diff --git a/backend/frontend/frontend_excluded.go b/backend/frontend/frontend_excluded.go
index 2e0e35ef..d7c652f6 100644
--- a/backend/frontend/frontend_excluded.go
+++ b/backend/frontend/frontend_excluded.go
@@ -2,8 +2,11 @@
 
 package frontend
 
-import "github.com/gin-gonic/gin"
+import (
+	"github.com/gin-gonic/gin"
+	"github.com/pocket-id/pocket-id/backend/internal/service"
+)
 
-func RegisterFrontend(router *gin.Engine, rateLimitMiddleware gin.HandlerFunc) error {
+func RegisterFrontend(router *gin.Engine, oidcService *service.OidcService) error {
 	return ErrFrontendNotIncluded
 }
diff --git a/backend/frontend/frontend_included.go b/backend/frontend/frontend_included.go
index 6486671b..c7eb88a6 100644
--- a/backend/frontend/frontend_included.go
+++ b/backend/frontend/frontend_included.go
@@ -10,13 +10,14 @@ import (
 	"io/fs"
 	"mime"
 	"net/http"
-	"os"
 	"path"
 	"strings"
 	"time"
 
 	"github.com/gin-gonic/gin"
 	"github.com/pocket-id/pocket-id/backend/internal/middleware"
+	"github.com/pocket-id/pocket-id/backend/internal/service"
+	"golang.org/x/time/rate"
 )
 
 //go:embed all:dist/*
@@ -54,7 +55,7 @@ func init() {
 	}
 }
 
-func RegisterFrontend(router *gin.Engine, rateLimitMiddleware gin.HandlerFunc) error {
+func RegisterFrontend(router *gin.Engine, oidcService *service.OidcService) error {
 	distFS, err := fs.Sub(frontendFS, "dist")
 	if err != nil {
 		return fmt.Errorf("failed to create sub FS: %w", err)
@@ -83,16 +84,19 @@ func RegisterFrontend(router *gin.Engine, rateLimitMiddleware gin.HandlerFunc) e
 			return
 		}
 
-		// If path is / or does not exist, serve index.html
-		if path == "" {
-			path = "index.html"
-		} else if _, err := fs.Stat(distFS, path); os.IsNotExist(err) {
-			path = "index.html"
-		}
-
-		if path == "index.html" {
+		if isSPARequest(path, distFS) {
 			nonce := middleware.GetCSPNonce(c)
 
+			if isOAuth2AuthorizationPostRequest(c) {
+				// In that case, we need to validate and allow form submissions to the redirect_uri
+				redirectURI := c.Query("redirect_uri")
+				clientID := c.Query("client_id")
+				validatedRedirectURI, err := oidcService.ResolveAllowedCallbackURL(c.Request.Context(), clientID, redirectURI)
+				if err == nil {
+					c.Header("Content-Security-Policy", middleware.BuildCSP(nonce, validatedRedirectURI))
+				}
+			}
+
 			// Do not cache the HTML shell, as it embeds a per-request nonce
 			c.Header("Content-Type", "text/html; charset=utf-8")
 			c.Header("Cache-Control", "no-store")
@@ -108,11 +112,46 @@ func RegisterFrontend(router *gin.Engine, rateLimitMiddleware gin.HandlerFunc) e
 		fileServer.ServeHTTP(c.Writer, c.Request)
 	}
 
-	router.NoRoute(rateLimitMiddleware, handler)
+	rateLimitMiddleware := middleware.NewRateLimitMiddleware().Add(rate.Every(300*time.Millisecond), 50)
+	router.NoRoute(rateLimitOnlyForOAuth2AuthorizationPostRequest(rateLimitMiddleware, distFS), handler)
 
 	return nil
 }
 
+func rateLimitOnlyForOAuth2AuthorizationPostRequest(rateLimitMiddleware gin.HandlerFunc, distFS fs.FS) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		path := strings.TrimPrefix(c.Request.URL.Path, "/")
+		if isSPARequest(path, distFS) && isOAuth2AuthorizationPostRequest(c) {
+			rateLimitMiddleware(c)
+			return
+		}
+
+		c.Next()
+	}
+}
+
+// isOAuth2AuthorizationRequest checks if this is an OAuth2 authorization request with response_mode=form_post
+// In that case, we need to validate and allow form submissions to the redirect_uri
+func isOAuth2AuthorizationPostRequest(c *gin.Context) bool {
+	responseMode := c.Query("response_mode")
+	redirectURI := c.Query("redirect_uri")
+	clientID := c.Query("client_id")
+
+	return responseMode == "form_post" && redirectURI != "" && clientID != ""
+}
+
+func isSPARequest(path string, distFS fs.FS) bool {
+	if path == "" {
+		return true
+	}
+
+	if _, err := fs.Stat(distFS, path); err != nil {
+		return true
+	}
+
+	return false
+}
+
 // FileServerWithCaching wraps http.FileServer to add caching headers
 type FileServerWithCaching struct {
 	root                    http.FileSystem
diff --git a/backend/frontend/frontend_included_test.go b/backend/frontend/frontend_included_test.go
new file mode 100644
index 00000000..5629ca09
--- /dev/null
+++ b/backend/frontend/frontend_included_test.go
@@ -0,0 +1,111 @@
+//go:build !exclude_frontend
+
+package frontend
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"testing/fstest"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestIsSPARequest(t *testing.T) {
+	distFS := fstest.MapFS{
+		"assets/app.js": &fstest.MapFile{Data: []byte("console.log('test')")},
+	}
+
+	t.Run("root path is spa request", func(t *testing.T) {
+		assert.True(t, isSPARequest("", distFS))
+	})
+
+	t.Run("existing bundled asset is not spa request", func(t *testing.T) {
+		assert.False(t, isSPARequest("assets/app.js", distFS))
+	})
+
+	t.Run("unknown path is spa request", func(t *testing.T) {
+		assert.True(t, isSPARequest("authorize", distFS))
+	})
+}
+
+func TestRateLimitOnlyForOAuth2AuthorizationPostRequest(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	distFS := fstest.MapFS{
+		"assets/app.js": &fstest.MapFile{Data: []byte("console.log('test')")},
+	}
+
+	t.Run("rate limits spa form_post request", func(t *testing.T) {
+		rateLimited := false
+		nextCalled := false
+		middleware := rateLimitOnlyForOAuth2AuthorizationPostRequest(func(c *gin.Context) {
+			rateLimited = true
+			c.Abort()
+		}, distFS)
+
+		router := gin.New()
+		router.NoRoute(
+			middleware,
+			func(c *gin.Context) {
+				nextCalled = true
+			},
+		)
+
+		recorder := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, "/authorize?response_mode=form_post&client_id=test&redirect_uri=https://example.com/callback", nil)
+		router.ServeHTTP(recorder, req)
+
+		assert.True(t, rateLimited)
+		assert.False(t, nextCalled)
+	})
+
+	t.Run("does not rate limit page request with no form_post params", func(t *testing.T) {
+		rateLimited := false
+		nextCalled := false
+		middleware := rateLimitOnlyForOAuth2AuthorizationPostRequest(func(c *gin.Context) {
+			rateLimited = true
+			c.Abort()
+		}, distFS)
+
+		router := gin.New()
+		router.NoRoute(
+			middleware,
+			func(c *gin.Context) {
+				nextCalled = true
+			},
+		)
+
+		recorder := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, "/authorize", nil)
+		router.ServeHTTP(recorder, req)
+
+		assert.False(t, rateLimited)
+		assert.True(t, nextCalled)
+	})
+
+	t.Run("does not rate limit static asset request with form_post params", func(t *testing.T) {
+		rateLimited := false
+		nextCalled := false
+		middleware := rateLimitOnlyForOAuth2AuthorizationPostRequest(func(c *gin.Context) {
+			rateLimited = true
+			c.Abort()
+		}, distFS)
+
+		router := gin.New()
+		router.NoRoute(
+			middleware,
+			func(c *gin.Context) {
+				nextCalled = true
+			},
+		)
+
+		recorder := httptest.NewRecorder()
+		req := httptest.NewRequest(http.MethodGet, "/assets/app.js?response_mode=form_post&client_id=test&redirect_uri=https://example.com/callback", nil)
+		router.ServeHTTP(recorder, req)
+
+		assert.False(t, rateLimited)
+		assert.True(t, nextCalled)
+	})
+}
diff --git a/backend/internal/bootstrap/router_bootstrap.go b/backend/internal/bootstrap/router_bootstrap.go
index 4cf7e3ea..30b3521e 100644
--- a/backend/internal/bootstrap/router_bootstrap.go
+++ b/backend/internal/bootstrap/router_bootstrap.go
@@ -40,7 +40,10 @@ func initRouter(db *gorm.DB, svc *services) (utils.Service, error) {
 	if err != nil {
 		return nil, err
 	}
-	registerRoutes(r, db, svc)
+	err = registerRoutes(r, db, svc)
+	if err != nil {
+		return nil, err
+	}
 
 	serverConfig, err := initServer(r)
 	if err != nil {
@@ -70,15 +73,6 @@ func initEngine() (*gin.Engine, error) {
 	configureEngine(r)
 	registerGlobalMiddleware(r)
 
-	frontendRateLimitMiddleware := middleware.NewRateLimitMiddleware().Add(rate.Every(100*time.Millisecond), 300)
-	if err := frontend.RegisterFrontend(r, frontendRateLimitMiddleware); err != nil {
-		if errors.Is(err, frontend.ErrFrontendNotIncluded) {
-			slog.Warn("Frontend is not included in the build. Skipping frontend registration.")
-			return r, nil
-		}
-		return nil, fmt.Errorf("failed to register frontend: %w", err)
-	}
-
 	return r, nil
 }
 
@@ -116,7 +110,16 @@ func registerGlobalMiddleware(r *gin.Engine) {
 	r.Use(middleware.NewErrorHandlerMiddleware().Add())
 }
 
-func registerRoutes(r *gin.Engine, db *gorm.DB, svc *services) {
+func registerRoutes(r *gin.Engine, db *gorm.DB, svc *services) error {
+
+	err := frontend.RegisterFrontend(r, svc.oidcService)
+	if errors.Is(err, frontend.ErrFrontendNotIncluded) {
+		slog.Warn("Frontend is not included in the build. Skipping frontend registration.")
+	} else if err != nil {
+		return fmt.Errorf("failed to register frontend: %w", err)
+	}
+
+	// Initialize middleware for specific routes
 	authMiddleware := middleware.NewAuthMiddleware(svc.apiKeyService, svc.userService, svc.jwtService)
 	fileSizeLimitMiddleware := middleware.NewFileSizeLimitMiddleware()
 	apiRateLimitMiddleware := middleware.NewRateLimitMiddleware().Add(rate.Every(time.Second), 100)
@@ -142,6 +145,8 @@ func registerRoutes(r *gin.Engine, db *gorm.DB, svc *services) {
 
 	// These are not rate-limited.
 	controller.NewHealthzController(r)
+
+	return nil
 }
 
 func registerTestRoutes(apiGroup *gin.RouterGroup, db *gorm.DB, svc *services) {
diff --git a/backend/internal/dto/oidc_dto.go b/backend/internal/dto/oidc_dto.go
index 4d8187e0..cc4a5b51 100644
--- a/backend/internal/dto/oidc_dto.go
+++ b/backend/internal/dto/oidc_dto.go
@@ -72,6 +72,7 @@ type AuthorizeOidcClientRequestDto struct {
 	CodeChallengeMethod   string `json:"codeChallengeMethod"`
 	ReauthenticationToken string `json:"reauthenticationToken"`
 	Prompt                string `json:"prompt"`
+	ResponseMode          string `json:"responseMode" binding:"omitempty,response_mode"`
 }
 
 type AuthorizeOidcClientResponseDto struct {
diff --git a/backend/internal/dto/validations.go b/backend/internal/dto/validations.go
index 8408df0f..2380775c 100644
--- a/backend/internal/dto/validations.go
+++ b/backend/internal/dto/validations.go
@@ -47,6 +47,9 @@ func init() {
 		"callback_url_pattern": func(fl validator.FieldLevel) bool {
 			return ValidateCallbackURLPattern(fl.Field().String())
 		},
+		"response_mode": func(fl validator.FieldLevel) bool {
+			return ValidateResponseMode(fl.Field().String())
+		},
 	}
 	for k, v := range validators {
 		err := engine.RegisterValidation(k, v)
@@ -87,3 +90,17 @@ func ValidateCallbackURLPattern(raw string) bool {
 	err := utils.ValidateCallbackURLPattern(raw)
 	return err == nil
 }
+
+// ValidateResponseMode validates response_mode parameter
+// If responseMode is present, it must be "form_post" or "query"
+// Empty responseMode is allowed (field not provided, use default)
+func ValidateResponseMode(responseMode string) bool {
+	switch responseMode {
+	case "form_post", "query":
+		return true
+	case "":
+		return true
+	default:
+		return false
+	}
+}
diff --git a/backend/internal/dto/validations_test.go b/backend/internal/dto/validations_test.go
index 9ef2af78..5f9d595d 100644
--- a/backend/internal/dto/validations_test.go
+++ b/backend/internal/dto/validations_test.go
@@ -58,6 +58,25 @@ func TestValidateClientID(t *testing.T) {
 	}
 }
 
+func TestValidateResponseMode(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected bool
+	}{
+		{"valid form_post", "form_post", true},
+		{"valid query", "query", true},
+		{"valid empty", "", true},
+		{"invalid fragment", "fragment", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.expected, ValidateResponseMode(tt.input))
+		})
+	}
+}
+
 func TestValidateCallbackURL(t *testing.T) {
 	tests := []struct {
 		name     string
diff --git a/backend/internal/middleware/csp_middleware.go b/backend/internal/middleware/csp_middleware.go
index b39b13f1..ae64528e 100644
--- a/backend/internal/middleware/csp_middleware.go
+++ b/backend/internal/middleware/csp_middleware.go
@@ -3,6 +3,7 @@ package middleware
 import (
 	"crypto/rand"
 	"encoding/base64"
+	"strings"
 
 	"github.com/gin-gonic/gin"
 )
@@ -28,22 +29,39 @@ func (m *CspMiddleware) Add() gin.HandlerFunc {
 		// Generate a random base64 nonce for this request
 		nonce := generateNonce()
 		c.Set("csp_nonce", nonce)
+		c.Writer.Header().Set("Content-Security-Policy", BuildCSP(nonce))
 
-		csp := "default-src 'self'; " +
-			"base-uri 'self'; " +
-			"object-src 'none'; " +
-			"frame-ancestors 'none'; " +
-			"form-action 'self'; " +
-			"img-src * blob:;" +
-			"font-src 'self'; " +
-			"style-src 'self' 'unsafe-inline'; " +
-			"script-src 'self' 'nonce-" + nonce + "'"
-
-		c.Writer.Header().Set("Content-Security-Policy", csp)
 		c.Next()
 	}
 }
 
+func BuildCSP(nonce string, formActionExtra ...string) string {
+	formAction := "'self'"
+
+	if len(formActionExtra) > 0 {
+		b := strings.Builder{}
+
+		for _, extra := range formActionExtra {
+			if extra != "" {
+				b.WriteByte(' ')
+				b.WriteString(extra)
+			}
+		}
+
+		formAction += b.String()
+	}
+
+	return "default-src 'self'; " +
+		"base-uri 'self'; " +
+		"object-src 'none'; " +
+		"frame-ancestors 'none'; " +
+		"form-action " + formAction + "; " +
+		"img-src * blob:;" +
+		"font-src 'self'; " +
+		"style-src 'self' 'unsafe-inline'; " +
+		"script-src 'self' 'nonce-" + nonce + "'"
+}
+
 func generateNonce() string {
 	b := make([]byte, 16)
 	if _, err := rand.Read(b); err != nil {
diff --git a/backend/internal/middleware/csp_middleware_test.go b/backend/internal/middleware/csp_middleware_test.go
new file mode 100644
index 00000000..67f90464
--- /dev/null
+++ b/backend/internal/middleware/csp_middleware_test.go
@@ -0,0 +1,24 @@
+package middleware
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBuildCSP(t *testing.T) {
+	t.Run("uses self form action by default", func(t *testing.T) {
+		csp := BuildCSP("test-nonce")
+
+		assert.Contains(t, csp, "form-action 'self';")
+		assert.Contains(t, csp, "script-src 'self' 'nonce-test-nonce'")
+	})
+
+	t.Run("adds validated form action targets", func(t *testing.T) {
+		csp := BuildCSP("test-nonce", "https://example.com/callback")
+
+		assert.Contains(t, csp, "form-action 'self' https://example.com/callback;")
+		assert.Equal(t, 1, strings.Count(csp, "form-action"))
+	})
+}
diff --git a/backend/internal/service/oidc_service.go b/backend/internal/service/oidc_service.go
index 6c09cb8c..53824588 100644
--- a/backend/internal/service/oidc_service.go
+++ b/backend/internal/service/oidc_service.go
@@ -754,6 +754,27 @@ func (s *OidcService) GetClient(ctx context.Context, clientID string) (model.Oid
 	return s.getClientInternal(ctx, clientID, s.db, false)
 }
 
+func (s *OidcService) ResolveAllowedCallbackURL(ctx context.Context, clientID, inputCallbackURL string) (string, error) {
+	client, err := s.GetClient(ctx, clientID)
+	if err != nil {
+		return "", err
+	}
+
+	if inputCallbackURL == "" || len(client.CallbackURLs) == 0 {
+		return "", &common.OidcMissingCallbackURLError{}
+	}
+
+	matched, err := utils.GetCallbackURLFromList(client.CallbackURLs, inputCallbackURL)
+	if err != nil {
+		return "", err
+	}
+	if matched == "" {
+		return "", &common.OidcInvalidCallbackURLError{}
+	}
+
+	return matched, nil
+}
+
 func (s *OidcService) getClientInternal(ctx context.Context, clientID string, tx *gorm.DB, forUpdate bool) (model.OidcClient, error) {
 	var client model.OidcClient
 	q := tx.
diff --git a/frontend/src/lib/services/oidc-service.ts b/frontend/src/lib/services/oidc-service.ts
index abed878a..34579a53 100644
--- a/frontend/src/lib/services/oidc-service.ts
+++ b/frontend/src/lib/services/oidc-service.ts
@@ -23,6 +23,7 @@ class OidcService extends APIService {
 		codeChallenge?: string,
 		codeChallengeMethod?: string,
 		reauthenticationToken?: string,
+		responseMode?: string,
 		prompt?: string
 	) => {
 		const res = await this.api.post('/oidc/authorize', {
@@ -33,6 +34,7 @@ class OidcService extends APIService {
 			codeChallenge,
 			codeChallengeMethod,
 			reauthenticationToken,
+			responseMode,
 			prompt
 		});
 
diff --git a/frontend/src/routes/authorize/+page.svelte b/frontend/src/routes/authorize/+page.svelte
index e9a99ba2..e35285ae 100644
--- a/frontend/src/routes/authorize/+page.svelte
+++ b/frontend/src/routes/authorize/+page.svelte
@@ -28,7 +28,8 @@
 		codeChallenge,
 		codeChallengeMethod,
 		authorizeState,
-		prompt
+		prompt,
+		responseMode
 	} = data;
 
 	let isLoading = $state(false);
@@ -118,6 +119,7 @@
 				codeChallenge,
 				codeChallengeMethod,
 				reauthToken,
+				responseMode,
 				prompt
 			);
 
@@ -164,7 +166,46 @@
 
 		success = true;
 		setTimeout(() => {
-			window.location.href = redirectURL.toString();
+			if (responseMode === 'form_post') {
+				// Create a hidden form and submit it via POST
+				const form = document.createElement('form');
+				form.method = 'POST';
+				form.action = callbackURL;
+
+				// Add code parameter
+				const codeInput = document.createElement('input');
+				codeInput.type = 'hidden';
+				codeInput.name = 'code';
+				codeInput.value = code;
+				form.appendChild(codeInput);
+
+				// Add state parameter
+				if (authorizeState) {
+					const stateInput = document.createElement('input');
+					stateInput.type = 'hidden';
+					stateInput.name = 'state';
+					stateInput.value = authorizeState;
+					form.appendChild(stateInput);
+				}
+
+				// Add issuer parameter
+				const issInput = document.createElement('input');
+				issInput.type = 'hidden';
+				issInput.name = 'iss';
+				issInput.value = issuer;
+				form.appendChild(issInput);
+
+				document.body.appendChild(form);
+				form.submit();
+			} else {
+				// Default query parameter redirect (response_mode=query or not specified)
+				const redirectURL = new URL(callbackURL);
+				redirectURL.searchParams.append('code', code);
+				redirectURL.searchParams.append('state', authorizeState);
+				redirectURL.searchParams.append('iss', issuer);
+
+				window.location.href = redirectURL.toString();
+			}
 		}, 1000);
 	}
 </script>
@@ -196,7 +237,7 @@
 				/>
 			</p>
 		{:else if authorizationRequired}
-			<div class="w-full max-w-[450px]" transition:slide={{ duration: 300 }}>
+			<div class="w-full max-w-md" transition:slide={{ duration: 300 }}>
 				<Card.Root class="mt-6 mb-10">
 					<Card.Header>
 						<p class="text-muted-foreground text-start">
@@ -212,7 +253,7 @@
 			</div>
 		{/if}
 		<!-- Flex flow is reversed so the sign in button, which has auto-focus, is the first one in the DOM, for a11y -->
-		<div class="flex w-full max-w-[450px] flex-row-reverse gap-2">
+		<div class="flex w-full max-w-md flex-row-reverse gap-2">
 			{#if !errorMessage}
 				<Button class="flex-1" {isLoading} onclick={authorize} autofocus={true}>
 					{m.sign_in()}
diff --git a/frontend/src/routes/authorize/+page.ts b/frontend/src/routes/authorize/+page.ts
index 80e807cd..10ccae1b 100644
--- a/frontend/src/routes/authorize/+page.ts
+++ b/frontend/src/routes/authorize/+page.ts
@@ -15,6 +15,7 @@ export const load: PageLoad = async ({ url }) => {
 		client,
 		codeChallenge: url.searchParams.get('code_challenge')!,
 		codeChallengeMethod: url.searchParams.get('code_challenge_method')!,
-		prompt: url.searchParams.get('prompt') || undefined
+		prompt: url.searchParams.get('prompt') || undefined,
+		responseMode: url.searchParams.get('response_mode') || undefined
 	};
 };
diff --git a/tests/specs/oidc.spec.ts b/tests/specs/oidc.spec.ts
index 05c70f34..6ffdd68e 100644
--- a/tests/specs/oidc.spec.ts
+++ b/tests/specs/oidc.spec.ts
@@ -1,4 +1,4 @@
-import test, { expect } from '@playwright/test';
+import test, { expect, type Page, type Request } from '@playwright/test';
 import { oidcClients, refreshTokens, users } from '../data';
 import { cleanupBackend } from '../utils/cleanup.util';
 import { generateIdToken, generateOauthAccessToken } from '../utils/jwt.util';
@@ -625,6 +625,49 @@ test('Forces reauthentication when client requires it', async ({ page, request }
 	expect(webauthnStartCalled).toBe(true);
 });
 
+test('Authorize existing client while not signed in with response_mode=form_post', async ({
+	page
+}) => {
+	const oidcClient = oidcClients.nextcloud;
+	const urlParams = createUrlParams(oidcClient);
+	urlParams.set('response_mode', 'form_post');
+	await page.context().clearCookies();
+
+	const formPostRequestPromise = waitForFormPostRequest(page, oidcClient.callbackUrl);
+	await page.goto(`/authorize?${urlParams.toString()}`);
+
+	await (await passkeyUtil.init(page)).addPasskey();
+	await page.getByRole('button', { name: 'Sign in' }).click();
+
+	await expectFormPostRequest(formPostRequestPromise);
+});
+
+test('Authorize existing client with response_mode=form_post', async ({ page }) => {
+	const oidcClient = oidcClients.nextcloud;
+	const urlParams = createUrlParams(oidcClient);
+	urlParams.set('response_mode', 'form_post');
+
+	const formPostRequestPromise = waitForFormPostRequest(page, oidcClient.callbackUrl);
+	await page.goto(`/authorize?${urlParams.toString()}`);
+
+	await expectFormPostRequest(formPostRequestPromise);
+});
+
+function waitForFormPostRequest(page: Page, callbackUrl: string): Promise<Request> {
+	return page.waitForRequest(
+		(request) => request.method() === 'POST' && request.url() === callbackUrl
+	);
+}
+
+async function expectFormPostRequest(formPostRequestPromise: Promise<Request>) {
+	const request = await formPostRequestPromise;
+	const formData = new URLSearchParams(request.postData() ?? '');
+
+	expect(formData.get('code')).toBeTruthy();
+	expect(formData.get('state')).toBe('nXx-6Qr-owc1SHBa');
+	expect(formData.get('iss')).toBeTruthy();
+}
+
 test.describe('OIDC prompt parameter', () => {
 	test('prompt=none redirects with login_required when user not authenticated', async ({
 		page
@@ -783,4 +826,4 @@ test.describe('OIDC prompt parameter', () => {
 		expect(redirectUrl.searchParams.get('error')).toBe('interaction_required');
 		expect(redirectUrl.searchParams.get('state')).toBe('nXx-6Qr-owc1SHBa');
 	});
-});
+});
\ No newline at end of file