diff --git a/infra/sandboxes/computer/bin/rakazo-broker b/infra/sandboxes/computer/bin/rakazo-broker
index 1fde5e2..f304bd3 100755
--- a/infra/sandboxes/computer/bin/rakazo-broker
+++ b/infra/sandboxes/computer/bin/rakazo-broker
@@ -25,14 +25,19 @@ for line in open("/proc/net/route").readlines()[1:]:
 sys.exit(1)
 ' "$RAKAZO_GITHUB_BROKER_URL"
 }
+# Names each path that failed so the user can pass the exact cause on.
 unreachable() {
-  echo "evOo could not reach its GitHub connection from this computer (curl exit $status). Tell the user GitHub access is broken on the evOo server; do not ask for a token." >&2
+  echo "evOo could not reach its GitHub connection from this computer (${attempts}curl exit $status). Tell the user GitHub access is broken on the evOo server and quote this line; do not ask for a token." >&2
   exit 1
 }
 status=0
+attempts=""
 socket=/run/rakazo-github-broker/broker.sock
 if [[ -S "$socket" ]]; then
   response="$(post http://broker/internal/github-broker --unix-socket "$socket" 2>/dev/null)" || status=$?
+  [[ "$status" == 0 ]] || attempts="socket exit $status; "
+else
+  attempts="no socket; "
 fi
 # 7: nothing listening on the socket either; then try the network.
 if [[ ! -S "$socket" || "$status" == 7 ]]; then
diff --git a/infra/sandboxes/supervisor/src/computer-git-shim.test.ts b/infra/sandboxes/supervisor/src/computer-git-shim.test.ts
index 49eb267..0622de3 100644
--- a/infra/sandboxes/supervisor/src/computer-git-shim.test.ts
+++ b/infra/sandboxes/supervisor/src/computer-git-shim.test.ts
@@ -1,4 +1,4 @@
-import { execFileSync } from "node:child_process";
+import { execFileSync, spawnSync } from "node:child_process";
 import { mkdtempSync, writeFileSync } from "node:fs";
 import { tmpdir } from "node:os";
 import path from "node:path";
@@ -52,4 +52,18 @@ describe("computer git and gh shims", () => {
     expect(endpoint).toBe("v1/gh");
     expect(body.argv).toEqual(["gh", "pr", "list"]);
   });
+
+  it("names the failed path when the broker is unreachable", () => {
+    // Port 1 refuses at once, and a plain loopback URL has no gateway fallback.
+    const result = spawnSync("bash", [shimPath("rakazo-broker"), "v1/git", "{}"], {
+      encoding: "utf8",
+      env: {
+        PATH: process.env.PATH,
+        RAKAZO_GITHUB_BROKER_URL: "http://127.0.0.1:1/internal/github-broker",
+        RAKAZO_GITHUB_BROKER_RUN_TOKEN: "rgb1.test",
+      },
+    });
+    expect(result.status).toBe(1);
+    expect(result.stderr).toContain("(no socket; curl exit 7)");
+  });
 });
diff --git a/infra/sandboxes/supervisor/src/computer-spec.test.ts b/infra/sandboxes/supervisor/src/computer-spec.test.ts
index 2b09b80..99b02a4 100644
--- a/infra/sandboxes/supervisor/src/computer-spec.test.ts
+++ b/infra/sandboxes/supervisor/src/computer-spec.test.ts
@@ -21,6 +21,7 @@ import {
   containerCreateOptions,
   containerNameFor,
   controlPortPublicationMatches,
+  githubBrokerSocketMounted,
   homeVolumeMatches,
   hostComputerUser,
   legacyNetworkOwnedSolelyBy,
@@ -131,6 +132,16 @@ describe("graphical computer spec", () => {
     });
   });
 
+  it("recognizes computers that already carry the GitHub broker socket", () => {
+    const mount = (Destination: string) =>
+      ({ Destination }) as Docker.ContainerInspectInfo["Mounts"][number];
+    expect(
+      githubBrokerSocketMounted([mount("/home/rakazo"), mount("/run/rakazo-github-broker")]),
+    ).toBe(true);
+    expect(githubBrokerSocketMounted([mount("/home/rakazo")])).toBe(false);
+    expect(githubBrokerSocketMounted(undefined)).toBe(false);
+  });
+
   it("still publishes host ports when NetworkMode is a per-bot isolated network", () => {
     const networkMode = computerNetworkNameFor("bot_isolation");
     const options = containerCreateOptions({
diff --git a/infra/sandboxes/supervisor/src/computer-spec.ts b/infra/sandboxes/supervisor/src/computer-spec.ts
index 56b182f..b085782 100644
--- a/infra/sandboxes/supervisor/src/computer-spec.ts
+++ b/infra/sandboxes/supervisor/src/computer-spec.ts
@@ -252,6 +252,13 @@ export interface ComputerCreateInput {
 /** Where computers find the GitHub broker socket; the git shim prefers it over the network. */
 export const GITHUB_BROKER_SOCKET_MOUNT = "/run/rakazo-github-broker";
 
+/** Computers created before the socket existed lack the mount and must be recreated to get it. */
+export function githubBrokerSocketMounted(
+  mounts: Docker.ContainerInspectInfo["Mounts"] | undefined,
+) {
+  return mounts?.some((mount) => mount.Destination === GITHUB_BROKER_SOCKET_MOUNT) ?? false;
+}
+
 interface PointerInput {
   kind: "pointer";
   x: number;
diff --git a/infra/sandboxes/supervisor/src/index.test.ts b/infra/sandboxes/supervisor/src/index.test.ts
index c6a8cc7..84d1753 100644
--- a/infra/sandboxes/supervisor/src/index.test.ts
+++ b/infra/sandboxes/supervisor/src/index.test.ts
@@ -8,6 +8,7 @@ import {
   MAX_SUPERVISOR_REQUEST_BYTES,
   needsBridgeBrokerListener,
   resolveDockerSocketPath,
+  servesGithubBrokerSocket,
   supervisorApp,
   supervisorRequestBodyLimit,
   waitForScreenReady,
@@ -202,6 +203,12 @@ describe("sandbox supervisor HTTP boundary", () => {
     expect(needsBridgeBrokerListener("0.0.0.0", "linux", false)).toBe(false);
   });
 
+  it("shares the GitHub broker socket from any Linux host supervisor", () => {
+    expect(servesGithubBrokerSocket("linux", false)).toBe(true);
+    expect(servesGithubBrokerSocket("linux", true)).toBe(false);
+    expect(servesGithubBrokerSocket("darwin", false)).toBe(false);
+  });
+
   it("keeps the larger file-write allowance scoped to that exact POST route", () => {
     expect(supervisorRequestBodyLimit("POST", "/computers/id/files")).toBe(
       MAX_SUPERVISOR_FILE_REQUEST_BYTES,
diff --git a/infra/sandboxes/supervisor/src/index.ts b/infra/sandboxes/supervisor/src/index.ts
index e641c5a..1986396 100644
--- a/infra/sandboxes/supervisor/src/index.ts
+++ b/infra/sandboxes/supervisor/src/index.ts
@@ -36,6 +36,7 @@ import {
   containerCreateOptions,
   containerNameFor,
   controlPortPublicationMatches,
+  githubBrokerSocketMounted,
   homeVolumeMatches,
   hostComputerUser,
   legacyNetworkOwnedSolelyBy,
@@ -153,6 +154,18 @@ export function needsBridgeBrokerListener(
   );
 }
 
+// Streamed file writes copy whole repository archives, and brokered clones of large repositories
+// run as long; Node's 5-minute default would cut them.
+const serverOptions = { requestTimeout: 2 * 60 * 60 * 1000 };
+
+// Docker binds host paths, so only a supervisor running directly on a Linux host can share one.
+export function servesGithubBrokerSocket(
+  platform: NodeJS.Platform = process.platform,
+  inContainer = existsSync("/.dockerenv"),
+): boolean {
+  return platform === "linux" && !inContainer;
+}
+
 // A socket bind-mounted into each computer reaches the broker without crossing the host firewall.
 const githubBrokerSocketDir = path.resolve(
   process.env.SANDBOX_GITHUB_BROKER_SOCKET_DIR ??
@@ -165,7 +178,7 @@ function listenGithubBrokerSocket(logger: ReturnType<typeof createRootLogger>) {
   mkdirSync(githubBrokerSocketDir, { recursive: true, mode: 0o755 });
   const socketPath = path.join(githubBrokerSocketDir, "broker.sock");
   rmSync(socketPath, { force: true });
-  const server = createAdaptorServer({ fetch: brokerApp.fetch });
+  const server = createAdaptorServer({ fetch: brokerApp.fetch, serverOptions });
   server.on("error", (error) => {
     logger.warn("github broker socket unavailable", { "error.message": error.message });
   });
@@ -256,7 +269,8 @@ app.post("/computers", async (c) => {
           (!networkMode || info.HostConfig.NetworkMode === networkMode) &&
           info.Config.User === computerUser &&
           controlPublishOk &&
-          (!storage.homeVolume || homeVolumeMatches(info.HostConfig.Mounts, storage.homeVolume))
+          (!storage.homeVolume || homeVolumeMatches(info.HostConfig.Mounts, storage.homeVolume)) &&
+          (!githubBrokerSocketReady || githubBrokerSocketMounted(info.Mounts))
         ) {
           if (!info.State.Running) await existing.start();
           return c.json({
@@ -896,22 +910,24 @@ function startSupervisor() {
   computerResourceLimits();
   const port = Number(process.env.SUPERVISOR_PORT ?? 7091);
   const hostname = process.env.SUPERVISOR_HOST ?? "127.0.0.1";
-  // Streamed file writes copy whole repository archives; Node's 5-minute default would cut them.
-  const serverOptions = { requestTimeout: 2 * 60 * 60 * 1000 };
   const server = serve({ fetch: app.fetch, hostname, port, serverOptions }, () => {
     logger.info("supervisor listening", { "http.host": hostname, "http.port": port });
   });
   let brokerServer: ReturnType<typeof serve> | undefined;
-  const brokerSocketServer = needsBridgeBrokerListener(hostname)
+  // The socket helps whatever the bind address: a host firewall can drop bridge traffic to 0.0.0.0 too.
+  const brokerSocketServer = servesGithubBrokerSocket()
     ? listenGithubBrokerSocket(logger)
     : undefined;
   if (needsBridgeBrokerListener(hostname)) {
     void bridgeGatewayAddress()
       .then((gateway) => {
         if (!gateway) throw new Error("the default bridge network has no gateway");
-        brokerServer = serve({ fetch: brokerApp.fetch, hostname: gateway, port }, () => {
-          logger.info("github broker listening", { "http.host": gateway, "http.port": port });
-        });
+        brokerServer = serve(
+          { fetch: brokerApp.fetch, hostname: gateway, port, serverOptions },
+          () => {
+            logger.info("github broker listening", { "http.host": gateway, "http.port": port });
+          },
+        );
       })
       .catch((error: unknown) => {
         logger.warn("github broker unavailable to computers", {