diff --git a/crates/acp-tunnel/src/lib.rs b/crates/acp-tunnel/src/lib.rs
index 139edb8..9a01e4f 100644
--- a/crates/acp-tunnel/src/lib.rs
+++ b/crates/acp-tunnel/src/lib.rs
@@ -29,6 +29,16 @@ pub mod config;
 /// The ACP sub-protocol token the server echoes back on the `/acp` upgrade.
 pub const ACP_SUBPROTOCOL: &str = "acp.v1";
 
+/// The ACP protocol version Studio negotiates on the `/acp` `initialize`.
+///
+/// ACP versions the wire protocol with a **u16 integer** — unlike MCP, which
+/// uses a date string (e.g. `"2024-11-05"`). The gateway deserializes this
+/// field as `u16`, so sending the MCP string fails with
+/// `invalid type: string, expected u16`. The *tunnelled* MCP `initialize`
+/// (what Studio relays to the `oab-mcp` sidecar) still uses the MCP date string —
+/// only the outer `/acp` handshake uses this integer.
+pub const ACP_PROTOCOL_VERSION: u16 = 1;
+
 /// 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
@@ -350,6 +360,17 @@ mod tests {
         );
     }
 
+    #[test]
+    fn acp_protocol_version_is_an_integer_not_an_mcp_date_string() {
+        // ACP versions the protocol with a u16; MCP uses a date string. The
+        // `/acp` gateway deserializes `protocolVersion` as u16, so it must
+        // serialize to a JSON number — a string regresses to
+        // `invalid type: string, expected u16`.
+        let v = json!({ "protocolVersion": ACP_PROTOCOL_VERSION });
+        assert!(v["protocolVersion"].is_number());
+        assert_eq!(v["protocolVersion"], json!(1));
+    }
+
     #[test]
     fn acp_subprotocol_is_a_single_clean_token() {
         // Native transports offer this verbatim as one `Sec-WebSocket-Protocol`
diff --git a/src-tauri/src/remote.rs b/src-tauri/src/remote.rs
index a59faab..286fe37 100644
--- a/src-tauri/src/remote.rs
+++ b/src-tauri/src/remote.rs
@@ -164,8 +164,12 @@ async fn run_once<R: Runtime>(
     let mut session = Session::new(vec![acp::oab_server(&conn_id)]);
 
     // Drive the client handshake: initialize, then session/new declaring "oab".
+    // This is the outer **ACP** handshake — `protocolVersion` is a u16 integer
+    // (the gateway deserializes it as `u16`). Do not copy MCP's date string here;
+    // the MCP date string is correct only for the *tunnelled* inner `initialize`
+    // in `handle_inner` below.
     let (_id, init) = session.initialize(json!({
-        "protocolVersion": "2024-11-05",
+        "protocolVersion": acp::ACP_PROTOCOL_VERSION,
         "clientCapabilities": {},
         "clientInfo": { "name": "oab-studio", "version": env!("CARGO_PKG_VERSION") }
     }));