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refactor: make oauth discovery reactive #1052

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110 changes: 82 additions & 28 deletions conformance/src/bin/client.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -195,25 +195,73 @@ const REDIRECT_URI: &str = "http://localhost:3000/callback";
const SCOPE_STEP_UP_INITIAL_SCOPES: &[&str] = &["mcp:basic"];
const SCOPE_STEP_UP_ESCALATED_SCOPES: &[&str] = &["mcp:basic", "mcp:write"];

/// Perform the headless OAuth authorization-code flow.
/// Attempt the real connection unauthenticated and return the server's
/// `WWW-Authenticate` challenge from the 401 — the reactive discovery
/// trigger.
///
/// 1. Discover metadata, register (or use CIMD), get auth URL
/// 2. Fetch the auth URL with redirect:manual → extract code from Location header
/// 3. Exchange code for token
/// 4. Return an `AuthClient` wrapping `reqwest::Client`
/// `None` (server accepted the unauthenticated connection, which is then
/// closed cleanly) is a legitimate outcome, not an error: the scope-step-up
/// and scope-retry-limit mocks allow unauthenticated `initialize` and only
/// enforce authorization on tool calls.
async fn initialize_challenge(
server_url: &str,
lifecycle: ClientLifecycleMode,
) -> anyhow::Result<Option<String>> {
let transport = StreamableHttpClientTransport::from_uri(server_url);
match BasicClientHandler
.serve_with_lifecycle(transport, lifecycle)
.await
{
Ok(client) => {
client.cancel().await.ok();
Ok(None)
}
Err(error) => match error.auth_challenge() {
Some(challenge) => Ok(Some(challenge.to_string())),
None => Err(error.into()),
},
}
}

fn with_optional_challenge(
request: AuthorizationRequest,
challenge: Option<String>,
) -> AuthorizationRequest {
match challenge {
Some(challenge) => request.with_challenge(challenge),
None => request,
}
}

/// Perform the headless OAuth authorization-code flow, reactively:
///
/// 1. Attempt the real connection; take the 401's WWW-Authenticate challenge
/// 2. Discover from the challenge, register (or use CIMD), get auth URL
/// 3. Fetch the auth URL with redirect:manual → extract code from Location header
/// 4. Exchange code for token
/// 5. Return an `AuthClient` wrapping `reqwest::Client`
async fn perform_oauth_flow(
server_url: &str,
_ctx: &ConformanceContext,
) -> anyhow::Result<AuthClient<reqwest::Client>> {
// Always the discover lifecycle here (not `conformance_lifecycle()`):
// this flow serves `run_auth_client`, whose 2026-07-28 auth mocks require
// the per-request MCP-Protocol-Version negotiation.
let challenge = initialize_challenge(
server_url,
ClientLifecycleMode::Discover {
preferred_versions: preferred_protocol_versions(),
},
)
.await?;
let mut oauth = OAuthState::new(server_url, None).await?;

// Discover + register + get auth URL
let request = AuthorizationRequest::new(REDIRECT_URI)
.with_client_name("conformance-client")
.with_client_metadata_url(CIMD_CLIENT_METADATA_URL);
oauth
.start_authorization(
AuthorizationRequest::new(REDIRECT_URI)
.with_client_name("conformance-client")
.with_client_metadata_url(CIMD_CLIENT_METADATA_URL),
)
.start_authorization(with_optional_challenge(request, challenge))
.await?;

let auth_url = oauth.get_authorization_url().await?;
Expand Down Expand Up @@ -255,14 +303,14 @@ async fn perform_oauth_flow_preregistered(
client_id: &str,
client_secret: &str,
) -> anyhow::Result<AuthClient<reqwest::Client>> {
let challenge = initialize_challenge(server_url, conformance_lifecycle()).await?;
let mut oauth = OAuthState::new(server_url, None).await?;

let request = AuthorizationRequest::new(REDIRECT_URI)
.with_preregistered_client(client_id)
.with_client_secret(client_secret);
oauth
.start_authorization(
AuthorizationRequest::new(REDIRECT_URI)
.with_preregistered_client(client_id)
.with_client_secret(client_secret),
)
.start_authorization(with_optional_challenge(request, challenge))
.await?;

let auth_url = oauth.get_authorization_url().await?;
Expand Down Expand Up @@ -325,14 +373,14 @@ async fn run_auth_scope_step_up_client(
server_url: &str,
_ctx: &ConformanceContext,
) -> anyhow::Result<()> {
let challenge = initialize_challenge(server_url, conformance_lifecycle()).await?;
let mut oauth = OAuthState::new(server_url, None).await?;
let request = AuthorizationRequest::new(REDIRECT_URI)
.with_scopes(SCOPE_STEP_UP_INITIAL_SCOPES.iter().copied())
.with_client_name("conformance-client")
.with_client_metadata_url(CIMD_CLIENT_METADATA_URL);
oauth
.start_authorization(
AuthorizationRequest::new(REDIRECT_URI)
.with_scopes(SCOPE_STEP_UP_INITIAL_SCOPES.iter().copied())
.with_client_name("conformance-client")
.with_client_metadata_url(CIMD_CLIENT_METADATA_URL),
)
.start_authorization(with_optional_challenge(request, challenge))
.await?;

let auth_url = oauth.get_authorization_url().await?;
Expand Down Expand Up @@ -427,15 +475,15 @@ async fn run_auth_scope_retry_limit_client(
) -> anyhow::Result<()> {
let max_retries = 3u32;
let mut attempt = 0u32;
let challenge = initialize_challenge(server_url, conformance_lifecycle()).await?;

loop {
let mut oauth = OAuthState::new(server_url, None).await?;
let request = AuthorizationRequest::new(REDIRECT_URI)
.with_client_name("conformance-client")
.with_client_metadata_url(CIMD_CLIENT_METADATA_URL);
oauth
.start_authorization(
AuthorizationRequest::new(REDIRECT_URI)
.with_client_name("conformance-client")
.with_client_metadata_url(CIMD_CLIENT_METADATA_URL),
)
.start_authorization(with_optional_challenge(request, challenge.clone()))
.await?;
let auth_url = oauth.get_authorization_url().await?;
let callback = headless_authorize(&auth_url).await?;
Expand Down Expand Up @@ -509,7 +557,10 @@ async fn migration_token(
return Ok(manager.get_access_token().await?);
}

let resolution = manager.resolve_metadata().await?;
let challenge = initialize_challenge(server_url, conformance_lifecycle()).await?;
let resolution = manager
.resolve_metadata_from_challenge(challenge.as_deref())
.await?;
manager.set_metadata(resolution.metadata);
manager
.register_client("conformance-client", REDIRECT_URI, &[])
Expand Down Expand Up @@ -609,7 +660,10 @@ async fn run_client_credentials_basic(
.unwrap_or("conformance-test-secret");

let mut manager = AuthorizationManager::new(server_url).await?;
let resolution = manager.resolve_metadata().await?;
let challenge = initialize_challenge(server_url, conformance_lifecycle()).await?;
let resolution = manager
.resolve_metadata_from_challenge(challenge.as_deref())
.await?;
let token_endpoint = resolution.metadata.token_endpoint.clone();
manager.set_metadata(resolution.metadata);

Expand Down
25 changes: 25 additions & 0 deletions crates/rmcp/src/service/client.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -86,6 +86,31 @@ impl ClientInitializeError {
context: context.into(),
}
}

/// The `WWW-Authenticate` challenge from the 401/403 the transport hit
/// during initialization, if that is why initialization failed.
///
/// This is the trigger of the reactive OAuth flow: feed the challenge to
/// `AuthorizationRequest::with_challenge` to authorize, then reconnect.
#[cfg(feature = "transport-streamable-http-client")]
pub fn auth_challenge(&self) -> Option<&str> {
use crate::transport::streamable_http_client::{AuthRequiredError, InsufficientScopeError};

let Self::TransportError { error, .. } = self else {
return None;
};
let mut source: Option<&(dyn std::error::Error + 'static)> = Some(error.error.as_ref());
while let Some(current) = source {
if let Some(auth_required) = current.downcast_ref::<AuthRequiredError>() {
return Some(&auth_required.www_authenticate_header);
}
if let Some(insufficient_scope) = current.downcast_ref::<InsufficientScopeError>() {
return Some(&insufficient_scope.www_authenticate_header);
}
source = current.source();
}
None
}
}

/// Helper function to get the next message from the stream
Expand Down
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