diff --git a/codex-rs/core/src/client.rs b/codex-rs/core/src/client.rs
index 8ec68d02..ae7904b8 100644
--- a/codex-rs/core/src/client.rs
+++ b/codex-rs/core/src/client.rs
@@ -15,6 +15,7 @@ use tokio_util::io::ReaderStream;
 use tracing::debug;
 use tracing::trace;
 use tracing::warn;
+use uuid::Uuid;
 
 use crate::chat_completions::AggregateStreamExt;
 use crate::chat_completions::stream_chat_completions;
@@ -44,6 +45,7 @@ pub struct ModelClient {
     config: Arc<Config>,
     client: reqwest::Client,
     provider: ModelProviderInfo,
+    session_id: Uuid,
     effort: ReasoningEffortConfig,
     summary: ReasoningSummaryConfig,
 }
@@ -54,11 +56,13 @@ impl ModelClient {
         provider: ModelProviderInfo,
         effort: ReasoningEffortConfig,
         summary: ReasoningSummaryConfig,
+        session_id: Uuid,
     ) -> Self {
         Self {
             config,
             client: reqwest::Client::new(),
             provider,
+            session_id,
             effort,
             summary,
         }
@@ -143,6 +147,7 @@ impl ModelClient {
                 .provider
                 .create_request_builder(&self.client)?
                 .header("OpenAI-Beta", "responses=experimental")
+                .header("session_id", self.session_id.to_string())
                 .header(reqwest::header::ACCEPT, "text/event-stream")
                 .json(&payload);
 
diff --git a/codex-rs/core/src/codex.rs b/codex-rs/core/src/codex.rs
index d4e73b2e..246198c0 100644
--- a/codex-rs/core/src/codex.rs
+++ b/codex-rs/core/src/codex.rs
@@ -591,6 +591,7 @@ async fn submission_loop(
                     provider.clone(),
                     model_reasoning_effort,
                     model_reasoning_summary,
+                    session_id,
                 );
 
                 // abort any current running session and clone its state
diff --git a/codex-rs/core/tests/client.rs b/codex-rs/core/tests/client.rs
new file mode 100644
index 00000000..f4fb58f5
--- /dev/null
+++ b/codex-rs/core/tests/client.rs
@@ -0,0 +1,117 @@
+use std::time::Duration;
+
+use codex_core::Codex;
+use codex_core::ModelProviderInfo;
+use codex_core::exec::CODEX_SANDBOX_NETWORK_DISABLED_ENV_VAR;
+use codex_core::protocol::EventMsg;
+use codex_core::protocol::InputItem;
+use codex_core::protocol::Op;
+use codex_core::protocol::SessionConfiguredEvent;
+mod test_support;
+use tempfile::TempDir;
+use test_support::load_default_config_for_test;
+use test_support::load_sse_fixture_with_id;
+use tokio::time::timeout;
+use wiremock::Mock;
+use wiremock::MockServer;
+use wiremock::ResponseTemplate;
+use wiremock::matchers::method;
+use wiremock::matchers::path;
+
+/// Build minimal SSE stream with completed marker using the JSON fixture.
+fn sse_completed(id: &str) -> String {
+    load_sse_fixture_with_id("tests/fixtures/completed_template.json", id)
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn includes_session_id_and_model_headers_in_request() {
+    #![allow(clippy::unwrap_used)]
+
+    if std::env::var(CODEX_SANDBOX_NETWORK_DISABLED_ENV_VAR).is_ok() {
+        println!(
+            "Skipping test because it cannot execute when network is disabled in a Codex sandbox."
+        );
+        return;
+    }
+
+    // Mock server
+    let server = MockServer::start().await;
+
+    // First request – must NOT include `previous_response_id`.
+    let first = ResponseTemplate::new(200)
+        .insert_header("content-type", "text/event-stream")
+        .set_body_raw(sse_completed("resp1"), "text/event-stream");
+
+    Mock::given(method("POST"))
+        .and(path("/v1/responses"))
+        .respond_with(first)
+        .expect(1)
+        .mount(&server)
+        .await;
+
+    // Environment
+    // Update environment – `set_var` is `unsafe` starting with the 2024
+    // edition so we group the calls into a single `unsafe { … }` block.
+    unsafe {
+        std::env::set_var("OPENAI_REQUEST_MAX_RETRIES", "0");
+        std::env::set_var("OPENAI_STREAM_MAX_RETRIES", "0");
+    }
+    let model_provider = ModelProviderInfo {
+        name: "openai".into(),
+        base_url: format!("{}/v1", server.uri()),
+        // Environment variable that should exist in the test environment.
+        // ModelClient will return an error if the environment variable for the
+        // provider is not set.
+        env_key: Some("PATH".into()),
+        env_key_instructions: None,
+        wire_api: codex_core::WireApi::Responses,
+        query_params: None,
+        http_headers: Some(
+            [("originator".to_string(), "codex_cli_rs".to_string())]
+                .into_iter()
+                .collect(),
+        ),
+        env_http_headers: None,
+    };
+
+    // Init session
+    let codex_home = TempDir::new().unwrap();
+    let mut config = load_default_config_for_test(&codex_home);
+    config.model_provider = model_provider;
+    let ctrl_c = std::sync::Arc::new(tokio::sync::Notify::new());
+    let (codex, _init_id) = Codex::spawn(config, ctrl_c.clone()).await.unwrap();
+
+    codex
+        .submit(Op::UserInput {
+            items: vec![InputItem::Text {
+                text: "hello".into(),
+            }],
+        })
+        .await
+        .unwrap();
+
+    let mut current_session_id = None;
+    // Wait for TaskComplete
+    loop {
+        let ev = timeout(Duration::from_secs(1), codex.next_event())
+            .await
+            .unwrap()
+            .unwrap();
+
+        if let EventMsg::SessionConfigured(SessionConfiguredEvent { session_id, .. }) = ev.msg {
+            current_session_id = Some(session_id.to_string());
+        }
+        if matches!(ev.msg, EventMsg::TaskComplete(_)) {
+            break;
+        }
+    }
+
+    // get request from the server
+    let request = &server.received_requests().await.unwrap()[0];
+    let request_body = request.headers.get("session_id").unwrap();
+    let originator = request.headers.get("originator").unwrap();
+
+    assert!(current_session_id.is_some());
+    assert_eq!(request_body.to_str().unwrap(), &current_session_id.unwrap());
+    assert_eq!(originator.to_str().unwrap(), "codex_cli_rs");
+}