Ollama API: 'invalid format: expected "json" or a valid JSON Schema object'

{"error":"invalid format: \"\\\"xml\\\"\"; expected \"json\" or a valid JSON Schema object"} — the format field of an Ollama request takes only two things: the string "json" or a JSON Schema object, as the message says. The other values tested, "xml" and true, were refused. For structured output, send a schema object.

The fix is to put a real schema in format, for example {"type":"object","properties":{...},"required":[...]}. A schema with a typo in it fails differently, with a 400 and JSON schema conversion failed, and the message names the bad keyword.

The error

PS> curl.exe -s -i http://127.0.0.1:15190/api/generate -d '@fmt-xml.json'   (fmt-xml.json: {"model":"qwen2.5:0.5b","prompt":"Give the name and age of a fictional patient.","stream":false,"format":"xml"})
HTTP/1.1 500 Internal Server Error
{"error":"invalid format: \"\\\"xml\\\"\"; expected \"json\" or a valid JSON Schema object"}

The runner printed the status line and the last line of each reply; the other headers are left out. /api/chat answered the same body shape with the same 500 and message, and "format":true gave invalid format: \"true\". A schema object with a wrong type name:

PS> curl.exe -s -i http://127.0.0.1:15190/api/generate -d '@fmt-badschema.json'   (fmt-badschema.json: {"model":"qwen2.5:0.5b","prompt":"Give the name and age of a fictional patient.","stream":false,"format":{"type":"objekt","properties":{"name":{"type":"string"}}}})
HTTP/1.1 400 Bad Request
{"error":"Field 'json_schema': \"json_schema\": JSON schema conversion failed:\nJSON schema error at #: unrecognized type objekt"}

Why it happens

format is not a list of output formats. The message itself names the two accepted forms: "json", which asks for any valid JSON, or a JSON Schema object, which asks for JSON of that exact shape. The other values tested, "xml" and true, were refused. Two details from the test are worth knowing: the invalid value came back as HTTP 500, not 400, so code that treats 5xx as "server down, retry" will retry a request that can never work; and the doubled escaping in \"\\\"xml\\\"\" is your value "xml", quotes included, escaped inside the error string.

A schema object with an unknown type was refused with a 400 instead of a 500, and the message gave the path # (the root of the schema) and the word it did not recognise.

The fix

PS> curl.exe -s -i http://127.0.0.1:15190/api/generate -d '@fmt-schema.json'   (fmt-schema.json: {"model":"qwen2.5:0.5b","prompt":"Give the name and age of a fictional patient.","stream":false,"options":{"temperature":0},"format":{"type":"object","properties":{"name":{"type":"string"},"age":{"type":"integer"}},"required":["name","age"]}})
HTTP/1.1 200 OK
...
PS> (curl.exe -s http://127.0.0.1:15190/api/generate -d '@fmt-schema.json' | ConvertFrom-Json).response
{"name":"李华","age":28}

With a valid schema the reply's response is a JSON string of that shape, ready to deserialize. The schema fixes the shape, not the content: asked for a fictional patient, the small model chose a Chinese name, so say what you want in the prompt as well. The other accepted value, "format":"json", asks only for valid JSON of any shape; a schema is the better choice when your code expects particular fields.

How it was reproduced

Ollama 0.40.14 on Windows 11, against a separate test server started with ollama serve on 127.0.0.1:15190 with its own models folder and qwen2.5:0.5b. Five bodies (the file names are the test's own) were written to files by a PowerShell script and sent with curl.exe -i: "xml" and true to /api/generate, "xml" to /api/chat, a schema with "objekt", and a valid schema with temperature 0.

Frequently asked

What values does the Ollama format field accept?

The string "json" or a JSON Schema object, as the error message says. The other values tested, "xml" and true, were refused with HTTP 500 and 'invalid format' in the test.

How do I get structured output from Ollama?

Put a JSON Schema object in format, for example type object with properties and required fields. The response field then holds a JSON string of that shape, which you can deserialize.

Why does Ollama say JSON schema conversion failed?

The format object is not a valid schema. In the test a misspelled type, objekt, gave HTTP 400 with 'unrecognized type objekt' and the path of the bad part, # for the root.

More decoded errors in the Fixes category. JSON mode from curl and .NET is part of the Ollama API post.