diff --git a/containers/api-proxy/provider-env-constants.js b/containers/api-proxy/provider-env-constants.js
index c6aa8053..d22430e8 100644
--- a/containers/api-proxy/provider-env-constants.js
+++ b/containers/api-proxy/provider-env-constants.js
@@ -3,10 +3,8 @@
 /**
  * Environment variable name constants for the API proxy provider adapters.
  *
- * This is the single source of truth for env var names on the container JS side.
  * The TypeScript equivalent lives in src/api-proxy-env-constants.ts.
- *
- * Both files must be kept in sync when adding or renaming env vars.
+ * Keep both files in sync; src/api-proxy-env-constants-sync.test.ts enforces parity.
  */
 
 /** Environment variable names for the OpenAI provider adapter. */
diff --git a/src/api-proxy-env-constants-sync.test.ts b/src/api-proxy-env-constants-sync.test.ts
new file mode 100644
index 00000000..2d41906a
--- /dev/null
+++ b/src/api-proxy-env-constants-sync.test.ts
@@ -0,0 +1,18 @@
+import { ANTHROPIC_ENV, COPILOT_ENV, GEMINI_ENV, OPENAI_ENV } from './api-proxy-env-constants';
+
+// eslint-disable-next-line @typescript-eslint/no-require-imports
+const providerEnvConstants = require('../containers/api-proxy/provider-env-constants.js') as {
+  OPENAI_ENV: typeof OPENAI_ENV;
+  ANTHROPIC_ENV: typeof ANTHROPIC_ENV;
+  GEMINI_ENV: typeof GEMINI_ENV;
+  COPILOT_ENV: typeof COPILOT_ENV;
+};
+
+describe('API proxy provider env constants', () => {
+  it('stays in sync between TypeScript host code and JS sidecar code', () => {
+    expect(providerEnvConstants.OPENAI_ENV).toEqual(OPENAI_ENV);
+    expect(providerEnvConstants.ANTHROPIC_ENV).toEqual(ANTHROPIC_ENV);
+    expect(providerEnvConstants.GEMINI_ENV).toEqual(GEMINI_ENV);
+    expect(providerEnvConstants.COPILOT_ENV).toEqual(COPILOT_ENV);
+  });
+});
diff --git a/src/api-proxy-env-constants.ts b/src/api-proxy-env-constants.ts
index 39909881..e9c3fb72 100644
--- a/src/api-proxy-env-constants.ts
+++ b/src/api-proxy-env-constants.ts
@@ -1,10 +1,8 @@
 /**
  * Environment variable name constants for the API proxy provider adapters.
  *
- * This is the single source of truth for env var names on the TypeScript host side.
  * The CommonJS equivalent lives in containers/api-proxy/provider-env-constants.js.
- *
- * Both files must be kept in sync when adding or renaming env vars.
+ * Keep both files in sync; src/api-proxy-env-constants-sync.test.ts enforces parity.
  */
 
 /** Environment variable names for the OpenAI provider adapter. */