diff --git a/.changeset/ai-tokens-source.md b/.changeset/ai-tokens-source.md
new file mode 100644
index 0000000000..e3c9f9bb2d
--- /dev/null
+++ b/.changeset/ai-tokens-source.md
@@ -0,0 +1,5 @@
+---
+'@posthog/ai': patch
+---
+
+Add `$ai_tokens_source` property ("sdk" or "passthrough") to all `$ai_generation` events to detect when token values are externally overridden via `posthogProperties`
diff --git a/packages/ai/src/utils.ts b/packages/ai/src/utils.ts
index 35390c01ff..a5a13e94ea 100644
--- a/packages/ai/src/utils.ts
+++ b/packages/ai/src/utils.ts
@@ -18,6 +18,22 @@ type EmbeddingCreateParams = OpenAIOrignal.EmbeddingCreateParams
 type TranscriptionCreateParams = OpenAIOrignal.Audio.Transcriptions.TranscriptionCreateParams
 type AnthropicTool = AnthropicOriginal.Tool
 
+const TOKEN_PROPERTY_KEYS = new Set([
+  '$ai_input_tokens',
+  '$ai_output_tokens',
+  '$ai_cache_read_input_tokens',
+  '$ai_cache_creation_input_tokens',
+  '$ai_total_tokens',
+  '$ai_reasoning_tokens',
+])
+
+export function getTokensSource(posthogProperties?: Record<string, unknown>): string {
+  if (posthogProperties && Object.keys(posthogProperties).some((key) => TOKEN_PROPERTY_KEYS.has(key))) {
+    return 'passthrough'
+  }
+  return 'sdk'
+}
+
 // limit large outputs by truncating to 200kb (approx 200k bytes)
 export const MAX_OUTPUT_SIZE = 200000
 const STRING_FORMAT = 'utf8'
@@ -727,6 +743,7 @@ export const sendEventToPosthog = async ({
     $ai_trace_id: traceId,
     $ai_base_url: baseURL,
     ...params.posthogProperties,
+    $ai_tokens_source: getTokensSource(params.posthogProperties),
     ...(distinctId ? {} : { $process_person_profile: false }),
     ...(tools ? { $ai_tools: tools } : {}),
     ...errorData,
diff --git a/packages/ai/tests/anthropic.test.ts b/packages/ai/tests/anthropic.test.ts
index 06f3caf6dd..4bfcc5f499 100644
--- a/packages/ai/tests/anthropic.test.ts
+++ b/packages/ai/tests/anthropic.test.ts
@@ -389,6 +389,25 @@ describe('PostHogAnthropic', () => {
       const [captureArgs] = captureMock.mock.calls
       const { properties } = captureArgs[0]
       expect(properties['$ai_usage']).toBeDefined()
+      expect(properties['$ai_tokens_source']).toBe('sdk')
+    })
+
+    conditionalTest('should set tokens_source to passthrough when token properties are overridden', async () => {
+      const response = await client.messages.create({
+        model: 'claude-3-opus-20240229',
+        messages: [{ role: 'user', content: 'Hello Claude' }],
+        max_tokens: 100,
+        posthogDistinctId: 'test-user-123',
+        posthogProperties: { $ai_input_tokens: 99999 },
+      })
+
+      expect(response).toEqual(mockResponse)
+
+      const captureMock = mockPostHogClient.capture as jest.Mock
+      const [captureArgs] = captureMock.mock.calls
+      const { properties } = captureArgs[0]
+      expect(properties['$ai_tokens_source']).toBe('passthrough')
+      expect(properties['$ai_input_tokens']).toBe(99999)
     })
 
     conditionalTest('should handle system prompts correctly', async () => {
diff --git a/packages/ai/tests/utils.test.ts b/packages/ai/tests/utils.test.ts
index 71a8c17d47..8c99372a0a 100644
--- a/packages/ai/tests/utils.test.ts
+++ b/packages/ai/tests/utils.test.ts
@@ -1,4 +1,21 @@
-import { toContentString } from '../src/utils'
+import { toContentString, getTokensSource } from '../src/utils'
+
+describe('getTokensSource', () => {
+  it.each([
+    ['undefined properties', undefined, 'sdk'],
+    ['empty properties', {}, 'sdk'],
+    ['unrelated properties', { foo: 'bar' }, 'sdk'],
+    ['$ai_input_tokens override', { $ai_input_tokens: 999 }, 'passthrough'],
+    ['$ai_output_tokens override', { $ai_output_tokens: 999 }, 'passthrough'],
+    ['$ai_total_tokens override', { $ai_total_tokens: 999 }, 'passthrough'],
+    ['$ai_cache_read_input_tokens override', { $ai_cache_read_input_tokens: 500 }, 'passthrough'],
+    ['$ai_cache_creation_input_tokens override', { $ai_cache_creation_input_tokens: 200 }, 'passthrough'],
+    ['$ai_reasoning_tokens override', { $ai_reasoning_tokens: 300 }, 'passthrough'],
+    ['mixed override and custom', { $ai_input_tokens: 999, custom_key: 'value' }, 'passthrough'],
+  ])('%s → %s', (_name, props, expected) => {
+    expect(getTokensSource(props)).toBe(expected)
+  })
+})
 
 describe('toContentString', () => {
   describe('string inputs', () => {