fix: pass through server-v3 operation errors (#1937)

## Summary
- return underlying operation error messages from server-v3 streaming
handlers
- return underlying non-AppError messages from the top-level server-v3
error wrapper
- preserve existing status code behavior while making local
act/extract/observe 500s diagnosable

## Testing
- pnpm --filter @browserbasehq/stagehand-server-v3 typecheck

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Pass through underlying operation errors in
`@browserbasehq/stagehand-server-v3` so clients see real failure
messages during streaming and top-level responses, and force flow logs
on for easier debugging. Status codes stay the same, making local
act/extract/observe 500s diagnosable.

- **Bug Fixes**
- Streaming + error wrapper: return the thrown error’s message for
non-`AppError` cases; fall back to a default when not an `Error`,
preserving status codes.
- Observe route: remove unused `Variables` import/cast to satisfy lint;
no behavior change.

<sup>Written for commit ba08fd0.
Summary will update on new commits. <a
href="https://cubic.dev/pr/browserbase/stagehand/pull/1937">Review in
cubic</a></sup>

<!-- End of auto-generated description by cubic. -->

Author: monadoid

.changeset/wild-dryers-search.md

@@ -0,0 +1,5 @@
+---
+"@browserbasehq/stagehand-server-v3": patch
+---
+
+Improve server-v3 error passthrough for local operation failures

packages/server-v3/src/lib/errorHandler.ts

@@ -60,11 +60,12 @@ export function withErrorHandling<
         return error(reply, err.getClientMessage(), err.statusCode);
       }
 
-      return error(
-        reply,
-        "An internal server error occurred",
-        StatusCodes.INTERNAL_SERVER_ERROR,
-      );
+      const message =
+        err instanceof Error
+          ? err.message
+          : "An internal server error occurred";
+
+      return error(reply, message, StatusCodes.INTERNAL_SERVER_ERROR);
     }
   };
 }

packages/server-v3/src/lib/stream.ts

@@ -192,7 +192,7 @@ export async function createStreamingResponse<TV3>({
     const clientMessage =
       handlerError instanceof AppError
         ? handlerError.getClientMessage()
-        : `${operation ?? "operation"} failed`;
+        : handlerError.message;
 
     sendData("error", "system", { status: "error", error: clientMessage });
 

packages/server-v3/src/routes/v1/sessions/_id/observe.ts

@@ -1,10 +1,6 @@
 import type { RouteHandlerMethod, RouteOptions } from "fastify";
 import { StatusCodes } from "http-status-codes";
-import type {
-  Action,
-  ObserveOptions,
-  Variables,
-} from "@browserbasehq/stagehand";
+import type { Action, ObserveOptions } from "@browserbasehq/stagehand";
 import type { FastifyZodOpenApiSchema } from "fastify-zod-openapi";
 import { Api } from "@browserbasehq/stagehand";
 
@@ -57,7 +53,6 @@ const observeRouteHandler: RouteHandlerMethod = withErrorHandling(
 
         const safeOptions: ObserveOptions = {
           ...data.options,
-          variables: data.options?.variables as Variables | undefined,
           model:
             typeof data.options?.model === "string"
               ? { modelName: data.options.model }

packages/server-v3/src/sea-entry.ts

@@ -6,6 +6,8 @@ const normalizedArgv = argv[0]?.startsWith("--") ? argv : argv.slice(1);
 
 // otherwise, start the stagehand/server
 if (!__internalMaybeRunShutdownSupervisorFromArgv(normalizedArgv)) {
+  // Force flow logs on before the server module loads Stagehand core.
+  process.env.BROWSERBASE_FLOW_LOGS = "1";
   void import("./server.js").catch((err) => {
     console.error("Failed to start server:", err);
     process.exit(1);