diff --git a/Cargo.lock b/Cargo.lock
index aaf2a66..1f02212 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -55,6 +55,7 @@ dependencies = [
  "async-trait",
  "futures",
  "irpc",
+ "parking_lot",
  "serde",
  "serde_json",
  "thiserror 2.0.18",
diff --git a/crates/alknet-call/Cargo.toml b/crates/alknet-call/Cargo.toml
index a9adfd6..c5ec659 100644
--- a/crates/alknet-call/Cargo.toml
+++ b/crates/alknet-call/Cargo.toml
@@ -22,4 +22,5 @@ async-trait = "0.1"
 tracing = "0.1"
 thiserror = "2"
 uuid = { version = "1", features = ["v4"] }
-futures = "0.3"
\ No newline at end of file
+futures = "0.3"
+parking_lot = "0.12"
\ No newline at end of file
diff --git a/crates/alknet-call/src/protocol/connection.rs b/crates/alknet-call/src/protocol/connection.rs
index e7b3b07..e42862e 100644
--- a/crates/alknet-call/src/protocol/connection.rs
+++ b/crates/alknet-call/src/protocol/connection.rs
@@ -5,4 +5,573 @@
 //! See `docs/architecture/crates/call/call-protocol.md` for the full
 //! specification.
 
-// TODO: implement
+use std::collections::HashMap;
+use std::pin::Pin;
+use std::sync::Arc;
+use std::task::{Context, Poll};
+use std::time::{Duration, Instant};
+
+use alknet_core::types::Connection;
+use futures::stream::Stream;
+use parking_lot::{Mutex, RwLock};
+use serde_json::Value;
+use tokio::sync::mpsc;
+
+use super::pending::PendingRequestMap;
+use super::wire::{
+    CallError, EventEnvelope, FrameFramedReader, FrameFramedWriter, EVENT_ABORTED, EVENT_COMPLETED,
+    EVENT_ERROR, EVENT_RESPONDED,
+};
+use crate::protocol::wire::ResponseEnvelope;
+use crate::registry::context::{
+    generate_request_id, AbortPolicy, OperationContext, ScopedOperationEnv,
+};
+use crate::registry::env::OperationEnv;
+use crate::registry::registration::{Handler, HandlerRegistration};
+
+const DEFAULT_CALL_TIMEOUT: Duration = Duration::from_secs(30);
+
+pub struct CallConnection {
+    connection: Arc<Connection>,
+    imported_operations: Arc<RwLock<HashMap<String, HandlerRegistration>>>,
+    pending: Arc<Mutex<PendingRequestMap>>,
+}
+
+impl CallConnection {
+    pub fn new(connection: Connection) -> Self {
+        Self {
+            connection: Arc::new(connection),
+            imported_operations: Arc::new(RwLock::new(HashMap::new())),
+            pending: Arc::new(Mutex::new(PendingRequestMap::new())),
+        }
+    }
+
+    pub fn connection(&self) -> &Arc<Connection> {
+        &self.connection
+    }
+
+    pub fn register_imported(&self, registration: HandlerRegistration) {
+        let name = registration.spec.name.clone();
+        self.imported_operations.write().insert(name, registration);
+    }
+
+    pub fn register_imported_all(&self, registrations: Vec<HandlerRegistration>) {
+        let mut overlay = self.imported_operations.write();
+        for reg in registrations {
+            overlay.insert(reg.spec.name.clone(), reg);
+        }
+    }
+
+    pub fn overlay_env(&self) -> Arc<dyn OperationEnv + Send + Sync> {
+        Arc::new(OverlayOperationEnv {
+            overlay: Arc::clone(&self.imported_operations),
+        })
+    }
+
+    pub async fn call(&self, operation_id: &str, input: Value) -> ResponseEnvelope {
+        let request_id = generate_request_id();
+        let payload = serde_json::json!({
+            "operationId": operation_id,
+            "input": input,
+        });
+
+        let (send, recv) = match self.connection.open_bi().await {
+            Ok(pair) => pair,
+            Err(err) => {
+                let call_error = CallError::internal(format!("failed to open stream: {err}"));
+                return ResponseEnvelope::error(request_id, call_error);
+            }
+        };
+
+        let receiver = {
+            let mut pending = self.pending.lock();
+            pending.register_call(request_id.clone(), Instant::now() + DEFAULT_CALL_TIMEOUT)
+        };
+
+        if let Err(err) = self.write_request(send, &request_id, payload).await {
+            let call_error = CallError::internal(err);
+            self.pending
+                .lock()
+                .handle_error(&request_id, call_error.clone());
+            return ResponseEnvelope::error(request_id, call_error);
+        }
+
+        let pending = Arc::clone(&self.pending);
+        tokio::spawn(async move {
+            read_stream_until_closed(recv, &pending).await;
+        });
+
+        match receiver.await {
+            Ok(Ok(value)) => ResponseEnvelope::ok(request_id, value),
+            Ok(Err(error)) => ResponseEnvelope::error(request_id, error),
+            Err(_) => ResponseEnvelope::error(request_id, CallError::internal("request cancelled")),
+        }
+    }
+
+    pub async fn subscribe(
+        &self,
+        operation_id: &str,
+        input: Value,
+    ) -> impl Stream<Item = ResponseEnvelope> {
+        let request_id = generate_request_id();
+        let payload = serde_json::json!({
+            "operationId": operation_id,
+            "input": input,
+        });
+
+        let (send, recv) = match self.connection.open_bi().await {
+            Ok(pair) => pair,
+            Err(err) => {
+                let call_error = CallError::internal(format!("failed to open stream: {err}"));
+                return SubscriptionStream::closed(request_id, call_error);
+            }
+        };
+
+        let receiver = {
+            let mut pending = self.pending.lock();
+            pending.register_subscribe(request_id.clone(), None)
+        };
+
+        if let Err(err) = self.write_request(send, &request_id, payload).await {
+            let call_error = CallError::internal(err);
+            self.pending
+                .lock()
+                .handle_error(&request_id, call_error.clone());
+            return SubscriptionStream::closed(request_id, call_error);
+        }
+
+        let pending = Arc::clone(&self.pending);
+        tokio::spawn(async move {
+            read_stream_until_closed(recv, &pending).await;
+        });
+
+        SubscriptionStream::new(request_id, receiver)
+    }
+
+    pub async fn abort(&self, request_id: &str) {
+        let envelope = EventEnvelope::aborted(request_id);
+        if let Err(err) = self.write_envelope(&envelope).await {
+            tracing::warn!(error = %err, request_id, "failed to send call.aborted");
+            return;
+        }
+        self.pending.lock().handle_aborted(request_id);
+    }
+
+    async fn write_request(
+        &self,
+        send: alknet_core::types::SendStream,
+        request_id: &str,
+        payload: Value,
+    ) -> Result<(), String> {
+        let envelope = EventEnvelope::requested(request_id, payload);
+        let mut writer = FrameFramedWriter::new(send);
+        writer
+            .write_frame(&envelope)
+            .await
+            .map_err(|e| format!("failed to write frame: {e}"))
+    }
+
+    async fn write_envelope(&self, envelope: &EventEnvelope) -> Result<(), String> {
+        let (send, _recv) = self
+            .connection
+            .open_bi()
+            .await
+            .map_err(|e| format!("failed to open stream: {e}"))?;
+        let mut writer = FrameFramedWriter::new(send);
+        writer
+            .write_frame(envelope)
+            .await
+            .map_err(|e| format!("failed to write frame: {e}"))
+    }
+}
+
+async fn read_stream_until_closed(
+    recv: alknet_core::types::RecvStream,
+    pending: &Arc<Mutex<PendingRequestMap>>,
+) {
+    let mut reader = FrameFramedReader::new(recv);
+    while let Ok(envelope) = reader.read_frame().await {
+        dispatch_envelope(pending, envelope);
+    }
+}
+
+fn dispatch_envelope(pending: &Arc<Mutex<PendingRequestMap>>, envelope: EventEnvelope) {
+    let request_id = envelope.id.clone();
+    match envelope.r#type.as_str() {
+        EVENT_RESPONDED => {
+            let output = envelope
+                .payload
+                .get("output")
+                .cloned()
+                .unwrap_or(Value::Null);
+            pending.lock().handle_responded(&request_id, output);
+        }
+        EVENT_COMPLETED => {
+            pending.lock().handle_completed(&request_id);
+        }
+        EVENT_ABORTED => {
+            pending.lock().handle_aborted(&request_id);
+        }
+        EVENT_ERROR => {
+            if let Ok(error) = serde_json::from_value::<CallError>(envelope.payload) {
+                pending.lock().handle_error(&request_id, error);
+            }
+        }
+        _ => {}
+    }
+}
+
+struct OverlayOperationEnv {
+    overlay: Arc<RwLock<HashMap<String, HandlerRegistration>>>,
+}
+
+#[async_trait::async_trait]
+impl OperationEnv for OverlayOperationEnv {
+    async fn invoke_with_policy(
+        &self,
+        namespace: &str,
+        operation: &str,
+        input: Value,
+        parent: &OperationContext,
+        policy: AbortPolicy,
+    ) -> ResponseEnvelope {
+        let name = format!("{namespace}/{operation}");
+
+        if !parent.scoped_env.allows(&name) {
+            return ResponseEnvelope::not_found(parent.request_id.clone(), &name);
+        }
+
+        let handler: Handler;
+        let composition_authority;
+        let scoped_env;
+        {
+            let overlay = self.overlay.read();
+            let Some(registration) = overlay.get(&name) else {
+                return ResponseEnvelope::not_found(parent.request_id.clone(), &name);
+            };
+            handler = Arc::clone(&registration.handler);
+            composition_authority = registration.composition_authority.clone();
+            scoped_env = registration
+                .scoped_env
+                .clone()
+                .unwrap_or_else(ScopedOperationEnv::empty);
+        }
+
+        let context = OperationContext {
+            request_id: generate_request_id(),
+            parent_request_id: Some(parent.request_id.clone()),
+            identity: parent
+                .handler_identity
+                .as_ref()
+                .and_then(|ca| ca.as_identity()),
+            handler_identity: composition_authority,
+            capabilities: parent.capabilities.clone(),
+            metadata: HashMap::new(),
+            abort_policy: policy,
+            deadline: parent.deadline,
+            scoped_env,
+            env: parent.env.clone(),
+            internal: true,
+        };
+
+        handler(input, context).await
+    }
+
+    fn contains(&self, name: &str) -> bool {
+        self.overlay.read().contains_key(name)
+    }
+}
+
+pub struct SubscriptionStream {
+    request_id: String,
+    receiver: mpsc::Receiver<Result<Value, CallError>>,
+    done: bool,
+}
+
+impl SubscriptionStream {
+    fn new(request_id: String, receiver: mpsc::Receiver<Result<Value, CallError>>) -> Self {
+        Self {
+            request_id,
+            receiver,
+            done: false,
+        }
+    }
+
+    fn closed(request_id: String, error: CallError) -> Self {
+        let (tx, rx) = mpsc::channel(1);
+        let _ = tx.try_send(Err(error));
+        Self {
+            request_id,
+            receiver: rx,
+            done: false,
+        }
+    }
+}
+
+impl Stream for SubscriptionStream {
+    type Item = ResponseEnvelope;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        if self.done {
+            return Poll::Ready(None);
+        }
+        let this = self.get_mut();
+        match this.receiver.poll_recv(cx) {
+            Poll::Ready(None) => {
+                this.done = true;
+                Poll::Ready(None)
+            }
+            Poll::Ready(Some(Ok(value))) => {
+                Poll::Ready(Some(ResponseEnvelope::ok(this.request_id.clone(), value)))
+            }
+            Poll::Ready(Some(Err(error))) => {
+                this.done = true;
+                Poll::Ready(Some(ResponseEnvelope::error(
+                    this.request_id.clone(),
+                    error,
+                )))
+            }
+            Poll::Pending => Poll::Pending,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::registry::context::CompositionAuthority;
+    use crate::registry::registration::{make_handler, OperationProvenance};
+    use crate::registry::spec::{AccessControl, OperationSpec, OperationType, Visibility};
+    use alknet_core::types::{Capabilities, MockConnection};
+    use std::collections::HashMap;
+    use std::net::{IpAddr, Ipv4Addr, SocketAddr};
+    use std::sync::Mutex as StdMutex;
+    use std::time::Duration;
+
+    struct StubConnection {
+        alpn: &'static [u8],
+        addr: Option<SocketAddr>,
+        closed: StdMutex<Option<(u32, String)>>,
+    }
+
+    impl MockConnection for StubConnection {
+        fn remote_alpn(&self) -> &[u8] {
+            self.alpn
+        }
+        fn remote_addr(&self) -> Option<SocketAddr> {
+            self.addr
+        }
+        fn close(&self, code: u32, reason: &str) {
+            *self.closed.lock().unwrap() = Some((code, reason.to_string()));
+        }
+    }
+
+    fn stub_connection() -> Connection {
+        Connection::from_mock(Arc::new(StubConnection {
+            alpn: b"alknet/call",
+            addr: Some(SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 4321)),
+            closed: StdMutex::new(None),
+        }))
+    }
+
+    fn external_spec(name: &str) -> OperationSpec {
+        OperationSpec::new(
+            name,
+            OperationType::Query,
+            Visibility::External,
+            serde_json::json!({}),
+            serde_json::json!({}),
+            vec![],
+            AccessControl::default(),
+        )
+    }
+
+    fn echo_handler() -> Handler {
+        make_handler(
+            |input, context| async move { ResponseEnvelope::ok(context.request_id, input) },
+        )
+    }
+
+    fn imported_registration(name: &str) -> HandlerRegistration {
+        HandlerRegistration::new(
+            external_spec(name),
+            echo_handler(),
+            OperationProvenance::FromCall,
+            None,
+            None,
+            Capabilities::new(),
+        )
+    }
+
+    fn root_context(
+        request_id: &str,
+        scoped_env: ScopedOperationEnv,
+        env: Arc<dyn OperationEnv + Send + Sync>,
+    ) -> OperationContext {
+        OperationContext {
+            request_id: request_id.to_string(),
+            parent_request_id: None,
+            identity: None,
+            handler_identity: Some(CompositionAuthority::new("agent", ["fs:read".to_string()])),
+            capabilities: Capabilities::new(),
+            metadata: HashMap::new(),
+            scoped_env,
+            env,
+            abort_policy: AbortPolicy::default(),
+            deadline: Some(Instant::now() + Duration::from_secs(30)),
+            internal: true,
+        }
+    }
+
+    #[test]
+    fn register_imported_adds_to_overlay_and_contains_returns_true() {
+        let conn = CallConnection::new(stub_connection());
+        let env = conn.overlay_env();
+
+        assert!(!env.contains("worker/exec"));
+
+        conn.register_imported(imported_registration("worker/exec"));
+
+        assert!(env.contains("worker/exec"));
+        assert!(!env.contains("worker/missing"));
+    }
+
+    #[test]
+    fn register_imported_all_bulk_adds_to_overlay() {
+        let conn = CallConnection::new(stub_connection());
+        let env = conn.overlay_env();
+
+        conn.register_imported_all(vec![
+            imported_registration("worker/exec"),
+            imported_registration("worker/status"),
+            imported_registration("fs/readFile"),
+        ]);
+
+        assert!(env.contains("worker/exec"));
+        assert!(env.contains("worker/status"));
+        assert!(env.contains("fs/readFile"));
+        assert!(!env.contains("worker/missing"));
+    }
+
+    #[tokio::test]
+    async fn overlay_env_dispatches_to_imported_op() {
+        let conn = CallConnection::new(stub_connection());
+        conn.register_imported(imported_registration("worker/exec"));
+        let env = conn.overlay_env();
+
+        let scoped = ScopedOperationEnv::new(["worker/exec"]);
+        let ctx = root_context("root-1", scoped, env.clone());
+
+        let response = env
+            .invoke("worker", "exec", serde_json::json!({"hi": 1}), &ctx)
+            .await;
+
+        assert!(response.result.is_ok());
+        assert_eq!(response.result.unwrap(), serde_json::json!({"hi": 1}));
+    }
+
+    #[tokio::test]
+    async fn overlay_env_contains_returns_false_for_non_imported_op() {
+        let conn = CallConnection::new(stub_connection());
+        conn.register_imported(imported_registration("worker/exec"));
+        let env = conn.overlay_env();
+
+        assert!(!env.contains("worker/missing"));
+
+        let scoped = ScopedOperationEnv::new(["worker/missing"]);
+        let ctx = root_context("root-2", scoped, env.clone());
+
+        let response = env
+            .invoke("worker", "missing", serde_json::json!({}), &ctx)
+            .await;
+
+        match response.result {
+            Err(e) => assert_eq!(e.code, "NOT_FOUND"),
+            other => panic!("expected NOT_FOUND, got {other:?}"),
+        }
+    }
+
+    #[tokio::test]
+    async fn overlay_env_reachability_check_returns_not_found_for_disallowed_op() {
+        let conn = CallConnection::new(stub_connection());
+        conn.register_imported(imported_registration("worker/exec"));
+        let env = conn.overlay_env();
+
+        let scoped = ScopedOperationEnv::empty();
+        let ctx = root_context("root-3", scoped, env.clone());
+
+        let response = env
+            .invoke("worker", "exec", serde_json::json!({}), &ctx)
+            .await;
+
+        match response.result {
+            Err(e) => assert_eq!(e.code, "NOT_FOUND"),
+            other => panic!("expected NOT_FOUND, got {other:?}"),
+        }
+    }
+
+    #[tokio::test]
+    async fn overlay_env_dispatched_child_has_internal_true_and_parent_set() {
+        let conn = CallConnection::new(stub_connection());
+        let inspect_handler = make_handler(|_input, context| async move {
+            let internal = context.is_internal();
+            let parent_set = context.parent_request_id.is_some();
+            ResponseEnvelope::ok(
+                context.request_id,
+                serde_json::json!({
+                    "internal": internal,
+                    "parent_set": parent_set,
+                }),
+            )
+        });
+        conn.register_imported(HandlerRegistration::new(
+            external_spec("worker/exec"),
+            inspect_handler,
+            OperationProvenance::FromCall,
+            None,
+            None,
+            Capabilities::new(),
+        ));
+        let env = conn.overlay_env();
+
+        let scoped = ScopedOperationEnv::new(["worker/exec"]);
+        let ctx = root_context("root-4", scoped, env.clone());
+
+        let response = env
+            .invoke("worker", "exec", serde_json::json!({}), &ctx)
+            .await;
+        let out = response.result.expect("ok");
+        assert_eq!(out["internal"], Value::Bool(true));
+        assert_eq!(out["parent_set"], Value::Bool(true));
+    }
+
+    #[test]
+    fn connection_accessor_returns_underlying_connection() {
+        let conn = CallConnection::new(stub_connection());
+        assert_eq!(conn.connection().remote_alpn(), b"alknet/call");
+    }
+
+    #[test]
+    fn empty_overlay_contains_nothing() {
+        let conn = CallConnection::new(stub_connection());
+        let env = conn.overlay_env();
+        assert!(!env.contains("anything"));
+        assert!(!env.contains(""));
+    }
+
+    #[test]
+    fn overlay_drops_with_connection() {
+        let captured: Arc<RwLock<HashMap<String, HandlerRegistration>>> =
+            Arc::new(RwLock::new(HashMap::new()));
+        {
+            let conn = CallConnection::new(stub_connection());
+            conn.register_imported(imported_registration("worker/exec"));
+            assert!(conn.overlay_env().contains("worker/exec"));
+            std::mem::swap(
+                &mut *captured.write(),
+                &mut *conn.imported_operations.write(),
+            );
+        }
+        assert!(captured.read().contains_key("worker/exec"));
+    }
+}
diff --git a/crates/alknet-call/src/registry/env.rs b/crates/alknet-call/src/registry/env.rs
index 346ce73..54899f2 100644
--- a/crates/alknet-call/src/registry/env.rs
+++ b/crates/alknet-call/src/registry/env.rs
@@ -199,30 +199,6 @@ mod tests {
         }
     }
 
-    fn external_spec(name: &str, acl: AccessControl) -> OperationSpec {
-        OperationSpec::new(
-            name,
-            OperationType::Query,
-            Visibility::External,
-            serde_json::json!({}),
-            serde_json::json!({}),
-            vec![],
-            acl,
-        )
-    }
-
-    fn internal_spec(name: &str, acl: AccessControl) -> OperationSpec {
-        OperationSpec::new(
-            name,
-            OperationType::Query,
-            Visibility::Internal,
-            serde_json::json!({}),
-            serde_json::json!({}),
-            vec![],
-            acl,
-        )
-    }
-
     fn echo_handler() -> crate::registry::registration::Handler {
         make_handler(
             |input, context| async move { ResponseEnvelope::ok(context.request_id, input) },
diff --git a/crates/alknet-core/src/types.rs b/crates/alknet-core/src/types.rs
index 5dc832f..e5ab82b 100644
--- a/crates/alknet-core/src/types.rs
+++ b/crates/alknet-core/src/types.rs
@@ -368,7 +368,7 @@ enum ConnectionKind {
 }
 
 #[allow(dead_code)]
-pub(crate) trait MockConnection: Send + Sync {
+pub trait MockConnection: Send + Sync {
     fn remote_alpn(&self) -> &[u8];
     fn remote_addr(&self) -> Option<SocketAddr>;
     fn close(&self, code: u32, reason: &str);
@@ -406,7 +406,7 @@ impl Connection {
     }
 
     #[allow(dead_code)]
-    pub(crate) fn from_mock(mock: Arc<dyn MockConnection + Send + Sync>) -> Self {
+    pub fn from_mock(mock: Arc<dyn MockConnection + Send + Sync>) -> Self {
         let alpn = mock.remote_alpn().to_vec();
         Self {
             kind: ConnectionKind::Mock(mock),
@@ -523,6 +523,7 @@ mod tests {
         closed: std::sync::Mutex<Option<(u32, String)>>,
     }
 
+    #[allow(dead_code)]
     impl MockConnection for MockConn {
         fn remote_alpn(&self) -> &[u8] {
             self.alpn