Parameter Compatibility

Which parameters work with which models.

Guaranteed Compatible (all models)

Parameter	Description
`model`	Model name
`messages`	Message array
`stream`	Enable SSE streaming
`max_tokens` / `max_completion_tokens`	Max output tokens
`temperature`	Sampling temperature
`top_p`	Nucleus sampling
`stop`	Stop sequences
`seed`	Deterministic sampling

Widely Supported (most models)

Parameter	Description
`tools` / `tool_choice`	Function calling
`response_format`	JSON mode / structured outputs
`frequency_penalty`	Repetition reduction
`presence_penalty`	Topic diversity
`logprobs` / `top_logprobs`	Token probabilities

Model-Specific

Parameter	Models
`top_k`	Anthropic, some open models
`repetition_penalty`	Open models (Llama, Mistral)
`min_p`	Some open models
`reasoning_effort` / `reasoning`	Reasoning models only. See Reasoning Controls

Check supported_parameters on GET /v1/models for the resolved model before relying on any of these.

How It Works

On the chat wire, parameters are forwarded by a deny-list: anything that is not a credential, an SSRF vector, or money and tenancy truth is passed through as sent, so a brand-new provider knob reaches the wire without a change here. The Anthropic and Responses wires behave differently: both rebuild the request from a fixed set of fields, so a knob outside that set is dropped rather than forwarded. Unsupported params are silently ignored by most providers (the same behavior as OpenRouter).

Several things modify a request on the way out:

A reviewed adapter policy removes or rewrites parameters a specific provider or model is known to reject. The reasoning-related rules are listed in Reasoning Controls.
Reactive repair. When a provider returns a 4xx that names its own fix ("Unsupported parameter: 'stop'", "does not support 'max' with this model"), the marketplace can apply that fix and retry the same seller instead of failing you. This is gated by ADAPTER_REPAIR, which ships off; until an operator arms it the provider's error surfaces instead. model, messages, tools, tool_choice and response_format are protected from the generic strip, though a few narrowly-scoped rules do rewrite messages and tool_choice where a provider names them specifically.
Prompt-cache injection, which can add a prompt_cache_key and Anthropic cache_control breakpoints.
stream_options: {"include_usage": true}, auto-injected for metering on streaming chat requests.

Most of these are named in the X-SI-Adapted-Params response header, which is the best available record of what changed. It is not exhaustive: when you send no stream_options of your own the injection is deliberately silent, and fields dropped by an Anthropic or Responses wire rebuild are not itemised. (If you did send stream_options and it was overwritten, that one is disclosed.)

A provider 4xx the marketplace cannot interpret is returned to you as a 400. It does not automatically fail over to another seller.