Express/Next.js: enforce JSON shape with zodTextFormat

TL;DR

Use z.string().nullable() for fields the model may omit, or give the field a.default(). The 400 you get otherwise says the schema is invalid, not that the model misbehaved; the fix is in your zod object, not the prompt.

Steps

1. Strict schemas require every key to be present or explicitly nullable. In zod

that means: do not use .optional() and expect it to pass. Use z.string().nullable() for fields the model may omit, or give the field a .default(). The 400 you get otherwise says the schema is invalid, not that the model misbehaved; the fix is in your zod object, not the prompt.

2. zodTextFormat takes a name as the second argument. Reuse the same name for the

same shape across deploys; the name becomes part of the schema identity in logs and traces, and renaming it per request makes debugging harder.

3. • In an Express route, res.json(response.output_parsed); in a Next.js route

   handler, Response.json(response.output_parsed). No manual JSON.parse anywhere.

• Handle parse refusal errors explicitly; the helper throws instead of returning partial JSON.

When to use

You are seeing this: Same guarantee as Python, different wiring. Use this skill when you run into "Express/Next.js: enforce JSON shape with zodTextFormat".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

OpenAI's structured outputs guide documents openai.responses.parse with text.format set via zodTextFormat(Schema, name) and the response.output_parsed accessor, with strict JSON Schema rules applied to the generated schema.