diff --git a/scripts/release-prepare.ts b/scripts/release-prepare.ts
index fae05224266e..b0637858a91f 100644
--- a/scripts/release-prepare.ts
+++ b/scripts/release-prepare.ts
@@ -166,6 +166,7 @@ export function createReleasePrepareSteps(
 
 export function runReleasePrepareSteps(params: {
   cwd: string;
+  json?: boolean;
   mode: ReleasePrepareMode;
   runStep?: (step: ReleasePrepareStep, cwd: string) => number;
   steps: ReleasePrepareStep[];
@@ -174,7 +175,10 @@ export function runReleasePrepareSteps(params: {
     return params.steps.map((step) => ({ ...step, durationMs: 0, status: "planned" }));
   }
 
-  const runStep = params.runStep ?? runReleasePrepareStep;
+  const runStep =
+    params.runStep ??
+    ((step: ReleasePrepareStep, cwd: string) =>
+      runReleasePrepareStep(step, cwd, { json: params.json ?? false }));
   const results: ReleasePrepareStepResult[] = [];
   let blocked = false;
   for (const step of params.steps) {
@@ -236,6 +240,7 @@ export function main(argv = process.argv.slice(2)): number {
   const before = readWorktreeState(args.rootDir);
   const results = runReleasePrepareSteps({
     cwd: args.rootDir,
+    json: args.json,
     mode: args.mode,
     steps,
   });
@@ -265,16 +270,31 @@ export function main(argv = process.argv.slice(2)): number {
   return manifest.status === "failed" ? 1 : 0;
 }
 
-function runReleasePrepareStep(step: ReleasePrepareStep, cwd: string): number {
-  process.stdout.write(`\n[release-prepare] ${step.name}\n`);
+export function runReleasePrepareStep(
+  step: ReleasePrepareStep,
+  cwd: string,
+  options: { json?: boolean } = {},
+): number {
+  const json = options.json ?? false;
+  const progressStream = json ? process.stderr : process.stdout;
+  progressStream.write(`\n[release-prepare] ${step.name}\n`);
   const result = spawnSync(step.command, step.args, {
     cwd,
+    encoding: json ? "utf8" : undefined,
     env: process.env,
-    stdio: "inherit",
+    stdio: json ? ["ignore", "pipe", "pipe"] : "inherit",
   });
   if (result.error) {
     throw result.error;
   }
+  if (json) {
+    if (typeof result.stdout === "string" && result.stdout) {
+      process.stderr.write(result.stdout);
+    }
+    if (typeof result.stderr === "string" && result.stderr) {
+      process.stderr.write(result.stderr);
+    }
+  }
   return result.status ?? 1;
 }
 
diff --git a/test/scripts/release-prepare.test.ts b/test/scripts/release-prepare.test.ts
index 27f9365e25b3..d5424256783f 100644
--- a/test/scripts/release-prepare.test.ts
+++ b/test/scripts/release-prepare.test.ts
@@ -1,9 +1,10 @@
 // Release prepare tests cover shadow planning, cutover commands, and candidate manifests.
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, vi } from "vitest";
 import {
   buildReleasePreparationManifest,
   createReleasePrepareSteps,
   parseReleasePrepareArgs,
+  runReleasePrepareStep,
   runReleasePrepareSteps,
 } from "../../scripts/release-prepare.ts";
 
@@ -117,6 +118,43 @@ describe("release preparation plan", () => {
 
     expect(results.map((result) => result.status)).toEqual(["failed", "skipped"]);
   });
+
+  it("keeps JSON mode child output off stdout", () => {
+    const stdout: string[] = [];
+    const stderr: string[] = [];
+    const stdoutSpy = vi.spyOn(process.stdout, "write").mockImplementation((chunk) => {
+      stdout.push(String(chunk));
+      return true;
+    });
+    const stderrSpy = vi.spyOn(process.stderr, "write").mockImplementation((chunk) => {
+      stderr.push(String(chunk));
+      return true;
+    });
+    try {
+      const status = runReleasePrepareStep(
+        {
+          args: [
+            "-e",
+            'process.stdout.write("child stdout\\n"); process.stderr.write("child stderr\\n");',
+          ],
+          command: process.execPath,
+          id: "release-version",
+          name: "JSON child",
+        },
+        process.cwd(),
+        { json: true },
+      );
+
+      expect(status).toBe(0);
+      expect(stdout.join("")).toBe("");
+      expect(stderr.join("")).toContain("[release-prepare] JSON child");
+      expect(stderr.join("")).toContain("child stdout");
+      expect(stderr.join("")).toContain("child stderr");
+    } finally {
+      stdoutSpy.mockRestore();
+      stderrSpy.mockRestore();
+    }
+  });
 });
 
 describe("release preparation manifest", () => {