Parameter mapping

A lane is the wire format the gateway uses to reach one kind of provider. OpenAI, Azure OpenAI and OpenAI-compatible custom endpoints share one lane; Anthropic, Bedrock, and Gemini with Vertex have a lane each. On the OpenAI-compatible lane the gateway forwards your POST /v1/chat/completions body as written. Two fields are exceptions: it removes the gateway’s own drop_tuning_params, and the organization’s cache rules can strip or add a top-level cache_control, see Cache control. Whatever the provider accepts, you get. On the other lanes the gateway builds the provider-native request itself, so every parameter has one of three dispositions:

mapped: translated to the provider’s own field.
dropped: a tuning parameter the lane has no field for. The gateway removes it, the request proceeds, and the drop is reported on the response and on the trace.
refused: the request depends on the parameter, and serving it without the parameter would give you a different answer than you asked for. The request fails with 400 unsupported_parameter, naming the parameter, the lane and the reason.

drop_tuning_params

Send "drop_tuning_params": false in the request body for strict mode. A parameter the lane cannot map is then refused instead of dropped. The default is true. The gateway removes the field before the request leaves, so no provider sees it.

{
  "model": "anthropic/claude-haiku-4-5",
  "messages": [{"role": "user", "content": "hi"}],
  "seed": 42,
  "drop_tuning_params": false
}

{
  "error": {
    "type": "unsupported_parameter",
    "code": "unsupported_parameter",
    "message": "refusing to drop 'seed' for anthropic/claude-haiku-4-5: drop_tuning_params is false and Anthropic has no sampling seed. Remove it, set drop_tuning_params to true, or use a model that supports it"
  }
}

Drop signals

A dropped parameter is reported in three places:

Response body: extra_fields.params_dropped lists the dropped parameter names. On a stream it is on the final usage-bearing chunk.
Response header: X-LangWatch-Params-Dropped carries the same list, comma separated.
Trace: the gateway span carries langwatch.gateway.params_dropped.

The table

Parameter	anthropic	bedrock	gemini / vertex	Notes
`audio`	dropped	dropped	dropped
`frequency_penalty`	dropped	dropped	mapped
`function_call`	refused	refused	refused
`functions`	refused	refused	refused
`logit_bias`	dropped	dropped	dropped
`logprobs`	refused	refused	mapped
`metadata`	dropped	dropped	dropped
`modalities`	dropped	dropped	dropped
`n`	dropped	dropped	dropped	n: 1 passes; n > 1 is reduced to one completion and signaled
`parallel_tool_calls`	dropped	dropped	dropped
`prediction`	dropped	dropped	dropped
`presence_penalty`	dropped	dropped	mapped
`prompt_cache_key`	dropped	dropped	dropped
`prompt_cache_retention`	dropped	dropped	dropped
`reasoning_effort`	mapped	mapped	mapped	bedrock: mapped for Anthropic (thinking) and Nova; refused for other families; a customer VPC endpoint maps Anthropic only
`response_format`	mapped	mapped	mapped	anthropic: json_schema enforced; json_object refused (no parseable-JSON guarantee); bedrock: json_schema enforced; json_object refused (no parseable-JSON guarantee); a customer VPC endpoint enforces json_schema for Anthropic models only; gemini: json_object and json_schema both enforced
`safety_identifier`	dropped	dropped	dropped
`seed`	dropped	dropped	dropped
`service_tier`	dropped	dropped	dropped	anthropic: “auto” passes natively; other values dropped; bedrock: dropped (AWS validates a different enum); gemini: dropped
`stop`	mapped	mapped	mapped
`store`	dropped	dropped	dropped
`stream_options`	mapped	mapped	mapped
`temperature`	mapped	mapped	mapped
`tool_choice`	mapped	mapped	mapped	anthropic: none/auto/required/named mapped; named must exist in tools; allowed_tools refused; bedrock: none maps by removing tools (Converse has no none mode); named must exist in tools; allowed_tools refused; gemini: none/auto/required/named mapped; named must exist in tools; allowed_tools refused
`tools`	mapped	mapped	mapped
`top_logprobs`	refused	refused	mapped
`top_p`	mapped	mapped	mapped	anthropic: dropped with a signal when temperature is also set (Anthropic models reject the pair); bedrock: dropped with a signal when temperature is also set on Anthropic models (they reject the pair); other families take both
`user`	dropped	dropped	dropped
`verbosity`	dropped	dropped	dropped
`web_search_options`	dropped	dropped	dropped
any other key	dropped	dropped	dropped	vendor params with no mapping on translated lanes

The codex lane

Codex models run on your own ChatGPT subscription under the model prefix openai_codex/, and they speak the Responses API. The gateway builds the outgoing body itself, so the table below gives every field the same three dispositions as above. The codex backend accepts a fixed set of fields and answers 400 Unsupported parameter to anything else. That is why every key the table does not list is dropped instead of forwarded. The gateway pins three values the backend requires: the bare model name, stream on, and store off. Dropped fields are reported on the X-LangWatch-Params-Dropped header and on the gateway span. The response is the backend’s own SSE stream, so extra_fields.params_dropped is not added to it. drop_tuning_params works on this lane too.

Parameter	codex	Notes
`background`	refused	the background response id the caller expects to poll would be silently absent
`include`	mapped
`input`	mapped
`instructions`	mapped
`max_output_tokens`	dropped	the codex backend refuses an output cap
`max_tool_calls`	refused	the tool-call cap would be silently unenforced
`metadata`	dropped	the codex backend refuses request metadata
`model`	mapped	pinned to the bare model name
`parallel_tool_calls`	mapped
`previous_response_id`	refused	the codex lane pins store: false, so the response chain the caller asked to continue would silently not continue
`prompt_cache_key`	mapped
`prompt_cache_options`	dropped	the codex backend manages its own prompt cache
`prompt_cache_retention`	dropped	the codex backend manages its own prompt cache
`reasoning`	mapped
`safety_identifier`	dropped	the codex backend refuses caller identifiers
`service_tier`	dropped	the codex backend has no service tiers
`store`	mapped	pinned off (the backend is stateless)
`stream`	mapped	pinned on (the backend is SSE-only)
`stream_options`	mapped
`temperature`	dropped	the codex backend refuses sampling options
`text`	mapped
`tool_choice`	mapped
`tools`	mapped
`top_logprobs`	refused	the requested log probabilities would be silently absent from the response
`top_p`	dropped	the codex backend refuses sampling options
`truncation`	dropped	the codex backend manages its own context truncation
`user`	dropped	the codex backend refuses caller identifiers
any other key	dropped	the codex backend accepts a fixed set of fields

reasoning_effort budgets

Claude models take a thinking budget in tokens. On the anthropic lane and on the bedrock lane, the gateway derives that budget from your output cap:

budget = 1024 + ratio x (max_tokens - 1024)

`reasoning_effort`	ratio
`minimal`	0.025
`low`	0.15
`medium`	0.425
`high`	0.80
`xhigh`	0.92
`max`	1.0

"reasoning_effort": "none" sends no thinking budget at all. Claude Opus and Sonnet from 4.6 on take the effort itself, through the provider’s adaptive thinking. minimal is sent as low, because Anthropic accepts low, medium, high and max. Gemini 3 and later take the effort as a native thinking level. Earlier Gemini models get a budget from the same formula, against the model’s own budget range.

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drop_tuning_params

Drop signals

The table

The codex lane

reasoning_effort budgets

​drop_tuning_params

​Drop signals

​The table

​The codex lane

​reasoning_effort budgets

drop_tuning_params

Drop signals

The table

The codex lane

reasoning_effort budgets