fix(ai): name langchain spans from the bare runName string

.changeset/langchain-run-name-string.md

@@ -0,0 +1,5 @@
+---
+'@posthog/ai': patch
+---
+
+Fix LangChain spans being named after their class instead of the runnable. LangChain passes `runName` as a bare string, which the name resolver skipped because it only inspected object arguments, so every tool span was captured as `DynamicStructuredTool` rather than the tool's own name.

packages/ai/src/langchain/callbacks.ts

@@ -562,10 +562,17 @@ export class LangChainCallbackHandler extends BaseCallbackHandler {
   private _getLangchainRunName(serialized: any, ...args: any): string | undefined {
     if (args && args.length > 0) {
       for (const arg of args) {
-        if (arg && typeof arg === 'object' && 'name' in arg) {
-          return arg.name
-        } else if (arg && typeof arg === 'object' && 'runName' in arg) {
-          return arg.runName
+        // LangChain hands runName through as a bare string, not wrapped in an object
+        if (typeof arg === 'string' && arg) {
+          return arg
+        }
+        if (arg && typeof arg === 'object') {
+          if (arg.name) {
+            return arg.name
+          }
+          if (arg.runName) {
+            return arg.runName
+          }
         }
       }
     }

packages/ai/tests/callbacks.test.ts

@@ -687,6 +687,29 @@ describe('LangChainCallbackHandler', () => {
   })
 })
 
+describe('LangChainCallbackHandler span naming', () => {
+  it('names a tool span from runName rather than the serialized class', () => {
+    const handler = new LangChainCallbackHandler({ client: mockPostHogClient })
+    jest.clearAllMocks()
+
+    const serialized = {
+      lc: 1,
+      type: 'constructor' as const,
+      id: ['langchain', 'tools', 'DynamicStructuredTool'],
+      kwargs: {},
+    }
+    const runId = 'run_tool_name'
+
+    // LangChain calls this with (tool, input, runId, parentRunId, tags, metadata, runName)
+    handler.handleToolStart(serialized, '{"city":"Paris"}', runId, 'parent_run', [], {}, 'get_weather')
+    handler.handleToolEnd('sunny', runId, 'parent_run')
+
+    const [captureCall] = (mockPostHogClient.capture as jest.Mock).mock.calls
+    expect(captureCall[0].event).toBe('$ai_span')
+    expect(captureCall[0].properties['$ai_span_name']).toBe('get_weather')
+  })
+})
+
 describe('LangChainCallbackHandler trace/span state sanitization', () => {
   it('redacts base64 data URLs from $ai_input_state and $ai_output_state', () => {
     const handler = new LangChainCallbackHandler({ client: mockPostHogClient })