diff --git a/Cargo.lock b/Cargo.lock
index 4d99642..21f8a92 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -531,7 +531,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "39cab71617ae0d63f51a36d69f866391735b51691dbda63cf6f96d042b63efeb"
 dependencies = [
  "libc",
- "windows-sys 0.61.2",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -1775,9 +1775,9 @@ dependencies = [
 
 [[package]]
 name = "rmcp"
-version = "3.5.0"
+version = "3.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "fae7019994ae0fe4ada40b732f798f3ff26f0f04facb1477f1bf37eb4f18a2d3"
+checksum = "ab5023804cbe0304cbe06197ac16e645a0a0d0ffa2953bf735c15c2fa0213350"
 dependencies = [
  "async-trait",
  "base64 0.23.1",
@@ -1807,9 +1807,9 @@ dependencies = [
 
 [[package]]
 name = "rmcp-macros"
-version = "3.5.0"
+version = "3.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c7e66877136e969d5a2d561ee5841b17717a0619aebd64abb47a2a2f46e34683"
+checksum = "a1da498700030856ba63114bceefa30ba21f5e278cb975e5b9de3529fe842b40"
 dependencies = [
  "darling",
  "proc-macro2",
@@ -1837,7 +1837,7 @@ dependencies = [
  "errno",
  "libc",
  "linux-raw-sys",
- "windows-sys 0.61.2",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -1894,7 +1894,7 @@ dependencies = [
  "security-framework",
  "security-framework-sys",
  "webpki-root-certs",
- "windows-sys 0.61.2",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -2333,7 +2333,7 @@ dependencies = [
  "getrandom 0.3.4",
  "once_cell",
  "rustix",
- "windows-sys 0.61.2",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -2833,7 +2833,7 @@ version = "0.1.11"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c2a7b1c03c876122aa43f3020e6c3c3ee5c05081c9a00739faf7503aeba10d22"
 dependencies = [
- "windows-sys 0.61.2",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
diff --git a/Cargo.toml b/Cargo.toml
index 8429a64..7dcbefa 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -32,7 +32,7 @@ hex = "0.4"
 http = "1"
 rand = "0.10"
 reqwest = { version = "0.13", default-features = false, features = ["json", "query", "stream", "rustls-no-provider"] }
-rmcp = { version = "3", features = ["server", "transport-io", "transport-streamable-http-server", "transport-streamable-http-server-session"] }
+rmcp = { version = ">=3.5.1,<4", features = ["server", "transport-io", "transport-streamable-http-server", "transport-streamable-http-server-session"] }
 rustls = { version = "0.23", default-features = false, features = ["ring", "std", "tls12", "logging"] }
 schemars = "1"
 serde = { version = "1", features = ["derive"] }
diff --git a/crates/rustsdcmcp/tests/stdio_discover_before_initialize.rs b/crates/rustsdcmcp/tests/stdio_discover_before_initialize.rs
new file mode 100644
index 0000000..d66404a
--- /dev/null
+++ b/crates/rustsdcmcp/tests/stdio_discover_before_initialize.rs
@@ -0,0 +1,181 @@
+//! Regression test for rustsdcmcp#217: a `server/discover` probe sent before
+//! `initialize` must not poison the rest of the session.
+//!
+//! Before rmcp 3.5.1, the stdio session latched `request_metadata_required`
+//! on any client that opened with `server/discover`, and never cleared it even
+//! after the client fell back to the classic (`initialize`-negotiated)
+//! lifecycle. Every later request without `_meta` -- including a completely
+//! ordinary `tools/list` -- was then rejected with -32602 "request _meta is
+//! missing or has malformed required fields", so the client loaded zero
+//! tools. Upstream fix: modelcontextprotocol/rust-sdk#1248.
+//!
+//! This test drives the exact wire sequence from the issue's repro over an
+//! in-memory stdio-shaped transport: `server/discover` (with the 2026-07-28
+//! draft's required `_meta`) -> `initialize` (protocolVersion 2025-11-25) ->
+//! `notifications/initialized` -> `tools/list` with no `_meta` at all.
+
+use rmcp::ServiceExt as _;
+use rustsdcmcp::{KNOWN_TOOLS, SdcHandler};
+use rustsdcmcp_core::{ChangeManager, SdcClient, SdcConfig};
+use serde_json::{Value, json};
+use std::{collections::BTreeSet, sync::Arc, time::Duration};
+use tokio::io::{AsyncBufRead, AsyncBufReadExt, AsyncWrite, AsyncWriteExt, BufReader};
+use tokio_util::sync::CancellationToken;
+
+async fn write_line(write: &mut (impl AsyncWrite + Unpin), value: Value) {
+    let mut bytes = serde_json::to_vec(&value).expect("serialize JSON-RPC message");
+    bytes.push(b'\n');
+    write
+        .write_all(&bytes)
+        .await
+        .expect("write JSON-RPC message");
+    write.flush().await.expect("flush JSON-RPC message");
+}
+
+async fn read_line(reader: &mut (impl AsyncBufRead + Unpin)) -> Value {
+    let mut line = String::new();
+    reader
+        .read_line(&mut line)
+        .await
+        .expect("read JSON-RPC message");
+    serde_json::from_str(line.trim_end()).expect("parse JSON-RPC message")
+}
+
+fn test_handler() -> SdcHandler {
+    let _ = rustls::crypto::ring::default_provider().install_default();
+    let config: SdcConfig = serde_json::from_value(json!({
+        "version": 1,
+        "tenant": "test",
+        "expected_tenant_id": "test",
+        "credential_env": "SDC_TEST_CREDENTIAL",
+        "auth_scheme": "api_key",
+    }))
+    .expect("config");
+    let client = SdcClient::new(
+        &config,
+        mecmcp_secret::OutboundSecret::new_unchecked("test-credential".to_owned()),
+    )
+    .expect("client");
+    let changes = Arc::new(
+        ChangeManager::load(
+            client.clone(),
+            "test",
+            config.endpoint.clone(),
+            None,
+            Duration::from_secs(60),
+            false,
+            None,
+            None,
+        )
+        .expect("changes"),
+    );
+    SdcHandler::new(Arc::<str>::from("test"), client, changes)
+}
+
+#[tokio::test]
+async fn discover_before_initialize_does_not_poison_tools_list() {
+    let handler = test_handler();
+    let (server_stream, client_stream) = tokio::io::duplex(64 * 1024);
+    let shutdown = CancellationToken::new();
+
+    let serve_shutdown = shutdown.clone();
+    let serving = tokio::spawn(async move {
+        if let Ok(service) = handler.serve_with_ct(server_stream, serve_shutdown).await {
+            let _ = service.waiting().await;
+        }
+    });
+
+    let (client_read, mut client_write) = tokio::io::split(client_stream);
+    let mut client_read = BufReader::new(client_read);
+
+    // 1. `server/discover`, carrying the 2026-07-28 draft's required `_meta`
+    //    keys -- the exact probe Claude Code (and similar clients) send
+    //    before deciding whether to use the modern or classic lifecycle.
+    write_line(
+        &mut client_write,
+        json!({
+            "jsonrpc": "2.0",
+            "id": 1,
+            "method": "server/discover",
+            "params": {
+                "_meta": {
+                    "io.modelcontextprotocol/protocolVersion": "2026-07-28",
+                    "io.modelcontextprotocol/clientCapabilities": {},
+                },
+            },
+        }),
+    )
+    .await;
+    let discover_response = read_line(&mut client_read).await;
+    assert!(
+        discover_response.get("error").is_none(),
+        "server/discover should succeed: {discover_response:?}"
+    );
+
+    // 2. Classic `initialize` negotiating 2025-11-25, with no `_meta`.
+    write_line(
+        &mut client_write,
+        json!({
+            "jsonrpc": "2.0",
+            "id": 2,
+            "method": "initialize",
+            "params": {
+                "protocolVersion": "2025-11-25",
+                "capabilities": {},
+                "clientInfo": {"name": "mec-2045-regression-client", "version": "0.0.0"},
+            },
+        }),
+    )
+    .await;
+    let initialize_response = read_line(&mut client_read).await;
+    assert!(
+        initialize_response.get("error").is_none(),
+        "initialize should succeed: {initialize_response:?}"
+    );
+
+    // 3. `notifications/initialized` -- no response expected.
+    write_line(
+        &mut client_write,
+        json!({
+            "jsonrpc": "2.0",
+            "method": "notifications/initialized",
+        }),
+    )
+    .await;
+
+    // 4. `tools/list` with no `_meta` at all: the shape any classic client
+    //    sends, and the one the pre-3.5.1 rmcp pin rejected with -32602
+    //    because `server/discover` had permanently required it.
+    write_line(
+        &mut client_write,
+        json!({
+            "jsonrpc": "2.0",
+            "id": 3,
+            "method": "tools/list",
+        }),
+    )
+    .await;
+    let tools_response = read_line(&mut client_read).await;
+    assert!(
+        tools_response.get("error").is_none(),
+        "tools/list with no _meta must succeed under the classic lifecycle, got: {tools_response:?}"
+    );
+
+    let returned_tools: BTreeSet<String> = tools_response["result"]["tools"]
+        .as_array()
+        .expect("tools array")
+        .iter()
+        .map(|tool| tool["name"].as_str().expect("tool name").to_owned())
+        .collect();
+    let expected_tools: BTreeSet<String> =
+        KNOWN_TOOLS.iter().map(|name| (*name).to_owned()).collect();
+    assert_eq!(
+        returned_tools, expected_tools,
+        "tools/list must return every known tool"
+    );
+
+    shutdown.cancel();
+    drop(client_write);
+    drop(client_read);
+    let _ = serving.await;
+}