Leaving mdx out of pageExtensions broke only the dev server

✓solved

Problem

A Next.js dev server fails on MDX pages with a React internals error while the production build passes.

Approach

Set pageExtensions to only ts and tsx so MDX files outside app/ would not become routes; found the dev build compiled .mdx against userland React because Next's alias condition is built from pageExtensions. Adding mdx to pageExtensions held.

Outcome

✓ solved

Conditions

Next.js App Router site with MDX content under content/ outside app/, pageExtensions set in next.config.ts.

Evidence

Fix in next.config.ts (pageExtensions: ["ts", "tsx", "mdx"]); alias rules in node_modules/next/dist/build/create-compiler-aliases.js

Implication

When a change touches next.config.ts, open one page of each kind in the dev server before calling it done.

Topics

build-tooling

Lessons live under content/, outside app/, so pageExtensions was set to ["ts", "tsx"] to keep MDX files from becoming routes by accident. next build passed and rendered every page. next dev failed on every lesson with Cannot read properties of undefined (reading 'recentlyCreatedOwnerStacks'), an error from React's development JSX runtime that says nothing about MDX or configuration.

The compiled lesson chunk imported node_modules/react/jsx-dev-runtime.js, the userland React, while the page around it ran on the React that Next vendors. Next aliases react/jsx-runtime to its own copy only for files that match its code condition, and that condition is built from pageExtensions. An .mdx file outside the list was compiled but never aliased. Production uses the plain JSX runtime, which does not touch the field, so only development broke.

Evidence: .next/server/_rsc_content_lessons_..._index_mdx.js importing (rsc)/./node_modules/react/jsx-runtime.js; the alias rules in node_modules/next/dist/build/create-compiler-aliases.js; the fix in next.config.ts (pageExtensions: ["ts", "tsx", "mdx"]).

Implications: a green next build does not show that next dev works. When a change touches next.config.ts, open one page of each kind in the dev server before calling it done. An error from deep inside React that mentions internals usually means two copies of React met, so check which react each chunk imports before reading the stack.