Create a model response

POST https://api.fastapi.ai/v1/responses

Creates a model response. Provide text or image inputs to generate text or JSON outputs. Have the model call your own custom code or use built-in tools like web search or file search to use your own data as input for the model's response.

Request body

background boolean Optional Defaults to false
Whether to run the model response in the background.

conversation string or object Optional Defaults to null
The conversation that this response belongs to. Items from this conversation are prepended to input_items for this response request. Input items and output items from this response are automatically added to this conversation after this response completes.

include array Optional
Specify additional output data to include in the model response. Currently supported values are:

web_search_call.action.sources: Include the sources of the web search tool call.
code_interpreter_call.outputs: Includes the outputs of python code execution in code interpreter tool call items.
computer_call_output.output.image_url: Include image urls from the computer call output.
file_search_call.results: Include the search results of the file search tool call.
message.input_image.image_url: Include image urls from the input message.
message.output_text.logprobs: Include logprobs with assistant messages.
reasoning.encrypted_content: Includes an encrypted version of reasoning tokens in reasoning item outputs. This enables reasoning items to be used in multi-turn conversations when using the Responses API statelessly (like when the store parameter is set to false, or when an organization is enrolled in the zero data retention program).

`input` string or array Optional
Text, image, or file inputs to the model, used to generate a response

instructions string Optional
A system (or developer) message inserted into the model's context.
When using along with previous_response_id, the instructions from a previous response will not be carried over to the next response. This makes it simple to swap out system (or developer) messages in new responses.

max_output_tokens integer Optional
An upper bound for the number of tokens that can be generated for a response, including visible output tokens and reasoning tokens.

max_tool_calls integer Optional
The maximum number of total calls to built-in tools that can be processed in a response. This maximum number applies across all built-in tool calls, not per individual tool. Any further attempts to call a tool by the model will be ignored.

metadata map Optional
Set of 16 key-value pairs that can be attached to an object. This can be useful for storing additional information about the object in a structured format, and querying for objects via API or the dashboard.
Keys are strings with a maximum length of 64 characters. Values are strings with a maximum length of 512 characters.

model string Optional
Model ID used to generate the response, like gpt-4o or o3. OpenAI offers a wide range of models with different capabilities, performance characteristics, and price points. Refer to the model guide to browse and compare available models.

parallel_tool_calls boolean Optional Defaults to true
Whether to allow the model to run tool calls in parallel.

previous_response_id string Optional
The unique ID of the previous response to the model. Use this to create multi-turn conversations. Cannot be used in conjunction with conversation.

prompt object Optional
Reference to a prompt template and its variables.

prompt_cache_key string Optional
Used by OpenAI to cache responses for similar requests to optimize your cache hit rates. Replaces the user field.

prompt_cache_retention string Optional
The retention policy for the prompt cache. Set to 24h to enable extended prompt caching, which keeps cached prefixes active for longer, up to a maximum of 24 hours.

reasoning object Optional
gpt-5 and o-series models only. Configuration options for reasoning models.

reasoning object

effort string Optional
Controls reasoning effort.

summary string Optional
Request a concise reasoning summary. Supported value: concise.

safety_identifier string Optional
A stable identifier used to help detect users of your application that may be violating OpenAI's usage policies. The IDs should be a string that uniquely identifies each user. We recommend hashing their username or email address, in order to avoid sending us any identifying information.

service_tier string Optional Defaults to auto
Specifies the processing type used for serving the request.

If set to auto, then the request will be processed with the service tier configured in the Project settings. Unless otherwise configured, the Project will use default.
If set to default, then the request will be processed with the standard pricing and performance for the selected model.
If set to flex or priority, then the request will be processed with the corresponding service tier.
When not set, the default behavior is auto.
When the service_tier parameter is set, the response body will include the service_tier value based on the processing mode actually used to serve the request. This response value may be different from the value set in the parameter.

store boolean Optional Defaults to true
Whether to store the generated model response for later retrieval via API.

stream boolean Optional Defaults to false
If set to true, the model response data will be streamed to the client as it is generated using server-sent events. See the Streaming events section for more information.

stream_options object Optional Defaults to null
Options for streaming responses. Only set this when you set stream: true.

stream options

include_usage boolean Optional
If set, an additional chunk is streamed before the data: [DONE] message containing the overall token usage.

include_obfuscation boolean Optional
If set, obfuscation fields are included in streaming delta events.

temperature number Optional Defaults to 1
What sampling temperature to use, between 0 and 2. Higher values like 0.8 will make the output more random, while lower values like 0.2 will make it more focused and deterministic. We generally recommend altering this or top_p but not both.

text object Optional
Configuration options for a text response from the model. Can be plain text or structured JSON data.

text object

format object Optional
Output format configuration.

type string Required
The type of response format: text, json_object, or json_schema.

json_schema object Required
The schema definition for the response format (required when type is json_schema).

name string Required
The name of the response format. Must be a-z, A-Z, 0-9, or contain underscores and dashes, with a maximum length of 64.

description string Optional
A description of what the response format is for.

schema object Optional
The JSON Schema for the response format.

strict boolean or null Optional Defaults to false
Whether to enable strict schema adherence when generating the output.

tool_choice string or object Optional
How the model should select which tool (or tools) to use when generating a response.

tools array Optional
An array of tools the model may call while generating a response. You can specify which tool to use by setting the tool_choice parameter.
We support the following categories of tools:

Built-in tools: Tools that are provided by OpenAI that extend the model's capabilities, like web search or file search.
MCP Tools: Integrations with third-party systems via custom MCP servers or predefined connectors such as Google Drive and SharePoint.
Function calls (custom tools): Functions that are defined by you, enabling the model to call your own code with strongly typed arguments and outputs. You can also use custom tools to call your own code.

top_logprobs integer Optional
An integer between 0 and 20 specifying the number of most likely tokens to return at each token position, each with an associated log probability.

top_p number Optional Defaults to 1
An alternative to sampling with temperature, called nucleus sampling, where the model considers the results of the tokens with top_p probability mass. So 0.1 means only the tokens comprising the top 10% probability mass are considered. We generally recommend altering this or temperature but not both.

truncation string Optional Defaults to disabled
The truncation strategy to use for the model response.

auto: If the input to this Response exceeds the model's context window size, the model will truncate the response to fit the context window by dropping items from the beginning of the conversation.
disabled (default): If the input size will exceed the context window size for a model, the request will fail with a 400 error.

user string Optional Deprecated
This field is being replaced by safety_identifier and prompt_cache_key. Use prompt_cache_key instead to maintain caching optimizations. A stable identifier for your end-users. Used to boost cache hit rates by better bucketing similar requests and to help OpenAI detect and prevent abuse.

Returns

Returns a Response object.

Returns the Response object.

Example

bash

curl https://api.fastapi.ai/v1/responses \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $FAST_API_KEY" \
  -d '{"model":"gpt-4.1","input":"Hello"}'

Create a model response ​

Request body ​

input string or array Optional Text, image, or file inputs to the model, used to generate a response ​

Returns ​

Example ​

Create a model response

Request body

`input` string or array Optional
Text, image, or file inputs to the model, used to generate a response

Returns

Example