---
title: Go Server SDK
---

# Go Server SDK

Official Go server-side SDK — `github.com/Captcha-La/captchala-go`.

Three jobs the SDK handles for you:

1. **Validate** — verify a `pt_` pass token from the browser SDK.
2. **Issue** — mint a one-time `sct_` server token to bind the upcoming challenge to a specific action / IP / UID.
3. **Moderate** — multi-modal (text + image) content moderation against the same OpenAI-compatible pipeline the dashboard uses.

## Install

```bash
go get github.com/Captcha-La/captchala-go
```

Stdlib only. No external dependencies.

## Validate a token

Pass the end-user's IP (from `CF-Connecting-IP` / `X-Forwarded-For`, falling
back to `RemoteAddr`). Optional, but **recommended** — it's used for additional
risk checks.

```go
import captchala "github.com/Captcha-La/captchala-go"

client := captchala.NewClient(appKey, appSecret)

func handler(w http.ResponseWriter, r *http.Request) {
    token := r.FormValue("captcha_token")

    result, err := client.ValidateWithClientIP(token, false, clientIP(r))
    if err != nil || !result.Valid {
        http.Error(w, result.Error, http.StatusBadRequest)
        return
    }
    // Verification passed; proceed with the request.
    // result.CaptchaArgs has Platform / UserIP / Referer / Pkg / SolvedAt / RiskScore
}

// Real end-user IP behind a CDN / proxy
func clientIP(r *http.Request) string {
    if ip := r.Header.Get("CF-Connecting-IP"); ip != "" {
        return ip
    }
    if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
        return strings.TrimSpace(strings.Split(xff, ",")[0])
    }
    host, _, _ := net.SplitHostPort(r.RemoteAddr)
    return host
}
```

No IP handy? Use `client.Validate(token)` — verification still works, just
without the extra IP-based risk signal.

If the original `server_token` was issued with `bind_uid`, compare:

```go
if result.UID != expectedUserID {
    http.Error(w, "user mismatch", http.StatusBadRequest)
    return
}
```

The solve-time context is also returned in `result.CaptchaArgs` for reference:

```go
ip := result.CaptchaArgs.UserIP   // visitor IP recorded at solve time
```

## Issue a server token

Recommended for high-value flows (login, register, payment): backend mints a
one-time `sct_` token, hands it to the browser, browser uses it as the
`serverToken` prop. Single-use, action-scoped, optionally IP/UID-bound.

```go
issue, err := client.IssueServerTokenWithOptions("login", captchala.IssueOptions{
    BindingIP: userIP,    // backend rejects token if a different IP redeems it
    TTL:       300,       // seconds
    MaxUses:   5,         // SDK retry budget
    BindUID:   userID,    // pair with ValidateResult.UID on verify
})
if err != nil || !issue.OK {
    http.Error(w, issue.Error, http.StatusBadRequest)
    return
}
// hand issue.Token to the browser
```

Defaults-only shortcut:

```go
issue, err := client.IssueServerToken("login")
```

## Moderate content

Multi-modal — accepts a slice of `ModerationItem` (text and/or image_url in
OpenAI-compatible format):

```go
result, err := client.ModerationCheck([]captchala.ModerationItem{
    captchala.TextItem(userComment),
    captchala.ImageURLItem(uploadedImageURL),
}, userID)

if err != nil || !result.OK {
    http.Error(w, result.Error, http.StatusBadRequest)
    return
}

if result.Flagged {
    if result.HasCategory("violence", "csam") {
        // hard block
    }
}
```

Plain-text shortcut:

```go
result, err := client.ModerationText("user comment here", userID)
```

Categories are model-defined; iterate `result.Categories` defensively rather
than hard-coding a fixed set.

## Types

| Type | Fields |
|---|---|
| `ValidateResult` | `Valid`, `Error`, `UID`, `ChallengeID`, `Action`, `Offline`, `ClientOnly`, `Warning`, `CaptchaArgs` |
| `CaptchaArgs` | `Platform`, `UserIP`, `Referer`, `Pkg`, `SolvedAt`, `RiskScore` (informational) |
| `IssueResult` | `OK`, `Token`, `ExpiresIn`, `IssuedAt`, `Error`, `Message` |
| `IssueOptions` | `BindingIP`, `TTL`, `MaxUses`, `BindUID` |
| `ModerationItem` | `Type`, `Text`, `ImageURL` (use `TextItem()` / `ImageURLItem()` helpers) |
| `ModerationResult` | `OK`, `Flagged`, `Categories`, `ContentType`, `Raw`, `Error`, `Message`; method `HasCategory(...names)` |

## Links

- [pkg.go.dev](https://pkg.go.dev/github.com/Captcha-La/captchala-go) · [GitHub](https://github.com/Captcha-La/captchala-go)
- [Web SDK overview](/web-sdk) · [API Reference](/api-reference)