Hono 4.12.0: c.json(undefined) throws "Value is not JSON serializable"

Hono's c.json() builds a JSON response from any value. In February 2026, honojs/hono#4707 added a fast path matching an existing optimisation in c.text(): when there is no status and there are no extra headers, return Response.json(object) directly instead of stringifying the value and building the response by hand. It was a 14-line performance change, and it shipped in Hono 4.12.0 the same day.

On 2026-10-07 we replayed the pull request through Warpway as it stood before merge. The Testing lens reported that one call the repository explicitly supports, c.json(undefined), now throws. In reality, the fast path was reverted four days after merge.

The change

return this.#useFastPath() && !arg && !headers
  ? Response.json(object)
  : this.#newResponse(JSON.stringify(object), arg, setDefaultContentType('application/json', headers));

How the bug works

JSON.stringify(undefined) returns undefined, not a string. The old path tolerated that and returned an empty application/json response. Response.json(undefined) is stricter: the Fetch standard requires it to throw a TypeError when serialization produces nothing. On Node, a route like this fails:

app.get('/user', (c) => c.json(findUser(c.req.query('id')))); // findUser returns undefined

TypeError: Value is not JSON serializable

The behavior also depends on the runtime and on whether you passed a status: c.json(value, 200) still takes the old path and succeeds, so the same value works in one route and fails in the next.

Are you affected?

You are affected on Hono 4.12.0 and 4.12.1 if any route can call c.json() with undefined and no status or headers. On Node it returns a 500 with the error above; other runtimes behave differently, as described in #4756.

Fix or workaround

Upgrade to Hono 4.12.2 or later, which reverted the fast path. On an affected version, pass a status (c.json(value, 200)) or return null instead of undefined (the body is then the JSON null). We verified both on 4.12.0.