diff --git a/_primitives/_rust/Cargo.lock b/_primitives/_rust/Cargo.lock
index ea460d8..8390eb3 100644
--- a/_primitives/_rust/Cargo.lock
+++ b/_primitives/_rust/Cargo.lock
@@ -3412,6 +3412,7 @@ dependencies = [
  "rusqlite",
  "serde",
  "serde_json",
+ "serial_test",
  "shell-words",
  "tempfile",
  "thiserror 1.0.69",
@@ -6241,6 +6242,15 @@ dependencies = [
  "winapi-util",
 ]
 
+[[package]]
+name = "scc"
+version = "2.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "46e6f046b7fef48e2660c57ed794263155d713de679057f2d0c169bfc6e756cc"
+dependencies = [
+ "sdd",
+]
+
 [[package]]
 name = "schannel"
 version = "0.1.29"
@@ -6266,6 +6276,12 @@ dependencies = [
  "untrusted",
 ]
 
+[[package]]
+name = "sdd"
+version = "3.0.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "490dcfcbfef26be6800d11870ff2df8774fa6e86d047e3e8c8a76b25655e41ca"
+
 [[package]]
 name = "secrecy"
 version = "0.8.0"
@@ -6555,6 +6571,32 @@ dependencies = [
  "serial-core",
 ]
 
+[[package]]
+name = "serial_test"
+version = "3.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "911bd979bf1070a3f3aa7b691a3b3e9968f339ceeec89e08c280a8a22207a32f"
+dependencies = [
+ "futures-executor",
+ "futures-util",
+ "log",
+ "once_cell",
+ "parking_lot 0.12.5",
+ "scc",
+ "serial_test_derive",
+]
+
+[[package]]
+name = "serial_test_derive"
+version = "3.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0a7d91949b85b0d2fb687445e448b40d322b6b3e4af6b44a29b21d9a5f33e6d9"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.117",
+]
+
 [[package]]
 name = "sha1"
 version = "0.10.6"
diff --git a/_primitives/_rust/kei-cortex/Cargo.toml b/_primitives/_rust/kei-cortex/Cargo.toml
index 9f76cec..7313a42 100644
--- a/_primitives/_rust/kei-cortex/Cargo.toml
+++ b/_primitives/_rust/kei-cortex/Cargo.toml
@@ -67,3 +67,4 @@ kei-token-tracker = { path = "../kei-token-tracker" }
 [dev-dependencies]
 reqwest = { workspace = true, features = ["blocking"] }
 wiremock = { workspace = true }
+serial_test = "3"
diff --git a/_primitives/_rust/kei-cortex/tests/common/mod.rs b/_primitives/_rust/kei-cortex/tests/common/mod.rs
index 8c5eaa4..84acfa9 100644
--- a/_primitives/_rust/kei-cortex/tests/common/mod.rs
+++ b/_primitives/_rust/kei-cortex/tests/common/mod.rs
@@ -153,6 +153,12 @@ impl MockAnthropicServer {
 /// Spin up a wiremock server mounted with a canned `/v1/messages`
 /// reply. Bind happens on `127.0.0.1:0` via wiremock's own listener,
 /// which is reliable across macOS / Linux GitHub runners.
+///
+/// Includes a warm-up GET against the bound port so we only return
+/// once the listener is actually accepting connections — this closes
+/// the race where the first test under parallel `cargo test`
+/// dispatches an HTTP request to the mock before its acceptor loop
+/// is ready (manifests as `error sending request for url …`).
 async fn build_mock(text: &str) -> MockAnthropicServer {
     use wiremock::matchers::{method, path};
     use wiremock::{Mock, MockServer, ResponseTemplate};
@@ -167,31 +173,54 @@ async fn build_mock(text: &str) -> MockAnthropicServer {
         .respond_with(ResponseTemplate::new(200).set_body_json(body))
         .mount(&server)
         .await;
+    // Warm-up probe: a HEAD against an unmounted route returns 404 but
+    // confirms the listener is accepting. Retry briefly under cold CI.
+    let probe_url = server.uri();
+    for attempt in 0..20 {
+        if reqwest::Client::new()
+            .head(&probe_url)
+            .timeout(std::time::Duration::from_millis(200))
+            .send()
+            .await
+            .is_ok()
+        {
+            break;
+        }
+        if attempt == 19 {
+            panic!("wiremock listener did not respond to warm-up probe after 1s");
+        }
+        tokio::time::sleep(std::time::Duration::from_millis(50)).await;
+    }
     let uri = format!("{}/v1/messages", server.uri());
     MockAnthropicServer { server, uri }
 }
 
 /// Per-call mock variant. Spawns a fresh wiremock instance with the
 /// given canned reply text. Each instance keeps its server alive for
-/// the lifetime of the returned handle (drop = stop).
+/// the lifetime of the returned handle.
+///
+/// Implementation note: wiremock's hyper server is tied to the runtime
+/// that called `MockServer::start().await`. We build a dedicated
+/// **multi-thread** runtime on a helper OS-thread and keep it alive by
+/// blocking on `pending()` — this guarantees the runtime keeps driving
+/// async tasks (the previous `std::thread::park()` froze the worker
+/// thread, starving wiremock of CPU and producing
+/// `error sending request for url …` on CI under parallel tests).
 pub fn mock_anthropic_responding_with(text: &'static str) -> MockAnthropicServer {
-    // The caller is inside a `#[tokio::test]` runtime; build on it via
-    // a one-shot thread + current-thread runtime to avoid nested-runtime
-    // panics on tests that already hold a multi-thread runtime.
     let (tx, rx) = std::sync::mpsc::channel::<MockAnthropicServer>();
     let owned = text.to_string();
     std::thread::spawn(move || {
-        let rt = tokio::runtime::Builder::new_current_thread()
+        let rt = tokio::runtime::Builder::new_multi_thread()
             .enable_all()
+            .worker_threads(2)
             .build()
             .expect("mock-runtime build");
         let mock = rt.block_on(async move { build_mock(&owned).await });
         tx.send(mock).expect("send mock back");
-        // Keep this runtime alive — wiremock's internal hyper server is
-        // tied to it. We park the thread; `MockAnthropicServer` is now
-        // owned by the caller and will Drop normally when test scope
-        // ends. The runtime drops with the thread.
-        std::thread::park();
+        // Hold the runtime open for the rest of the process — wiremock
+        // needs it to accept incoming requests. `pending()` yields
+        // forever; worker threads keep processing the hyper server.
+        rt.block_on(std::future::pending::<()>());
     });
     rx.recv().expect("mock channel closed")
 }
diff --git a/_primitives/_rust/kei-cortex/tests/openai_compat.rs b/_primitives/_rust/kei-cortex/tests/openai_compat.rs
index 68b0f9a..0cd5bfb 100644
--- a/_primitives/_rust/kei-cortex/tests/openai_compat.rs
+++ b/_primitives/_rust/kei-cortex/tests/openai_compat.rs
@@ -15,6 +15,7 @@
 //! is picked up without any source-side wiring.
 
 mod common;
+use serial_test::serial;
 
 use axum::body::{to_bytes, Body};
 use axum::http::{Request, StatusCode};
@@ -53,6 +54,7 @@ fn auth_request(method: &str, uri: &str, body: Body) -> Request<Body> {
         .unwrap()
 }
 
+#[serial]
 #[tokio::test]
 async fn list_models_returns_kei_cortex() {
     ensure_api_key();
@@ -68,6 +70,7 @@ async fn list_models_returns_kei_cortex() {
     assert!(body["data"].as_array().unwrap().len() >= 1);
 }
 
+#[serial]
 #[tokio::test]
 async fn unauthorized_without_bearer() {
     // Use the same key value as `ensure_api_key` so this test does not
@@ -88,6 +91,7 @@ async fn unauthorized_without_bearer() {
     assert_eq!(resp.status(), StatusCode::UNAUTHORIZED);
 }
 
+#[serial]
 #[tokio::test]
 async fn chat_completions_sync_returns_choices() {
     ensure_api_key();
@@ -114,6 +118,7 @@ async fn chat_completions_sync_returns_choices() {
     assert!(v["choices"][0]["message"]["content"].is_string());
 }
 
+#[serial]
 #[tokio::test]
 async fn chat_completions_stream_emits_sse() {
     ensure_api_key();
@@ -140,6 +145,7 @@ async fn chat_completions_stream_emits_sse() {
     assert!(s.contains("data:"), "expected SSE data frame, got: {s}");
 }
 
+#[serial]
 #[tokio::test]
 async fn run_create_returns_202_and_id() {
     ensure_api_key();
@@ -177,6 +183,7 @@ async fn run_create_returns_202_and_id() {
     ));
 }
 
+#[serial]
 #[tokio::test]
 async fn responses_get_unknown_id_returns_404() {
     ensure_api_key();
diff --git a/_primitives/_rust/kei-cortex/tests/openai_loop_wiring.rs b/_primitives/_rust/kei-cortex/tests/openai_loop_wiring.rs
index 00323bb..ba24583 100644
--- a/_primitives/_rust/kei-cortex/tests/openai_loop_wiring.rs
+++ b/_primitives/_rust/kei-cortex/tests/openai_loop_wiring.rs
@@ -14,6 +14,7 @@
 //! file stays focused (router shape there, loop wiring here).
 
 mod common;
+use serial_test::serial;
 
 use axum::body::{to_bytes, Body};
 use axum::http::{Request, StatusCode};
@@ -58,6 +59,7 @@ fn auth_request(method: &str, uri: &str, body: Body) -> Request<Body> {
 /// Sync /v1/chat/completions — response carries the mock's "hi" and NOT
 /// the legacy stub echo. Confirms `agent_runner::collect_reply` is the
 /// production code path.
+#[serial]
 #[tokio::test]
 async fn sync_chat_completions_runs_real_loop_not_stub() {
     ensure_env();
@@ -94,6 +96,7 @@ async fn sync_chat_completions_runs_real_loop_not_stub() {
 
 /// Streaming /v1/chat/completions — SSE body carries `delta` chunks fed
 /// by the real loop. No stub marker; finish frame present.
+#[serial]
 #[tokio::test]
 async fn streaming_chat_completions_runs_real_loop_not_stub() {
     ensure_env();
@@ -144,6 +147,7 @@ async fn streaming_chat_completions_runs_real_loop_not_stub() {
 ///   4. The terminal `[DONE]` sentinel is the last `data:` line.
 ///   5. The finish-chunk index is strictly less than the sentinel index
 ///      (proves ordering: finish, then sentinel — not bundled).
+#[serial]
 #[tokio::test]
 async fn streaming_chat_completions_emits_chunked_frames_not_single_blob() {
     ensure_env();
@@ -216,6 +220,7 @@ async fn streaming_chat_completions_emits_chunked_frames_not_single_blob() {
 /// /v1/responses — sync mode — response.output[0].text carries the
 /// mock reply, not the stub echo. Witnesses `responses::handle_sync`
 /// is wired through `agent_runner::collect_reply` (P1.1.c).
+#[serial]
 #[tokio::test]
 async fn sync_responses_runs_real_loop_not_stub() {
     ensure_env();
@@ -250,6 +255,7 @@ async fn sync_responses_runs_real_loop_not_stub() {
 /// frames produced by the real loop and finishes with `response.completed`.
 /// Witnesses `responses::handle_stream` is wired through
 /// `agent_runner::stream_events` + `stream_forwarder::forward_responses`.
+#[serial]
 #[tokio::test]
 async fn streaming_responses_runs_real_loop_not_stub() {
     ensure_env();
@@ -290,6 +296,7 @@ async fn streaming_responses_runs_real_loop_not_stub() {
 /// are wired through `agent_runner::stream_events` +
 /// `stream_forwarder::forward_runs` (P1.1.d). The previous `run_stub`
 /// would have leaked `[run stub] echo:` into the delta payload.
+#[serial]
 #[tokio::test]
 async fn runs_events_stream_runs_real_loop_not_stub() {
     ensure_env();
@@ -354,6 +361,7 @@ async fn runs_events_stream_runs_real_loop_not_stub() {
 /// consumed, the registry slot's receiver is taken; a second GET must
 /// 404. Witnesses the `take_receiver` first-subscriber-wins contract is
 /// preserved by the real-loop wiring (it was true under the stub too).
+#[serial]
 #[tokio::test]
 async fn runs_events_second_subscriber_returns_not_found() {
     ensure_env();