z.xor() works fine with strict objects but breaks with regular ones

[RareScrap]

For some reason the code below fails in a really weird way. With the given input both gentleXor and strictXor must successfully parse the input but gentleXor fails.

import z from "zod"

const input = {
    "os": {
        "name": "windows",
        "version": "^10\\."
    }
}

const nameOsRule = z.object({
    name: z.enum(["windows", "osx", "linux"]).nonoptional()
})

const versionOsRule = nameOsRule.extend({
    version: z.string().nonempty().nonoptional()
})

// This will break
const gentleXor = z.xor([
    nameOsRule,
    versionOsRule
])

// The strict version works fine
const strictXor = z.xor([
    z.strictObject(nameOsRule.shape),
    z.strictObject(versionOsRule.shape)
])

gentleXor.parse(input.os, { reportInput: true })
strictXor.parse(input.os, { reportInput: true })

The error doesn't give any clue and looks uninformative:

ZodError: [
  {
    "code": "invalid_union",
    "errors": [],
    "inclusive": false,
    "path": [],
    "message": "Invalid input",
    "input": {
      "name": "windows",
      "version": "^10\\."
    }
  }
]

Can someone explain me why this breaks?

Zod version is 4.4.3

[pullfrog]

Behaving as intended — the variable names in your repro are swapped, but the version that fails is the one built from plain z.object().

z.xor() requires exactly one branch to match. Plain object schemas strip unknown keys instead of rejecting them, so nameOsRule happily parses { name, version } (returning { name: "windows" }) and versionOsRule parses it too. Two matches, so the XOR fails. z.strictObject() makes the first branch reject the extra version key, leaving exactly one match.

You can see it directly:

const input = { name: "windows", version: "^10\\." };

nameOsRule.safeParse(input);     // { success: true, data: { name: "windows" } }  <- also matches
versionOsRule.safeParse(input);  // { success: true, data: { name: "windows", version: "^10\\." } }

Only the narrower branch needs to be strict for this to disambiguate:

z.xor([nameOsRule.strict(), versionOsRule]).parse(input); // ok

The uninformative error is a fair complaint, though. When an exclusive union fails because multiple branches matched, the issue is emitted with errors: [] and inclusive: false, and the English error map has no case for it, so you get the generic "Invalid input". I'll look at giving that a distinct message.

  ^{｜ View workflow run ｜ via Pullfrog ｜ Using Claude Opus (free via Pullfrog for OSS) ｜ 𝕏}

[RareScrap]

Edited the description because I messed up a little