diff --git a/.changeset/posthog-no-implicit-local-evaluation.md b/.changeset/posthog-no-implicit-local-evaluation.md
new file mode 100644
index 00000000..9264537d
--- /dev/null
+++ b/.changeset/posthog-no-implicit-local-evaluation.md
@@ -0,0 +1,9 @@
+---
+"@flags-sdk/posthog": patch
+---
+
+Stop the default `postHogAdapter` from enabling local evaluation when `POSTHOG_PERSONAL_API_KEY` is set.
+
+`POSTHOG_PERSONAL_API_KEY` is only needed for provider data discovery (`getProviderData`), which reads it independently. Passing it into the runtime `posthog-node` client also enabled local evaluation, which starts a per-process feature-flag poller (`featureFlagsPollingInterval: 10_000`). On serverless deployments each warm process ran its own poller, generating large numbers of PostHog feature flag requests unrelated to actual traffic.
+
+The default adapter no longer forwards the personal API key or the polling interval to the runtime client. To use local evaluation, create the adapter explicitly with `createPostHogAdapter` and pass `personalApiKey` and `featureFlagsPollingInterval` in `postHogOptions`.
diff --git a/packages/adapter-posthog/src/index.test.ts b/packages/adapter-posthog/src/index.test.ts
index b2364a9f..af2a828e 100644
--- a/packages/adapter-posthog/src/index.test.ts
+++ b/packages/adapter-posthog/src/index.test.ts
@@ -1,4 +1,5 @@
 import type { ReadonlyHeaders, ReadonlyRequestCookies } from 'flags';
+import { PostHog } from 'posthog-node';
 import { beforeAll, describe, expect, it, vi } from 'vitest';
 import { type PostHogEntities, postHogAdapter } from '.';
 
@@ -30,6 +31,9 @@ describe('postHogAdapter', () => {
     beforeAll(() => {
       process.env.NEXT_PUBLIC_POSTHOG_KEY = 'test-posthog-key';
       process.env.NEXT_PUBLIC_POSTHOG_HOST = 'https://us.i.posthog.com';
+      // Present because provider data discovery needs it, but it must not leak
+      // into the runtime client. See issue #393.
+      process.env.POSTHOG_PERSONAL_API_KEY = 'phx_test_personal_api_key';
     });
 
     describe('isFeatureEnabled', () => {
@@ -90,5 +94,17 @@ describe('postHogAdapter', () => {
         expect(postHogClientMock.getFeatureFlagPayload).toHaveBeenCalled();
       });
     });
+
+    // Regression test for issue #393: the default adapter must not forward
+    // `POSTHOG_PERSONAL_API_KEY` into the runtime `posthog-node` client, because
+    // that enables local evaluation and starts a per-process feature-flag poller.
+    describe('default runtime client options', () => {
+      it('should not enable local evaluation from POSTHOG_PERSONAL_API_KEY', () => {
+        expect(PostHog).toHaveBeenCalled();
+        const options = vi.mocked(PostHog).mock.calls[0]?.[1] ?? {};
+        expect(options).not.toHaveProperty('personalApiKey');
+        expect(options).not.toHaveProperty('featureFlagsPollingInterval');
+      });
+    });
   });
 });
diff --git a/packages/adapter-posthog/src/index.ts b/packages/adapter-posthog/src/index.ts
index 4cd28242..b030d287 100644
--- a/packages/adapter-posthog/src/index.ts
+++ b/packages/adapter-posthog/src/index.ts
@@ -116,8 +116,12 @@ function getOrCreateDefaultAdapter() {
       postHogKey: assertEnv('NEXT_PUBLIC_POSTHOG_KEY'),
       postHogOptions: {
         host: assertEnv('NEXT_PUBLIC_POSTHOG_HOST'),
-        personalApiKey: process.env.POSTHOG_PERSONAL_API_KEY,
-        featureFlagsPollingInterval: 10_000,
+        // `POSTHOG_PERSONAL_API_KEY` is only needed for provider data discovery
+        // (see `getProviderData`), which reads it independently of this client.
+        // Passing it here would enable local evaluation in `posthog-node`, which
+        // starts a per-process feature-flag poller. To use local evaluation,
+        // create the adapter explicitly with `createPostHogAdapter` and pass
+        // `personalApiKey`/`featureFlagsPollingInterval` in `postHogOptions`.
         // Presumption: Server IP is likely not a good proxy for user location
         disableGeoip: true,
       },