### Build and Run OAuth2 Server

Source: https://github.com/go-oauth2/oauth2/blob/master/example/README.md

Navigate to the server example directory, build the executable, and run the server.

```bash
cd example/server
go build server.go
./server
```

--------------------------------

### Build and Run OAuth2 Client

Source: https://github.com/go-oauth2/oauth2/blob/master/example/README.md

Navigate to the client example directory, build the executable, and run the client.

```bash
cd example/client
go build client.go
./client
```

--------------------------------

### Install OAuth 2.0 Package

Source: https://github.com/go-oauth2/oauth2/blob/master/README.md

Download and install the Go OAuth 2.0 package using the go get command.

```bash
go get -u -v github.com/go-oauth2/oauth2/v4/...

```

--------------------------------

### Build and Run Server

Source: https://github.com/go-oauth2/oauth2/blob/master/README.md

Compile the Go server file and then run the executable to start the OAuth 2.0 server.

```bash
go build server.go

./server

```

--------------------------------

### Client Credentials Grant Flow Example

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Demonstrates how to obtain an access token using client credentials, suitable for machine-to-machine authentication. Supports POST requests and GET requests if AllowGetAccessRequest is enabled.

```curl
curl -X POST http://localhost:9096/token \
  -d "grant_type=client_credentials" \
  -d "client_id=my-client-id" \
  -d "client_secret=my-client-secret" \
  -d "scope=read"
```

```json
{
  "access_token": "OA6ITALNMDOGD58C0SN-MG",
  "token_type": "Bearer",
  "expires_in": 7200,
  "scope": "read"
}
```

```http
GET http://localhost:9096/token?grant_type=client_credentials&client_id=my-client-id&client_secret=my-client-secret&scope=read
```

--------------------------------

### Client Request with Bearer Token

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Example using curl to make a request to a protected resource, including the Authorization header with a bearer token.

```bash
# Client request with bearer token
# curl -H "Authorization: Bearer J86XVRYSNFCFI233KXDL0Q" http://localhost:9096/api/resource
```

--------------------------------

### Token Response Example

Source: https://github.com/go-oauth2/oauth2/blob/master/README.md

A sample JSON response received after a successful token request, containing the access token, expiration time, scope, and token type.

```json
{
  "access_token": "J86XVRYSNFCFI233KXDL0Q",
  "expires_in": 7200,
  "scope": "read",
  "token_type": "Bearer"
}

```

--------------------------------

### Authorization Request URL

Source: https://github.com/go-oauth2/oauth2/blob/master/README.md

Example URL to initiate an authorization code grant flow. Ensure the client_id and response_type parameters are correctly set.

```http
http://localhost:9096/authorize?client_id=000000&response_type=code

```

--------------------------------

### Grant Token Request URL

Source: https://github.com/go-oauth2/oauth2/blob/master/README.md

Example URL to request an access token using client credentials. Includes grant_type, client_id, client_secret, and scope.

```http
http://localhost:9096/token?grant_type=client_credentials&client_id=000000&client_secret=999999&scope=read

```

--------------------------------

### Authorization Code Grant Flow Example

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Illustrates the steps for the Authorization Code Grant flow, including the initial authorization request, user redirection, and exchanging the code for tokens. This is a common flow for web applications.

```http
GET http://localhost:9096/authorize?client_id=my-client-id&response_type=code&redirect_uri=http://localhost:8080/callback&scope=read&state=xyz
```

```http
http://localhost:8080/callback?code=AUTHORIZATION_CODE&state=xyz
```

```curl
curl -X POST http://localhost:9096/token \
  -d "grant_type=authorization_code" \
  -d "code=AUTHORIZATION_CODE" \
  -d "client_id=my-client-id" \
  -d "client_secret=my-client-secret" \
  -d "redirect_uri=http://localhost:8080/callback"
```

```json
{
  "access_token": "J86XVRYSNFCFI233KXDL0Q",
  "token_type": "Bearer",
  "refresh_token": "5FBLXQ47XJ2MGTY8YRZQ8W",
  "expires_in": 7200
}
```

--------------------------------

### Request Token with Password Grant

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Example using curl to request an access token using the password grant type. This requires sending user credentials directly.

```bash
# Request token using password grant
# curl -X POST http://localhost:9096/token \
#   -d "grant_type=password" \
#   -d "username=test" \
#   -d "password=test" \
#   -d "client_id=my-client-id" \
#   -d "client_secret=my-client-secret" \
#   -d "scope=read write"
```

--------------------------------

### Refresh Token Flow Request

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Example using curl to obtain a new access token using a refresh token. This is used when an existing access token has expired.

```bash
# Refresh an expired access token
# curl -X POST http://localhost:9096/token \
#   -d "grant_type=refresh_token" \
#   -d "refresh_token=5FBLXQ47XJ2MGTY8YRZQ8W" \
#   -d "client_id=my-client-id" \
#   -d "client_secret=my-client-secret"
```

--------------------------------

### Create and Configure In-Memory Client Store

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Initialize an in-memory client store and register clients with their IDs, secrets, and domains. Public clients do not require a secret.

```go
import (
	"github.com/go-oauth2/oauth2/v4/models"
	"github.com/go-oauth2/oauth2/v4/store"
)

// Create in-memory client store
clientStore := store.NewClientStore()

// Register clients
clientStore.Set("web-app", &models.Client{
	ID:     "web-app",
	Secret: "web-app-secret",
	Domain: "https://myapp.example.com",
	Public: false,
	UserID: "owner-user-id",
})

// Public client (mobile/SPA - no secret)
clientStore.Set("mobile-app", &models.Client{
	ID:     "mobile-app",
	Secret: "",
	Domain: "myapp://callback",
	Public: true,
})

manager.MapClientStorage(clientStore)
```

--------------------------------

### Initialize Golang OAuth 2.0 Server

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Sets up the OAuth 2.0 server with in-memory token storage, a test client, and configures handlers for authorization and token requests. Ensure to replace in-memory storage with a persistent solution for production.

```go
package main

import (
	"context"
	"log"
	"net/http"

	"github.com/go-oauth2/oauth2/v4/errors"
	"github.com/go-oauth2/oauth2/v4/manage"
	"github.com/go-oauth2/oauth2/v4/models"
	"github.com/go-oauth2/oauth2/v4/server"
	"github.com/go-oauth2/oauth2/v4/store"
)

func main() {
	// Create a manager with default token generators
	manager := manage.NewDefaultManager()

	// Configure token storage (in-memory for development)
	manager.MustTokenStorage(store.NewMemoryTokenStore())

	// Configure client storage with a test client
	clientStore := store.NewClientStore()
	clientStore.Set("my-client-id", &models.Client{
		ID:     "my-client-id",
		Secret: "my-client-secret",
		Domain: "http://localhost:8080",
	})
	manager.MapClientStorage(clientStore)

	// Create the OAuth 2.0 server
	srv := server.NewDefaultServer(manager)
	srv.SetAllowGetAccessRequest(true)
	srv.SetClientInfoHandler(server.ClientFormHandler)

	// Configure user authorization handler
	srv.UserAuthorizationHandler = func(w http.ResponseWriter, r *http.Request) (userID string, err error) {
		// In production, authenticate the user and return their ID
		return "user-123", nil
	}

	// Configure error handlers
	srv.SetInternalErrorHandler(func(err error) (re *errors.Response) {
		log.Println("Internal Error:", err.Error())
		return
	})

	srv.SetResponseErrorHandler(func(re *errors.Response) {
		log.Println("Response Error:", re.Error.Error())
	})

	// Mount OAuth endpoints
	http.HandleFunc("/authorize", func(w http.ResponseWriter, r *http.Request) {
		err := srv.HandleAuthorizeRequest(w, r)
		if err != nil {
			http.Error(w, err.Error(), http.StatusBadRequest)
		}
	})

	http.HandleFunc("/token", func(w http.ResponseWriter, r *http.Request) {
		srv.HandleTokenRequest(w, r)
	})

	log.Println("OAuth server running on :9096")
	log.Fatal(http.ListenAndServe(":9096", nil))
}
```

--------------------------------

### Create OAuth 2.0 Server

Source: https://github.com/go-oauth2/oauth2/blob/master/README.md

Sets up a default OAuth 2.0 server with memory token and client stores, handling authorization and token requests. Requires specific handlers for user authorization and error responses.

```go
package main

import (
	"log"
	"net/http"

	"github.com/go-oauth2/oauth2/v4/errors"
	"github.com/go-oauth2/oauth2/v4/manage"
	"github.com/go-oauth2/oauth2/v4/models"
	"github.com/go-oauth2/oauth2/v4/server"
	"github.com/go-oauth2/oauth2/v4/store"
)

func main() {
	manager := manage.NewDefaultManager()
	// token memory store
	manager.MustTokenStorage(store.NewMemoryTokenStore())

	// client memory store
	clientStore := store.NewClientStore()
	clientStore.Set("000000", &models.Client{
		ID:     "000000",
		Secret: "999999",
		Domain: "http://localhost",
	})
	manager.MapClientStorage(clientStore)

	srv := server.NewDefaultServer(manager)
	srv.SetAllowGetAccessRequest(true)
	srv.SetClientInfoHandler(server.ClientFormHandler)

	srv.UserAuthorizationHandler = func(w http.ResponseWriter, r *http.Request) (userID string, err error) {
		return "000000", nil
	}

	srv.SetInternalErrorHandler(func(err error) (re *errors.Response) {
		log.Println("Internal Error:", err.Error())
		return
	})

	srv.SetResponseErrorHandler(func(re *errors.Response) {
		log.Println("Response Error:", re.Error.Error())
	})

	http.HandleFunc("/authorize", func(w http.ResponseWriter, r *http.Request) {
		err := srv.HandleAuthorizeRequest(w, r)
		if err != nil {
			http.Error(w, err.Error(), http.StatusBadRequest)
		}
	})

	http.HandleFunc("/token", func(w http.ResponseWriter, r *http.Request) {
		srv.HandleTokenRequest(w, r)
	})

	log.Fatal(http.ListenAndServe(":9096", nil))
}

```

--------------------------------

### Configure File-Based Token Store

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Initialize a file-based token store for persistent token storage. Specify the database file name.

```go
import "github.com/go-oauth2/oauth2/v4/store"

// File-based token store (persistent)
fileStore, err := store.NewFileTokenStore("oauth_tokens.db")
if err != nil {
	log.Fatal(err)
}
manager.MapTokenStorage(fileStore)
```

--------------------------------

### Configure In-Memory Token Store

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Set up an in-memory token store for development or testing purposes. Handles errors during initialization.

```go
import "github.com/go-oauth2/oauth2/v4/store"

// In-memory token store (development)
memStore, err := store.NewMemoryTokenStore()
if err != nil {
	log.Fatal(err)
}
manager.MustTokenStorage(memStore, nil)
```

--------------------------------

### Complete OAuth 2.0 Server in Go

Source: https://context7.com/go-oauth2/oauth2/llms.txt

This Go program implements a full OAuth 2.0 server. It requires setting up a token manager, client store, and defining handlers for authorization, token requests, and protected resources. Use this as a foundation for building your own authorization server.

```go
package main

import (
	"context"
	"encoding/json"
	"log"
	"net/http"
	"time"

	"github.com/go-oauth2/oauth2/v4/errors"
	"github.com/go-oauth2/oauth2/v4/manage"
	"github.com/go-oauth2/oauth2/v4/models"
	"github.com/go-oauth2/oauth2/v4/server"
	"github.com/go-oauth2/oauth2/v4/store"
)

func main() {
	manager := manage.NewDefaultManager()
	manager.MustTokenStorage(store.NewMemoryTokenStore())

	clientStore := store.NewClientStore()
	clientStore.Set("client-id", &models.Client{
		ID:     "client-id",
		Secret: "client-secret",
		Domain: "http://localhost:8080",
	})
	manager.MapClientStorage(clientStore)

	srv := server.NewDefaultServer(manager)
	srv.SetAllowGetAccessRequest(true)
	srv.SetClientInfoHandler(server.ClientFormHandler)

	srv.UserAuthorizationHandler = func(w http.ResponseWriter, r *http.Request) (string, error) {
		// Implement your user authentication logic
		return "user-id", nil
	}

	srv.SetPasswordAuthorizationHandler(func(ctx context.Context, clientID, username, password string) (string, error) {
		if username == "demo" && password == "demo" {
			return username, nil
		}
		return "", errors.ErrAccessDenied
	})

	// Authorization endpoint
	http.HandleFunc("/oauth/authorize", func(w http.ResponseWriter, r *http.Request) {
		if err := srv.HandleAuthorizeRequest(w, r); err != nil {
			http.Error(w, err.Error(), http.StatusBadRequest)
		}
	})

	// Token endpoint
	http.HandleFunc("/oauth/token", func(w http.ResponseWriter, r *http.Request) {
		srv.HandleTokenRequest(w, r)
	})

	// Protected resource endpoint
	http.HandleFunc("/api/me", func(w http.ResponseWriter, r *http.Request) {
		token, err := srv.ValidationBearerToken(r)
		if err != nil {
			w.WriteHeader(http.StatusUnauthorized)
			json.NewEncoder(w).Encode(map[string]string{"error": err.Error()})
			return
		}
		json.NewEncoder(w).Encode(map[string]interface{}{
			"client_id":  token.GetClientID(),
			"user_id":    token.GetUserID(),
			"scope":      token.GetScope(),
			"expires_in": int64(token.GetAccessCreateAt().Add(token.GetAccessExpiresIn()).Sub(time.Now()).Seconds()),
		})
	})

	log.Println("OAuth 2.0 Server running on :9096")
	log.Println("Authorization endpoint: http://localhost:9096/oauth/authorize")
	log.Println("Token endpoint: http://localhost:9096/oauth/token")
	log.Fatal(http.ListenAndServe(":9096", nil))
}

```

--------------------------------

### Configure Password Grant Server

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Set up the server to handle user credentials directly for token exchange. Ensure user credentials are validated against your user store.

```go
// Server configuration for password grant
srv.SetPasswordAuthorizationHandler(func(ctx context.Context, clientID, username, password string) (userID string, err error) {
	// Validate user credentials against your user store
	if username == "test" && password == "test" {
		return "user-123", nil
	}
	return "", errors.ErrAccessDenied
})
```

--------------------------------

### Enable PKCE Requirement on Server

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Configure the OAuth2 server to enforce PKCE for authorization code grants. Allows plain and S256 challenge methods.

```go
import (
	"github.com/go-oauth2/oauth2/v4"
	"github.com/go-oauth2/oauth2/v4/server"
)

// Server configuration - enable PKCE requirement
config := server.NewConfig()
config.ForcePKCE = true  // Require PKCE for all authorization code grants
config.AllowedCodeChallengeMethods = []oauth2.CodeChallengeMethod{
	oauth2.CodeChallengePlain,
	oauth2.CodeChallengeS256,
}
srv := server.NewServer(config, manager)
```

--------------------------------

### Retrieve Client Information

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Fetch client details from the ClientStore using the client ID. Handles errors if the client is not found.

```go
import (
	"fmt"
	"log"
)

// Retrieve client info
clientInfo, err := clientStore.GetByID(ctx, "web-app")
if err != nil {
	log.Fatal("Client not found")
}
fmt.Printf("Client domain: %s\n", clientInfo.GetDomain())
```

--------------------------------

### Configure JWT Access Token Generation (HMAC)

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Set up JWT access token generation using HMAC signing. This involves specifying a key ID, a secret key, and the desired signing method.

```go
import (
	"github.com/go-oauth2/oauth2/v4/generates"
	"github.com/golang-jwt/jwt/v5"
)

// Configure JWT access token generation with HMAC
manager.MapAccessGenerate(generates.NewJWTAccessGenerate(
	"key-id-1",                    // Key ID (optional, for key rotation)
	[]byte("my-secret-key-32byte"), // Secret key
	jwt.SigningMethodHS512,         // Signing method
))
```

--------------------------------

### Generate PKCE Code Challenge

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Client-side implementation for PKCE: generate a code verifier, then compute the code challenge using SHA256.

```go
import "crypto/sha256"

// Client implementation:
// 1. Generate code_verifier (43-128 character random string)
codeVerifier := "dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk"

// 2. Generate code_challenge using S256 method
hash := sha256.Sum256([]byte(codeVerifier))
codeChallenge := base64.RawURLEncoding.EncodeToString(hash[:])
```

--------------------------------

### Configure Custom URI Validation

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Implement a custom handler to validate redirect URIs against the base URI. Ensures that the hostnames match.

```go
import (
	"errors"
	"net/url"
)

// Configure custom URI validation
manager.SetValidateURIHandler(func(baseURI, redirectURI string) error {
	base, _ := url.Parse(baseURI)
	redirect, _ := url.Parse(redirectURI)
	if base.Host != redirect.Host {
		return errors.ErrInvalidRedirectURI
	}
	return nil
})
```

--------------------------------

### Configure OAuth2 Manager Settings

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Set token expiration times and token configuration for different grant types. Default authorization code expiration is 10 minutes.

```go
import (
	t"time"
	"github.com/go-oauth2/oauth2/v4/manage"
)

manager := manage.NewDefaultManager()

// Configure authorization code expiration (default: 10 minutes)
manager.SetAuthorizeCodeExp(time.Minute * 5)

// Configure token expiration for each grant type
manager.SetAuthorizeCodeTokenCfg(&manage.Config{
	AccessTokenExp:    time.Hour * 2,
	RefreshTokenExp:   time.Hour * 24 * 3,
	IsGenerateRefresh: true,
})

manager.SetImplicitTokenCfg(&manage.Config{
	AccessTokenExp: time.Hour * 1,  // No refresh token for implicit grant
})

manager.SetPasswordTokenCfg(&manage.Config{
	AccessTokenExp:    time.Hour * 2,
	RefreshTokenExp:   time.Hour * 24 * 7,
	IsGenerateRefresh: true,
})

manager.SetClientTokenCfg(&manage.Config{
	AccessTokenExp: time.Hour * 2,  // No refresh token for client credentials
})
```

--------------------------------

### Client Credentials Grant - Access Token Response

Source: https://github.com/go-oauth2/oauth2/blob/master/example/README.md

Sample JSON response for obtaining an access token using the Client Credentials Grant, suitable for machine-to-machine authentication.

```json
{
  "access_token": "OA6ITALNMDOGD58C0SN-MG",
  "token_type": "Bearer",
  "expiry": "2019-02-12T11:10:35.864838+08:00"
}
```

--------------------------------

### Configure Refresh Token Behavior

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Customize the behavior of refresh tokens, including expiry times and whether to generate new refresh tokens or remove old ones.

```go
# Configure refresh token behavior
manager.SetRefreshTokenCfg(&manage.RefreshingConfig{
	AccessTokenExp:     time.Hour * 2,
	RefreshTokenExp:    time.Hour * 24 * 7,
	IsGenerateRefresh:  true,          // Generate new refresh token
	IsResetRefreshTime: true,          // Reset refresh token expiry
	IsRemoveAccess:     true,          // Remove old access token
	IsRemoveRefreshing: true,          // Remove old refresh token
})
```

--------------------------------

### Parse and Verify JWT Access Token

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Implement a function to parse and verify JWT access tokens on the resource server. This function checks the signing method and token validity.

```go
// Parse and verify JWT access token on resource server
func validateJWT(accessToken string) (*generates.JWTAccessClaims, error) {

token, err := jwt.ParseWithClaims(accessToken, &generates.JWTAccessClaims{}, func(t *jwt.Token) (interface{}, error) {
		// Verify signing method
		if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
			return nil, fmt.Errorf("unexpected signing method: %v", t.Header["alg"])
		}
		return []byte("my-secret-key-32byte"), nil
	})
	if err != nil {
		return nil, err
	}

	claims, ok := token.Claims.(*generates.JWTAccessClaims)
	if !ok || !token.Valid {
		return nil, errors.New("invalid token")
	}
	return claims, nil
}
```

--------------------------------

### Authorization Code Grant - Try Access Token Response

Source: https://github.com/go-oauth2/oauth2/blob/master/example/README.md

Sample JSON response when trying to access a resource with an access token obtained via the Authorization Code Grant.

```json
{
  "client_id": "222222",
  "expires_in": 7195,
  "user_id": "000000"
}
```

--------------------------------

### Password Credentials Grant - Access Token Response

Source: https://github.com/go-oauth2/oauth2/blob/master/example/README.md

Sample JSON response for obtaining an access token using the Password Credentials Grant, which typically involves username and password.

```json
{
  "access_token": "87JT3N6WOWANXVDNZFHY7Q",
  "token_type": "Bearer",
  "refresh_token": "LDIS6PXAVY-BXHPEDESWNG",
  "expiry": "2019-02-12T10:58:43.734902+08:00"
}
```

--------------------------------

### Generate JWT Access Tokens

Source: https://github.com/go-oauth2/oauth2/blob/master/README.md

Configures the OAuth2 manager to use JWT for generating access tokens with a specified signing method and secret key. Also shows how to parse and verify a JWT access token.

```go
import (
	"github.com/go-oauth2/oauth2/v4/generates"
	"github.com/dgrijalva/jwt-go"
)

// ...
manager.MapAccessGenerate(generates.NewJWTAccessGenerate("", []byte("00000000"), jwt.SigningMethodHS512))

// Parse and verify jwt access token
token, err := jwt.ParseWithClaims(access, &generates.JWTAccessClaims{}, func(t *jwt.Token) (interface{}, error) {
	if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
		return nil, fmt.Errorf("parse error")
	}
	return []byte("00000000"), nil
})
if err != nil {
	// panic(err)
}

claims, ok := token.Claims.(*generates.JWTAccessClaims)
if !ok || !token.Valid {
	// panic("invalid token")
}

```

--------------------------------

### Set Pre-Redirect Error Handler

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Customize error handling specifically for scenarios that involve redirects. This handler can intercept errors and provide alternative responses, such as custom error pages.

```go
// Pre-redirect error handler - custom handling before redirect
srv.PreRedirectErrorHandler = func(w http.ResponseWriter, req *AuthorizeRequest, err error) error {
	// Custom error page instead of redirect
	if err == errors.ErrAccessDenied {
		http.Redirect(w, req.Request, "/access-denied", http.StatusFound)
		return nil  // Prevent default redirect
	}
	return err  // Use default redirect behavior
}
```

--------------------------------

### Authorization Code Grant - Access Token Response

Source: https://github.com/go-oauth2/oauth2/blob/master/example/README.md

Sample JSON response received after a successful Authorization Code Grant flow, containing the access token and related details.

```json
{
  "access_token": "GIGXO8XWPQSAUGOYQGTV8Q",
  "token_type": "Bearer",
  "refresh_token": "5FBLXQ47XJ2MGTY8YRZQ8W",
  "expiry": "2019-01-08T01:53:45.868194+08:00"
}
```

--------------------------------

### Refresh Token Grant - Access Token Response

Source: https://github.com/go-oauth2/oauth2/blob/master/example/README.md

Sample JSON response after successfully refreshing an access token using a refresh token. This response includes a new access token and potentially a new refresh token.

```json
{
  "access_token": "0IIL4_AJN2-SR0JEYZVQWG",
  "token_type": "Bearer",
  "refresh_token": "AG6-63MLXUEFUV2Q_BLYIW",
  "expiry": "2019-01-09T23:03:16.374062+08:00"
}
```

--------------------------------

### Customize Access Token Expiration Handler

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Set custom lifetimes for access tokens. This handler allows for dynamic expiration durations based on request parameters like scope.

```go
// Access token expiration handler - custom token lifetime
srv.AccessTokenExpHandler = func(w http.ResponseWriter, r *http.Request) (time.Duration, error) {
	// Different expiration based on scope
	if strings.Contains(r.FormValue("scope"), "remember") {
		return time.Hour * 24 * 30, nil  // 30 days
	}
	return time.Hour * 2, nil  // Default 2 hours
}
```

--------------------------------

### Restrict Grant Types Per Client

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Control which grant types are allowed for specific clients. This handler enforces client-specific authorization rules.

```go
// Client authorization handler - restrict grant types per client
srv.ClientAuthorizedHandler = func(clientID string, grant oauth2.GrantType) (bool, error) {
	// Only allow password grant for specific clients
	if grant == oauth2.PasswordCredentials && clientID != "trusted-client" {
		return false, nil
	}
	return true, nil
}
```

--------------------------------

### Validate Bearer Token for Protected Resources

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Implement a handler for protected endpoints that validates the incoming bearer token using `ValidationBearerToken`. This ensures only authorized clients can access resources.

```go
# Protected endpoint handler
http.HandleFunc("/api/resource", func(w http.ResponseWriter, r *http.Request) {
	// Validate the bearer token

tokenInfo, err := srv.ValidationBearerToken(r)
	if err != nil {
		http.Error(w, err.Error(), http.StatusUnauthorized)
		return
	}

	// Token is valid - access token info

response := map[string]interface{}{
		"client_id":  tokenInfo.GetClientID(),
		"user_id":    tokenInfo.GetUserID(),
		"scope":      tokenInfo.GetScope(),
		"expires_in": int64(tokenInfo.GetAccessCreateAt().Add(tokenInfo.GetAccessExpiresIn()).Sub(time.Now()).Seconds()),
	}
	json.NewEncoder(w).Encode(response)
})
```

--------------------------------

### Set Internal Error Handler

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Customize how internal OAuth2 errors are handled. This allows for logging errors and optionally overriding the default error response format.

```go
import "github.com/go-oauth2/oauth2/v4/errors"

// Internal error handler - log errors, optionally override response
srv.SetInternalErrorHandler(func(err error) *errors.Response {
	log.Printf("Internal OAuth error: %v", err)
	// Return nil to use default error response
	// Or return custom response:
	return &errors.Response{
		Error:       errors.ErrServerError,
		Description: "An internal error occurred",
		StatusCode:  500,
	}
})
```

--------------------------------

### Add Extension Fields to Token Response

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Include custom fields in the OAuth2 token response. This handler receives token information and returns a map of additional key-value pairs.

```go
// Extension fields handler - add custom fields to token response
srv.ExtensionFieldsHandler = func(ti oauth2.TokenInfo) map[string]interface{} {
	return map[string]interface{}{
		"custom_field": "custom_value",
		"user_id":      ti.GetUserID(),
	}
}
```

--------------------------------

### Validate Client Scope

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Validate if a client is permitted to request a specific scope. This handler checks client ID against allowed scopes.

```go
// Client scope handler - validate scope for client
srv.ClientScopeHandler = func(tgr *oauth2.TokenGenerateRequest) (bool, error) {
	// Check if client is allowed to request this scope
	if tgr.Scope == "admin" && tgr.ClientID != "admin-client" {
		return false, nil
	}
	return true, nil
}
```

--------------------------------

### Customize Authorization Scope Handler

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Modify or validate the requested scope during authorization. This handler can deny specific scopes or alter the requested scope before proceeding.

```go
// Authorize scope handler - modify or validate requested scope
srv.AuthorizeScopeHandler = func(w http.ResponseWriter, r *http.Request) (scope string, err error) {
	// Optionally modify or restrict scope
	requestedScope := r.FormValue("scope")
	if requestedScope == "admin" {
		return "", errors.ErrInvalidScope  // Deny admin scope
	}
	return requestedScope, nil
}
```

--------------------------------

### Set Response Error Handler

Source: https://context7.com/go-oauth2/oauth2/llms.txt

Modify OAuth2 error responses before they are sent to the client. This handler can add custom headers or log detailed error information.

```go
// Response error handler - modify error before sending
srv.SetResponseErrorHandler(func(re *errors.Response) {
	log.Printf("OAuth error response: %s - %s", re.Error, re.Description)
	// Optionally modify the response
	re.SetHeader("X-Error-ID", generateErrorID())
})
```

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