Zen x-preview-f-free returns string "null" for nullable string tool arguments

Description

The OpenCode Zen x-preview-f-free route returns the string "null" for a
tool argument declared as string | null. The same request to
nemotron-3-ultra-free returns JSON null.

Ox Alpha response:

{"nullableValue":"null"}

Nemotron response:

{"nullableValue":null}

The wrong type is present in the raw
choices[0].message.tool_calls[0].function.arguments response from Zen, before
an OpenCode client or AI SDK parses it.

I also tested strict: true, anyOf, oneOf, reversed union order, and
OpenAPI-style nullable: true. Ox Alpha returned the string "null" in every
case. It returned JSON null for a null-only schema and for unions of null with
integer, boolean, number, object, or array. The failure appears specific to
nullable strings on this route.

This breaks any tool that uses null as the no-value form for a string field,
including paths and object IDs.

Plugins

None. The reproduction calls the public Zen endpoint directly.

OpenCode version

The client integration uses OpenCode 1.18.16. The minimal reproduction below
does not use the client.

Steps to reproduce

Run this keyless request:

curl -sS https://opencode.ai/zen/v1/chat/completions \
  -H 'Authorization: Bearer public' \
  -H 'Content-Type: application/json' \
  --data-binary '{
    "model": "x-preview-f-free",
    "messages": [{
      "role": "user",
      "content": "Call probe_tool exactly once. nullableValue is JSON null. Preserve JSON types."
    }],
    "tools": [{
      "type": "function",
      "function": {
        "name": "probe_tool",
        "description": "Schema compatibility probe",
        "parameters": {
          "type": "object",
          "properties": {
            "nullableValue": {
              "type": ["string", "null"]
            }
          },
          "required": ["nullableValue"],
          "additionalProperties": false
        }
      }
    }],
    "tool_choice": {
      "type": "function",
      "function": {"name": "probe_tool"}
    },
    "stream": false
  }' | jq -r '.choices[0].message.tool_calls[0].function.arguments'

Observed:

{"nullableValue":"null"}

Change only model to nemotron-3-ultra-free. The result is:

{"nullableValue":null}

A client compatibility lowering works on Ox Alpha: advertise the field as an
optional string and decode omission back to null. In two no-value probes, Ox
Alpha returned {}. When given a string, it returned
{"nullableValue":"abc123"}.

Screenshot and/or share link

None.

Operating System

macOS

Terminal

zsh

Start with the keyless curl reproduction against the OpenCode Zen chat completions endpoint and compare x-preview-f-free with nemotron-3-ultra-free. Inspect the raw tool-call arguments before client parsing, including the nullable string schema cases described. Done means determining the route-level cause and returning JSON null rather than the string "null".