When server-side logs that use printf-style formatting (e.g. Nitro's request logger calling console.log('%s - %s info GET %s %d in %dms', ts, level, method, status, ms)) are piped to the browser via the virtual-console SSE bridge, the format substitution is lost and the literal %s tokens appear in the browser console.

Example output in browser:

[Server] %s - %s info GET /build-version 200 in 1ms

Cause

In packages/devtools-vite/src/virtual-console.ts, the browser-side handler reconstructs the call as:

var prefix = '%c[Server]%c';
logMethod.apply(console, [prefix, prefixStyle, resetStyle].concat(transformedArgs));

The browser console only treats the first argument as a format string. Since the prefix '%c[Server]%c' is now in position 0, the original server-side format string (which is at transformedArgs[0]) is no longer interpreted — its %s tokens print literally and the remaining args are appended as separate values rather than substituted.

Reproduction

Any TanStack Start app with devtools() in the Vite plugins. Hit any route while the dev server is running; the Nitro request log lands in the browser console with literal %s.

Suggested fix

Either:

• (a) Pre-format the server args with util.format on the server before serializing, so the browser receives a single already-formatted string; or
• (b) On the browser side, if transformedArgs[0] is a string containing format specifiers, merge it into the prefix format string (e.g. '%c[Server]%c ' + transformedArgs[0]) and shift the remaining args accordingly.

Environment

• @tanstack/[email protected]
• Vite 8
• TanStack Start
• Node 18+