diff --git a/packages/cli/src/commands/dev.ts b/packages/cli/src/commands/dev.ts
index 080d8acd0..d1568a0bd 100644
--- a/packages/cli/src/commands/dev.ts
+++ b/packages/cli/src/commands/dev.ts
@@ -9,7 +9,7 @@ import { resolveConfigFilePathOption } from "../lib/config-file-path.js";
 import { devEnvStatePath, ensureLocalDashboardSecret, readDevEnvState, recordLocalDashboardProcess } from "../lib/dev-env-state.js";
 import { CliError } from "../lib/errors.js";
 import { cliVersion } from "../lib/own-package.js";
-import { maybeReexecToLatest } from "../lib/self-update.js";
+import { maybeReexecToLatest, REEXEC_MARKER_ENV } from "../lib/self-update.js";
 
 type ChildCommand = {
   command: string,
@@ -693,8 +693,6 @@ const childEnv = { ...process.env };
 delete childEnv.HEXCLAVE_DEV_APP_COMMAND_PARENT_PID;
 delete childEnv.HEXCLAVE_DEV_APP_COMMAND;
 delete childEnv.HEXCLAVE_DEV_APP_COMMAND_ARGS_JSON;
-// Internal to the npx auto-update handshake; never meant for the user's command.
-delete childEnv.HEXCLAVE_CLI_REEXEC_MARKER;
 
 const child = spawn(command, args, {
   env: childEnv,
@@ -751,14 +749,20 @@ child.on("error", (error) => {
 `;
 
 function runChildProcess(command: ChildCommand, env: NodeJS.ProcessEnv): Promise<number> {
+  // The npx auto-update handshake marker is internal to the parent<->child
+  // re-exec; never leak it into the user's command. Scrub it here (rather than in
+  // the wrapper script) so it uses the exported constant and covers both the
+  // Windows-direct and POSIX-wrapper spawn paths.
+  const childEnv = { ...env };
+  delete childEnv[REEXEC_MARKER_ENV];
   return new Promise((resolvePromise, reject) => {
     const child = process.platform === "win32"
-      ? spawn(command.command, command.args, { stdio: "inherit", env })
+      ? spawn(command.command, command.args, { stdio: "inherit", env: childEnv })
       : spawn(process.execPath, ["-e", APP_COMMAND_WRAPPER_SCRIPT], {
         detached: true,
         stdio: "inherit",
         env: {
-          ...env,
+          ...childEnv,
           [APP_COMMAND_WRAPPER_PARENT_PID_ENV_VAR]: String(process.pid),
           [APP_COMMAND_WRAPPER_COMMAND_ENV_VAR]: command.command,
           [APP_COMMAND_WRAPPER_ARGS_ENV_VAR]: JSON.stringify(command.args),
diff --git a/packages/cli/src/lib/self-update.test.ts b/packages/cli/src/lib/self-update.test.ts
index 69faae175..a01e2f6af 100644
--- a/packages/cli/src/lib/self-update.test.ts
+++ b/packages/cli/src/lib/self-update.test.ts
@@ -336,4 +336,24 @@ describe("maybeReexecToLatest fallback wiring", () => {
     await promise;
     expect(exitSpy).toHaveBeenCalledWith(1);
   });
+
+  it("propagates a nonzero, non-NaN code when killed by a signal missing from os.constants", async () => {
+    const exitSpy = vi.spyOn(process, "exit").mockImplementation(((() => {
+      throw new Error("__exit__");
+    }) as never));
+    const child = mockNpxChild({ writeMarker: false });
+
+    const promise = maybeReexecToLatest({ forwardArgs: ["dev"] });
+    // SIGSTKFLT is absent from os.constants.signals on macOS (present on Linux).
+    // Either way the abort must surface as a real nonzero code — never NaN (which
+    // process.exit coerces to 0, masking the abort as success).
+    child.emit("close", null, "SIGSTKFLT");
+
+    await promise;
+    expect(exitSpy).toHaveBeenCalledTimes(1);
+    const code = exitSpy.mock.calls[0][0];
+    expect(typeof code).toBe("number");
+    expect(Number.isNaN(code as number)).toBe(false);
+    expect(code as number).toBeGreaterThan(0);
+  });
 });
diff --git a/packages/cli/src/lib/self-update.ts b/packages/cli/src/lib/self-update.ts
index f9e33f8ad..86392ce10 100644
--- a/packages/cli/src/lib/self-update.ts
+++ b/packages/cli/src/lib/self-update.ts
@@ -138,11 +138,16 @@ function runReexec(invocation: NpxInvocation, markerFile: string | null): Promis
     child.on("close", (code, signal) => {
       cleanup();
       if (signal != null) {
-        // Killed by a signal we forwarded (e.g. Ctrl-C). Report it with the
-        // conventional 128 + signal-number exit code so the caller can both
-        // recognize the abort and propagate a sensible code.
-        const signalNumber = osConstants.signals[signal];
-        resolvePromise({ exited: true, code: 128 + signalNumber, signal });
+        // Killed by a signal (a Ctrl-C we forwarded, or an external kill). Report
+        // the conventional 128 + signal-number exit code so the caller can both
+        // recognize the abort and propagate a sensible code. `os.constants.signals`
+        // doesn't list every NodeJS.Signals name on every platform, so the lookup
+        // can be undefined at runtime (the type says otherwise) — `128 + undefined`
+        // is NaN and `process.exit(NaN)` coerces to 0, masking the abort as
+        // success. Fall back to a generic nonzero code in that case.
+        const signalNumber = osConstants.signals[signal] as number | undefined;
+        const code = signalNumber != null ? 128 + signalNumber : 1;
+        resolvePromise({ exited: true, code, signal });
         return;
       }
       resolvePromise({ exited: true, code: code ?? 1, signal: null });