diff --git a/Cargo.toml b/Cargo.toml
index 954185a..15403b9 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -15,6 +15,11 @@ smarm-trace = []
 # (D6). The `ActorInfo.budget_cycles` field exists regardless; it just stays 0
 # unless this is enabled.
 budget-accounting = []
+# RFC 016 Chunk 4: the live observer gen_server (src/observer.rs). Off by
+# default (DECISION D10) — the read primitive (Chunks 1–3) is always present
+# and unflagged; only the optional gen_server transport sits behind this, so a
+# release build pays nothing for an observer it never starts.
+observer = []
 # Run-queue selection: exactly one, compile-time (see src/run_queue.rs).
 # Non-default variants need --no-default-features (features are additive).
 rq-mutex = []
diff --git a/src/lib.rs b/src/lib.rs
index f60056b..0552e09 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -28,6 +28,8 @@ pub mod pg;
 pub mod link;
 pub mod gen_server;
 pub mod introspect;
+#[cfg(feature = "observer")]
+pub mod observer;
 pub mod runtime;
 pub(crate) mod raw_mutex;
 pub(crate) mod slot_state;
@@ -59,6 +61,8 @@ pub use introspect::{
     actor_info, snapshot, tree, tree_from, ActorInfo, ActorState, RuntimeSnapshot, RuntimeTree,
     TreeNode, SNAPSHOT_FORMAT_VERSION,
 };
+#[cfg(feature = "observer")]
+pub use observer::{ObserverReply, ObserverRequest};
 pub use link::{link, trap_exit, unlink, ExitSignal};
 pub use monitor::{demonitor, monitor, Down, DownReason, Monitor, MonitorId};
 pub use mutex::{LockTimeout, Mutex, MutexGuard};
diff --git a/src/observer.rs b/src/observer.rs
new file mode 100644
index 0000000..0f77120
--- /dev/null
+++ b/src/observer.rs
@@ -0,0 +1,114 @@
+//! RFC 016 — runtime observability (Chunk 4: the observer gen_server).
+//!
+//! A thin [`GenServer`] that consumes the Chunk-1 read primitive
+//! ([`snapshot`](crate::snapshot) / [`tree`](crate::tree) /
+//! [`actor_info`](crate::actor_info)) over a message interface — the live
+//! `observer` process, in the OTP sense. It is a *transport*, not the
+//! mechanism: the synchronous internal read stays the primitive, and the
+//! observer is just one more consumer of it alongside the test suite. This is
+//! also the read half of the future RFC 003 control plane — the same actor
+//! gains write verbs there rather than a second consumer being spun up
+//! (DECISION D12).
+//!
+//! ## Why it is feature-gated (DECISION D10)
+//!
+//! The read primitive (Chunks 1–3) is always present and unflagged: it is pure
+//! reads and the test suite leans on it. The *gen_server* sits behind the
+//! `observer` Cargo feature, off by default, matching RFC 003's dev-only
+//! feature-flag stance — a release build pays nothing for a live observer it
+//! never starts.
+//!
+//! ## The protocol is the contract (DECISION D11)
+//!
+//! [`ObserverRequest`] / [`ObserverReply`] *are* the wire contract. They carry
+//! no version field of their own because the payloads already do:
+//! [`RuntimeSnapshot`](crate::RuntimeSnapshot) and
+//! [`RuntimeTree`](crate::RuntimeTree) each carry
+//! [`SNAPSHOT_FORMAT_VERSION`](crate::SNAPSHOT_FORMAT_VERSION) (D1). The owned
+//! snapshot — a potentially large `Vec<ActorInfo>` — travels over the call
+//! channel by value; that is intended, it is exactly what a remote observer
+//! (RFC 011) will serialize across a node boundary.
+
+use crate::gen_server::{GenServer, ServerBuilder, ServerRef};
+use crate::introspect::{actor_info, snapshot, tree};
+use crate::introspect::{ActorInfo, RuntimeSnapshot, RuntimeTree};
+use crate::pid::Pid;
+
+/// A read-only request to the observer. Each verb maps one-to-one onto a
+/// Chunk-1 read; there are deliberately no mutating verbs here (those are RFC
+/// 003, D12).
+#[derive(Debug, Clone)]
+pub enum ObserverRequest {
+    /// Whole-runtime [`snapshot`].
+    Snapshot,
+    /// Parentage forest, folded from a snapshot ([`tree`]).
+    Tree,
+    /// Coherent view of one actor ([`actor_info`]); `None` reply if the pid is
+    /// stale, forged, or names a vacant slot.
+    ActorInfo(Pid),
+}
+
+/// The observer's reply, tagged to match the [`ObserverRequest`] verb. Each
+/// variant wraps the owned Chunk-1 read result unchanged — the observer adds no
+/// interpretation, it is pure transport.
+#[derive(Debug, Clone)]
+pub enum ObserverReply {
+    Snapshot(RuntimeSnapshot),
+    Tree(RuntimeTree),
+    ActorInfo(Option<ActorInfo>),
+}
+
+/// The observer server. Stateless by construction (a ZST): every reply is
+/// derived freshly from the live runtime on each call, so there is nothing to
+/// keep between requests.
+pub struct Observer;
+
+impl GenServer for Observer {
+    type Call = ObserverRequest;
+    type Reply = ObserverReply;
+    /// No async verbs: the observer is request/reply only. `Infallible` is
+    /// uninhabited, so a `cast` can never be constructed and
+    /// [`handle_cast`](GenServer::handle_cast) is statically unreachable.
+    type Cast = core::convert::Infallible;
+    type Info = ();
+    type Timer = ();
+
+    fn handle_call(&mut self, request: ObserverRequest) -> ObserverReply {
+        match request {
+            ObserverRequest::Snapshot => ObserverReply::Snapshot(snapshot()),
+            ObserverRequest::Tree => ObserverReply::Tree(tree()),
+            ObserverRequest::ActorInfo(pid) => ObserverReply::ActorInfo(actor_info(pid)),
+        }
+    }
+
+    fn handle_cast(&mut self, request: core::convert::Infallible) {
+        // Uninhabited: this match has no arms because `Cast` cannot be
+        // constructed. It documents at the type level that the observer takes
+        // no fire-and-forget traffic.
+        match request {}
+    }
+}
+
+/// Spawn the observer under the current actor and hand back its [`ServerRef`].
+/// Shorthand for `ServerBuilder::new(Observer).start()`; use the builder
+/// directly (e.g. `.under(sup)`) to slot it into a supervision tree.
+///
+/// ```
+/// use smarm::run;
+/// use smarm::observer::{self, ObserverRequest, ObserverReply};
+///
+/// run(|| {
+///     let obs = observer::start();
+///
+///     // Ask for a whole-runtime snapshot over the call channel.
+///     let ObserverReply::Snapshot(snap) = obs.call(ObserverRequest::Snapshot).unwrap()
+///     else { panic!("snapshot verb must reply with a snapshot") };
+///
+///     // The observer is itself a scheduled actor, so it appears in the very
+///     // snapshot it produced — transport over the same read every consumer sees.
+///     assert!(snap.actors.iter().any(|a| a.pid == obs.pid()));
+/// });
+/// ```
+pub fn start() -> ServerRef<Observer> {
+    ServerBuilder::new(Observer).start()
+}
diff --git a/tests/observer.rs b/tests/observer.rs
new file mode 100644
index 0000000..909cf07
--- /dev/null
+++ b/tests/observer.rs
@@ -0,0 +1,121 @@
+//! RFC 016 Chunk 4 — the observer gen_server. The whole file is gated on the
+//! `observer` feature (run with `cargo test --features observer`); without it
+//! the module does not exist and there is nothing to compile.
+//!
+//! The point these prove: the observer is *transport over the same reads*. Each
+//! verb returns exactly what the corresponding Chunk-1 primitive would, just
+//! marshalled over the gen_server call channel — so a known spawned actor that
+//! `snapshot()` / `actor_info()` would see is equally visible through the
+//! observer.
+#![cfg(feature = "observer")]
+
+use smarm::observer::{self, ObserverReply, ObserverRequest};
+use smarm::{channel, run, ActorState, SNAPSHOT_FORMAT_VERSION};
+
+#[test]
+fn observer_relays_snapshot_tree_and_actor_info() {
+    run(|| {
+        // A worker parked on an empty gate: a known, stable actor for the
+        // observer to find across all three verbs.
+        let (ready_tx, ready_rx) = channel::<()>();
+        let (gate_tx, gate_rx) = channel::<()>();
+        let worker = smarm::spawn(move || {
+            ready_tx.send(()).unwrap();
+            gate_rx.recv().unwrap();
+        });
+        ready_rx.recv().unwrap();
+
+        let obs = observer::start();
+
+        // Snapshot: the worker is present, and so is the observer itself — it
+        // is a scheduled actor like any other.
+        let ObserverReply::Snapshot(snap) = obs.call(ObserverRequest::Snapshot).unwrap() else {
+            panic!("Snapshot verb must reply Snapshot");
+        };
+        assert_eq!(snap.format_version, SNAPSHOT_FORMAT_VERSION);
+        assert!(
+            snap.actors.iter().any(|a| a.pid == worker.pid()),
+            "observer's snapshot should contain the spawned worker"
+        );
+        assert!(
+            snap.actors.iter().any(|a| a.pid == obs.pid()),
+            "observer should appear in the snapshot it produced"
+        );
+
+        // Tree: same data folded into the parentage forest, same version.
+        let ObserverReply::Tree(t) = obs.call(ObserverRequest::Tree).unwrap() else {
+            panic!("Tree verb must reply Tree");
+        };
+        assert_eq!(t.format_version, SNAPSHOT_FORMAT_VERSION);
+        fn contains(nodes: &[smarm::TreeNode], pid: smarm::Pid) -> bool {
+            nodes
+                .iter()
+                .any(|n| n.info.pid == pid || contains(&n.children, pid))
+        }
+        assert!(
+            contains(&t.roots, worker.pid()),
+            "worker should appear somewhere in the observer's tree"
+        );
+
+        // ActorInfo: coherent single-actor view, matching a direct read.
+        let ObserverReply::ActorInfo(Some(info)) =
+            obs.call(ObserverRequest::ActorInfo(worker.pid())).unwrap()
+        else {
+            panic!("ActorInfo verb must reply ActorInfo(Some) for a live worker");
+        };
+        assert_eq!(info.pid, worker.pid());
+
+        gate_tx.send(()).unwrap();
+        worker.join().unwrap();
+    });
+}
+
+#[test]
+fn observer_reports_none_for_a_forged_pid() {
+    run(|| {
+        let obs = observer::start();
+        // An index that is not in the slab at all — the verb relays the
+        // primitive's `None` faithfully.
+        let forged = smarm::Pid::new(u32::MAX - 1, 0);
+        let ObserverReply::ActorInfo(none) =
+            obs.call(ObserverRequest::ActorInfo(forged)).unwrap()
+        else {
+            panic!("ActorInfo verb must reply ActorInfo");
+        };
+        assert!(none.is_none(), "a forged pid should relay as None");
+    });
+}
+
+#[test]
+fn observer_sees_a_parked_actor_as_parked() {
+    run(|| {
+        let (ready_tx, ready_rx) = channel::<()>();
+        let (gate_tx, gate_rx) = channel::<()>();
+        let worker = smarm::spawn(move || {
+            ready_tx.send(()).unwrap();
+            gate_rx.recv().unwrap(); // nothing to do but park on the gate
+        });
+        ready_rx.recv().unwrap();
+
+        let obs = observer::start();
+        // Bounded poll through the observer until the worker reaches Parked —
+        // proving the live state classification rides the call channel intact.
+        let mut parked = false;
+        for _ in 0..100_000 {
+            let ObserverReply::ActorInfo(info) =
+                obs.call(ObserverRequest::ActorInfo(worker.pid())).unwrap()
+            else {
+                panic!("ActorInfo verb must reply ActorInfo");
+            };
+            if matches!(info, Some(i) if i.state == ActorState::Parked) {
+                parked = true;
+                break;
+            }
+            smarm::yield_now();
+        }
+        assert!(parked, "observer should eventually report the worker as Parked");
+
+        gate_tx.send(()).unwrap();
+        worker.join().unwrap();
+    });
+}