Antimatter

POST/api/compareNo key

The chat board, the planet consoles, the bridge and the docs tester run on this path. No key. Rate limited per IP address, always streams, and forces max_tokens to 1024. Presets, aliases, BYOK, and credits do not apply.

Request body

application/json, required

Request body fields
Field	Type	What it is
modelrequired	string	A catalog id from GET /v1/models, one of your aliases, preset/<name>, or an autopilot policy id (antimatter/auto, antimatter/auto:cheap, antimatter/auto:fast, antimatter/auto:code, antimatter/auto:extract, antimatter/auto:reason, antimatter/auto:long, antimatter/auto:vision, antimatter/auto:tools, antimatter/auto:json). At most 200 characters.
models	array of string	Ordered fallback list tried in turn when a candidate fails with a retryable error (429 or 5xx). At most 4 entries, so at most 5 candidates including model. The x-antimatter-model response header names the id that actually answered.
messagesrequired	array of object
messages[].rolerequired	string	One of "system", "user", "assistant", "tool".
messages[].content	string or array of object or null
messages[].tool_call_id	string	At most 300 characters.
messages[].name	string	At most 300 characters.
messages[].tool_calls	array of object
stream	boolean	Server-sent events in the OpenAI chunk format, terminated by data: [DONE].
max_tokens	integer	Clamped rather than refused: to the model's own published max output where the catalog has it (max_output_tokens on its GET /v1/models entry), and to the deployment ceiling (ANTIMATTER_MAX_OUTPUT_TOKENS, 131072 by default). A clamp is announced as max_tokens=<sent> in x-antimatter-adjusted. Absent means 4096 (ANTIMATTER_DEFAULT_OUTPUT_TOKENS), or the model's max output if that is lower. At most 9007199254740991.
temperature	number	From 0 to 2.
top_p	number	From 0 to 1.
stop	string or array of string
tools	array of object	Passed through to the upstream verbatim.
tool_choice	string or object
parallel_tool_calls	boolean
response_format	object	Passed through to the upstream verbatim.
seed	integer	From -9007199254740991 to 9007199254740991.
frequency_penalty	number	From -2 to 2.
presence_penalty	number	From -2 to 2.
prompt	object	A prompt kept here: its version's messages come before these, and a request that names no model takes the version's. With no version or label, the production label when it is set, else the newest version.
prompt.idrequired	string
prompt.version	integer or string	A version number, or a label such as production.
prompt.label	string
prompt.variables	object	A value for every {{variable}} the version names, and no others.

Example

{
  "model": "antimatter/auto",
  "messages": [
    {
      "role": "user",
      "content": "Say hello in five words."
    }
  ]
}

Answer 200

SSE stream of OpenAI format chunks.

The answer's fields are not named in the document.

Errors

400OpenAI shaped error.
429Rate limited, or a spent budget or spend cap. The type distinguishes rate_limit_error, policy_exceeded, and spend_cap_exceeded.
502OpenAI shaped error.
503OpenAI shaped error.

Every error answers in one shape, with its type, code and the field it names. The whole list is in the docs' Errors section.

Request headers it reads (1)

Request headers
Field	Type	What it is
x-antimatter-session	string	A free form label that groups requests in the log. Printable ASCII, trimmed to 120 characters. At most 120 characters.

Try it

The same call with curl

curl -X POST https://useantimatter.com/api/compare \
  -H "Content-Type: application/json" \
  -d '{"model":"antimatter/auto","messages":[{"role":"user","content":"Say hello in five words."}]}'