feat(rust): add OAuth header provider

Cargo.lock

@@ -5384,6 +5384,7 @@ dependencies = [
  "tokenizers",
  "tokio",
  "url",
+ "urlencoding",
  "uuid",
  "walkdir",
 ]

rust/lancedb/Cargo.toml

@@ -50,7 +50,7 @@ lance-namespace = { workspace = true }
 lance-namespace-impls = { workspace = true }
 moka = { workspace = true }
 pin-project = { workspace = true }
-tokio = { version = "1.23", features = ["rt-multi-thread"] }
+tokio = { version = "1.23", features = ["rt-multi-thread", "sync"] }
 log.workspace = true
 async-trait = "0"
 bytes = "1"
@@ -75,6 +75,7 @@ reqwest = { version = "0.12.0", default-features = false, features = [
     "stream",
 ], optional = true }
 http = { version = "1", optional = true } # Matching what is in reqwest
+urlencoding = { version = "2", optional = true }
 uuid = { version = "1.7.0", features = ["v4", "v5"] }
 polars-arrow = { version = ">=0.37,<0.40.0", optional = true }
 polars = { version = ">=0.37,<0.40.0", optional = true }
@@ -93,6 +94,7 @@ semver = { workspace = true }
 anyhow = "1"
 tempfile = "3.5.0"
 random_word = { version = "0.4.3", features = ["en"] }
+tokio = { version = "1.23", features = ["io-util", "macros", "net", "rt-multi-thread", "sync"] }
 uuid = { version = "1.7.0", features = ["v4"] }
 walkdir = "2"
 aws-sdk-dynamodb = { version = "1.55.0" }
@@ -129,7 +131,13 @@ huggingface = [
     "lance-namespace-impls/dir-huggingface",
 ]
 dynamodb = ["lance/dynamodb", "aws"]
-remote = ["dep:reqwest", "dep:http", "lance-namespace-impls/rest", "lance-namespace-impls/rest-adapter"]
+remote = [
+    "dep:reqwest",
+    "dep:http",
+    "dep:urlencoding",
+    "lance-namespace-impls/rest",
+    "lance-namespace-impls/rest-adapter",
+]
 fp16kernels = ["lance-linalg/fp16kernels"]
 s3-test = []
 bedrock = ["dep:aws-sdk-bedrockruntime"]

rust/lancedb/src/connection.rs

@@ -667,6 +667,8 @@ pub struct ConnectRequest {
 pub struct ConnectBuilder {
     request: ConnectRequest,
     embedding_registry: Option<Arc<dyn EmbeddingRegistry>>,
+    #[cfg(feature = "remote")]
+    oauth_config: Option<crate::remote::OAuthConfig>,
 }
 
 #[cfg(feature = "remote")]
@@ -688,6 +690,8 @@ impl ConnectBuilder {
                 session: None,
             },
             embedding_registry: None,
+            #[cfg(feature = "remote")]
+            oauth_config: None,
         }
     }
 
@@ -776,6 +780,19 @@ impl ConnectBuilder {
         self
     }
 
+    /// Configure OAuth authentication for LanceDB Cloud/Enterprise.
+    ///
+    /// This creates an [`OAuthHeaderProvider`](crate::remote::OAuthHeaderProvider)
+    /// from the given config and sets it as the header provider. OAuth cannot
+    /// be combined with an API key or another header provider.
+    ///
+    /// Token acquisition and refresh are handled in Rust.
+    #[cfg(feature = "remote")]
+    pub fn oauth_config(mut self, config: crate::remote::OAuthConfig) -> Self {
+        self.oauth_config = Some(config);
+        self
+    }
+
     /// Provide a custom [`EmbeddingRegistry`] to use for this connection.
     pub fn embedding_registry(mut self, registry: Arc<dyn EmbeddingRegistry>) -> Self {
         self.embedding_registry = Some(registry);
@@ -921,9 +938,40 @@ impl ConnectBuilder {
         let region = options.region.ok_or_else(|| Error::InvalidInput {
             message: "A region is required when connecting to LanceDb Cloud".to_string(),
         })?;
-        let api_key = options.api_key.ok_or_else(|| Error::InvalidInput {
-            message: "An api_key is required when connecting to LanceDb Cloud".to_string(),
-        })?;
+        let api_key = match (&self.oauth_config, &options.api_key) {
+            (Some(_), None) => String::new(),
+            (Some(_), Some(_)) => {
+                return Err(Error::InvalidInput {
+                    message:
+                        "api_key and oauth_config cannot both be set when connecting to LanceDb Cloud"
+                            .to_string(),
+                });
+            }
+            (None, Some(key)) => key.clone(),
+            (None, None) => {
+                return Err(Error::InvalidInput {
+                    message:
+                        "An api_key or oauth_config is required when connecting to LanceDb Cloud"
+                            .to_string(),
+                });
+            }
+        };
+
+        if self.oauth_config.is_some() && self.request.client_config.header_provider.is_some() {
+            return Err(Error::InvalidInput {
+                message:
+                    "oauth_config and client_config.header_provider cannot both be set when connecting to LanceDb Cloud"
+                        .to_string(),
+            });
+        }
+
+        let mut client_config = self.request.client_config;
+
+        if let Some(oauth_config) = self.oauth_config {
+            let provider = crate::remote::OAuthHeaderProvider::new(oauth_config)?;
+            client_config.header_provider =
+                Some(Arc::new(provider) as Arc<dyn crate::remote::HeaderProvider>);
+        }
 
         let storage_options = StorageOptions(options.storage_options.clone());
         let internal = Arc::new(crate::remote::db::RemoteDatabase::try_new(
@@ -931,7 +979,7 @@ impl ConnectBuilder {
             &api_key,
             &region,
             options.host_override,
-            self.request.client_config,
+            client_config,
             storage_options.into(),
             self.request.read_consistency_interval,
         )?);
@@ -1240,6 +1288,83 @@ mod tests {
         assert_eq!(Some(&"EXPLICIT-VALUE".to_string()), options.get(opts_key));
     }
 
+    #[cfg(feature = "remote")]
+    #[tokio::test]
+    async fn test_connect_rejects_api_key_with_oauth_config() {
+        let oauth_config = crate::remote::OAuthConfig {
+            issuer_url: "https://issuer.example.com".to_string(),
+            client_id: "client-id".to_string(),
+            client_secret: Some("secret".to_string()),
+            scopes: vec!["scope".to_string()],
+            flow: crate::remote::OAuthFlow::ClientCredentials,
+            refresh_buffer_secs: None,
+        };
+
+        let result = ConnectBuilder::new("db://my-container/my-prefix")
+            .region("us-east-1")
+            .api_key("my-api-key")
+            .oauth_config(oauth_config)
+            .execute()
+            .await;
+
+        match result {
+            Err(Error::InvalidInput { message })
+                if message
+                    == "api_key and oauth_config cannot both be set when connecting to LanceDb Cloud" =>
+                {}
+            Err(err) => panic!("expected InvalidInput, got {err:?}"),
+            Ok(_) => panic!("expected api_key and oauth_config to be rejected"),
+        }
+    }
+
+    #[cfg(feature = "remote")]
+    #[tokio::test]
+    async fn test_connect_rejects_header_provider_with_oauth_config() {
+        #[derive(Debug)]
+        struct TestHeaderProvider;
+
+        #[async_trait::async_trait]
+        impl crate::remote::HeaderProvider for TestHeaderProvider {
+            async fn get_headers(&self) -> Result<HashMap<String, String>> {
+                Ok(HashMap::from([(
+                    "authorization".to_string(),
+                    "Bearer token".to_string(),
+                )]))
+            }
+        }
+
+        let oauth_config = crate::remote::OAuthConfig {
+            issuer_url: "https://issuer.example.com".to_string(),
+            client_id: "client-id".to_string(),
+            client_secret: Some("secret".to_string()),
+            scopes: vec!["scope".to_string()],
+            flow: crate::remote::OAuthFlow::ClientCredentials,
+            refresh_buffer_secs: None,
+        };
+        let client_config = crate::remote::ClientConfig {
+            header_provider: Some(
+                Arc::new(TestHeaderProvider) as Arc<dyn crate::remote::HeaderProvider>
+            ),
+            ..Default::default()
+        };
+
+        let result = ConnectBuilder::new("db://my-container/my-prefix")
+            .region("us-east-1")
+            .client_config(client_config)
+            .oauth_config(oauth_config)
+            .execute()
+            .await;
+
+        match result {
+            Err(Error::InvalidInput { message })
+                if message
+                    == "oauth_config and client_config.header_provider cannot both be set when connecting to LanceDb Cloud" =>
+                {}
+            Err(err) => panic!("expected InvalidInput, got {err:?}"),
+            Ok(_) => panic!("expected header_provider and oauth_config to be rejected"),
+        }
+    }
+
     #[cfg(not(windows))]
     #[tokio::test]
     async fn test_connect_relative() {

rust/lancedb/src/remote.rs

@@ -8,6 +8,7 @@
 
 pub(crate) mod client;
 pub(crate) mod db;
+pub mod oauth;
 mod retry;
 pub(crate) mod table;
 pub(crate) mod util;
@@ -20,3 +21,4 @@ const JSON_CONTENT_TYPE: &str = "application/json";
 
 pub use client::{ClientConfig, HeaderProvider, RetryConfig, TimeoutConfig, TlsConfig};
 pub use db::{RemoteDatabaseOptions, RemoteDatabaseOptionsBuilder};
+pub use oauth::{OAuthConfig, OAuthFlow, OAuthHeaderProvider};

rust/lancedb/src/remote/oauth.rs

@@ -0,0 +1,857 @@
+// SPDX-License-Identifier: Apache-2.0
+// SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+use std::collections::HashMap;
+use std::net::IpAddr;
+use std::sync::Arc;
+use std::time::{Duration, Instant};
+
+use log::debug;
+use reqwest::Client;
+use serde::Deserialize;
+use tokio::sync::RwLock;
+
+use crate::error::{Error, Result};
+use crate::remote::client::HeaderProvider;
+
+const DEFAULT_REFRESH_BUFFER_SECS: u64 = 300;
+const DEFAULT_TOKEN_TTL_SECS: u64 = 3600;
+const AZURE_IMDS_ENDPOINT: &str = "http://169.254.169.254/metadata/identity/oauth2/token";
+const AZURE_IMDS_API_VERSION: &str = "2018-02-01";
+
+/// OAuth authentication flow configuration.
+#[derive(Debug, Clone)]
+pub enum OAuthFlow {
+    /// Client Credentials grant (service-to-service / M2M).
+    /// Requires `client_secret` in [`OAuthConfig`].
+    ClientCredentials,
+
+    /// Azure Managed Identity via IMDS.
+    /// Works on Azure VMs, AKS, App Service, and Azure Functions.
+    AzureManagedIdentity {
+        /// Client ID for user-assigned managed identity.
+        /// Omit for system-assigned managed identity.
+        client_id: Option<String>,
+    },
+}
+
+/// OAuth configuration for LanceDB authentication.
+///
+/// All token acquisition and refresh is handled in the Rust layer.
+/// Python and TypeScript bindings expose this as a plain config object.
+#[derive(Clone)]
+pub struct OAuthConfig {
+    /// OIDC issuer URL or OAuth authority URL.
+    /// For Azure: `https://login.microsoftonline.com/{tenant_id}/v2.0`
+    pub issuer_url: String,
+
+    /// Application / Client ID.
+    pub client_id: String,
+
+    /// Client secret (required for `ClientCredentials`, optional for others).
+    pub client_secret: Option<String>,
+
+    /// OAuth scopes to request.
+    /// For Azure managed identity, exactly one scope or resource is required.
+    /// For example: `["api://{app_id}/.default"]`
+    pub scopes: Vec<String>,
+
+    /// Authentication flow to use.
+    pub flow: OAuthFlow,
+
+    /// Seconds before token expiry to trigger proactive refresh (default: 300).
+    /// Keep this well below the token TTL; if it is greater than or equal to
+    /// the TTL, each request refreshes the token.
+    pub refresh_buffer_secs: Option<u64>,
+}
+
+impl std::fmt::Debug for OAuthConfig {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("OAuthConfig")
+            .field("issuer_url", &self.issuer_url)
+            .field("client_id", &self.client_id)
+            .field(
+                "client_secret",
+                &self.client_secret.as_deref().map(|_| "<redacted>"),
+            )
+            .field("scopes", &self.scopes)
+            .field("flow", &self.flow)
+            .field("refresh_buffer_secs", &self.refresh_buffer_secs)
+            .finish()
+    }
+}
+
+// -- OIDC Discovery --
+
+#[derive(Debug, Deserialize)]
+struct OidcDiscovery {
+    token_endpoint: String,
+}
+
+// -- Token Response --
+
+#[derive(Deserialize)]
+struct TokenResponse {
+    access_token: String,
+    /// Token lifetime in seconds.
+    /// Some providers (Azure IMDS) return this as a string, so we accept both.
+    #[serde(default, deserialize_with = "deserialize_optional_u64_or_string")]
+    expires_in: Option<u64>,
+    #[serde(default)]
+    #[allow(dead_code)]
+    token_type: Option<String>,
+}
+
+impl std::fmt::Debug for TokenResponse {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("TokenResponse")
+            .field("access_token", &"<redacted>")
+            .field("expires_in", &self.expires_in)
+            .field("token_type", &self.token_type)
+            .finish()
+    }
+}
+
+fn deserialize_optional_u64_or_string<'de, D>(
+    deserializer: D,
+) -> std::result::Result<Option<u64>, D::Error>
+where
+    D: serde::Deserializer<'de>,
+{
+    use serde::de;
+
+    struct U64OrString;
+    impl<'de> de::Visitor<'de> for U64OrString {
+        type Value = Option<u64>;
+
+        fn expecting(&self, formatter: &mut std::fmt::Formatter) -> std::fmt::Result {
+            formatter.write_str("an integer, an integer-valued float, a numeric string, or null")
+        }
+
+        fn visit_u64<E: de::Error>(self, v: u64) -> std::result::Result<Self::Value, E> {
+            Ok(Some(v))
+        }
+
+        fn visit_i64<E: de::Error>(self, v: i64) -> std::result::Result<Self::Value, E> {
+            if v < 0 {
+                return Err(E::custom(format!("invalid expires_in value: {v}")));
+            }
+            Ok(Some(v as u64))
+        }
+
+        fn visit_f64<E: de::Error>(self, v: f64) -> std::result::Result<Self::Value, E> {
+            if !v.is_finite() || v < 0.0 || v.fract() != 0.0 || v > u64::MAX as f64 {
+                return Err(E::custom(format!("invalid expires_in value: {v}")));
+            }
+            Ok(Some(v as u64))
+        }
+
+        fn visit_str<E: de::Error>(self, v: &str) -> std::result::Result<Self::Value, E> {
+            v.parse::<u64>().map(Some).map_err(de::Error::custom)
+        }
+
+        fn visit_none<E: de::Error>(self) -> std::result::Result<Self::Value, E> {
+            Ok(None)
+        }
+
+        fn visit_unit<E: de::Error>(self) -> std::result::Result<Self::Value, E> {
+            Ok(None)
+        }
+    }
+
+    deserializer.deserialize_any(U64OrString)
+}
+
+// -- Internal Token State --
+
+struct TokenState {
+    access_token: Option<String>,
+    expires_at: Option<Instant>,
+}
+
+impl TokenState {
+    fn new() -> Self {
+        Self {
+            access_token: None,
+            expires_at: None,
+        }
+    }
+
+    fn is_expired(&self, buffer: Duration) -> bool {
+        match (self.access_token.as_ref(), self.expires_at) {
+            (Some(_), Some(expires_at)) => Instant::now() + buffer >= expires_at,
+            (None, _) => true,
+            (Some(_), None) => true,
+        }
+    }
+
+    fn update(&mut self, resp: &TokenResponse) {
+        self.access_token = Some(resp.access_token.clone());
+        let expires_in = resp.expires_in.unwrap_or(DEFAULT_TOKEN_TTL_SECS);
+        self.expires_at = Some(Instant::now() + Duration::from_secs(expires_in));
+    }
+}
+
+/// OAuth header provider that manages the full token lifecycle.
+///
+/// Implements [`HeaderProvider`] to inject `Authorization: Bearer <token>`
+/// headers into every LanceDB request, with automatic token refresh.
+pub struct OAuthHeaderProvider {
+    config: OAuthConfig,
+    http_client: Client,
+    token_state: Arc<RwLock<TokenState>>,
+    /// Cached OIDC discovery document
+    discovery: Arc<RwLock<Option<OidcDiscovery>>>,
+    refresh_buffer: Duration,
+}
+
+impl std::fmt::Debug for OAuthHeaderProvider {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("OAuthHeaderProvider")
+            .field("issuer_url", &self.config.issuer_url)
+            .field("client_id", &self.config.client_id)
+            .field("flow", &self.config.flow)
+            .finish()
+    }
+}
+
+impl OAuthHeaderProvider {
+    /// Create a new OAuth header provider from configuration.
+    pub fn new(config: OAuthConfig) -> Result<Self> {
+        // Validate config upfront
+        if matches!(config.flow, OAuthFlow::ClientCredentials) && config.client_secret.is_none() {
+            return Err(Error::InvalidInput {
+                message: "client_secret is required for ClientCredentials flow".to_string(),
+            });
+        }
+        if config.scopes.is_empty() {
+            return Err(Error::InvalidInput {
+                message: "At least one OAuth scope is required".to_string(),
+            });
+        }
+        if matches!(config.flow, OAuthFlow::AzureManagedIdentity { .. }) && config.scopes.len() != 1
+        {
+            return Err(Error::InvalidInput {
+                message: "AzureManagedIdentity flow requires exactly one OAuth scope or resource"
+                    .to_string(),
+            });
+        }
+        Self::validate_issuer_transport(&config)?;
+
+        let http_client = Client::builder()
+            .timeout(Duration::from_secs(30))
+            .build()
+            .map_err(|e| Error::Runtime {
+                message: format!("Failed to create HTTP client for OAuth: {e}"),
+            })?;
+
+        let refresh_buffer = Duration::from_secs(
+            config
+                .refresh_buffer_secs
+                .unwrap_or(DEFAULT_REFRESH_BUFFER_SECS),
+        );
+
+        Ok(Self {
+            config,
+            http_client,
+            token_state: Arc::new(RwLock::new(TokenState::new())),
+            discovery: Arc::new(RwLock::new(None)),
+            refresh_buffer,
+        })
+    }
+
+    fn validate_issuer_transport(config: &OAuthConfig) -> Result<()> {
+        if !matches!(config.flow, OAuthFlow::ClientCredentials) {
+            return Ok(());
+        }
+
+        let issuer = url::Url::parse(&config.issuer_url).map_err(|e| Error::InvalidInput {
+            message: format!("Invalid OAuth issuer_url: {e}"),
+        })?;
+
+        match issuer.scheme() {
+            "https" => Ok(()),
+            "http" if Self::is_loopback_issuer(&issuer) => Ok(()),
+            _ => Err(Error::InvalidInput {
+                message:
+                    "ClientCredentials OAuth issuer_url must use https, except for loopback hosts"
+                        .to_string(),
+            }),
+        }
+    }
+
+    fn is_loopback_issuer(issuer: &url::Url) -> bool {
+        let Some(host) = issuer.host_str() else {
+            return false;
+        };
+
+        host.eq_ignore_ascii_case("localhost")
+            || host
+                .parse::<IpAddr>()
+                .map(|addr| addr.is_loopback())
+                .unwrap_or(false)
+    }
+
+    /// Get a valid access token, refreshing if necessary.
+    async fn get_valid_token(&self) -> Result<String> {
+        // Fast path: check if current token is still valid
+        {
+            let state = self.token_state.read().await;
+            if !state.is_expired(self.refresh_buffer)
+                && let Some(ref token) = state.access_token
+            {
+                return Ok(token.clone());
+            }
+        }
+
+        // Slow path: acquire or refresh token
+        let mut state = self.token_state.write().await;
+
+        // Double-check after acquiring write lock
+        if !state.is_expired(self.refresh_buffer)
+            && let Some(ref token) = state.access_token
+        {
+            return Ok(token.clone());
+        }
+
+        debug!("Acquiring new OAuth token via {:?} flow", self.config.flow);
+        let resp = self.acquire_token().await?;
+
+        state.update(&resp);
+        Ok(resp.access_token)
+    }
+
+    /// Acquire a new token using the configured flow.
+    async fn acquire_token(&self) -> Result<TokenResponse> {
+        match &self.config.flow {
+            OAuthFlow::ClientCredentials => self.acquire_client_credentials().await,
+            OAuthFlow::AzureManagedIdentity { client_id } => {
+                self.acquire_managed_identity(client_id.as_deref()).await
+            }
+        }
+    }
+
+    // -- OIDC Discovery --
+
+    async fn get_discovery(&self) -> Result<OidcDiscovery> {
+        {
+            let cached = self.discovery.read().await;
+            if let Some(ref disc) = *cached {
+                return Ok(OidcDiscovery {
+                    token_endpoint: disc.token_endpoint.clone(),
+                });
+            }
+        }
+
+        let mut cache = self.discovery.write().await;
+        // Double-check
+        if let Some(ref disc) = *cache {
+            return Ok(OidcDiscovery {
+                token_endpoint: disc.token_endpoint.clone(),
+            });
+        }
+
+        let discovery_url = format!(
+            "{}/.well-known/openid-configuration",
+            self.config.issuer_url.trim_end_matches('/')
+        );
+
+        debug!("Fetching OIDC discovery from {}", discovery_url);
+
+        let resp = self
+            .http_client
+            .get(&discovery_url)
+            .send()
+            .await
+            .map_err(|e| Error::Runtime {
+                message: format!("Failed to fetch OIDC discovery document: {e}"),
+            })?;
+
+        if !resp.status().is_success() {
+            return Err(Error::Runtime {
+                message: format!(
+                    "OIDC discovery failed with status {}: {}",
+                    resp.status(),
+                    resp.text().await.unwrap_or_default()
+                ),
+            });
+        }
+
+        let disc: OidcDiscovery = resp.json().await.map_err(|e| Error::Runtime {
+            message: format!("Failed to parse OIDC discovery document: {e}"),
+        })?;
+
+        let result = OidcDiscovery {
+            token_endpoint: disc.token_endpoint.clone(),
+        };
+
+        *cache = Some(disc);
+        Ok(result)
+    }
+
+    async fn get_token_endpoint(&self) -> Result<String> {
+        self.get_discovery().await.map(|disc| disc.token_endpoint)
+    }
+
+    fn scopes_string(&self) -> String {
+        self.config.scopes.join(" ")
+    }
+
+    fn managed_identity_resource(&self) -> Result<String> {
+        let [scope] = self.config.scopes.as_slice() else {
+            return Err(Error::InvalidInput {
+                message: "AzureManagedIdentity flow requires exactly one OAuth scope or resource"
+                    .to_string(),
+            });
+        };
+
+        Ok(scope.strip_suffix("/.default").unwrap_or(scope).to_string())
+    }
+
+    // -- Client Credentials Flow --
+
+    async fn acquire_client_credentials(&self) -> Result<TokenResponse> {
+        let client_secret = self
+            .config
+            .client_secret
+            .as_ref()
+            .ok_or(Error::InvalidInput {
+                message: "client_secret is required for ClientCredentials flow".to_string(),
+            })?;
+
+        let token_endpoint = self.get_token_endpoint().await?;
+
+        let params = [
+            ("grant_type", "client_credentials"),
+            ("client_id", &self.config.client_id),
+            ("client_secret", client_secret),
+            ("scope", &self.scopes_string()),
+        ];
+
+        self.post_token_request(&token_endpoint, &params).await
+    }
+
+    // -- Azure Managed Identity Flow --
+
+    async fn acquire_managed_identity(&self, mi_client_id: Option<&str>) -> Result<TokenResponse> {
+        let resource = self.managed_identity_resource()?;
+
+        let mut url = format!(
+            "{AZURE_IMDS_ENDPOINT}?api-version={AZURE_IMDS_API_VERSION}&resource={}",
+            urlencoding::encode(&resource),
+        );
+        if let Some(cid) = mi_client_id {
+            url.push_str(&format!("&client_id={}", urlencoding::encode(cid)));
+        }
+
+        let resp = self
+            .http_client
+            .get(&url)
+            .header("Metadata", "true")
+            .send()
+            .await
+            .map_err(|e| Error::Runtime {
+                message: format!("Azure IMDS request failed: {e}"),
+            })?;
+
+        if !resp.status().is_success() {
+            return Err(Error::Runtime {
+                message: format!(
+                    "Azure IMDS returned status {}: {}",
+                    resp.status(),
+                    resp.text().await.unwrap_or_default()
+                ),
+            });
+        }
+
+        resp.json().await.map_err(|e| Error::Runtime {
+            message: format!("Failed to parse IMDS token response: {e}"),
+        })
+    }
+
+    // -- Shared Helpers --
+
+    async fn post_token_request(
+        &self,
+        endpoint: &str,
+        params: &[(&str, &str)],
+    ) -> Result<TokenResponse> {
+        let resp = self
+            .http_client
+            .post(endpoint)
+            .form(params)
+            .send()
+            .await
+            .map_err(|e| Error::Runtime {
+                message: format!("Token request to {endpoint} failed: {e}"),
+            })?;
+
+        if !resp.status().is_success() {
+            return Err(Error::Runtime {
+                message: format!(
+                    "Token request failed with status {}: {}",
+                    resp.status(),
+                    resp.text().await.unwrap_or_default()
+                ),
+            });
+        }
+
+        resp.json().await.map_err(|e| Error::Runtime {
+            message: format!("Failed to parse token response: {e}"),
+        })
+    }
+}
+
+#[async_trait::async_trait]
+impl HeaderProvider for OAuthHeaderProvider {
+    async fn get_headers(&self) -> Result<HashMap<String, String>> {
+        let token = self.get_valid_token().await?;
+        Ok(HashMap::from([(
+            "authorization".to_string(),
+            format!("Bearer {token}"),
+        )]))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::sync::atomic::{AtomicUsize, Ordering};
+
+    use tokio::io::{AsyncReadExt, AsyncWriteExt};
+    use tokio::net::{TcpListener, TcpStream};
+    use tokio::task::JoinHandle;
+
+    #[test]
+    fn test_token_state_expiry() {
+        let mut state = TokenState::new();
+        assert!(state.is_expired(Duration::from_secs(0)));
+
+        state.access_token = Some("tok".to_string());
+        state.expires_at = Some(Instant::now() + Duration::from_secs(600));
+        assert!(!state.is_expired(Duration::from_secs(300)));
+        assert!(state.is_expired(Duration::from_secs(601)));
+
+        state.expires_at = None;
+        assert!(state.is_expired(Duration::from_secs(0)));
+    }
+
+    #[test]
+    fn test_token_state_uses_default_expiry() {
+        let mut state = TokenState::new();
+        let response = TokenResponse {
+            access_token: "tok".to_string(),
+            expires_in: None,
+            token_type: None,
+        };
+
+        state.update(&response);
+
+        assert!(!state.is_expired(Duration::from_secs(DEFAULT_TOKEN_TTL_SECS - 1)));
+        assert!(state.is_expired(Duration::from_secs(DEFAULT_TOKEN_TTL_SECS + 1)));
+    }
+
+    #[test]
+    fn test_token_response_accepts_float_expires_in() {
+        let response: TokenResponse =
+            serde_json::from_str(r#"{"access_token":"tok","expires_in":3600.0}"#).unwrap();
+
+        assert_eq!(response.expires_in, Some(3600));
+    }
+
+    #[test]
+    fn test_token_response_rejects_negative_expires_in() {
+        let err =
+            serde_json::from_str::<TokenResponse>(r#"{"access_token":"tok","expires_in":-1}"#)
+                .unwrap_err();
+
+        assert!(err.to_string().contains("invalid expires_in value: -1"));
+    }
+
+    #[test]
+    fn test_token_response_debug_redacts_access_token() {
+        let response = TokenResponse {
+            access_token: "secret-token".to_string(),
+            expires_in: Some(3600),
+            token_type: Some("Bearer".to_string()),
+        };
+
+        let debug = format!("{response:?}");
+        assert!(!debug.contains("secret-token"));
+        assert!(debug.contains("access_token: \"<redacted>\""));
+    }
+
+    #[test]
+    fn test_scopes_string() {
+        let config = OAuthConfig {
+            issuer_url: "https://login.microsoftonline.com/tenant/v2.0".to_string(),
+            client_id: "app-id".to_string(),
+            client_secret: Some("secret".to_string()),
+            scopes: vec!["scope1".to_string(), "scope2".to_string()],
+            flow: OAuthFlow::ClientCredentials,
+            refresh_buffer_secs: None,
+        };
+        let provider = OAuthHeaderProvider::new(config).unwrap();
+        assert_eq!(provider.scopes_string(), "scope1 scope2");
+    }
+
+    #[test]
+    fn test_oauth_config_debug_redacts_client_secret() {
+        let config = OAuthConfig {
+            issuer_url: "https://issuer.example.com".to_string(),
+            client_id: "client-id".to_string(),
+            client_secret: Some("super-secret".to_string()),
+            scopes: vec!["scope".to_string()],
+            flow: OAuthFlow::ClientCredentials,
+            refresh_buffer_secs: None,
+        };
+
+        let debug = format!("{config:?}");
+        assert!(!debug.contains("super-secret"));
+        assert!(debug.contains("client_secret: Some(\"<redacted>\")"));
+    }
+
+    #[test]
+    fn test_managed_identity_resource_from_default_scope() {
+        let config = OAuthConfig {
+            issuer_url: "https://login.microsoftonline.com/tenant/v2.0".to_string(),
+            client_id: "app-id".to_string(),
+            client_secret: None,
+            scopes: vec!["api://test/.default".to_string()],
+            flow: OAuthFlow::AzureManagedIdentity { client_id: None },
+            refresh_buffer_secs: None,
+        };
+        let provider = OAuthHeaderProvider::new(config).unwrap();
+        assert_eq!(provider.managed_identity_resource().unwrap(), "api://test");
+    }
+
+    #[test]
+    fn test_managed_identity_resource_without_default_suffix() {
+        let config = OAuthConfig {
+            issuer_url: "https://login.microsoftonline.com/tenant/v2.0".to_string(),
+            client_id: "app-id".to_string(),
+            client_secret: None,
+            scopes: vec!["api://test".to_string()],
+            flow: OAuthFlow::AzureManagedIdentity { client_id: None },
+            refresh_buffer_secs: None,
+        };
+        let provider = OAuthHeaderProvider::new(config).unwrap();
+        assert_eq!(provider.managed_identity_resource().unwrap(), "api://test");
+    }
+
+    #[test]
+    fn test_managed_identity_rejects_multiple_scopes() {
+        let config = OAuthConfig {
+            issuer_url: "https://login.microsoftonline.com/tenant/v2.0".to_string(),
+            client_id: "app-id".to_string(),
+            client_secret: None,
+            scopes: vec![
+                "api://test-a/.default".to_string(),
+                "api://test-b/.default".to_string(),
+            ],
+            flow: OAuthFlow::AzureManagedIdentity { client_id: None },
+            refresh_buffer_secs: None,
+        };
+        assert!(OAuthHeaderProvider::new(config).is_err());
+    }
+
+    #[tokio::test]
+    async fn test_token_endpoint_requires_discovery_success() {
+        let (issuer_url, server) = spawn_discovery_error_server().await;
+        let config = OAuthConfig {
+            issuer_url,
+            client_id: "client-id".to_string(),
+            client_secret: Some("secret".to_string()),
+            scopes: vec!["scope".to_string()],
+            flow: OAuthFlow::ClientCredentials,
+            refresh_buffer_secs: None,
+        };
+        let provider = OAuthHeaderProvider::new(config).unwrap();
+
+        let err = provider.get_token_endpoint().await.unwrap_err();
+        assert!(matches!(
+            err,
+            Error::Runtime { message }
+                if message.contains("OIDC discovery failed with status 503")
+        ));
+        server.await.unwrap();
+    }
+
+    #[test]
+    fn test_client_credentials_requires_secret() {
+        let config = OAuthConfig {
+            issuer_url: "https://login.microsoftonline.com/tenant/v2.0".to_string(),
+            client_id: "app-id".to_string(),
+            client_secret: None,
+            scopes: vec!["scope".to_string()],
+            flow: OAuthFlow::ClientCredentials,
+            refresh_buffer_secs: None,
+        };
+        assert!(OAuthHeaderProvider::new(config).is_err());
+    }
+
+    #[test]
+    fn test_client_credentials_rejects_insecure_non_loopback_issuer() {
+        let config = OAuthConfig {
+            issuer_url: "http://issuer.example.com".to_string(),
+            client_id: "app-id".to_string(),
+            client_secret: Some("secret".to_string()),
+            scopes: vec!["scope".to_string()],
+            flow: OAuthFlow::ClientCredentials,
+            refresh_buffer_secs: None,
+        };
+
+        let err = OAuthHeaderProvider::new(config).unwrap_err();
+        assert!(matches!(
+            err,
+            Error::InvalidInput { message }
+                if message == "ClientCredentials OAuth issuer_url must use https, except for loopback hosts"
+        ));
+    }
+
+    #[test]
+    fn test_empty_scopes_rejected() {
+        let config = OAuthConfig {
+            issuer_url: "https://login.microsoftonline.com/tenant/v2.0".to_string(),
+            client_id: "app-id".to_string(),
+            client_secret: None,
+            scopes: vec![],
+            flow: OAuthFlow::AzureManagedIdentity { client_id: None },
+            refresh_buffer_secs: None,
+        };
+        assert!(OAuthHeaderProvider::new(config).is_err());
+    }
+
+    #[tokio::test]
+    async fn test_client_credentials_token_lifecycle() {
+        let (issuer_url, token_requests, server) = spawn_oauth_server().await;
+        let config = OAuthConfig {
+            issuer_url,
+            client_id: "client-id".to_string(),
+            client_secret: Some("secret".to_string()),
+            scopes: vec!["scope".to_string()],
+            flow: OAuthFlow::ClientCredentials,
+            refresh_buffer_secs: Some(0),
+        };
+        let provider = OAuthHeaderProvider::new(config).unwrap();
+
+        let headers = provider.get_headers().await.unwrap();
+        assert_eq!(headers.get("authorization").unwrap(), "Bearer token-1");
+        assert_eq!(token_requests.load(Ordering::SeqCst), 1);
+
+        let headers = provider.get_headers().await.unwrap();
+        assert_eq!(headers.get("authorization").unwrap(), "Bearer token-1");
+        assert_eq!(token_requests.load(Ordering::SeqCst), 1);
+
+        provider.token_state.write().await.expires_at =
+            Some(Instant::now() - Duration::from_secs(1));
+
+        let headers = provider.get_headers().await.unwrap();
+        assert_eq!(headers.get("authorization").unwrap(), "Bearer token-2");
+        assert_eq!(token_requests.load(Ordering::SeqCst), 2);
+
+        server.await.unwrap();
+    }
+
+    async fn spawn_oauth_server() -> (String, Arc<AtomicUsize>, JoinHandle<()>) {
+        let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
+        let addr = listener.local_addr().unwrap();
+        let issuer_url = format!("http://{addr}");
+        let token_requests = Arc::new(AtomicUsize::new(0));
+        let server_token_requests = Arc::clone(&token_requests);
+
+        let server = tokio::spawn(async move {
+            for _ in 0..3 {
+                let (mut stream, _) = listener.accept().await.unwrap();
+                let (request_line, body) = read_http_request(&mut stream).await;
+
+                if request_line.starts_with("GET /.well-known/openid-configuration ") {
+                    let discovery = format!(r#"{{"token_endpoint":"http://{addr}/token"}}"#);
+                    write_json_response(&mut stream, "200 OK", &discovery).await;
+                } else if request_line.starts_with("POST /token ") {
+                    assert!(body.contains("grant_type=client_credentials"));
+                    assert!(body.contains("client_id=client-id"));
+                    assert!(body.contains("client_secret=secret"));
+                    assert!(body.contains("scope=scope"));
+
+                    let token_num = server_token_requests.fetch_add(1, Ordering::SeqCst) + 1;
+                    let token = format!(
+                        r#"{{"access_token":"token-{token_num}","expires_in":3600,"token_type":"Bearer"}}"#
+                    );
+                    write_json_response(&mut stream, "200 OK", &token).await;
+                } else {
+                    write_json_response(&mut stream, "404 Not Found", "{}").await;
+                }
+            }
+        });
+
+        (issuer_url, token_requests, server)
+    }
+
+    async fn spawn_discovery_error_server() -> (String, JoinHandle<()>) {
+        let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
+        let addr = listener.local_addr().unwrap();
+        let issuer_url = format!("http://{addr}");
+
+        let server = tokio::spawn(async move {
+            let (mut stream, _) = listener.accept().await.unwrap();
+            let (request_line, _) = read_http_request(&mut stream).await;
+            assert!(request_line.starts_with("GET /.well-known/openid-configuration "));
+            write_json_response(&mut stream, "503 Service Unavailable", "{}").await;
+        });
+
+        (issuer_url, server)
+    }
+
+    async fn read_http_request(stream: &mut TcpStream) -> (String, String) {
+        let mut buffer = Vec::new();
+        let mut header_end = None;
+
+        while header_end.is_none() {
+            let mut chunk = [0; 1024];
+            let read = stream.read(&mut chunk).await.unwrap();
+            assert_ne!(read, 0, "connection closed before request headers");
+            buffer.extend_from_slice(&chunk[..read]);
+            header_end = find_subsequence(&buffer, b"\r\n\r\n").map(|pos| pos + 4);
+        }
+
+        let header_end = header_end.unwrap();
+        let headers = String::from_utf8_lossy(&buffer[..header_end]).to_string();
+        let request_line = headers.lines().next().unwrap_or_default().to_string();
+        let content_length = headers
+            .lines()
+            .find_map(|line| {
+                let (name, value) = line.split_once(':')?;
+                name.eq_ignore_ascii_case("content-length")
+                    .then(|| value.trim().parse::<usize>().ok())
+                    .flatten()
+            })
+            .unwrap_or(0);
+
+        while buffer.len() < header_end + content_length {
+            let mut chunk = [0; 1024];
+            let read = stream.read(&mut chunk).await.unwrap();
+            assert_ne!(read, 0, "connection closed before request body");
+            buffer.extend_from_slice(&chunk[..read]);
+        }
+
+        let body =
+            String::from_utf8_lossy(&buffer[header_end..header_end + content_length]).to_string();
+
+        (request_line, body)
+    }
+
+    fn find_subsequence(haystack: &[u8], needle: &[u8]) -> Option<usize> {
+        haystack
+            .windows(needle.len())
+            .position(|window| window == needle)
+    }
+
+    async fn write_json_response(stream: &mut TcpStream, status: &str, body: &str) {
+        let response = format!(
+            "HTTP/1.1 {status}\r\ncontent-type: application/json\r\ncontent-length: {}\r\nconnection: close\r\n\r\n{body}",
+            body.len()
+        );
+        stream.write_all(response.as_bytes()).await.unwrap();
+    }
+}