diff --git a/Cargo.toml b/Cargo.toml
index cd505c3..b7cf83d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -1,3 +1,3 @@
 [workspace]
-members = ["crates/agent-lifecycle", "crates/oabctl", "crates/studio-cp", "crates/oab-mcp"]
+members = ["crates/agent-lifecycle", "crates/oabctl", "crates/studio-cp", "crates/oab-mcp", "crates/acp-tunnel"]
 resolver = "2"
diff --git a/crates/acp-tunnel/Cargo.toml b/crates/acp-tunnel/Cargo.toml
new file mode 100644
index 0000000..976b4f7
--- /dev/null
+++ b/crates/acp-tunnel/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "acp-tunnel"
+version = "0.1.0"
+edition = "2021"
+description = "Studio-side reverse-MCP-over-ACP protocol: the /acp session lifecycle and tunnel framing Studio speaks as an ACP WebSocket client (reverse-MCP client ADR, Part B)"
+license = "MIT"
+
+[dependencies]
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
diff --git a/crates/acp-tunnel/src/lib.rs b/crates/acp-tunnel/src/lib.rs
new file mode 100644
index 0000000..a7b6360
--- /dev/null
+++ b/crates/acp-tunnel/src/lib.rs
@@ -0,0 +1,335 @@
+//! Studio-side reverse-MCP-over-ACP: the `/acp` **session lifecycle** and tunnel
+//! framing Studio speaks as an **ACP WebSocket client** (reverse-MCP client ADR,
+//! Part B; upstream openab #1447 + the MCP-over-ACP tunnel contract).
+//!
+//! This crate is **pure protocol** — JSON in, JSON out, a small state machine —
+//! with **no** transport, Tauri, or AWS dependency, so it is unit-tested in the
+//! root workspace (`cargo test --workspace`) and reused by:
+//!   - `src-tauri` — drives it over a real `tokio-tungstenite` `/acp` WS and
+//!     relays tunnel tool calls to the running `oab-mcp` sidecar;
+//!   - a future headless reverse-MCP client, if one is ever needed.
+//!
+//! **Scope of this module (slice 3): the session lifecycle** — dial-time framing
+//! (the bearer sub-protocol), the `initialize` → `session/new` → (`session/resume`
+//! on reconnect) handshake, and the `mcpServers` declaration that publishes
+//! Studio's `oab-mcp` surface. The inbound tunnel dispatch (`mcp/connect` /
+//! `mcp/message` / …) lands in slice 4.
+//!
+//! What is **contract-specified** (owned fully here): the `type:"acp"` server
+//! declaration `{id, name}`, the JSON-RPC envelope, id correlation, method names,
+//! and the [`limits`]. What is **standard ACP** and confirmed against the live
+//! endpoint when the transport is wired: the exact `initialize` params — passed
+//! through by the transport (see [`Session::initialize`]).
+
+use serde::{Deserialize, Serialize};
+use serde_json::{json, Value};
+
+/// The ACP sub-protocol token the server echoes back on the `/acp` upgrade.
+pub const ACP_SUBPROTOCOL: &str = "acp.v1";
+
+/// The WebSocket `Sec-WebSocket-Protocol` offer for a bearer-authed `/acp` dial:
+/// `openab.bearer.<token>, acp.v1` (tunnel contract §1). The server echoes only
+/// `acp.v1`. The token is a secret — this returns it for the transport to place
+/// in the handshake header; never log the result.
+pub fn bearer_subprotocol(token: &str) -> String {
+    format!("openab.bearer.{token}, {ACP_SUBPROTOCOL}")
+}
+
+/// Protocol limits Studio must respect (tunnel contract §7). The transport
+/// enforces them; they live here so both the transport and its tests share one
+/// source.
+pub mod limits {
+    /// One tunnelled `mcp/message` request; the server's default, under the ACP
+    /// idle ceiling.
+    pub const TUNNEL_REQUEST_TIMEOUT_SECS: u64 = 170;
+    /// `mcp/connect` + the `initialize` that follows it.
+    pub const CONNECT_HANDSHAKE_TIMEOUT_SECS: u64 = 30;
+    /// `type:acp` entries accepted per `session/new`.
+    pub const MAX_SERVERS_PER_SESSION: usize = 8;
+    /// Any inbound frame; exceeding it closes the connection.
+    pub const MAX_FRAME_BYTES: usize = 8 * 1024 * 1024;
+    /// A method-bearing frame (request or notification).
+    pub const MAX_METHOD_FRAME_BYTES: usize = 1024 * 1024;
+}
+
+/// A `type:acp` MCP-server declaration Studio publishes on `session/new`
+/// (tunnel contract §2). `id` is minted fresh **per connection** (a UUID, by the
+/// transport) and used as the `acpId` in `mcp/connect`; `name` is **stable**
+/// across reconnects and is what a tool prefix resolves by. For Studio there is
+/// a single declaration, `name = "oab"` (reverse-MCP client ADR, D1).
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub struct ServerDecl {
+    pub id: String,
+    pub name: String,
+}
+
+impl ServerDecl {
+    /// The wire form: `{ "type": "acp", "id": …, "name": … }`.
+    pub fn to_json(&self) -> Value {
+        json!({ "type": "acp", "id": self.id, "name": self.name })
+    }
+}
+
+/// The single server Studio declares (D1): one `oab` server, fleet-parameterized
+/// tools. `id` is the per-connection UUID the transport mints.
+pub fn oab_server(connection_id: &str) -> ServerDecl {
+    ServerDecl {
+        id: connection_id.to_string(),
+        name: "oab".to_string(),
+    }
+}
+
+// ---- JSON-RPC 2.0 envelope helpers ------------------------------------------
+// All `/acp` frames are JSON-RPC 2.0 (tunnel contract §1). Kept tiny and shared
+// so request/response/notification framing is written once.
+
+/// A JSON-RPC **request** (has an `id`, so a response is owed).
+pub fn request(id: u64, method: &str, params: Value) -> Value {
+    json!({ "jsonrpc": "2.0", "id": id, "method": method, "params": params })
+}
+
+/// A JSON-RPC **notification** (no `id`, fire-and-forget).
+pub fn notification(method: &str, params: Value) -> Value {
+    json!({ "jsonrpc": "2.0", "method": method, "params": params })
+}
+
+/// A successful JSON-RPC **response** to request `id`.
+pub fn response(id: Value, result: Value) -> Value {
+    json!({ "jsonrpc": "2.0", "id": id, "result": result })
+}
+
+/// A JSON-RPC **error** response to request `id`.
+pub fn error_response(id: Value, code: i64, message: &str) -> Value {
+    json!({ "jsonrpc": "2.0", "id": id, "error": { "code": code, "message": message } })
+}
+
+/// Where the session is in the `initialize → session/new` handshake.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum Phase {
+    /// Nothing sent yet (fresh, or after a disconnect).
+    New,
+    /// `initialize` sent, awaiting its result.
+    Initializing,
+    /// `initialize` acked; ready to open (or resume) a session.
+    Initialized,
+    /// A session exists; the `oab` server is declared and tools are reachable.
+    SessionActive,
+}
+
+/// The client-side session state machine. It **produces** the outbound frames
+/// (`initialize`, `session/new`, `session/resume`) and tracks the phase + the
+/// declaration set; the transport owns the socket and feeds results back in.
+/// One `Session` per logical connection; on reconnect the transport mints a new
+/// connection id (hence a new [`ServerDecl::id`]) and calls [`Session::resume`].
+#[derive(Debug, Clone)]
+pub struct Session {
+    servers: Vec<ServerDecl>,
+    next_id: u64,
+    session_id: Option<String>,
+    phase: Phase,
+}
+
+impl Session {
+    /// A fresh session that will declare `servers` (at most
+    /// [`limits::MAX_SERVERS_PER_SESSION`]; Studio declares exactly one).
+    pub fn new(servers: Vec<ServerDecl>) -> Self {
+        Session {
+            servers,
+            next_id: 1,
+            session_id: None,
+            phase: Phase::New,
+        }
+    }
+
+    pub fn phase(&self) -> Phase {
+        self.phase
+    }
+
+    pub fn session_id(&self) -> Option<&str> {
+        self.session_id.as_deref()
+    }
+
+    /// The declared servers in wire form — the `mcpServers` array reused by
+    /// `session/new` and every `session/resume`.
+    pub fn declarations(&self) -> Vec<Value> {
+        self.servers.iter().map(ServerDecl::to_json).collect()
+    }
+
+    /// Allocate the next JSON-RPC request id (monotonic per session).
+    fn alloc_id(&mut self) -> u64 {
+        let id = self.next_id;
+        self.next_id += 1;
+        id
+    }
+
+    /// Build the `initialize` request and advance to [`Phase::Initializing`].
+    /// `params` is the standard-ACP initialize payload (protocol version /
+    /// client capabilities) the transport supplies against the ACP version in
+    /// use — this crate owns the envelope + id + phase, not the ACP schema.
+    /// Returns `(id, frame)`.
+    pub fn initialize(&mut self, params: Value) -> (u64, Value) {
+        let id = self.alloc_id();
+        self.phase = Phase::Initializing;
+        (id, request(id, "initialize", params))
+    }
+
+    /// Record a successful `initialize` result and advance to
+    /// [`Phase::Initialized`].
+    pub fn on_initialized(&mut self) {
+        self.phase = Phase::Initialized;
+    }
+
+    /// Build the `session/new` request that **declares** Studio's servers
+    /// (tunnel contract §2). Returns `(id, frame)`.
+    pub fn open_session(&mut self, cwd: &str) -> (u64, Value) {
+        let id = self.alloc_id();
+        let frame = request(
+            id,
+            "session/new",
+            json!({ "cwd": cwd, "mcpServers": self.declarations() }),
+        );
+        (id, frame)
+    }
+
+    /// Record the created session id and advance to [`Phase::SessionActive`].
+    pub fn on_session_created(&mut self, session_id: impl Into<String>) {
+        self.session_id = Some(session_id.into());
+        self.phase = Phase::SessionActive;
+    }
+
+    /// Build the `session/resume` request after a reconnect. A resume
+    /// **re-presents the whole declaration set** — anything omitted is withdrawn
+    /// (tunnel contract §7) — so the transport must have refreshed the per-
+    /// connection [`ServerDecl::id`]s (via [`Session::redeclare`]) first. Returns
+    /// `None` if there is no session to resume yet.
+    pub fn resume(&mut self, cwd: &str) -> Option<(u64, Value)> {
+        let session_id = self.session_id.clone()?;
+        let id = self.alloc_id();
+        let frame = request(
+            id,
+            "session/resume",
+            json!({
+                "sessionId": session_id,
+                "cwd": cwd,
+                "mcpServers": self.declarations(),
+            }),
+        );
+        Some((id, frame))
+    }
+
+    /// Replace the declaration set — used on reconnect to swap in fresh per-
+    /// connection server ids before a [`Session::resume`], and to reset the phase
+    /// to [`Phase::New`] so a full `initialize` runs on the new socket.
+    pub fn redeclare(&mut self, servers: Vec<ServerDecl>) {
+        self.servers = servers;
+        self.phase = Phase::New;
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn bearer_subprotocol_pairs_the_token_with_acp_v1() {
+        assert_eq!(
+            bearer_subprotocol("tok123"),
+            "openab.bearer.tok123, acp.v1"
+        );
+    }
+
+    #[test]
+    fn server_decl_wire_form_is_type_acp() {
+        let d = oab_server("conn-uuid");
+        assert_eq!(
+            d.to_json(),
+            json!({ "type": "acp", "id": "conn-uuid", "name": "oab" })
+        );
+    }
+
+    #[test]
+    fn envelopes_match_jsonrpc_2_0() {
+        assert_eq!(
+            request(7, "initialize", json!({})),
+            json!({ "jsonrpc": "2.0", "id": 7, "method": "initialize", "params": {} })
+        );
+        // a notification carries no id
+        let n = notification("notifications/initialized", json!({}));
+        assert_eq!(n["jsonrpc"], "2.0");
+        assert_eq!(n["method"], "notifications/initialized");
+        assert!(n.get("id").is_none());
+        assert_eq!(
+            response(json!(3), json!({ "ok": true })),
+            json!({ "jsonrpc": "2.0", "id": 3, "result": { "ok": true } })
+        );
+        assert_eq!(
+            error_response(json!(3), -32601, "nope")["error"]["code"],
+            json!(-32601)
+        );
+    }
+
+    #[test]
+    fn lifecycle_walks_new_to_session_active_with_monotonic_ids() {
+        let mut s = Session::new(vec![oab_server("c1")]);
+        assert_eq!(s.phase(), Phase::New);
+
+        let (init_id, init) = s.initialize(json!({ "protocolVersion": "x" }));
+        assert_eq!(init_id, 1);
+        assert_eq!(init["method"], "initialize");
+        assert_eq!(init["params"]["protocolVersion"], "x");
+        assert_eq!(s.phase(), Phase::Initializing);
+
+        s.on_initialized();
+        assert_eq!(s.phase(), Phase::Initialized);
+
+        let (new_id, frame) = s.open_session("/work");
+        assert_eq!(new_id, 2); // monotonic
+        assert_eq!(frame["method"], "session/new");
+        assert_eq!(frame["params"]["cwd"], "/work");
+        // the oab server is declared, in wire form
+        assert_eq!(
+            frame["params"]["mcpServers"],
+            json!([{ "type": "acp", "id": "c1", "name": "oab" }])
+        );
+
+        s.on_session_created("sess-42");
+        assert_eq!(s.phase(), Phase::SessionActive);
+        assert_eq!(s.session_id(), Some("sess-42"));
+    }
+
+    #[test]
+    fn resume_redeclares_the_whole_set_with_the_fresh_connection_id() {
+        let mut s = Session::new(vec![oab_server("c1")]);
+        s.initialize(json!({}));
+        s.on_initialized();
+        s.open_session("/w");
+        s.on_session_created("sess-1");
+
+        // reconnect: fresh per-connection id, phase resets so initialize re-runs
+        s.redeclare(vec![oab_server("c2")]);
+        assert_eq!(s.phase(), Phase::New);
+        s.initialize(json!({}));
+        s.on_initialized();
+
+        let (_id, frame) = s.resume("/w").expect("a session exists to resume");
+        assert_eq!(frame["method"], "session/resume");
+        assert_eq!(frame["params"]["sessionId"], "sess-1");
+        // the whole set is re-presented, with the NEW connection id
+        assert_eq!(
+            frame["params"]["mcpServers"],
+            json!([{ "type": "acp", "id": "c2", "name": "oab" }])
+        );
+    }
+
+    #[test]
+    fn resume_is_none_before_a_session_exists() {
+        let mut s = Session::new(vec![oab_server("c1")]);
+        assert!(s.resume("/w").is_none());
+    }
+
+    #[test]
+    fn studios_single_declaration_is_within_the_server_cap() {
+        let s = Session::new(vec![oab_server("c1")]);
+        assert!(s.declarations().len() <= limits::MAX_SERVERS_PER_SESSION);
+    }
+}