diff --git a/README.md b/README.md
index aa5b4148..5b60e684 100644
--- a/README.md
+++ b/README.md
@@ -46,6 +46,11 @@ Engine is an open-source, backend HTTP server that provides a production-ready i
 - Other deployment options
   - [Deploy on Railway](https://railway.app/template/fcEVay)
 
+## Notes on custom chains
+
+- Chain overrides let Engine resolve custom EVM chains for standard reads, writes, and deployments.
+- Smart wallet / UserOp routes still depend on thirdweb bundler support for the target chain. A chain override alone does not enable hosted bundler or paymaster infrastructure for that chain.
+
 ## Contributing
 
 We welcome your contributions! See [how to contribute](./contributing.md).
diff --git a/src/server/middleware/error.ts b/src/server/middleware/error.ts
index 7b61b4ca..b29b444d 100644
--- a/src/server/middleware/error.ts
+++ b/src/server/middleware/error.ts
@@ -37,7 +37,7 @@ export const badAddressError = (address: string): CustomError =>
 
 export const badChainError = (chain: string | number): CustomError =>
   createCustomError(
-    `Invalid chain: ${chain}. If this is a custom chain, add it to chain overrides.`,
+    `Invalid chain: ${chain}. If this is a custom chain, add it to chain overrides. Smart wallet and UserOp routes also require bundler support for that chain.`,
     StatusCodes.BAD_REQUEST,
     "INVALID_CHAIN",
   );