tRPC v11 mergeRouters no longer merges middleware stacks automatically

Resolution

In tRPC v11, mergeRouters no longer automatically concatenates the middleware stacks of the routers being merged. To ensure middleware is correctly applied, you must manually re-attach the required middleware to the resulting merged router or apply it globally at the root level.

There is no configuration flag to revert to the v10 automatic merging behavior. This change was intentional to provide explicit control over the execution order and scope of middleware.

Implementation Strategy

Depending on whether your middleware is intended to be global or scoped, use one of the following patterns:

1. Global Middleware Application

If the middleware was previously applied to all sub-routers and should remain global, apply it once to the final merged router instance:

const mergedRouter = mergeRouters(routerA, routerB);

// Manually apply the middleware to the combined router
const finalRouter = mergedRouter.middleware([globalMiddleware]);

2. Explicit Scoped Middleware

If middleware must only run for specific subsets of procedures, ensure that middleware is defined within the sub-router's own procedure definitions before they are passed to mergeRouters. While the stack is not merged into the root, procedures that were already wrapped in middleware within their source router will still execute that middleware.

Behavioral Nuances

Error-Handling Middleware

Error-handling middleware is treated the same as standard middleware. Because it typically needs to catch exceptions from any procedure, it should be applied to the merged router last. If applied to sub-routers individually before merging, you may find that errors are not caught by a centralized handler unless that handler is also applied to the final merged output.

Middleware Types

This change affects all middleware types equally. Whether the middleware handles authentication, logging, or validation, the implicit merging mechanism has been removed across the board.

Verification

To verify your middleware is executing after the upgrade, you can use a simple logging check:

• Add a console.log to your middleware function.
• Invoke a procedure from a merged router.
• If the log does not appear in your server console, the middleware was not correctly re-attached to the merged router.

Diagnostic Note: To provide a more specific implementation example, please clarify if your middleware is defined as a standalone function or as part of a t.middleware factory.