diff --git a/scripts/test-projects.test-support.mjs b/scripts/test-projects.test-support.mjs
index 513de73eb8f..82d986c5337 100644
--- a/scripts/test-projects.test-support.mjs
+++ b/scripts/test-projects.test-support.mjs
@@ -397,6 +397,7 @@ const TOOLING_SOURCE_TEST_TARGETS = new Map([
   ["scripts/docker-e2e-timings.mjs", ["test/scripts/docker-e2e-helper-cli.test.ts"]],
   ["scripts/kova-ci-summary.mjs", ["test/scripts/kova-ci-summary.test.ts"]],
   ["scripts/test-extension-batch.mjs", ["test/scripts/test-extension.test.ts"]],
+  ["scripts/zai-fallback-repro.ts", ["test/scripts/zai-fallback-repro.test.ts"]],
   ["scripts/lib/extension-test-plan.mjs", ["test/scripts/test-extension.test.ts"]],
   ["scripts/lib/vitest-batch-runner.mjs", ["test/scripts/test-extension.test.ts"]],
   ["scripts/lib/ci-node-test-plan.mjs", ["test/scripts/ci-node-test-plan.test.ts"]],
@@ -461,6 +462,7 @@ const TOOLING_TEST_TARGETS = new Map([
     "test/scripts/vitest-local-scheduling.test.ts",
     ["test/scripts/vitest-local-scheduling.test.ts"],
   ],
+  ["test/scripts/zai-fallback-repro.test.ts", ["test/scripts/zai-fallback-repro.test.ts"]],
 ]);
 const GROUP_VISIBLE_REPLY_TEST_TARGETS = [
   "src/auto-reply/reply/dispatch-acp.test.ts",
diff --git a/scripts/zai-fallback-repro.ts b/scripts/zai-fallback-repro.ts
index 0585764e173..87945c73021 100644
--- a/scripts/zai-fallback-repro.ts
+++ b/scripts/zai-fallback-repro.ts
@@ -13,6 +13,11 @@ type RunResult = {
   stderr: string;
 };
 
+type OutputCapture = {
+  text: string;
+  truncatedChars: number;
+};
+
 type PnpmCommand = {
   args: string[];
   command: string;
@@ -29,11 +34,33 @@ type ResolvePnpmCommandOptions = {
   platform?: NodeJS.Platform;
 };
 
+const COMMAND_OUTPUT_MAX_CHARS = 512 * 1024;
+
 function resolveEnvValue(env: NodeJS.ProcessEnv, name: string): string | undefined {
   const key = Object.keys(env).find((candidate) => candidate.toLowerCase() === name.toLowerCase());
   return key === undefined ? undefined : env[key];
 }
 
+export function appendBoundedReproOutput(
+  capture: OutputCapture,
+  chunk: unknown,
+  maxChars = COMMAND_OUTPUT_MAX_CHARS,
+): OutputCapture {
+  const nextText = capture.text + String(chunk);
+  if (nextText.length <= maxChars) {
+    return { text: nextText, truncatedChars: capture.truncatedChars };
+  }
+  const truncatedChars = capture.truncatedChars + nextText.length - maxChars;
+  return { text: nextText.slice(-maxChars), truncatedChars };
+}
+
+function formatBoundedReproOutput(capture: OutputCapture): string {
+  if (capture.truncatedChars === 0) {
+    return capture.text;
+  }
+  return `[output truncated ${capture.truncatedChars} chars; showing tail]\n${capture.text}`;
+}
+
 export function resolveZaiFallbackPnpmCommand(
   args: string[],
   options: ResolvePnpmCommandOptions = {},
@@ -83,25 +110,31 @@ async function runCommand(
       stdio: ["ignore", "pipe", "pipe"],
       windowsVerbatimArguments: command.windowsVerbatimArguments,
     });
-    let stdout = "";
-    let stderr = "";
+    let stdout: OutputCapture = { text: "", truncatedChars: 0 };
+    let stderr: OutputCapture = { text: "", truncatedChars: 0 };
     child.stdout.on("data", (chunk) => {
       const text = String(chunk);
-      stdout += text;
+      stdout = appendBoundedReproOutput(stdout, text);
       process.stdout.write(text);
     });
     child.stderr.on("data", (chunk) => {
       const text = String(chunk);
-      stderr += text;
+      stderr = appendBoundedReproOutput(stderr, text);
       process.stderr.write(text);
     });
     child.on("error", (err) => reject(err));
     child.on("close", (code, signal) => {
+      const result = {
+        code,
+        signal,
+        stdout: formatBoundedReproOutput(stdout),
+        stderr: formatBoundedReproOutput(stderr),
+      };
       if (code === 0) {
-        resolve({ code, signal, stdout, stderr });
+        resolve(result);
         return;
       }
-      resolve({ code, signal, stdout, stderr });
+      resolve(result);
       const summary = signal
         ? `${label} exited with signal ${signal}`
         : `${label} exited with code ${code}`;
diff --git a/test/scripts/test-projects.test.ts b/test/scripts/test-projects.test.ts
index 70ec10801fd..c434c19d159 100644
--- a/test/scripts/test-projects.test.ts
+++ b/test/scripts/test-projects.test.ts
@@ -214,6 +214,13 @@ describe("scripts/test-projects changed-target routing", () => {
     });
   });
 
+  it("routes Z.AI fallback repro script changes through its regression test", () => {
+    expect(resolveChangedTestTargetPlan(["scripts/zai-fallback-repro.ts"])).toEqual({
+      mode: "targets",
+      targets: ["test/scripts/zai-fallback-repro.test.ts"],
+    });
+  });
+
   it("routes group visible reply config changes through channel delivery regressions", () => {
     expect(
       resolveChangedTestTargetPlan([
diff --git a/test/scripts/zai-fallback-repro.test.ts b/test/scripts/zai-fallback-repro.test.ts
index c56c8d7a010..bb19367ea7e 100644
--- a/test/scripts/zai-fallback-repro.test.ts
+++ b/test/scripts/zai-fallback-repro.test.ts
@@ -1,5 +1,8 @@
 import { describe, expect, it } from "vitest";
-import { resolveZaiFallbackPnpmCommand } from "../../scripts/zai-fallback-repro.ts";
+import {
+  appendBoundedReproOutput,
+  resolveZaiFallbackPnpmCommand,
+} from "../../scripts/zai-fallback-repro.ts";
 
 describe("zai fallback repro command resolution", () => {
   it("wraps Windows pnpm.cmd without Node shell argv", () => {
@@ -24,4 +27,12 @@ describe("zai fallback repro command resolution", () => {
       windowsVerbatimArguments: true,
     });
   });
+
+  it("keeps only a bounded child output tail", () => {
+    const first = appendBoundedReproOutput({ text: "", truncatedChars: 0 }, "abcdef", 5);
+    const second = appendBoundedReproOutput(first, "ghij", 5);
+
+    expect(first).toEqual({ text: "bcdef", truncatedChars: 1 });
+    expect(second).toEqual({ text: "fghij", truncatedChars: 5 });
+  });
 });