diff --git a/Cargo.toml b/Cargo.toml
index ce5431d1d..4a4909c76 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -4,3 +4,5 @@ members = [
   "oauth2-types",
   "matrix-authentication-service",
 ]
+
+resolver = "2"
diff --git a/matrix-authentication-service/src/config/oauth2.rs b/matrix-authentication-service/src/config/oauth2.rs
index d83e35930..015411b50 100644
--- a/matrix-authentication-service/src/config/oauth2.rs
+++ b/matrix-authentication-service/src/config/oauth2.rs
@@ -289,7 +289,7 @@ pub struct OAuth2ClientConfig {
     pub client_secret: Option<String>,
 
     #[serde(default)]
-    pub redirect_uris: Option<Vec<Url>>,
+    pub redirect_uris: Vec<Url>,
 }
 
 #[derive(Debug, Error)]
@@ -301,19 +301,20 @@ impl OAuth2ClientConfig {
         &'a self,
         suggested_uri: &'a Option<Url>,
     ) -> Result<&'a Url, InvalidRedirectUriError> {
-        match (suggested_uri, &self.redirect_uris) {
-            (None, None) => Err(InvalidRedirectUriError),
-            (None, Some(redirect_uris)) => {
-                redirect_uris.iter().next().ok_or(InvalidRedirectUriError)
-            }
-            (Some(suggested_uri), None) => Ok(suggested_uri),
-            (Some(suggested_uri), Some(redirect_uris)) => {
-                if redirect_uris.contains(suggested_uri) {
-                    Ok(suggested_uri)
-                } else {
-                    Err(InvalidRedirectUriError)
-                }
-            }
+        suggested_uri.as_ref().map_or_else(
+            || self.redirect_uris.get(0).ok_or(InvalidRedirectUriError),
+            |suggested_uri| self.check_redirect_uri(suggested_uri),
+        )
+    }
+
+    pub fn check_redirect_uri<'a>(
+        &self,
+        redirect_uri: &'a Url,
+    ) -> Result<&'a Url, InvalidRedirectUriError> {
+        if self.redirect_uris.contains(redirect_uri) {
+            Ok(redirect_uri)
+        } else {
+            Err(InvalidRedirectUriError)
         }
     }
 }
@@ -464,11 +465,11 @@ mod tests {
             assert_eq!(config.clients[0].client_id, "hello");
             assert_eq!(
                 config.clients[0].redirect_uris,
-                Some(vec!["https://exemple.fr/callback".parse().unwrap()])
+                vec!["https://exemple.fr/callback".parse().unwrap()]
             );
 
             assert_eq!(config.clients[1].client_id, "world");
-            assert_eq!(config.clients[1].redirect_uris, None);
+            assert_eq!(config.clients[1].redirect_uris, Vec::new());
 
             Ok(())
         });
diff --git a/matrix-authentication-service/src/handlers/oauth2/authorization.rs b/matrix-authentication-service/src/handlers/oauth2/authorization.rs
index deea237ea..c4c2c1c4c 100644
--- a/matrix-authentication-service/src/handlers/oauth2/authorization.rs
+++ b/matrix-authentication-service/src/handlers/oauth2/authorization.rs
@@ -18,7 +18,6 @@ use std::{
 };
 
 use chrono::Duration;
-use data_encoding::BASE64URL_NOPAD;
 use hyper::{
     header::LOCATION,
     http::uri::{Parts, PathAndQuery, Uri},
@@ -26,18 +25,21 @@ use hyper::{
 };
 use itertools::Itertools;
 use oauth2_types::{
+    errors::{ErrorResponse, InvalidRequest, OAuth2Error},
     pkce,
     requests::{
         AccessTokenResponse, AuthorizationRequest, AuthorizationResponse, ResponseMode,
         ResponseType,
     },
 };
-use rand::thread_rng;
+use rand::{distributions::Alphanumeric, thread_rng, Rng};
 use serde::{Deserialize, Serialize};
+use serde_json::Value;
 use sqlx::{PgPool, Postgres, Transaction};
 use url::Url;
 use warp::{
     redirect::see_other,
+    reject::InvalidQuery,
     reply::{html, with_header},
     Filter, Rejection, Reply,
 };
@@ -63,6 +65,26 @@ use crate::{
     tokens,
 };
 
+#[derive(Deserialize)]
+struct PartialParams {
+    client_id: Option<String>,
+    redirect_uri: Option<String>,
+    /*
+    response_type: Option<String>,
+    response_mode: Option<String>,
+    */
+}
+
+enum ReplyOrBackToClient {
+    Reply(Box<dyn Reply>),
+    BackToClient {
+        params: Value,
+        redirect_uri: Url,
+        response_mode: ResponseMode,
+    },
+    Error(Box<dyn OAuth2Error>),
+}
+
 fn back_to_client<T>(
     mut redirect_uri: Url,
     response_mode: ResponseMode,
@@ -173,7 +195,6 @@ pub fn filter(
         .and(warp::query())
         .and(with_optional_session(pool, cookies_config))
         .and(with_transaction(pool))
-        .and(with_templates(templates))
         .and_then(get);
 
     let step = warp::path!("oauth2" / "authorize" / "step")
@@ -181,10 +202,81 @@ pub fn filter(
         .and(warp::query().map(|s: StepRequest| s.id))
         .and(with_session(pool, cookies_config))
         .and(with_transaction(pool))
-        .and(with_templates(templates))
         .and_then(step);
 
-    authorize.or(step)
+    let clients = oauth2_config.clients.clone();
+    authorize
+        .or(step)
+        .unify()
+        .recover(recover)
+        .unify()
+        .and(warp::query())
+        .and(warp::any().map(move || clients.clone()))
+        .and(with_templates(templates))
+        .and_then(actually_reply)
+}
+
+async fn recover(rejection: Rejection) -> Result<ReplyOrBackToClient, Rejection> {
+    if rejection.find::<InvalidQuery>().is_some() {
+        Ok(ReplyOrBackToClient::Error(Box::new(InvalidRequest)))
+    } else {
+        Err(rejection)
+    }
+}
+
+async fn actually_reply(
+    rep: ReplyOrBackToClient,
+    q: PartialParams,
+    clients: Vec<OAuth2ClientConfig>,
+    templates: Templates,
+) -> Result<impl Reply, Rejection> {
+    let (redirect_uri, response_mode, params) = match rep {
+        ReplyOrBackToClient::Reply(r) => return Ok(r),
+        ReplyOrBackToClient::BackToClient {
+            redirect_uri,
+            response_mode,
+            params,
+        } => (redirect_uri, response_mode, params),
+        ReplyOrBackToClient::Error(error) => {
+            let PartialParams {
+                client_id,
+                redirect_uri,
+                ..
+            } = q;
+
+            // First, disover the client
+            let client = client_id.and_then(|client_id| {
+                clients
+                    .into_iter()
+                    .find(|client| client.client_id == client_id)
+            });
+
+            let client = match client {
+                Some(client) => client,
+                None => return Ok(Box::new(html(templates.render_error(&error.into())?))),
+            };
+
+            let redirect_uri: Result<Option<Url>, _> = redirect_uri.map(|r| r.parse()).transpose();
+            let redirect_uri = match redirect_uri {
+                Ok(r) => r,
+                Err(_) => return Ok(Box::new(html(templates.render_error(&error.into())?))),
+            };
+
+            let redirect_uri = client.resolve_redirect_uri(&redirect_uri);
+            let redirect_uri = match redirect_uri {
+                Ok(r) => r,
+                Err(_) => return Ok(Box::new(html(templates.render_error(&error.into())?))),
+            };
+
+            let reply: ErrorResponse = error.into();
+            let reply = serde_json::to_value(&reply).wrap_error()?;
+            // TODO: resolve response mode
+            (redirect_uri.clone(), ResponseMode::Query, reply)
+        }
+    };
+
+    // TODO: we should include the state param in errors
+    back_to_client(redirect_uri, response_mode, params, &templates).wrap_error()
 }
 
 async fn get(
@@ -192,8 +284,7 @@ async fn get(
     params: Params,
     maybe_session: Option<SessionInfo>,
     mut txn: Transaction<'_, Postgres>,
-    templates: Templates,
-) -> Result<Box<dyn Reply>, Rejection> {
+) -> Result<ReplyOrBackToClient, Rejection> {
     // First, find out what client it is
     let client = clients
         .into_iter()
@@ -201,11 +292,6 @@ async fn get(
         .ok_or_else(|| anyhow::anyhow!("could not find client"))
         .wrap_error()?;
 
-    // Then, figure out the redirect URI
-    let redirect_uri = client
-        .resolve_redirect_uri(&params.auth.redirect_uri)
-        .wrap_error()?;
-
     let maybe_session_id = maybe_session.as_ref().map(SessionInfo::key);
 
     let scope: String = {
@@ -213,6 +299,9 @@ async fn get(
         Itertools::intersperse(it, " ".to_string()).collect()
     };
 
+    let redirect_uri = client
+        .resolve_redirect_uri(&params.auth.redirect_uri)
+        .wrap_error()?;
     let response_type = &params.auth.response_type;
     let response_mode =
         resolve_response_mode(response_type, params.auth.response_mode).wrap_error()?;
@@ -234,9 +323,13 @@ async fn get(
 
     // Generate the code at this stage, since we have the PKCE params ready
     if response_type.contains(&ResponseType::Code) {
-        // 192bit random bytes encoded in base64, which gives a 32 character code
-        let code: [u8; 24] = rand::random();
-        let code = BASE64URL_NOPAD.encode(&code);
+        // 32 random alphanumeric characters, about 190bit of entropy
+        let code: String = thread_rng()
+            .sample_iter(&Alphanumeric)
+            .take(32)
+            .map(char::from)
+            .collect();
+
         oauth2_session
             .add_code(&mut txn, &code, &params.pkce)
             .await
@@ -247,7 +340,7 @@ async fn get(
     let user_session = oauth2_session.fetch_session(&mut txn).await.wrap_error()?;
 
     if let Some(user_session) = user_session {
-        step(oauth2_session.id, user_session, txn, templates).await
+        step(oauth2_session.id, user_session, txn).await
     } else {
         // If not, redirect the user to the login page
         txn.commit().await.wrap_error()?;
@@ -258,7 +351,7 @@ async fn get(
             .to_string();
 
         let destination = LoginRequest::new(Some(next)).build_uri().wrap_error()?;
-        Ok(Box::new(see_other(destination)))
+        Ok(ReplyOrBackToClient::Reply(Box::new(see_other(destination))))
     }
 }
 
@@ -288,8 +381,7 @@ async fn step(
     oauth2_session_id: i64,
     user_session: SessionInfo,
     mut txn: Transaction<'_, Postgres>,
-    templates: Templates,
-) -> Result<Box<dyn Reply>, Rejection> {
+) -> Result<ReplyOrBackToClient, Rejection> {
     let mut oauth2_session = get_session_by_id(&mut txn, oauth2_session_id)
         .await
         .wrap_error()?;
@@ -350,11 +442,16 @@ async fn step(
             todo!("id tokens are not implemented yet");
         }
 
-        back_to_client(redirect_uri, response_mode, params, &templates).wrap_error()?
+        let params = serde_json::to_value(&params).unwrap();
+        ReplyOrBackToClient::BackToClient {
+            redirect_uri,
+            response_mode,
+            params,
+        }
     } else {
         // Ask for a reauth
         // TODO: have the OAuth2 session ID in there
-        Box::new(see_other(Uri::from_static("/reauth")))
+        ReplyOrBackToClient::Reply(Box::new(see_other(Uri::from_static("/reauth"))))
     };
 
     txn.commit().await.wrap_error()?;
diff --git a/matrix-authentication-service/src/storage/oauth2/session.rs b/matrix-authentication-service/src/storage/oauth2/session.rs
index d620e7f38..18b30bc0d 100644
--- a/matrix-authentication-service/src/storage/oauth2/session.rs
+++ b/matrix-authentication-service/src/storage/oauth2/session.rs
@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use std::{collections::HashSet, convert::TryFrom, str::FromStr};
+use std::{collections::HashSet, convert::TryFrom, str::FromStr, string::ToString};
 
 use anyhow::Context;
 use chrono::{DateTime, Duration, Utc};
@@ -34,7 +34,7 @@ use super::{
 pub struct OAuth2Session {
     pub id: i64,
     user_session_id: Option<i64>,
-    client_id: String,
+    pub client_id: String,
     redirect_uri: String,
     scope: String,
     pub state: Option<String>,
@@ -143,6 +143,7 @@ pub async fn start_session(
     // Checked convertion of duration to i32, maxing at i32::MAX
     let max_age = max_age.map(|d| i32::try_from(d.num_seconds()).unwrap_or(i32::MAX));
     let response_mode = response_mode.to_string();
+    let redirect_uri = redirect_uri.to_string();
     let response_type: String = {
         let it = response_type.iter().map(ToString::to_string);
         Itertools::intersperse(it, " ".to_string()).collect()
@@ -162,7 +163,7 @@ pub async fn start_session(
         "#,
         optional_session_id,
         client_id,
-        redirect_uri.as_str(),
+        redirect_uri,
         scope,
         state,
         nonce,
diff --git a/matrix-authentication-service/src/templates.rs b/matrix-authentication-service/src/templates.rs
index 399e9392f..754ad9961 100644
--- a/matrix-authentication-service/src/templates.rs
+++ b/matrix-authentication-service/src/templates.rs
@@ -14,6 +14,7 @@
 
 use std::{collections::HashSet, string::ToString, sync::Arc};
 
+use oauth2_types::errors::OAuth2Error;
 use serde::Serialize;
 use tera::{Context, Error as TeraError, Tera};
 use thiserror::Error;
@@ -146,6 +147,9 @@ register_templates! {
 
     /// Render the form used by the form_post response mode
     pub fn render_form_post<T: Serialize>(FormPostContext<T>) { "form_post.html" }
+
+    /// Render the HTML error page
+    pub fn render_error(ErrorContext) { "error.html" }
 }
 
 /// Helper trait to construct context wrappers
@@ -254,3 +258,42 @@ impl<T> FormPostContext<T> {
         }
     }
 }
+
+#[derive(Default, Serialize)]
+pub struct ErrorContext {
+    code: Option<&'static str>,
+    description: Option<String>,
+    details: Option<String>,
+}
+
+impl ErrorContext {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn with_code(mut self, code: &'static str) -> Self {
+        self.code = Some(code);
+        self
+    }
+
+    pub fn with_description(mut self, description: String) -> Self {
+        self.description = Some(description);
+        self
+    }
+
+    #[allow(dead_code)]
+    pub fn with_details(mut self, details: String) -> Self {
+        self.details = Some(details);
+        self
+    }
+}
+
+impl From<Box<dyn OAuth2Error>> for ErrorContext {
+    fn from(err: Box<dyn OAuth2Error>) -> Self {
+        let mut ctx = ErrorContext::new().with_code(err.error());
+        if let Some(desc) = err.description() {
+            ctx = ctx.with_description(desc);
+        }
+        ctx
+    }
+}
diff --git a/matrix-authentication-service/src/tokens.rs b/matrix-authentication-service/src/tokens.rs
index 5c189212f..7e88c864c 100644
--- a/matrix-authentication-service/src/tokens.rs
+++ b/matrix-authentication-service/src/tokens.rs
@@ -91,7 +91,6 @@ pub enum TokenFormatError {
     InvalidCrc { expected: String, got: String },
 }
 
-#[allow(dead_code)]
 pub fn check(token: &str) -> Result<TokenType, TokenFormatError> {
     let split: Vec<&str> = token.split('_').collect();
     let [prefix, random_part, crc]: [&str; 3] = split
diff --git a/oauth2-types/src/errors.rs b/oauth2-types/src/errors.rs
index 92d588510..a7d6cf2f4 100644
--- a/oauth2-types/src/errors.rs
+++ b/oauth2-types/src/errors.rs
@@ -16,7 +16,7 @@ use http::status::StatusCode;
 use serde::ser::{Serialize, SerializeMap};
 use url::Url;
 
-pub trait OAuth2Error: std::fmt::Debug {
+pub trait OAuth2Error: std::fmt::Debug + Send + Sync {
     /// A single ASCII error code.
     ///
     /// Maps to the required "error" field.
@@ -41,11 +41,11 @@ pub trait OAuth2Error: std::fmt::Debug {
     }
 
     /// Wraps the error with an `ErrorResponse` to help serializing.
-    fn into_response(self) -> ErrorResponse<Self>
+    fn into_response(self) -> ErrorResponse
     where
-        Self: Sized,
+        Self: Sized + 'static,
     {
-        ErrorResponse(self)
+        ErrorResponse(Box::new(self))
     }
 }
 
@@ -55,15 +55,15 @@ trait OAuth2ErrorCode: OAuth2Error {
 }
 
 #[derive(Debug)]
-pub struct ErrorResponse<T: OAuth2Error>(T);
+pub struct ErrorResponse(Box<dyn OAuth2Error>);
 
-impl<T: OAuth2ErrorCode> OAuth2ErrorCode for ErrorResponse<T> {
-    fn status(&self) -> StatusCode {
-        self.0.status()
+impl From<Box<dyn OAuth2Error>> for ErrorResponse {
+    fn from(b: Box<dyn OAuth2Error>) -> Self {
+        Self(b)
     }
 }
 
-impl<T: OAuth2Error> OAuth2Error for ErrorResponse<T> {
+impl OAuth2Error for ErrorResponse {
     fn error(&self) -> &'static str {
         self.0.error()
     }
@@ -77,7 +77,7 @@ impl<T: OAuth2Error> OAuth2Error for ErrorResponse<T> {
     }
 }
 
-impl<T: OAuth2Error> Serialize for ErrorResponse<T> {
+impl Serialize for ErrorResponse {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where
         S: serde::Serializer,