Make Runtime.outputStreamsShareDevice required

packages/cli-engine/src/execution/rendering.ts

@@ -438,12 +438,11 @@ export function renderCompletedHuman(
    *  (amends the 2026-08-09 "always" ruling; any redirection of either
    *  stream keeps the mirror, so pipes still receive exactly the data
    *  lines). A harness that allocates two separate PTYs reports
-   *  outputStreamsShareDevice false and keeps its mirror; a host that
-   *  cannot tell is treated as one terminal. */
+   *  outputStreamsShareDevice false and keeps its mirror. */
   const oneScreen =
     runtime.isTty.stdout &&
     runtime.isTty.stderr &&
-    runtime.outputStreamsShareDevice !== false;
+    runtime.outputStreamsShareDevice;
   if (!oneScreen) {
     for (const line of presented.presentation.stdout) {
       runtime.stdout.write(`${line}\n`);

packages/cli-engine/src/runtime.ts

@@ -40,11 +40,15 @@ export interface Runtime {
    * human blocks and the machine stdout mirror would draw on one screen
    * as visible duplication. Consulted only when both streams are TTYs:
    * `false` there means two separate terminals, so the mirror is kept
-   * for whatever is reading stdout. Absent means the host cannot tell,
-   * which is treated as "same" — the overwhelmingly common case for two
-   * TTYs is one terminal.
+   * for whatever is reading stdout.
+   *
+   * Required, and deliberately so. It decides whether a command's stdout
+   * payload is written at all, so a host that says nothing is choosing to
+   * drop that payload — a choice that belongs at the call site, in the open,
+   * not in a default the host never sees. A bin that cannot tell answers
+   * `true`: two TTYs are one terminal far more often than not.
    */
-  readonly outputStreamsShareDevice?: boolean;
+  readonly outputStreamsShareDevice: boolean;
   /**
    * Forces the answer to "is this CI", where telemetry never reports.
    * Absent — the normal case — means the engine detects CI from `env`

packages/cli-engine/src/testing.ts

@@ -130,6 +130,11 @@ export interface TestCli {
       readonly onSettled?: (summary: RunSummary) => void;
       readonly cwd?: string;
       readonly isTty?: { stdin?: boolean; stdout?: boolean; stderr?: boolean };
+      /** Whether stdout and stderr are the same open device. Defaults to
+       *  true — two TTYs are one terminal far more often than not — which
+       *  suppresses the stdout mirror. Set false to assert a command's
+       *  stdout payload the way a caller with separate sinks receives it. */
+      readonly outputStreamsShareDevice?: boolean;
       /** Terminal width, as the stream would report it. Absent means
        *  not a terminal, which is what ui.width reads as unbounded. */
       readonly columns?: { stderr?: number };
@@ -361,6 +366,7 @@ export function createTestCli(spec: {
           stdout: opts?.isTty?.stdout ?? false,
           stderr: opts?.isTty?.stderr ?? false,
         },
+        outputStreamsShareDevice: opts?.outputStreamsShareDevice ?? true,
         isCIOverride: opts?.isCI ?? spec.isCI,
         exit: (code: number): never => {
           throw new Error(

packages/cli-engine/tests/clack-isolation.test.ts

@@ -115,6 +115,7 @@ describe("scripted and non-TTY paths are clack-free", () => {
       cwd: "/",
       env: {},
       isTty: { stdin: true, stdout: true, stderr: true },
+      outputStreamsShareDevice: true,
       exit: (code: number): never => {
         throw new Error(`runtime.exit(${code})`);
       },

packages/cli-engine/tests/clack-prompts.test.ts

@@ -103,6 +103,7 @@ async function runInteractive(
     cwd: "/",
     env: {},
     isTty: { stdin: true, stdout: true, stderr: true },
+    outputStreamsShareDevice: true,
     exit: (code: number): never => {
       throw new Error(`runtime.exit(${code})`);
     },

packages/cli-engine/tests/config.test.ts

@@ -864,6 +864,7 @@ describe("needs.config", { timeout: 60_000 }, () => {
       cwd,
       env: {},
       isTty: { stdin: false, stdout: false, stderr: false },
+      outputStreamsShareDevice: true,
       exit: (code: number): never => {
         throw new Error(`runtime.exit(${code})`);
       },

packages/cli-engine/tests/engine.type-test.ts

@@ -474,6 +474,7 @@ export const runtimeShape: Runtime = {
   cwd: "/",
   env: { CI: "1" },
   isTty: { stdin: true, stdout: true, stderr: true },
+  outputStreamsShareDevice: true,
   exit: (code: number): never => {
     throw new Error(String(code));
   },

packages/cli-engine/tests/environment-credential-manager.test.ts

@@ -167,6 +167,7 @@ describe("wired as a Runtime's manager", () => {
       cwd: "/",
       env,
       isTty: { stdin: false, stdout: false, stderr: false },
+      outputStreamsShareDevice: true,
       exit: (code: number): never => {
         throw new Error(`runtime.exit(${code})`);
       },

packages/cli-engine/tests/execution.test.ts

@@ -601,6 +601,7 @@ describe("needs preconditions", () => {
       cwd: process.cwd(),
       env: {},
       isTty: { stdin: opts.interactive, stdout: false, stderr: false },
+      outputStreamsShareDevice: true,
       exit: (code: number): never => {
         throw new Error(`runtime.exit(${code})`);
       },
@@ -821,6 +822,7 @@ describe("report() after the handler resolved", () => {
       cwd: "/",
       env: {},
       isTty: { stdin: false, stdout: false, stderr: false },
+      outputStreamsShareDevice: true,
       exit: (code: number): never => {
         throw new Error(`runtime.exit(${code})`);
       },
@@ -896,6 +898,7 @@ describe("credentials that cannot be read", () => {
       cwd: "/",
       env: {},
       isTty: { stdin: false, stdout: false, stderr: false },
+      outputStreamsShareDevice: true,
       exit: (code: number): never => {
         throw new Error(`runtime.exit(${code})`);
       },

packages/cli-engine/tests/lifetimes.test.ts

@@ -263,6 +263,7 @@ describe("the engine owns the double-signal policy", () => {
       cwd: "/",
       env: {},
       isTty: { stdin: false, stdout: false, stderr: false },
+      outputStreamsShareDevice: true,
       exit: (code: number): never => {
         exited.push(code);
         throw new Error(`runtime.exit(${code})`);

packages/cli-engine/tests/management-api.test.ts

@@ -103,6 +103,7 @@ function makeRuntime(overrides?: {
     cwd: "/",
     env: {},
     isTty: { stdin: false, stdout: false, stderr: false },
+    outputStreamsShareDevice: true,
     exit: (code: number): never => {
       throw new Error(`runtime.exit(${code})`);
     },

packages/cli-engine/tests/prompts.test.ts

@@ -524,6 +524,7 @@ describe("stdin cleanup", () => {
       cwd: "/",
       env: {},
       isTty: { stdin: true, stdout: true, stderr: true },
+      outputStreamsShareDevice: true,
       exit: (code: number): never => {
         throw new Error(`runtime.exit(${code})`);
       },

packages/cli-engine/tests/spawn.test.ts

@@ -1773,6 +1773,7 @@ function controllableRuntime() {
     cwd: "/",
     env: {},
     isTty: { stdin: false, stdout: false, stderr: false },
+    outputStreamsShareDevice: true,
     exit: (code: number): never => {
       exited.push(code);
       throw new Error(`runtime.exit(${code})`);

packages/cli/src/runtime.ts

@@ -83,15 +83,17 @@ function warnOnDeprecatedStateFileEnvVar(proc: HostProcess): void {
 
 /** Whether fd 1 and fd 2 are the same open device. Distinguishes one
  *  terminal (mirror suppressed) from a harness that allocated separate
- *  PTYs for the two streams (mirror kept). Undefined when the fds
- *  cannot be inspected — the engine then assumes one terminal. */
-function outputStreamsShareDevice(): boolean | undefined {
+ *  PTYs for the two streams (mirror kept). */
+function outputStreamsShareDevice(): boolean {
   try {
     const out = fstatSync(1);
     const err = fstatSync(2);
     return out.dev === err.dev && out.ino === err.ino && out.rdev === err.rdev;
   } catch {
-    return undefined;
+    // Cannot tell: answer "one terminal", which two TTYs are far more
+    // often than not. Same behaviour the engine used to apply to an
+    // absent field, now stated here rather than defaulted out of sight.
+    return true;
   }
 }