Add a usernameAvailable GraphQL query for anonymous lookups

Exposes a new query telling whether a given username is available for
registration, and if not, why: already taken, or rejected by the
registration policy. A username reserved on the homeserver side is
deliberately reported as taken, mirroring the ambiguity of the
registration form, which doesn't tell the two apart either.

Because the query reveals whether a username exists, it is rate-limited
per requester, configurable through the new
`rate_limiting.username_availability` option.

crates/config/src/sections/rate_limiting.rs

@@ -1,3 +1,4 @@
+// Copyright 2025, 2026 Element Creations Ltd.
 // Copyright 2024, 2025 New Vector Ltd.
 // Copyright 2024 The Matrix.org Foundation C.I.C.
 //
@@ -38,6 +39,14 @@ pub struct RateLimitingConfig {
     /// This can protect against brute-forcing the user code.
     #[serde(default = "default_device_code_link")]
     pub device_code_link: RateLimiterConfiguration,
+
+    /// Controls how often the registration form can check whether a username
+    /// is available, based on source address.
+    ///
+    /// The form checks the username as it is typed. This limit makes it
+    /// harder to use those checks to find out which usernames exist.
+    #[serde(default = "default_username_availability")]
+    pub username_availability: RateLimiterConfiguration,
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq)]
@@ -174,6 +183,10 @@ impl ConfigurationSection for RateLimitingConfig {
             return Err(error_on_field(error, "registration").into());
         }
 
+        if let Some(error) = error_on_limiter(&self.username_availability) {
+            return Err(error_on_field(error, "username_availability").into());
+        }
+
         if let Some(error) = error_on_limiter(&self.login.per_ip) {
             return Err(error_on_nested_field(error, "login", "per_ip").into());
         }
@@ -275,6 +288,13 @@ fn default_device_code_link() -> RateLimiterConfiguration {
     }
 }
 
+fn default_username_availability() -> RateLimiterConfiguration {
+    RateLimiterConfiguration {
+        burst: NonZeroU32::new(5).unwrap(),
+        per_second: 1.0,
+    }
+}
+
 impl Default for RateLimitingConfig {
     fn default() -> Self {
         RateLimitingConfig {
@@ -283,6 +303,7 @@ impl Default for RateLimitingConfig {
             account_recovery: AccountRecoveryRateLimitingConfig::default(),
             email_authentication: EmailauthenticationRateLimitingConfig::default(),
             device_code_link: default_device_code_link(),
+            username_availability: default_username_availability(),
         }
     }
 }

crates/handlers/src/graphql/query/user.rs

@@ -1,3 +1,4 @@
+// Copyright 2025, 2026 Element Creations Ltd.
 // Copyright 2024, 2025 New Vector Ltd.
 // Copyright 2024 The Matrix.org Foundation C.I.C.
 //
@@ -5,7 +6,7 @@
 // Please see LICENSE files in the repository root for full details.
 
 use async_graphql::{
-    Context, Enum, ID, Object,
+    Context, Enum, ID, Object, SimpleObject,
     connection::{Connection, Edge, OpaqueCursor, query},
 };
 use mas_storage::{Pagination, user::UserFilter};
@@ -16,6 +17,34 @@ use crate::graphql::{
     state::ContextExt as _,
 };
 
+/// Why a username is not available for registration.
+#[derive(Enum, Copy, Clone, Eq, PartialEq)]
+enum UsernameUnavailableReason {
+    /// The username is already in use, either by an existing MAS user or
+    /// reserved by the homeserver.
+    Taken,
+    /// The username does not pass the registration policy. See
+    /// `violationCodes` for the specific reasons.
+    Invalid,
+}
+
+/// The result of a username availability check.
+#[derive(SimpleObject)]
+struct UsernameAvailability {
+    /// Whether the username is available for registration.
+    available: bool,
+    /// If the username is not available, the reason why.
+    reason: Option<UsernameUnavailableReason>,
+    /// If the username is invalid, the list of policy violation codes (e.g.
+    /// `username-too-long`, `username-all-numeric`, `username-banned`,
+    /// `username-not-allowed`, `username-invalid-chars`), so that clients can
+    /// localize the error message.
+    ///
+    /// This may be an empty list, when the registration policy rejected the
+    /// username for a reason that has no code attached to it.
+    violation_codes: Option<Vec<String>>,
+}
+
 #[derive(Default)]
 pub struct UserQuery;
 
@@ -45,6 +74,99 @@ impl UserQuery {
         Ok(user.map(User))
     }
 
+    /// Check whether a username is available for registration.
+    ///
+    /// This query is accessible to anonymous users, as it is used during the
+    /// registration flow. Unlike `userByUsername`, it reveals whether a
+    /// username exists, so it is rate-limited per requester.
+    async fn username_available(
+        &self,
+        ctx: &Context<'_>,
+        username: String,
+    ) -> Result<UsernameAvailability, async_graphql::Error> {
+        let state = ctx.state();
+        let requester = ctx.requester();
+
+        if !state.site_config().password_registration_enabled {
+            return Err(async_graphql::Error::new(
+                "Password registration is disabled on this server",
+            ));
+        }
+
+        if let Err(e) = state
+            .limiter()
+            .check_username_availability(requester.fingerprint())
+        {
+            tracing::warn!(error = &e as &dyn std::error::Error);
+            return Err(async_graphql::Error::new("Too many requests"));
+        }
+
+        // Check if the username exists in the MAS database
+        let mut repo = state.repository().await?;
+        let exists = repo.user().exists(&username).await?;
+        repo.cancel().await?;
+
+        if exists {
+            return Ok(UsernameAvailability {
+                available: false,
+                reason: Some(UsernameUnavailableReason::Taken),
+                violation_codes: None,
+            });
+        }
+
+        // Run the same registration policy the register views use, so that this
+        // query doesn't report a username as available when registration would
+        // actually reject it
+        let mut policy = state.policy().await?;
+        let res = policy
+            .evaluate_register(mas_policy::RegisterInput {
+                registration_method: mas_policy::RegistrationMethod::Password,
+                username: &username,
+                email: None,
+                requester: requester.for_policy(),
+            })
+            .await?;
+
+        // Any violation means registration would be rejected, even if it isn't
+        // about the username itself (e.g. a banned requester) or has no code we
+        // know about. The codes we do recognise are only there to let clients
+        // localize the error, so they may well be empty.
+        if !res.violations.is_empty() {
+            let violation_codes: Vec<String> = res
+                .violations
+                .into_iter()
+                .filter(|violation| violation.field.as_deref() == Some("username"))
+                .filter_map(|violation| {
+                    violation.variant.map(|variant| variant.as_str().to_owned())
+                })
+                .collect();
+
+            return Ok(UsernameAvailability {
+                available: false,
+                reason: Some(UsernameUnavailableReason::Invalid),
+                violation_codes: Some(violation_codes),
+            });
+        }
+
+        // Check if the username is available on the homeserver
+        let homeserver = state.homeserver_connection();
+        let homeserver_available = homeserver.is_localpart_available(&username).await?;
+
+        if !homeserver_available {
+            return Ok(UsernameAvailability {
+                available: false,
+                reason: Some(UsernameUnavailableReason::Taken),
+                violation_codes: None,
+            });
+        }
+
+        Ok(UsernameAvailability {
+            available: true,
+            reason: None,
+            violation_codes: None,
+        })
+    }
+
     /// Fetch a user by its username.
     async fn user_by_username(
         &self,

crates/handlers/src/graphql/tests.rs

@@ -1,3 +1,4 @@
+// Copyright 2025, 2026 Element Creations Ltd.
 // Copyright 2024, 2025 New Vector Ltd.
 // Copyright 2023, 2024 The Matrix.org Foundation C.I.C.
 //
@@ -22,7 +23,12 @@ use oauth2_types::{
 use sqlx::PgPool;
 use zeroize::Zeroizing;
 
-use crate::test_utils::{self, CookieHelper, RequestBuilderExt, ResponseExt, TestState, setup};
+use crate::{
+    SiteConfig,
+    test_utils::{
+        self, CookieHelper, RequestBuilderExt, ResponseExt, TestState, setup, test_site_config,
+    },
+};
 
 async fn create_test_client(state: &TestState) -> Client {
     let mut repo = state.repository().await.unwrap();
@@ -1084,3 +1090,386 @@ async fn test_deactivate_user_rejected_wrong_password(pool: PgPool) {
         response.data
     );
 }
+
+const USERNAME_AVAILABLE_QUERY: &str = r"
+    query($username: String!) {
+        usernameAvailable(username: $username) {
+            available
+            reason
+            violationCodes
+        }
+    }
+";
+
+/// Test that the `usernameAvailable` query reports an available username.
+#[sqlx::test(migrator = "mas_storage_pg::MIGRATOR")]
+async fn test_username_available(pool: PgPool) {
+    setup();
+    let state = TestState::from_pool(pool).await.unwrap();
+
+    let request = Request::post("/graphql").json(serde_json::json!({
+        "query": USERNAME_AVAILABLE_QUERY,
+        "variables": { "username": "carol" },
+    }));
+
+    let response = state.request(request).await;
+    response.assert_status(StatusCode::OK);
+    let response: GraphQLResponse = response.json();
+    assert!(response.errors.is_empty(), "{:?}", response.errors);
+    assert_eq!(
+        response.data["usernameAvailable"],
+        serde_json::json!({
+            "available": true,
+            "reason": null,
+            "violationCodes": null,
+        })
+    );
+}
+
+/// Test that the `usernameAvailable` query reports a username already taken
+/// by an existing MAS user.
+#[sqlx::test(migrator = "mas_storage_pg::MIGRATOR")]
+async fn test_username_available_taken(pool: PgPool) {
+    setup();
+    let state = TestState::from_pool(pool).await.unwrap();
+    create_test_user(&state, "alice").await;
+
+    let request = Request::post("/graphql").json(serde_json::json!({
+        "query": USERNAME_AVAILABLE_QUERY,
+        "variables": { "username": "alice" },
+    }));
+
+    let response = state.request(request).await;
+    response.assert_status(StatusCode::OK);
+    let response: GraphQLResponse = response.json();
+    assert!(response.errors.is_empty(), "{:?}", response.errors);
+    assert_eq!(
+        response.data["usernameAvailable"],
+        serde_json::json!({
+            "available": false,
+            "reason": "TAKEN",
+            "violationCodes": null,
+        })
+    );
+}
+
+/// Test that the `usernameAvailable` query matches existing MAS users
+/// case-insensitively.
+///
+/// The MAS-side existence check is a `LOWER(username) = LOWER($1)` match and
+/// runs before the registration policy, so a differently-cased spelling of an
+/// existing username reports `TAKEN` rather than the `username-invalid-chars`
+/// violation the policy would raise for the uppercase letter.
+#[sqlx::test(migrator = "mas_storage_pg::MIGRATOR")]
+async fn test_username_available_different_case(pool: PgPool) {
+    setup();
+    let state = TestState::from_pool(pool).await.unwrap();
+    create_test_user(&state, "alice").await;
+
+    let request = Request::post("/graphql").json(serde_json::json!({
+        "query": USERNAME_AVAILABLE_QUERY,
+        "variables": { "username": "Alice" },
+    }));
+
+    let response = state.request(request).await;
+    response.assert_status(StatusCode::OK);
+    let response: GraphQLResponse = response.json();
+    assert!(response.errors.is_empty(), "{:?}", response.errors);
+    assert_eq!(
+        response.data["usernameAvailable"],
+        serde_json::json!({
+            "available": false,
+            "reason": "TAKEN",
+            "violationCodes": null,
+        })
+    );
+}
+
+/// Test that the `usernameAvailable` query rejects an empty username.
+#[sqlx::test(migrator = "mas_storage_pg::MIGRATOR")]
+async fn test_username_available_empty(pool: PgPool) {
+    setup();
+    let state = TestState::from_pool(pool).await.unwrap();
+
+    let request = Request::post("/graphql").json(serde_json::json!({
+        "query": USERNAME_AVAILABLE_QUERY,
+        "variables": { "username": "" },
+    }));
+
+    let response = state.request(request).await;
+    response.assert_status(StatusCode::OK);
+    let response: GraphQLResponse = response.json();
+    assert!(response.errors.is_empty(), "{:?}", response.errors);
+    assert_eq!(
+        response.data["usernameAvailable"]["available"],
+        serde_json::json!(false)
+    );
+    assert_eq!(
+        response.data["usernameAvailable"]["reason"].as_str(),
+        Some("INVALID")
+    );
+}
+
+/// Test that the `usernameAvailable` query reports a username reserved on
+/// the homeserver as `TAKEN`, like an existing MAS user.
+#[sqlx::test(migrator = "mas_storage_pg::MIGRATOR")]
+async fn test_username_available_reserved(pool: PgPool) {
+    setup();
+    let state = TestState::from_pool(pool).await.unwrap();
+    state.homeserver_connection.reserve_localpart("bob").await;
+
+    let request = Request::post("/graphql").json(serde_json::json!({
+        "query": USERNAME_AVAILABLE_QUERY,
+        "variables": { "username": "bob" },
+    }));
+
+    let response = state.request(request).await;
+    response.assert_status(StatusCode::OK);
+    let response: GraphQLResponse = response.json();
+    assert!(response.errors.is_empty(), "{:?}", response.errors);
+    assert_eq!(
+        response.data["usernameAvailable"],
+        serde_json::json!({
+            "available": false,
+            "reason": "TAKEN",
+            "violationCodes": null,
+        })
+    );
+}
+
+/// Test that the `usernameAvailable` query runs the registration policy and
+/// reports the violation codes for an all-numeric username.
+#[sqlx::test(migrator = "mas_storage_pg::MIGRATOR")]
+async fn test_username_available_invalid_all_numeric(pool: PgPool) {
+    setup();
+    let state = TestState::from_pool(pool).await.unwrap();
+
+    let request = Request::post("/graphql").json(serde_json::json!({
+        "query": USERNAME_AVAILABLE_QUERY,
+        "variables": { "username": "1234567" },
+    }));
+
+    let response = state.request(request).await;
+    response.assert_status(StatusCode::OK);
+    let response: GraphQLResponse = response.json();
+    assert!(response.errors.is_empty(), "{:?}", response.errors);
+    assert_eq!(
+        response.data["usernameAvailable"]["reason"].as_str(),
+        Some("INVALID")
+    );
+    let codes = response.data["usernameAvailable"]["violationCodes"]
+        .as_array()
+        .expect("violationCodes to be an array");
+    assert!(
+        codes
+            .iter()
+            .any(|c| c.as_str() == Some("username-all-numeric")),
+        "{:?}",
+        codes
+    );
+}
+
+/// Test that the `usernameAvailable` query reports a username as unavailable
+/// when the registration policy rejects the registration for a reason that
+/// isn't about the username, and therefore has no violation code.
+#[sqlx::test(migrator = "mas_storage_pg::MIGRATOR")]
+async fn test_username_available_banned_requester(pool: PgPool) {
+    setup();
+    let state = TestState::from_pool_with_site_config_and_policy_data(
+        pool,
+        test_site_config(),
+        serde_json::json!({
+            "requester": {
+                "banned_ips": ["1.2.3.4"],
+            },
+        }),
+    )
+    .await
+    .unwrap();
+
+    // The policy crashes on a null user agent when formatting the requester,
+    // so make sure we send one
+    let request = Request::post("/graphql")
+        .client_ip([1, 2, 3, 4].into())
+        .header("user-agent", "Test/1.0")
+        .json(serde_json::json!({
+            "query": USERNAME_AVAILABLE_QUERY,
+            "variables": { "username": "frank" },
+        }));
+
+    let response = state.request(request).await;
+    response.assert_status(StatusCode::OK);
+    let response: GraphQLResponse = response.json();
+    assert!(response.errors.is_empty(), "{:?}", response.errors);
+    assert_eq!(
+        response.data["usernameAvailable"],
+        serde_json::json!({
+            "available": false,
+            "reason": "INVALID",
+            "violationCodes": [],
+        })
+    );
+}
+
+/// Test that the `usernameAvailable` query reports an overly long username as
+/// invalid, with the violation code the registration policy raises.
+#[sqlx::test(migrator = "mas_storage_pg::MIGRATOR")]
+async fn test_username_available_too_long(pool: PgPool) {
+    setup();
+    let state = TestState::from_pool(pool).await.unwrap();
+
+    let username = "a".repeat(300);
+    let request = Request::post("/graphql").json(serde_json::json!({
+        "query": USERNAME_AVAILABLE_QUERY,
+        "variables": { "username": username },
+    }));
+
+    let response = state.request(request).await;
+    response.assert_status(StatusCode::OK);
+    let response: GraphQLResponse = response.json();
+    assert!(response.errors.is_empty(), "{:?}", response.errors);
+    assert_eq!(
+        response.data["usernameAvailable"]["reason"].as_str(),
+        Some("INVALID")
+    );
+    assert_eq!(
+        response.data["usernameAvailable"]["violationCodes"],
+        serde_json::json!(["username-too-long"])
+    );
+}
+
+/// Test that the `usernameAvailable` query errors out when password
+/// registration is disabled on the server.
+#[sqlx::test(migrator = "mas_storage_pg::MIGRATOR")]
+async fn test_username_available_registration_disabled(pool: PgPool) {
+    setup();
+    let state = TestState::from_pool_with_site_config(
+        pool,
+        SiteConfig {
+            password_registration_enabled: false,
+            ..test_site_config()
+        },
+    )
+    .await
+    .unwrap();
+
+    let request = Request::post("/graphql").json(serde_json::json!({
+        "query": USERNAME_AVAILABLE_QUERY,
+        "variables": { "username": "dave" },
+    }));
+
+    let response = state.request(request).await;
+    response.assert_status(StatusCode::OK);
+    let response: GraphQLResponse = response.json();
+    assert_eq!(
+        response.errors[0]["message"].as_str(),
+        Some("Password registration is disabled on this server"),
+        "{:?}",
+        response.errors
+    );
+    assert!(response.data["usernameAvailable"].is_null());
+}
+
+/// Test that the `usernameAvailable` query errors out when the homeserver call
+/// fails, rather than reporting the username as available.
+#[sqlx::test(migrator = "mas_storage_pg::MIGRATOR")]
+async fn test_username_available_homeserver_failure(pool: PgPool) {
+    setup();
+    let state = TestState::from_pool(pool).await.unwrap();
+    state.homeserver_connection.fail_localpart_availability();
+
+    let request = Request::post("/graphql").json(serde_json::json!({
+        "query": USERNAME_AVAILABLE_QUERY,
+        "variables": { "username": "grace" },
+    }));
+
+    let response = state.request(request).await;
+    response.assert_status(StatusCode::OK);
+    let response: GraphQLResponse = response.json();
+    assert!(!response.errors.is_empty());
+    assert!(response.data["usernameAvailable"].is_null());
+}
+
+/// Test that the `usernameAvailable` query is rate-limited per requester.
+#[sqlx::test(migrator = "mas_storage_pg::MIGRATOR")]
+async fn test_username_available_rate_limited(pool: PgPool) {
+    setup();
+    let state = TestState::from_pool(pool).await.unwrap();
+
+    // The default configuration allows a burst of 5 requests.
+    for _ in 0..5 {
+        let request = Request::post("/graphql").json(serde_json::json!({
+            "query": USERNAME_AVAILABLE_QUERY,
+            "variables": { "username": "eve" },
+        }));
+        let response = state.request(request).await;
+        response.assert_status(StatusCode::OK);
+        let response: GraphQLResponse = response.json();
+        assert!(response.errors.is_empty(), "{:?}", response.errors);
+    }
+
+    // The next one should be rate limited.
+    let request = Request::post("/graphql").json(serde_json::json!({
+        "query": USERNAME_AVAILABLE_QUERY,
+        "variables": { "username": "eve" },
+    }));
+    let response = state.request(request).await;
+    response.assert_status(StatusCode::OK);
+    let response: GraphQLResponse = response.json();
+    assert_eq!(
+        response.errors[0]["message"].as_str(),
+        Some("Too many requests"),
+        "{:?}",
+        response.errors
+    );
+}
+
+/// Test that the `usernameAvailable` rate limit is keyed on the client IP, so
+/// that one client can't exhaust another client's budget.
+#[sqlx::test(migrator = "mas_storage_pg::MIGRATOR")]
+async fn test_username_available_rate_limited_per_ip(pool: PgPool) {
+    setup();
+    let state = TestState::from_pool(pool).await.unwrap();
+
+    // Exhaust the burst of 5 requests for the first client
+    for _ in 0..5 {
+        let request = Request::post("/graphql")
+            .client_ip([1, 2, 3, 4].into())
+            .json(serde_json::json!({
+                "query": USERNAME_AVAILABLE_QUERY,
+                "variables": { "username": "eve" },
+            }));
+        let response = state.request(request).await;
+        response.assert_status(StatusCode::OK);
+        let response: GraphQLResponse = response.json();
+        assert!(response.errors.is_empty(), "{:?}", response.errors);
+    }
+
+    let request = Request::post("/graphql")
+        .client_ip([1, 2, 3, 4].into())
+        .json(serde_json::json!({
+            "query": USERNAME_AVAILABLE_QUERY,
+            "variables": { "username": "eve" },
+        }));
+    let response = state.request(request).await;
+    response.assert_status(StatusCode::OK);
+    let response: GraphQLResponse = response.json();
+    assert_eq!(
+        response.errors[0]["message"].as_str(),
+        Some("Too many requests"),
+        "{:?}",
+        response.errors
+    );
+
+    // The second client still has its own budget
+    let request = Request::post("/graphql")
+        .client_ip([5, 6, 7, 8].into())
+        .json(serde_json::json!({
+            "query": USERNAME_AVAILABLE_QUERY,
+            "variables": { "username": "eve" },
+        }));
+    let response = state.request(request).await;
+    response.assert_status(StatusCode::OK);
+    let response: GraphQLResponse = response.json();
+    assert!(response.errors.is_empty(), "{:?}", response.errors);
+}

crates/handlers/src/rate_limit.rs

@@ -39,6 +39,12 @@ pub enum RegistrationLimitedError {
     Requester(RequesterFingerprint),
 }
 
+#[derive(Debug, Clone, thiserror::Error)]
+pub enum UsernameAvailabilityLimitedError {
+    #[error("Too many username availability checks for requester {0}")]
+    Requester(RequesterFingerprint),
+}
+
 #[derive(Debug, Clone, thiserror::Error)]
 pub enum EmailAuthenticationLimitedError {
     #[error("Too many email authentication requests for requester {0}")]
@@ -129,6 +135,7 @@ struct LimiterInner {
     email_authentication_emails_per_session: KeyedRateLimiter<Ulid>,
     email_authentication_attempt_per_session: KeyedRateLimiter<Ulid>,
     device_code_link_per_requester: KeyedRateLimiter<RequesterFingerprint>,
+    username_availability_per_requester: KeyedRateLimiter<RequesterFingerprint>,
 }
 
 impl LimiterInner {
@@ -156,6 +163,9 @@ impl LimiterInner {
                 config.email_authentication.attempt_per_session.to_quota()?,
             ),
             device_code_link_per_requester: RateLimiter::keyed(config.device_code_link.to_quota()?),
+            username_availability_per_requester: RateLimiter::keyed(
+                config.username_availability.to_quota()?,
+            ),
         })
     }
 }
@@ -202,6 +212,9 @@ impl Limiter {
                     .email_authentication_attempt_per_session
                     .retain_recent();
                 this.inner.device_code_link_per_requester.retain_recent();
+                this.inner
+                    .username_availability_per_requester
+                    .retain_recent();
 
                 interval.tick().await;
             }
@@ -275,6 +288,23 @@ impl Limiter {
         Ok(())
     }
 
+    /// Check if a username availability check can be performed
+    ///
+    /// # Errors
+    ///
+    /// Returns an error if the operation is rate limited.
+    pub fn check_username_availability(
+        &self,
+        requester: RequesterFingerprint,
+    ) -> Result<(), UsernameAvailabilityLimitedError> {
+        self.inner
+            .username_availability_per_requester
+            .check_key(&requester)
+            .map_err(|_| UsernameAvailabilityLimitedError::Requester(requester))?;
+
+        Ok(())
+    }
+
     /// Check if an email can be sent to the address for an email
     /// authentication session
     ///

crates/handlers/src/test_utils.rs

@@ -170,6 +170,17 @@ impl TestState {
     pub async fn from_pool_with_site_config(
         pool: PgPool,
         site_config: SiteConfig,
+    ) -> Result<Self, anyhow::Error> {
+        Self::from_pool_with_site_config_and_policy_data(pool, site_config, serde_json::json!({}))
+            .await
+    }
+
+    /// Create a new test state from the given database pool, site config and
+    /// policy data
+    pub async fn from_pool_with_site_config_and_policy_data(
+        pool: PgPool,
+        site_config: SiteConfig,
+        policy_data: serde_json::Value,
     ) -> Result<Self, anyhow::Error> {
         // Make sure the rustls crypto provider and tracing are set up, as
         // building the HTTP client below requires a process-level provider to
@@ -224,7 +235,7 @@ impl TestState {
                 server_name: site_config.server_name.clone(),
                 session_limit: site_config.session_limit.clone(),
             },
-            serde_json::json!({}),
+            policy_data,
         )
         .await?;
 

crates/matrix/src/mock.rs

@@ -1,10 +1,14 @@
+// Copyright 2025, 2026 Element Creations Ltd.
 // Copyright 2024, 2025 New Vector Ltd.
 // Copyright 2023, 2024 The Matrix.org Foundation C.I.C.
 //
 // SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 // Please see LICENSE files in the repository root for full details.
 
-use std::collections::{HashMap, HashSet};
+use std::{
+    collections::{HashMap, HashSet},
+    sync::atomic::{AtomicBool, Ordering},
+};
 
 use anyhow::Context;
 use async_trait::async_trait;
@@ -24,12 +28,13 @@ pub struct MockUser {
     pub locked: bool,
 }
 
-/// A mock implementation of a [`HomeserverConnection`], which never fails and
-/// doesn't do anything.
+/// A mock implementation of a [`HomeserverConnection`], which doesn't do
+/// anything and only fails when explicitly told to.
 pub struct HomeserverConnection {
     homeserver: String,
     users: RwLock<HashMap<String, MockUser>>,
     reserved_localparts: RwLock<HashSet<&'static str>>,
+    fail_localpart_availability: AtomicBool,
 }
 
 impl HomeserverConnection {
@@ -46,6 +51,7 @@ impl HomeserverConnection {
             homeserver: homeserver.into(),
             users: RwLock::new(HashMap::new()),
             reserved_localparts: RwLock::new(HashSet::new()),
+            fail_localpart_availability: AtomicBool::new(false),
         }
     }
 
@@ -53,6 +59,13 @@ impl HomeserverConnection {
         self.reserved_localparts.write().await.insert(localpart);
     }
 
+    /// Make [`crate::HomeserverConnection::is_localpart_available`] fail, to
+    /// exercise homeserver failures.
+    pub fn fail_localpart_availability(&self) {
+        self.fail_localpart_availability
+            .store(true, Ordering::Relaxed);
+    }
+
     /// Like `query_user` but get the raw test state of the user.
     ///
     /// # Errors
@@ -125,6 +138,11 @@ impl crate::HomeserverConnection for HomeserverConnection {
     }
 
     async fn is_localpart_available(&self, localpart: &str) -> Result<bool, anyhow::Error> {
+        anyhow::ensure!(
+            !self.fail_localpart_availability.load(Ordering::Relaxed),
+            "Mock homeserver failure while checking localpart availability"
+        );
+
         if self.reserved_localparts.read().await.contains(localpart) {
             return Ok(false);
         }

docs/config.schema.json

@@ -2033,6 +2033,18 @@
               "$ref": "#/definitions/RateLimiterConfiguration"
             }
           ]
+        },
+        "username_availability": {
+          "description": "Controls how often the registration form can check whether a username\nis available, based on source address.\n\nThe form checks the username as it is typed. This limit makes it\nharder to use those checks to find out which usernames exist.",
+          "default": {
+            "burst": 5,
+            "per_second": 1.0
+          },
+          "allOf": [
+            {
+              "$ref": "#/definitions/RateLimiterConfiguration"
+            }
+          ]
         }
       }
     },

docs/reference/configuration.md

@@ -648,6 +648,14 @@ rate_limiting:
   device_code_link:
     burst: 10
     per_second: 0.016666
+
+  # Limits how often the registration form can check whether a username is
+  # available, based on source address.
+  # The form checks the username as it is typed. This limit makes it harder to
+  # use those checks to find out which usernames exist.
+  username_availability:
+    burst: 5
+    per_second: 1.0
 ```
 
 ## `telemetry`

frontend/schema.graphql

@@ -1185,6 +1185,14 @@ type Query {
   """
   user(id: ID!): User
   """
+  Check whether a username is available for registration.
+
+  This query is accessible to anonymous users, as it is used during the
+  registration flow. Unlike `userByUsername`, it reveals whether a
+  username exists, so it is rate-limited per requester.
+  """
+  usernameAvailable(username: String!): UsernameAvailability!
+  """
   Fetch a user by its username.
   """
   userByUsername(username: String!): User
@@ -2470,6 +2478,46 @@ enum UserState {
   LOCKED
 }
 
+"""
+The result of a username availability check.
+"""
+type UsernameAvailability {
+  """
+  Whether the username is available for registration.
+  """
+  available: Boolean!
+  """
+  If the username is not available, the reason why.
+  """
+  reason: UsernameUnavailableReason
+  """
+  If the username is invalid, the list of policy violation codes (e.g.
+  `username-too-long`, `username-all-numeric`, `username-banned`,
+  `username-not-allowed`, `username-invalid-chars`), so that clients can
+  localize the error message.
+
+  This may be an empty list, when the registration policy rejected the
+  username for a reason that has no code attached to it.
+  """
+  violationCodes: [String!]
+}
+
+"""
+Why a username is not available for registration.
+"""
+enum UsernameUnavailableReason {
+  """
+  The username is already in use, either by an existing MAS user or
+  reserved by the homeserver.
+  """
+  TAKEN
+  """
+  The username does not pass the registration policy. See
+  `violationCodes` for the specific reasons.
+  """
+  INVALID
+}
+
 """
 Represents the current viewer
 """