed458bf8d26c8219c331daecfdbd079001"
+dependencies = [
+ "proc-macro2",
+]
+
+[[package]]
+name = "serde"
+version = "1.0.229"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4148590afebada386688f18773da617792bf2ef03ffc1e4cbd2b1d45b023e0ba"
+dependencies = [
+ "serde_core",
+]
+
+[[package]]
+name = "serde_core"
+version = "1.0.229"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "67dca2c9c51e58a4791a4b1ed58308b39c64224d349a935ab5039aa360942a48"
+dependencies = [
+ "serde_derive",
+]
+
+[[package]]
+name = "serde_derive"
+version = "1.0.229"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e7a5d71263a5a7d47b41f6b3f06ba276f10cc18b0931f1799f710578e2309348"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "serde_json"
+version = "1.0.154"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e7e9cc8b1b85264074fbcc02a88680c4096b1e47df8f739dceb03bf482f04bd6"
+dependencies = [
+ "itoa",
+ "memchr",
+ "serde",
+ "serde_core",
+ "zmij",
+]
+
+[[package]]
+name = "syn"
+version = "3.0.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d78c8dee4c7bf0e14673097256fed6142ce9d3b85a408189d07482442145823b"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
+[[package]]
+name = "unicode-ident"
+version = "1.0.27"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a2c754d6c33795a1c324727428e5a7dedb5b06195f9890bdbcba760d3e246563"
+
+[[package]]
+name = "zmij"
+version = "1.0.23"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "29666d0abbfad1e3dc4dcf6144730dd3a3ab225bbbdac83319345b1b44ccfc1b"