diff --git a/crates/gateway/src/channel.rs b/crates/gateway/src/channel.rs
index ffc049601a..f3d8df2973 100644
--- a/crates/gateway/src/channel.rs
+++ b/crates/gateway/src/channel.rs
@@ -815,6 +815,12 @@ impl ChannelService for LiveChannelService {
             return Err("redirect_uri is required".into());
         }
 
+        tracing::debug!(
+            account_id = %account_id,
+            redirect_uri = %redirect_uri,
+            "channels.oauth_start called"
+        );
+
         // Merge caller-provided config (ownership_mode, policies, etc.) with
         // the required OIDC fields.
         let mut config = params
diff --git a/crates/matrix/src/oidc.rs b/crates/matrix/src/oidc.rs
index f378d706c6..1c756bf10b 100644
--- a/crates/matrix/src/oidc.rs
+++ b/crates/matrix/src/oidc.rs
@@ -5,7 +5,7 @@
 //! (`Client::oauth()`), which handles PKCE, dynamic client registration,
 //! token exchange, and automatic refresh.
 
-use std::path::PathBuf;
+use std::{error::Error as StdError, fmt, path::PathBuf};
 
 use {
     matrix_sdk::{
@@ -20,7 +20,7 @@ use {
     moltis_common::secret_serde,
     secrecy::{ExposeSecret, Secret},
     serde::{Deserialize, Serialize},
-    tracing::{info, instrument, warn},
+    tracing::{debug, info, instrument, warn},
     url::Url,
 };
 
@@ -52,8 +52,8 @@ struct PersistedOidcSession {
     refresh_token: Option<Secret<String>>,
 }
 
-impl std::fmt::Debug for PersistedOidcSession {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl fmt::Debug for PersistedOidcSession {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("PersistedOidcSession")
             .field("client_id", &self.client_id)
             .field("user_id", &self.user_id)
@@ -150,6 +150,85 @@ async fn load_oidc_session(account_id: &str) -> ChannelResult<Option<PersistedOi
 /// MAS validates this URL and rejects loopback addresses.
 const MOLTIS_CLIENT_URI: &str = "https://moltis.org/";
 
+#[derive(Clone, Debug, Eq, PartialEq)]
+struct ClientRegistrationDiagnostics {
+    original_redirect_uri: String,
+    registration_redirect_uri: String,
+    is_loopback: bool,
+    application_type: String,
+    issuer: String,
+    registration_endpoint: Option<String>,
+    client_metadata_json: String,
+}
+
+impl ClientRegistrationDiagnostics {
+    fn new(
+        original_redirect_uri: &Url,
+        registration_redirect_uri: &Url,
+        metadata: &ClientMetadata,
+        issuer: &Url,
+        registration_endpoint: Option<&Url>,
+        raw_metadata: &Raw<ClientMetadata>,
+    ) -> Self {
+        Self {
+            original_redirect_uri: original_redirect_uri.to_string(),
+            registration_redirect_uri: registration_redirect_uri.to_string(),
+            is_loopback: is_loopback_uri(original_redirect_uri),
+            application_type: metadata.application_type.to_string(),
+            issuer: issuer.to_string(),
+            registration_endpoint: registration_endpoint.map(ToString::to_string),
+            client_metadata_json: raw_metadata.json().get().to_string(),
+        }
+    }
+}
+
+impl fmt::Display for ClientRegistrationDiagnostics {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "original_redirect_uri={}, registration_redirect_uri={}, is_loopback={}, \
+             application_type={}, issuer={}, registration_endpoint={}, client_metadata_json={}",
+            self.original_redirect_uri,
+            self.registration_redirect_uri,
+            self.is_loopback,
+            self.application_type,
+            self.issuer,
+            self.registration_endpoint.as_deref().unwrap_or("none"),
+            self.client_metadata_json
+        )
+    }
+}
+
+#[derive(Debug)]
+struct ClientRegistrationFailure {
+    diagnostics: ClientRegistrationDiagnostics,
+    source: Box<dyn StdError + Send + Sync>,
+}
+
+impl ClientRegistrationFailure {
+    fn new(
+        diagnostics: ClientRegistrationDiagnostics,
+        source: impl StdError + Send + Sync + 'static,
+    ) -> Self {
+        Self {
+            diagnostics,
+            source: Box::new(source),
+        }
+    }
+}
+
+impl fmt::Display for ClientRegistrationFailure {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}; diagnostics: {}", self.source, self.diagnostics)
+    }
+}
+
+impl StdError for ClientRegistrationFailure {
+    fn source(&self) -> Option<&(dyn StdError + 'static)> {
+        Some(self.source.as_ref())
+    }
+}
+
 fn is_loopback_uri(uri: &Url) -> bool {
     let host = uri.host_str().unwrap_or_default();
     if host == "localhost" || host == "::1" || host.ends_with(".localhost") {
@@ -177,22 +256,18 @@ fn normalize_loopback_redirect(redirect_uri: &Url) -> Url {
     }
 }
 
+/// Build OIDC client metadata for dynamic registration.
+///
+/// `redirect_uri` must already be normalized (loopback https -> http) via
+/// [`normalize_loopback_redirect`] before calling this.
 fn build_client_metadata(redirect_uri: &Url) -> ChannelResult<ClientMetadata> {
     let client_uri_url: Url = MOLTIS_CLIENT_URI
         .parse()
         .map_err(|error| ChannelError::external("matrix oidc parse client uri", error))?;
     let client_uri = Localized::new(client_uri_url, std::iter::empty());
-    let is_loopback = is_loopback_uri(redirect_uri);
-    let registration_redirect = if is_loopback && redirect_uri.scheme() == "https" {
-        let mut normalized = redirect_uri.clone();
-        let _ = normalized.set_scheme("http");
-        normalized
-    } else {
-        redirect_uri.clone()
-    };
     // MAS requires `Native` for loopback redirect URIs (RFC 8252) and `Web`
     // for non-loopback URIs (e.g. behind a reverse proxy).
-    let app_type = if is_loopback {
+    let app_type = if is_loopback_uri(redirect_uri) {
         ApplicationType::Native
     } else {
         ApplicationType::Web
@@ -200,7 +275,7 @@ fn build_client_metadata(redirect_uri: &Url) -> ChannelResult<ClientMetadata> {
     Ok(ClientMetadata::new(
         app_type,
         vec![OAuthGrantType::AuthorizationCode {
-            redirect_uris: vec![registration_redirect],
+            redirect_uris: vec![redirect_uri.clone()],
         }],
         client_uri,
     ))
@@ -218,16 +293,50 @@ pub(crate) async fn start_oidc_login(
     device_id: Option<&str>,
 ) -> ChannelResult<OidcLoginPending> {
     // Verify the homeserver supports OIDC.
-    client
-        .oauth()
-        .server_metadata()
-        .await
-        .map_err(|error| ChannelError::external("matrix oidc server metadata discovery", error))?;
+    let server_metadata =
+        client.oauth().server_metadata().await.map_err(|error| {
+            ChannelError::external("matrix oidc server metadata discovery", error)
+        })?;
+
+    debug!(
+        account_id,
+        issuer = %server_metadata.issuer,
+        registration_endpoint = ?server_metadata.registration_endpoint,
+        "matrix OIDC server metadata discovered"
+    );
 
     let registration_redirect = normalize_loopback_redirect(redirect_uri);
-    let metadata = build_client_metadata(redirect_uri)?;
+    let metadata = build_client_metadata(&registration_redirect)?;
+
+    let is_loopback = is_loopback_uri(redirect_uri);
+    debug!(
+        account_id,
+        original_redirect_uri = %redirect_uri,
+        registration_redirect_uri = %registration_redirect,
+        is_loopback,
+        application_type = ?metadata.application_type,
+        "matrix OIDC client registration parameters"
+    );
+
     let raw_metadata: Raw<ClientMetadata> = Raw::new(&metadata)
         .map_err(|error| ChannelError::external("matrix oidc serialize client metadata", error))?;
+    let diagnostics = ClientRegistrationDiagnostics::new(
+        redirect_uri,
+        &registration_redirect,
+        &metadata,
+        &server_metadata.issuer,
+        server_metadata.registration_endpoint.as_ref(),
+        &raw_metadata,
+    );
+
+    // Log the serialized metadata so operators can see exactly what is sent
+    // to the MAS registration endpoint.
+    debug!(
+        account_id,
+        client_metadata = %diagnostics.client_metadata_json,
+        "matrix OIDC client metadata for dynamic registration"
+    );
+
     let registration_data = ClientRegistrationData::new(raw_metadata);
 
     let device_id_owned = device_id
@@ -247,7 +356,25 @@ pub(crate) async fn start_oidc_login(
         )
         .build()
         .await
-        .map_err(|error| ChannelError::external("matrix oidc authorization code build", error))?;
+        .map_err(|error| {
+            let failure = ClientRegistrationFailure::new(diagnostics.clone(), error);
+            warn!(
+                account_id,
+                original_redirect_uri = %diagnostics.original_redirect_uri,
+                registration_redirect_uri = %diagnostics.registration_redirect_uri,
+                is_loopback = diagnostics.is_loopback,
+                application_type = %diagnostics.application_type,
+                issuer = %diagnostics.issuer,
+                registration_endpoint = %diagnostics.registration_endpoint.as_deref().unwrap_or("none"),
+                client_metadata = %diagnostics.client_metadata_json,
+                error = %failure,
+                error_debug = ?failure,
+                "matrix OIDC client registration failed, \
+                 check that redirect_uri is reachable and the homeserver's \
+                 MAS allows dynamic client registration with this URI"
+            );
+            ChannelError::external("matrix oidc authorization code build", failure)
+        })?;
 
     info!(account_id, auth_url = %url, "matrix OIDC login started");
 
@@ -398,7 +525,18 @@ fn spawn_session_persistence_task(client: &Client, account_id: &str) {
 
 #[cfg(test)]
 mod tests {
-    use super::*;
+    use {
+        crate::oidc::{
+            ClientRegistrationDiagnostics, MOLTIS_CLIENT_URI, PersistedOidcSession,
+            build_client_metadata, is_loopback_uri, normalize_loopback_redirect, oidc_session_path,
+        },
+        matrix_sdk::{
+            authentication::oauth::registration::{ApplicationType, OAuthGrantType},
+            ruma::serde::Raw,
+        },
+        secrecy::{ExposeSecret, Secret},
+        url::Url,
+    };
 
     #[test]
     fn oidc_session_path_returns_expected_path() {
@@ -437,11 +575,13 @@ mod tests {
     }
 
     #[test]
-    fn build_client_metadata_normalizes_loopback_and_uses_project_client_uri() {
+    fn build_client_metadata_uses_pre_normalized_loopback_and_project_client_uri() {
+        // Caller must normalize before passing to build_client_metadata.
         let redirect: Url = "https://localhost:52979/auth/callback"
             .parse()
             .unwrap_or_else(|error| panic!("{error}"));
-        let metadata = build_client_metadata(&redirect).unwrap_or_else(|error| panic!("{error}"));
+        let normalized = normalize_loopback_redirect(&redirect);
+        let metadata = build_client_metadata(&normalized).unwrap_or_else(|error| panic!("{error}"));
         match &metadata.grant_types[0] {
             OAuthGrantType::AuthorizationCode { redirect_uris } => {
                 assert_eq!(
@@ -528,6 +668,99 @@ mod tests {
         );
     }
 
+    #[test]
+    fn registration_diagnostics_include_metadata_json_and_redirect_context() {
+        let original_redirect: Url = "https://localhost:52979/auth/callback"
+            .parse()
+            .unwrap_or_else(|error| panic!("{error}"));
+        let registration_redirect = normalize_loopback_redirect(&original_redirect);
+        let metadata =
+            build_client_metadata(&registration_redirect).unwrap_or_else(|error| panic!("{error}"));
+        let raw_metadata =
+            Raw::new(&metadata).unwrap_or_else(|error| panic!("raw metadata: {error}"));
+        let issuer: Url = "https://matrix.org/"
+            .parse()
+            .unwrap_or_else(|error| panic!("{error}"));
+        let registration_endpoint: Url =
+            "https://matrix.org/_matrix/client/unstable/org.matrix.msc2965/auth_issuer/register"
+                .parse()
+                .unwrap_or_else(|error| panic!("{error}"));
+
+        let diagnostics = ClientRegistrationDiagnostics::new(
+            &original_redirect,
+            &registration_redirect,
+            &metadata,
+            &issuer,
+            Some(&registration_endpoint),
+            &raw_metadata,
+        );
+
+        assert_eq!(
+            diagnostics.original_redirect_uri,
+            "https://localhost:52979/auth/callback"
+        );
+        assert_eq!(
+            diagnostics.registration_redirect_uri,
+            "http://localhost:52979/auth/callback"
+        );
+        assert!(diagnostics.is_loopback);
+        assert_eq!(diagnostics.application_type, "native");
+        assert_eq!(diagnostics.issuer, "https://matrix.org/");
+        assert_eq!(
+            diagnostics.registration_endpoint.as_deref(),
+            Some(registration_endpoint.as_str())
+        );
+        assert!(diagnostics.client_metadata_json.contains("redirect_uris"));
+        assert!(
+            diagnostics
+                .client_metadata_json
+                .contains("http://localhost:52979/auth/callback")
+        );
+
+        let display = diagnostics.to_string();
+        assert!(display.contains("original_redirect_uri=https://localhost:52979/auth/callback"));
+        assert!(display.contains("registration_redirect_uri=http://localhost:52979/auth/callback"));
+        assert!(display.contains("application_type=native"));
+        assert!(display.contains("registration_endpoint=https://matrix.org/"));
+        assert!(display.contains("client_metadata_json="));
+    }
+
+    #[test]
+    fn registration_failure_display_includes_source_and_diagnostics() {
+        let original_redirect: Url = "https://moltis.example.com/auth/callback"
+            .parse()
+            .unwrap_or_else(|error| panic!("{error}"));
+        let metadata =
+            build_client_metadata(&original_redirect).unwrap_or_else(|error| panic!("{error}"));
+        let raw_metadata =
+            Raw::new(&metadata).unwrap_or_else(|error| panic!("raw metadata: {error}"));
+        let issuer: Url = "https://matrix.org/"
+            .parse()
+            .unwrap_or_else(|error| panic!("{error}"));
+        let diagnostics = ClientRegistrationDiagnostics::new(
+            &original_redirect,
+            &original_redirect,
+            &metadata,
+            &issuer,
+            None,
+            &raw_metadata,
+        );
+        let failure = crate::oidc::ClientRegistrationFailure::new(
+            diagnostics,
+            std::io::Error::other("invalid_redirect_uri: invalid redirect_uri"),
+        );
+
+        let display = failure.to_string();
+        assert!(display.contains("invalid_redirect_uri: invalid redirect_uri"));
+        assert!(display.contains("original_redirect_uri=https://moltis.example.com/auth/callback"));
+        assert!(
+            display.contains("registration_redirect_uri=https://moltis.example.com/auth/callback")
+        );
+        assert!(display.contains("is_loopback=false"));
+        assert!(display.contains("application_type=web"));
+        assert!(display.contains("registration_endpoint=none"));
+    }
+
     #[test]
     fn is_loopback_uri_covers_full_127_range() {
         let url_127_0_0_2: Url = "http://127.0.0.2:8080/auth/callback"