> ## Documentation Index
> Fetch the complete documentation index at: https://phaseo.app/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Usage

> Create a Rust client, send Gateway requests, and handle responses and errors.

Use `Phaseo` when your Rust service needs a synchronous Phaseo Gateway client.

## Create a client

Load the API key and optional base URL from the environment:

```rust theme={null}
use phaseo::Phaseo;

let client = Phaseo::from_env()?;
```

Or pass the key directly:

```rust theme={null}
use phaseo::Phaseo;

let client = Phaseo::new("phaseo_v1_sk_...")?;
```

`Phaseo` redacts the API key and custom header values from its `Debug` output.

## Send a Responses request

```rust theme={null}
use phaseo::Phaseo;
use serde_json::{json, Value};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let client = Phaseo::from_env()?;
    let response = client.responses(&json!({
        "model": "openai/gpt-5.6-sol",
        "input": "Summarize why provider fallbacks improve reliability."
    }))?;

    let text = response.body
        .get("output")
        .and_then(Value::as_array)
        .into_iter()
        .flatten()
        .filter(|item| item.get("type").and_then(Value::as_str) == Some("message"))
        .flat_map(|item| {
            item.get("content")
                .and_then(Value::as_array)
                .into_iter()
                .flatten()
        })
        .find_map(|part| {
            (part.get("type").and_then(Value::as_str) == Some("output_text"))
                .then(|| part.get("text").and_then(Value::as_str))
                .flatten()
        })
        .unwrap_or("");

    println!("{text}");
    Ok(())
}
```

The complete JSON response remains available in `response.body`.

## Send a Chat Completions request

```rust theme={null}
let response = client.chat_completions(&json!({
    "model": "openai/gpt-5.6-sol",
    "messages": [{
        "role": "user",
        "content": "Reply with exactly: chat works"
    }]
}))?;

println!("{}", response.body);
```

## Call another JSON endpoint

Use `post(...)` for a Phaseo endpoint that does not yet have a high-level helper:

```rust theme={null}
let response = client.post("/embeddings", &json!({
    "model": "openai/text-embedding-3-small",
    "input": "Phaseo routes across AI providers"
}))?;
```

The path may include or omit its leading slash.

## Add a request header

```rust theme={null}
let client = Phaseo::from_env()?
    .with_header("x-phaseo-workspace-id", "workspace_123");
```

## Handle Gateway errors

```rust theme={null}
let request = serde_json::json!({
    "model": "openai/gpt-5.6-sol",
    "input": "Handle this request"
});

match client.responses(&request) {
    Ok(response) => println!("{}", response.body),
    Err(error) => {
        eprintln!("message: {}", error.message);
        eprintln!("status: {:?}", error.status);
        eprintln!("body: {:?}", error.body);
    }
}
```

`PhaseoError` contains an HTTP status and parsed JSON body when the Gateway returned an error. Transport and configuration failures have no HTTP status.

## Next

* [Responses API](./responses)
* [Chat Completions](./chat-completions)
* [Rust API reference](./api-reference)