diff --git a/src/model/output.test.ts b/src/model/output.test.ts
index e49e6bbc..ba82bfd6 100644
--- a/src/model/output.test.ts
+++ b/src/model/output.test.ts
@@ -1,9 +1,48 @@
+import { afterEach, describe, expect, it } from 'bun:test';
+import { mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
 import { Output } from './output.ts';
 
+const originalGitHubOutput = process.env['GITHUB_OUTPUT'];
+
+let directory: string | undefined;
+
+afterEach(() => {
+  if (originalGitHubOutput === undefined) {
+    delete process.env['GITHUB_OUTPUT'];
+  } else {
+    process.env['GITHUB_OUTPUT'] = originalGitHubOutput;
+  }
+
+  if (directory) {
+    rmSync(directory, { recursive: true, force: true });
+    directory = undefined;
+  }
+});
+
 describe('Output', () => {
   describe('setBuildVersion', () => {
     it('does not throw', () => {
       expect(() => Output.setBuildVersion('1.0.0')).not.toThrow();
     });
+
+    // The end of the chain unity-builder reads: `game-ci build` publishes a
+    // version, and a wrapping action gets it out of $GITHUB_OUTPUT.
+    it('reaches $GITHUB_OUTPUT', () => {
+      directory = mkdtempSync(join(tmpdir(), 'game-ci-outputs-'));
+      const filePath = join(directory, 'outputs');
+      writeFileSync(filePath, '');
+      process.env['GITHUB_OUTPUT'] = filePath;
+
+      Output.setBuildVersion('1.166.0');
+      Output.setAndroidVersionCode('1166000');
+
+      const written = readFileSync(filePath, 'utf8');
+      expect(written).toContain('buildVersion<<');
+      expect(written).toContain('1.166.0');
+      expect(written).toContain('androidVersionCode<<');
+      expect(written).toContain('1166000');
+    });
   });
 });
diff --git a/src/module/actions/core.test.ts b/src/module/actions/core.test.ts
new file mode 100644
index 00000000..c9922456
--- /dev/null
+++ b/src/module/actions/core.test.ts
@@ -0,0 +1,84 @@
+import { afterEach, describe, expect, it } from 'bun:test';
+import { mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
+import { EOL, tmpdir } from 'node:os';
+import { join } from 'node:path';
+import { core } from './index.ts';
+
+const originalGitHubOutput = process.env['GITHUB_OUTPUT'];
+const originalConsoleLog = console.log;
+
+let directory: string | undefined;
+
+/** An empty $GITHUB_OUTPUT file, as a runner would hand one to a step. */
+function useOutputFile(): string {
+  directory = mkdtempSync(join(tmpdir(), 'game-ci-outputs-'));
+  const filePath = join(directory, 'outputs');
+  writeFileSync(filePath, '');
+  process.env['GITHUB_OUTPUT'] = filePath;
+  return filePath;
+}
+
+afterEach(() => {
+  if (originalGitHubOutput === undefined) {
+    delete process.env['GITHUB_OUTPUT'];
+  } else {
+    process.env['GITHUB_OUTPUT'] = originalGitHubOutput;
+  }
+
+  console.log = originalConsoleLog;
+
+  if (directory) {
+    rmSync(directory, { recursive: true, force: true });
+    directory = undefined;
+  }
+});
+
+describe('core.setOutput', () => {
+  // The regression unity-builder v6 shipped with: the CLI printed a version
+  // and never wrote the file, so buildVersion/androidVersionCode arrived
+  // empty on every `unity-builder` run that used the CLI.
+  it('writes a real record to $GITHUB_OUTPUT when a runner provides one', () => {
+    const filePath = useOutputFile();
+
+    core.setOutput('buildVersion', '1.166.0');
+
+    const written = readFileSync(filePath, 'utf8');
+    expect(written).toMatch(
+      /^buildVersion<<ghadelimiter_[\w-]+\r?\n1\.166\.0\r?\nghadelimiter_[\w-]+\r?\n$/,
+    );
+  });
+
+  // `key=value` would truncate at the first newline, so the delimiter form is
+  // load-bearing rather than cosmetic.
+  it('round-trips a value containing newlines', () => {
+    const filePath = useOutputFile();
+
+    core.setOutput('buildVersion', `1.166.0${EOL}second-line`);
+
+    expect(readFileSync(filePath, 'utf8')).toContain(`1.166.0${EOL}second-line`);
+  });
+
+  it('appends instead of overwriting, so several outputs survive', () => {
+    const filePath = useOutputFile();
+
+    core.setOutput('buildVersion', '1.166.0');
+    core.setOutput('androidVersionCode', '1166000');
+
+    const written = readFileSync(filePath, 'utf8');
+    expect(written).toContain('buildVersion<<');
+    expect(written).toContain('androidVersionCode<<');
+    expect(written).toContain('1166000');
+  });
+
+  it('falls back to printing when there is no $GITHUB_OUTPUT', () => {
+    delete process.env['GITHUB_OUTPUT'];
+    const printed: string[] = [];
+    console.log = (message?: unknown) => {
+      printed.push(String(message));
+    };
+
+    core.setOutput('buildVersion', '1.166.0');
+
+    expect(printed).toEqual(['(mock) Output "buildVersion" is set to "1.166.0"']);
+  });
+});
diff --git a/src/module/actions/core.ts b/src/module/actions/core.ts
index 46146c14..17142e7b 100644
--- a/src/module/actions/core.ts
+++ b/src/module/actions/core.ts
@@ -1,5 +1,9 @@
 /* eslint-disable no-console */
 
+import { appendFileSync } from 'node:fs';
+import { randomUUID } from 'node:crypto';
+import { EOL } from 'node:os';
+
 // Adapted from: https://github.com/actions/toolkit/blob/9b7bcb1567c9b7f134eb3c2d6bbf409a5106a956/packages/core/src/core.ts#L13
 /**
  * Interface for getInput options
@@ -22,6 +26,20 @@ export const core = {
   },
 
   setOutput: (key: string, value: string) => {
+    // Same split as the real @actions/core: write the file GitHub hands us,
+    // and only fall back to stdout when there isn't one. Printing was
+    // previously the only branch, so this never reached $GITHUB_OUTPUT at all
+    // - which silently dropped the buildVersion/androidVersionCode that
+    // `game-ci build` publishes for unity-builder to consume.
+    const filePath = process.env['GITHUB_OUTPUT'];
+    if (filePath) {
+      // The delimiter form, not `key=value`: the value is user-supplied (a
+      // version string), and `key=value` cannot carry a newline.
+      const delimiter = `ghadelimiter_${randomUUID()}`;
+      appendFileSync(filePath, `${key}<<${delimiter}${EOL}${value}${EOL}${delimiter}${EOL}`);
+      return;
+    }
+
     console.log(`(mock) Output "${key}" is set to "${value}"`);
   },