diff --git a/datadog-sidecar-ffi/src/lib.rs b/datadog-sidecar-ffi/src/lib.rs
index f3c5c97254..18245ed6f9 100644
--- a/datadog-sidecar-ffi/src/lib.rs
+++ b/datadog-sidecar-ffi/src/lib.rs
@@ -755,6 +755,43 @@ pub unsafe extern "C" fn ddog_sidecar_session_set_process_tags(
     MaybeError::None
 }
 
+/// Records the tracer's auto-resolved default service name for the session
+/// (process-bound; sidecar emits `svc.auto:<name>` when `DD_SERVICE` is not
+/// currently set for the active request). Pass an empty `CharSlice` to clear.
+#[no_mangle]
+#[allow(clippy::missing_safety_doc)]
+pub unsafe extern "C" fn ddog_sidecar_session_set_default_service_name(
+    transport: &mut Box<SidecarTransport>,
+    default_service_name: ffi::CharSlice,
+) -> MaybeError {
+    let name = if default_service_name.is_empty() {
+        None
+    } else {
+        Some(default_service_name.to_utf8_lossy().into_owned())
+    };
+    try_c!(blocking::set_session_default_service_name(transport, name));
+
+    MaybeError::None
+}
+
+/// Records whether `DD_SERVICE` is currently set for the session (per-request
+/// mutable; refresh on each RINIT). When `true` the sidecar emits
+/// `svc.user:true`; when `false` it falls back to the previously-recorded
+/// `svc.auto:<name>` (if any).
+#[no_mangle]
+#[allow(clippy::missing_safety_doc)]
+pub unsafe extern "C" fn ddog_sidecar_session_set_user_service_defined(
+    transport: &mut Box<SidecarTransport>,
+    is_user_defined: bool,
+) -> MaybeError {
+    try_c!(blocking::set_session_user_service_defined(
+        transport,
+        is_user_defined,
+    ));
+
+    MaybeError::None
+}
+
 #[repr(C)]
 pub struct TracerHeaderTags<'a> {
     pub lang: ffi::CharSlice<'a>,
diff --git a/datadog-sidecar/src/service/blocking.rs b/datadog-sidecar/src/service/blocking.rs
index 80a9257384..9d7eb32cac 100644
--- a/datadog-sidecar/src/service/blocking.rs
+++ b/datadog-sidecar/src/service/blocking.rs
@@ -271,6 +271,22 @@ pub fn set_session_process_tags(
     Ok(())
 }
 
+pub fn set_session_default_service_name(
+    transport: &mut SidecarTransport,
+    name: Option<String>,
+) -> io::Result<()> {
+    lock_sender(transport)?.set_session_default_service_name(name);
+    Ok(())
+}
+
+pub fn set_session_user_service_defined(
+    transport: &mut SidecarTransport,
+    is_defined: bool,
+) -> io::Result<()> {
+    lock_sender(transport)?.set_session_user_service_defined(is_defined);
+    Ok(())
+}
+
 /// Sends a trace as bytes.
 pub fn send_trace_v04_bytes(
     transport: &mut SidecarTransport,
diff --git a/datadog-sidecar/src/service/mod.rs b/datadog-sidecar/src/service/mod.rs
index a7019781ff..3f12030916 100644
--- a/datadog-sidecar/src/service/mod.rs
+++ b/datadog-sidecar/src/service/mod.rs
@@ -101,3 +101,4 @@ pub enum SidecarAction {
         metrics: Vec<FfeEvaluationMetric>,
     },
 }
+
diff --git a/datadog-sidecar/src/service/runtime_info.rs b/datadog-sidecar/src/service/runtime_info.rs
index 25128d1a50..0c95c7d450 100644
--- a/datadog-sidecar/src/service/runtime_info.rs
+++ b/datadog-sidecar/src/service/runtime_info.rs
@@ -144,6 +144,9 @@ impl ActiveApplication {
             .expect("Expecting remote config invariants to be set early")
             .clone();
 
+        // Target is hashed on the sidecar side and on the PHP read side
+        // (sidecar.c:ddog_remote_configs_service_env_change). PHP passes the
+        // bare process_tags Vec, so we must too — otherwise SHM lookups miss.
         let process_tags = session.process_tags.lock_or_panic().clone();
 
         if *session.remote_config_enabled.lock_or_panic() {
diff --git a/datadog-sidecar/src/service/sender.rs b/datadog-sidecar/src/service/sender.rs
index e88720e23f..bb88a7b85d 100644
--- a/datadog-sidecar/src/service/sender.rs
+++ b/datadog-sidecar/src/service/sender.rs
@@ -35,6 +35,8 @@ use tracing::trace;
 struct SidecarOutbox {
     set_session_config: Option<SidecarInterfaceRequest>,
     set_session_process_tags: Option<SidecarInterfaceRequest>,
+    set_session_default_service_name: Option<SidecarInterfaceRequest>,
+    set_session_user_service_defined: Option<SidecarInterfaceRequest>,
     set_universal_service_tags: Option<SidecarInterfaceRequest>,
     set_request_config: Option<SidecarInterfaceRequest>,
     clear_queue_id: Option<SidecarInterfaceRequest>,
@@ -43,10 +45,12 @@ struct SidecarOutbox {
 }
 
 impl SidecarOutbox {
-    fn slots_mut(&mut self) -> [&mut Option<SidecarInterfaceRequest>; 7] {
+    fn slots_mut(&mut self) -> [&mut Option<SidecarInterfaceRequest>; 9] {
         [
             &mut self.set_session_config,
             &mut self.set_session_process_tags,
+            &mut self.set_session_default_service_name,
+            &mut self.set_session_user_service_defined,
             &mut self.set_universal_service_tags,
             &mut self.set_request_config,
             &mut self.clear_queue_id,
@@ -122,6 +126,12 @@ fn coalesce(outbox: &mut SidecarOutbox, incoming: SidecarInterfaceRequest) {
         SidecarInterfaceRequest::SetSessionProcessTags { .. } => {
             outbox.set_session_process_tags = Some(incoming);
         }
+        SidecarInterfaceRequest::SetSessionDefaultServiceName { .. } => {
+            outbox.set_session_default_service_name = Some(incoming);
+        }
+        SidecarInterfaceRequest::SetSessionUserServiceDefined { .. } => {
+            outbox.set_session_user_service_defined = Some(incoming);
+        }
         SidecarInterfaceRequest::SetUniversalServiceTags { .. } => {
             outbox.set_universal_service_tags = Some(incoming);
         }
@@ -236,6 +246,22 @@ impl SidecarSender {
         self.try_drain_outbox();
     }
 
+    pub fn set_session_default_service_name(&mut self, name: Option<String>) {
+        coalesce(
+            &mut self.outbox,
+            SidecarInterfaceRequest::SetSessionDefaultServiceName { name },
+        );
+        self.try_drain_outbox();
+    }
+
+    pub fn set_session_user_service_defined(&mut self, is_defined: bool) {
+        coalesce(
+            &mut self.outbox,
+            SidecarInterfaceRequest::SetSessionUserServiceDefined { is_defined },
+        );
+        self.try_drain_outbox();
+    }
+
     #[allow(clippy::too_many_arguments)]
     pub fn set_universal_service_tags(
         &mut self,
diff --git a/datadog-sidecar/src/service/session_info.rs b/datadog-sidecar/src/service/session_info.rs
index 565f516fb9..fbe88b206e 100644
--- a/datadog-sidecar/src/service/session_info.rs
+++ b/datadog-sidecar/src/service/session_info.rs
@@ -45,6 +45,8 @@ pub(crate) struct SessionInfo {
     pub(crate) pid: Arc<AtomicI32>,
     pub(crate) remote_config_enabled: Arc<Mutex<bool>>,
     pub(crate) process_tags: Arc<Mutex<Vec<Tag>>>,
+    pub(crate) auto_resolved_service_name: Arc<Mutex<Option<String>>>,
+    pub(crate) user_service_defined: Arc<Mutex<bool>>,
     pub(crate) stats_config: Arc<Mutex<Option<crate::service::stats_flusher::StatsConfig>>>,
     otlp_metrics_endpoint: Arc<Mutex<Option<Endpoint>>>,
 }
@@ -131,6 +133,31 @@ impl SessionInfo {
         self.runtimes.lock_or_panic()
     }
 
+    pub(crate) fn process_tags_with_svc_source(&self) -> Vec<Tag> {
+        let mut tags = self.process_tags.lock_or_panic().clone();
+        if *self.user_service_defined.lock_or_panic() {
+            if let Ok(tag) = Tag::new("svc.user", "true") {
+                tags.push(tag);
+            }
+        } else if let Some(name) = self.auto_resolved_service_name.lock_or_panic().as_ref() {
+            if let Ok(tag) = Tag::new("svc.auto", name.clone()) {
+                tags.push(tag);
+            }
+        }
+        tags
+    }
+
+    pub(crate) fn refresh_stats_process_tags(&self) {
+        if let Some(stats) = self.stats_config.lock_or_panic().as_mut() {
+            stats.process_tags = self
+                .process_tags_with_svc_source()
+                .iter()
+                .map(|t| t.to_string())
+                .collect::<Vec<_>>()
+                .join(",");
+        }
+    }
+
     pub(crate) fn get_telemetry_config(
         &self,
     ) -> MutexGuard<'_, Option<libdd_telemetry::config::Config>> {
diff --git a/datadog-sidecar/src/service/sidecar_interface.rs b/datadog-sidecar/src/service/sidecar_interface.rs
index 6f6244b258..67265f203e 100644
--- a/datadog-sidecar/src/service/sidecar_interface.rs
+++ b/datadog-sidecar/src/service/sidecar_interface.rs
@@ -69,6 +69,15 @@ pub trait SidecarInterface {
     /// * `process_tags` - The process tags.
     async fn set_session_process_tags(process_tags: Vec<Tag>);
 
+    /// Records the auto-resolved default service name for the session
+    /// (process-bound; the tracer's fallback when `DD_SERVICE` is unset).
+    /// Pass `None` to clear it.
+    async fn set_session_default_service_name(name: Option<String>);
+
+    /// Records whether `DD_SERVICE` is currently set for the session
+    /// (per-request mutable; tracer should refresh on RINIT).
+    async fn set_session_user_service_defined(is_defined: bool);
+
     /// Removes the application entry for the given queue ID from the instance.
     ///
     /// # Arguments
diff --git a/datadog-sidecar/src/service/sidecar_server.rs b/datadog-sidecar/src/service/sidecar_server.rs
index 5b2d27b801..c5121f4b66 100644
--- a/datadog-sidecar/src/service/sidecar_server.rs
+++ b/datadog-sidecar/src/service/sidecar_server.rs
@@ -478,7 +478,7 @@ impl SidecarInterface for ConnectionSidecarHandler {
                 .unwrap_or("unknown-service");
             let env = entry.get().env.as_deref().unwrap_or("none");
 
-            let process_tags = session.process_tags.lock_or_panic().clone();
+            let process_tags = session.process_tags_with_svc_source();
 
             // Pre-compute session config so both the primary and retry get_or_create calls
             // can use it without re-locking the session.
@@ -763,8 +763,8 @@ impl SidecarInterface for ConnectionSidecarHandler {
             } else {
                 config.hostname.clone()
             },
-            process_tags: config
-                .process_tags
+            process_tags: session
+                .process_tags_with_svc_source()
                 .iter()
                 .map(|t| t.to_string())
                 .collect::<Vec<_>>()
@@ -828,6 +828,37 @@ impl SidecarInterface for ConnectionSidecarHandler {
             .unwrap_or_default();
         let session = self.server.get_session(session_id);
         *session.process_tags.lock_or_panic() = process_tags;
+        session.refresh_stats_process_tags();
+    }
+
+    async fn set_session_default_service_name(
+        &self,
+        _peer: PeerCredentials,
+        name: Option<String>,
+    ) {
+        let session_id = self
+            .session_id
+            .get()
+            .map(|s| s.as_str())
+            .unwrap_or_default();
+        let session = self.server.get_session(session_id);
+        *session.auto_resolved_service_name.lock_or_panic() = name;
+        session.refresh_stats_process_tags();
+    }
+
+    async fn set_session_user_service_defined(
+        &self,
+        _peer: PeerCredentials,
+        is_defined: bool,
+    ) {
+        let session_id = self
+            .session_id
+            .get()
+            .map(|s| s.as_str())
+            .unwrap_or_default();
+        let session = self.server.get_session(session_id);
+        *session.user_service_defined.lock_or_panic() = is_defined;
+        session.refresh_stats_process_tags();
     }
 
     async fn shutdown_runtime(&self, _peer: PeerCredentials, instance_id: InstanceId) {
diff --git a/datadog-sidecar/src/service/telemetry.rs b/datadog-sidecar/src/service/telemetry.rs
index 9954f6eb41..980d8f127b 100644
--- a/datadog-sidecar/src/service/telemetry.rs
+++ b/datadog-sidecar/src/service/telemetry.rs
@@ -703,7 +703,7 @@ fn get_telemetry_client(
         return None;
     };
 
-    let process_tags = session.process_tags.lock_or_panic().clone();
+    let process_tags = session.process_tags_with_svc_source();
 
     Some(sidecar.telemetry_clients.get_or_create(
         service_name,