diff --git a/src/openai/mod.rs b/src/openai/mod.rs
index a18bf89..81efcce 100644
--- a/src/openai/mod.rs
+++ b/src/openai/mod.rs
@@ -2,3 +2,4 @@ pub mod chat;
 pub mod decisions;
 pub mod embeddings;
 pub mod models;
+pub mod system_one;
diff --git a/src/openai/system_one.rs b/src/openai/system_one.rs
new file mode 100644
index 0000000..f48e053
--- /dev/null
+++ b/src/openai/system_one.rs
@@ -0,0 +1,265 @@
+use serde::{Deserialize, Serialize};
+use std::collections::HashMap;
+
+/// The top-level request structure for the system one endpoint.
+///
+/// This represents the complete request body that can be sent to the API.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct SystemOneRequest<Model> {
+    /// The content to evaluate. Can be a string, object, or array.
+    ///
+    /// For text content, use a plain string. For structured data (like chat logs or application state),
+    /// use an object or array.
+    pub state: State,
+
+    /// The model that handles the request.
+    pub model: Model,
+
+    /// A map of typed questions to evaluate against the state.
+    ///
+    /// Each key is a question ID you choose, and the value is a [Question] object.
+    /// The answers will be returned under the same keys. The question ID is not sent to the model.
+    pub questions: HashMap<String, Question>,
+}
+
+/// Represents the possible state formats that can be evaluated.
+///
+/// This is a convenience enum that wraps the different possible state formats:
+/// - Plain text (String)
+/// - Structured data (Object/Array)
+/// - Null (for models that support it)
+#[derive(Debug, Clone, Deserialize, Serialize)]
+#[serde(untagged)]
+pub enum State {
+    /// Plain text state
+    Text(String),
+
+    /// Structured state (object or array)
+    Structured(serde_json::Value),
+
+    /// Null state (for models that support it)
+    Null,
+}
+
+/// The main question type enum that represents all possible question types.
+///
+/// Each variant corresponds to a different question type (Noul, Choice, Score)
+/// and contains the appropriate fields for that type.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+#[serde(tag = "type", rename_all = "lowercase")]
+pub enum Question {
+    /// Yes/no question type
+    Noul(NoulQuestion),
+
+    /// Multiple choice question type
+    Choice(ChoiceQuestion),
+
+    /// Scoring question type
+    Score(ScoreQuestion),
+}
+
+/// Represents a yes/no question (Noul = "no/yes/uncertain/likely" type)
+///
+/// Returns the probability that the answer is yes (value near 1) or no (value near 0).
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct NoulQuestion {
+    /// The yes/no question to evaluate.
+    ///
+    /// Can be a plain string, or a structured object containing the question and referenced data.
+    pub instructions: QuestionInstructions,
+
+    /// Optional descriptions of what a yes and no mean.
+    ///
+    /// Provides additional context for interpreting the yes/no answer.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub criteria: Option<NoulCriteria>,
+}
+
+/// Criteria for a Noul question, describing what true/false mean.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct NoulCriteria {
+    /// What a yes (value near 1) means.
+    pub r#true: QuestionInstructions,
+
+    /// What a no (value near 0) means.
+    pub r#false: QuestionInstructions,
+}
+
+/// Represents a multiple choice question.
+///
+/// Returns the chosen option and the full probability distribution.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct ChoiceQuestion {
+    /// What the model should decide.
+    ///
+    /// Can be a plain string, or a structured object containing the question and referenced data.
+    pub instructions: QuestionInstructions,
+
+    /// A map of options to their descriptions.
+    ///
+    /// Each key is an option you define, and the value is its description.
+    /// Use null for options that don't need additional description.
+    /// Maximum of 255 options per Choice question.
+    pub criteria: HashMap<String, Option<QuestionInstructions>>,
+}
+
+/// Represents a scoring question.
+///
+/// Returns a probability-weighted value across the defined levels.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct ScoreQuestion {
+    /// What the model should rate.
+    ///
+    /// Can be a plain string, or a structured object containing the question and referenced data.
+    pub instructions: QuestionInstructions,
+
+    /// An ordered array of level descriptions.
+    ///
+    /// Must have at least 2 levels, maximum of 10. The first element is the lowest level,
+    /// the last is the highest.
+    pub criteria: Vec<QuestionInstructions>,
+}
+
+/// Represents the possible formats for question instructions and criteria.
+///
+/// This is a convenience enum that wraps different possible formats:
+/// - Plain text (String)
+/// - Structured data (Object/Array)
+#[derive(Debug, Clone, Deserialize, Serialize)]
+#[serde(untagged)]
+pub enum QuestionInstructions {
+    /// Plain text instructions
+    Text(String),
+
+    /// Structured instructions (object or array)
+    Structured(serde_json::Value),
+}
+
+/// The top-level response structure from the system one endpoint.
+///
+/// This represents the complete response body returned by the API.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct SystemOneResponse<Model> {
+    /// The model that performed the evaluation.
+    pub model: Model,
+
+    /// A map of answers, keyed by the same IDs used in the request questions.
+    pub answers: HashMap<String, Answer>,
+
+    /// Token usage information for the request.
+    pub usage: Usage,
+}
+
+/// Represents the possible answer types that can be returned.
+///
+/// Each variant corresponds to a different question type (Noul, Choice, Score)
+/// and contains the appropriate fields for that type.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+#[serde(tag = "type", rename_all = "lowercase")]
+pub enum Answer {
+    /// Answer to a Noul question
+    Noul(NoulAnswer),
+
+    /// Answer to a Choice question
+    Choice(ChoiceAnswer),
+
+    /// Answer to a Score question
+    Score(ScoreAnswer),
+}
+
+/// Answer to a Noul question (yes/no question).
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct NoulAnswer {
+    /// The yes/no answer on a scale from 0 (no) to 1 (yes).
+    pub noul: f32,
+}
+
+/// Answer to a Choice question (multiple choice).
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct ChoiceAnswer {
+    /// The option with the highest probability.
+    pub choice: String,
+
+    /// The probability of each option, they sum to 1.
+    pub probabilities: HashMap<String, f32>,
+
+    /// How certain the model is (0 to 1).
+    ///
+    /// 0 means all options are equally likely, 1 means maximum certainty.
+    pub confidence: f32,
+}
+
+/// Answer to a Score question.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct ScoreAnswer {
+    /// The expected level index, weighted by probability.
+    ///
+    /// Can be between two levels (e.g., 1.5 would be halfway between level 1 and 2).
+    pub score: f32,
+
+    /// The description of each level index.
+    pub legend: HashMap<String, String>,
+
+    /// The probability of each level index, they sum to 1.
+    pub probabilities: HashMap<String, f32>,
+
+    /// How certain the model is (0 to 1).
+    ///
+    /// Derived from the probabilities distribution.
+    pub confidence: f32,
+}
+
+/// Token usage information for the request.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct Usage {
+    /// Number of input tokens (prompt tokens for all questions).
+    pub input_tokens: u32,
+
+    /// Number of output tokens (always 0 for this API).
+    pub output_tokens: u32,
+}
+
+/// Optional file input for multimodal models.
+///
+/// Represents an array of input files (images, audio, etc.) provided as data URLs.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct FileInput {
+    /// Array of data URLs for input files.
+    ///
+    /// For images: data:image/...;base64,...
+    /// For audio: data:audio/...;base64,...
+    pub files: Vec<String>,
+}
+
+/// Alternative name for file input (alias for `files`).
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct ImageInput {
+    /// Array of image data URLs.
+    ///
+    /// Same format as `files` but specifically for images.
+    pub images: Vec<String>,
+}
+
+/// Complete request structure that includes optional file inputs.
+///
+/// This is a convenience structure that combines the basic request with optional
+/// file inputs for multimodal models.
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct MultimodalRequest {
+    /// The content to evaluate (same as in Request).
+    pub state: State,
+
+    /// Optional file inputs (images, audio, etc.).
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub files: Option<Vec<String>>,
+
+    /// Alternative name for file inputs (alias for `files`).
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub images: Option<Vec<String>>,
+
+    /// The model that handles the request (same as in Request).
+    pub model: String,
+
+    /// A map of typed questions to evaluate (same as in Request).
+    pub questions: HashMap<String, Question>,
+}