diff --git a/CHANGELOG.md b/CHANGELOG.md
index 482f9e54..f5e4c841 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -39,6 +39,22 @@ NAT64 spellings of them — were refused whatever this switch said, before and a
 not moved. What changed is that it is no longer the only thing left standing in a production
 deployment that copied the example.
 
+### The supervisor answers on loopback, not on every address the host has
+
+The supervisor's port was published without an interface in front of it, so it bound every address
+the machine had and answered anything that could route to it. This is the service that holds the
+Docker socket, so reaching it is root on the host by way of four verbs, and `SUPERVISOR_TOKEN` is a
+shared secret rather than a network boundary. The documentation already said not to expose it
+outside the deployment network; the compose file did.
+
+It is now published on `127.0.0.1`, like the computer's own port and the two Bots'. If you reach the
+supervisor from another machine, that stops working and it was the thing worth stopping: put the
+caller on the host, or run the server inside the compose network, where it reaches the supervisor as
+`supervisor:4300` and never uses the published mapping at all. `SUPERVISOR_PORT` still chooses the
+host port.
+
+Nothing changes for a default deployment. `scripts/start.sh` already reached it on `localhost`.
+
 ## 0.0.4
 
 ### A click citing a ref this deployment cannot resolve is refused
diff --git a/README.md b/README.md
index c57b4af8..60b37f83 100644
--- a/README.md
+++ b/README.md
@@ -153,7 +153,7 @@ Leave `EMBEDDED_POSTGRES` off and set `DATABASE_URL` to point at a database you
 - **Decide who gets in**: `/admin/people` lists everybody who has signed in, promotes and demotes them, and removes access, which ends the session they are using and stops the next sign-in. Every change is on the audit trail.
 - **An audit trail you can read**: `/admin/audit` lists what was permitted, what was refused and what failed, and every refusal carries the rule that caused it.
 - **Credentials encrypted at rest**: stored through `/admin/credentials`, never returned by an API, and redacted from audit events.
-- **Loopback by default**: computers bind to `127.0.0.1` and require a per-container token, so nothing reaches a logged-in browser by knowing its port.
+- **Loopback by default**: computers bind to `127.0.0.1` and require a per-container token, so nothing reaches a logged-in browser by knowing its port. The supervisor binds there too, because it holds the Docker socket and its token is a shared secret rather than a network boundary.
 - **Durable threads and memory**: conversations survive restarts through CopilotKit Intelligence, and each deployment stamps the threads it owns.
 
 ## Bring your own agent
diff --git a/docker-compose.yml b/docker-compose.yml
index 6fcd2a52..a832ffc0 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -147,7 +147,16 @@ services:
       SPIRE_AGENT_SOCKET_VOLUME: ${COMPOSE_PROJECT_NAME:-openbot}_spire-agent-socket
     ports:
       # For the server on the host to ask for a Bot's computer.
-      - "${SUPERVISOR_PORT:-4500}:4300"
+      #
+      # Loopback only, like the computer's own port and for a stronger version of the same reason.
+      # This process holds the Docker socket, so reaching it is root on the host through four verbs,
+      # and SUPERVISOR_TOKEN is a shared secret rather than a network boundary. Published without an
+      # interface in front of it this answered every address the host has.
+      #
+      # A deployment running the server inside this network does not use this mapping at all: it sets
+      # COMPUTER_NETWORK and reaches the supervisor as `supervisor:4300`, which is unaffected because
+      # the process still listens on every interface inside its own container.
+      - "127.0.0.1:${SUPERVISOR_PORT:-4500}:4300"
     volumes:
       # Read-only because this service only ever needs to ask; it is still root-equivalent, which is
       # the whole reason nothing else here gets it.
diff --git a/docs/architecture.md b/docs/architecture.md
index 6751e95f..9a0759b7 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -73,7 +73,7 @@ With `COMPUTER_SUPERVISOR_URL`, each Bot gets its own computer container, worksp
 
 A command on the computer inherits PATH, locale and terminal names, and the proxy variables, not the rest of the process environment. Userinfo is stripped from a proxy URL. `COMPUTER_SHELL_ENV` names anything else a deployment wants passed.
 
-The supervisor exposes only ensure, stop, reset, and list operations. It holds the Docker socket, so do not expose it outside the deployment network. Set `COMPUTER_RUNTIME=runsc` to run computers under gVisor on hosts that support it.
+The supervisor exposes only ensure, stop, reset, and list operations. It holds the Docker socket, so do not expose it outside the deployment network: Docker Compose binds it to `127.0.0.1:4500`, and a deployment running the server inside the compose network reaches it as `supervisor:4300` and needs no published port at all. Set `COMPUTER_RUNTIME=runsc` to run computers under gVisor on hosts that support it.
 
 ## Human control and secrets
 
diff --git a/supervisor/src/index.ts b/supervisor/src/index.ts
index 779d19b6..41fc2e08 100644
--- a/supervisor/src/index.ts
+++ b/supervisor/src/index.ts
@@ -32,6 +32,13 @@ import { namesFor } from "./names";
  * processes on the same network from driving it, but even with the token the worst available action
  * is cycling a computer that already belongs to a Bot.
  *
+ * Neither is the network the boundary, but it is the layer in front of both. Compose publishes this
+ * port on `127.0.0.1` rather than on every address the host has, for the same reason the computer's
+ * own port is bound there: a secret in an environment variable is one leak away from being known,
+ * and this process holds the Docker socket. This listener stays on every interface inside its own
+ * container, which is what the published mapping forwards to and what a server running inside the
+ * compose network connects to as `supervisor:4300`.
+ *
  * Refusing to start without it matches the computer. This process holds the Docker socket, which is
  * root on the host, so missing authentication is a deployment failure.
  */
diff --git a/tests/compose.test.ts b/tests/compose.test.ts
index c80b6eba..ac18455f 100644
--- a/tests/compose.test.ts
+++ b/tests/compose.test.ts
@@ -38,6 +38,39 @@ test("publishes every service on a settable port with the documented default", (
   }
 });
 
+/**
+ * The services that answer to a secret are published to the host's loopback and no further.
+ *
+ * A published port with no interface in front of it binds every address the host has, so the
+ * service answers anything that can route to the machine. That is the wrong default for all of
+ * these and worst for the supervisor, which holds the Docker socket: reaching it is root on the
+ * host by way of four verbs, and `SUPERVISOR_TOKEN` is a shared secret rather than a network
+ * boundary. The computer says the same thing about itself in a comment beside its own port, and
+ * this is that reasoning applied to every service that has one.
+ *
+ * Named ports rather than a blanket rule, so adding a service is a decision about where it should
+ * answer rather than something this test quietly grants.
+ */
+test("publishes every service that holds a secret on loopback only", () => {
+  const compose = readFileSync(
+    join(import.meta.dir, "..", "docker-compose.yml"),
+    "utf8",
+  );
+
+  for (const name of [
+    "SUPERVISOR_PORT",
+    "COMPUTER_PORT",
+    "BOT_PORT",
+    "LANGGRAPH_PORT",
+  ]) {
+    const published = compose.match(
+      new RegExp(`^\\s*- "(.*)\\$\\{${name}:-\\d+\\}:\\d+"`, "m"),
+    );
+    expect(published).not.toBeNull();
+    expect(published?.[1]).toBe("127.0.0.1:");
+  }
+});
+
 /**
  * Both Bots are reachable at whatever `OPENAI_BASE_URL` names.
  *