diff --git a/tests/bug-2798-context-window-config-key.test.cjs b/tests/bug-2798-context-window-config-key.test.cjs
new file mode 100644
index 000000000..cb1d11b23
--- /dev/null
+++ b/tests/bug-2798-context-window-config-key.test.cjs
@@ -0,0 +1,89 @@
+/**
+ * Regression test for bug #2798
+ *
+ * `gsd-sdk query config-set context_window <n>` was rejected with
+ * "Unknown config key: context_window" because context_window was missing
+ * from VALID_CONFIG_KEYS in sdk/src/query/config-schema.ts.
+ *
+ * The fix added 'context_window' to the allowlist.
+ * This test prevents future drift where the key gets accidentally removed.
+ */
+
+'use strict';
+
+const { describe, test, beforeEach, afterEach } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+const { execFileSync } = require('node:child_process');
+const { createTempProject, cleanup } = require('./helpers.cjs');
+
+const REPO_ROOT = path.join(__dirname, '..');
+const SDK_CLI = path.join(REPO_ROOT, 'sdk', 'dist', 'cli.js');
+
+function runConfigSet(key, value, projectDir) {
+  const argv = ['query', 'config-set', key, String(value), '--project-dir', projectDir];
+  let stdout = '';
+  let exitCode = 0;
+  try {
+    stdout = execFileSync(process.execPath, [SDK_CLI, ...argv], {
+      encoding: 'utf-8',
+      stdio: ['pipe', 'pipe', 'pipe'],
+      env: { ...process.env, GSD_SESSION_KEY: '' },
+    });
+  } catch (err) {
+    exitCode = err.status ?? 1;
+    stdout = err.stdout?.toString() ?? '';
+  }
+  let json = null;
+  try { json = JSON.parse(stdout.trim()); } catch { /* ok */ }
+  return { exitCode, json };
+}
+
+describe('bug-2798: context_window is a valid config key', () => {
+  let tmpDir;
+
+  beforeEach(() => {
+    tmpDir = createTempProject('gsd-test-2798-');
+    fs.writeFileSync(
+      path.join(tmpDir, '.planning', 'config.json'),
+      JSON.stringify({ mode: 'balanced' })
+    );
+  });
+
+  afterEach(() => {
+    cleanup(tmpDir);
+  });
+
+  test('config-set context_window succeeds (not rejected as unknown key)', () => {
+    const result = runConfigSet('context_window', 1000000, tmpDir);
+
+    assert.strictEqual(result.exitCode, 0, 'should exit 0 (key is valid)');
+    assert.ok(result.json !== null, 'should emit JSON');
+    assert.strictEqual(result.json?.updated, true, 'updated should be true');
+    assert.strictEqual(result.json?.key, 'context_window');
+  });
+
+  test('context_window value is written to config.json', () => {
+    runConfigSet('context_window', 500000, tmpDir);
+
+    const config = JSON.parse(
+      fs.readFileSync(path.join(tmpDir, '.planning', 'config.json'), 'utf-8')
+    );
+    assert.strictEqual(config.context_window, 500000, 'context_window should be persisted');
+  });
+
+  test('config-schema CJS and SDK allowlists both include context_window', () => {
+    const cjsSchema = require(path.join(REPO_ROOT, 'get-shit-done', 'bin', 'lib', 'config-schema.cjs'));
+    const sdkSchema = require(path.join(REPO_ROOT, 'sdk', 'dist', 'query', 'config-schema.js'));
+
+    assert.ok(
+      cjsSchema.VALID_CONFIG_KEYS.has('context_window'),
+      'CJS VALID_CONFIG_KEYS must include context_window'
+    );
+    assert.ok(
+      sdkSchema.VALID_CONFIG_KEYS.has('context_window'),
+      'SDK VALID_CONFIG_KEYS must include context_window'
+    );
+  });
+});