diff --git a/typescript/src/sandbox.ts b/typescript/src/sandbox.ts
index 4eee955b..ac8a3252 100644
--- a/typescript/src/sandbox.ts
+++ b/typescript/src/sandbox.ts
@@ -4,7 +4,7 @@ import {
   Desktop,
 } from "./desktop.js";
 import * as defaults from "./defaults.js";
-import { SandboxError } from "./errors.js";
+import { SandboxError, SandboxConnectionError } from "./errors.js";
 import { type HttpRequestOptions, type Traced, HttpClient } from "./http.js";
 import {
   type CheckpointOptions,
@@ -719,6 +719,7 @@ export class Sandbox {
     const stdoutLines: string[] = [];
     const stderrLines: string[] = [];
     let exitCode = -1;
+    let sawExit = false;
 
     for await (const raw of parseSSEStream<Record<string, unknown>>(sseStream)) {
       if (typeof raw.line === "string") {
@@ -728,6 +729,7 @@ export class Sandbox {
           stdoutLines.push(raw.line);
         }
       } else if ("exit_code" in raw || "signal" in raw) {
+        sawExit = true;
         if (typeof raw.exit_code === "number") {
           exitCode = raw.exit_code;
         } else if (typeof raw.signal === "number") {
@@ -736,6 +738,16 @@ export class Sandbox {
       }
     }
 
+    // A completed run always terminates with an exit_code/signal event. If the
+    // stream ends without one, the command never finished — e.g. the sandbox was
+    // unreachable and the proxy closed the stream after a timeout. Surface that
+    // as an error instead of returning a misleading success with empty output.
+    if (!sawExit) {
+      throw new SandboxConnectionError(
+        `Command "${command}" did not complete: the process stream ended without an exit status.`,
+      );
+    }
+
     return Object.assign(
       { exitCode, stdout: stdoutLines.join("\n"), stderr: stderrLines.join("\n") },
       { traceId },
diff --git a/typescript/tests/sandbox.test.ts b/typescript/tests/sandbox.test.ts
index ead29048..7fd02264 100644
--- a/typescript/tests/sandbox.test.ts
+++ b/typescript/tests/sandbox.test.ts
@@ -87,6 +87,25 @@ describe("Sandbox", () => {
       await sbx.run("echo");
       sbx.close();
     });
+
+    it("throws when the stream ends without an exit event", async () => {
+      // Reproduces the unreachable-sandbox case: the proxy closes the run
+      // stream after a timeout without ever delivering an exit_code, so the
+      // command never actually executed. This must surface as an error rather
+      // than a silent success with empty output.
+      mockFetch((url) => {
+        if (url.includes("/api/v1/processes/run")) {
+          return sseResponse([{ pid: 42, started_at: 1700000000 }]);
+        }
+        return new Response("", { status: 404 });
+      });
+
+      const sbx = makeSandbox();
+      await expect(sbx.run("node", { args: ["-v"] })).rejects.toThrow(
+        /did not complete/,
+      );
+      sbx.close();
+    });
   });
 
   describe("listProcesses", () => {