diff --git a/backend/crates/providers/openai/src/transport/request.rs b/backend/crates/providers/openai/src/transport/request.rs
index 99a82daef..ecc0b23db 100644
--- a/backend/crates/providers/openai/src/transport/request.rs
+++ b/backend/crates/providers/openai/src/transport/request.rs
@@ -10,6 +10,8 @@ use sha2::{Digest, Sha256};
 use crate::transport::protocol::responses::CodexResponsesRequest;
 
 const PASSTHROUGH_HEADERS_CONTEXT_KEY: &str = "opaque_request_headers";
+const THREAD_SPAWN_SUBAGENT_KIND: &str = "thread_spawn";
+const THREAD_SPAWN_CONVERSATION_PREFIX: &str = "thread-spawn:";
 
 const CROSS_ACCOUNT_IDENTITY_KEYS: &[&str] = &[
     "authorization",
@@ -160,28 +162,52 @@ fn string_value(value: Option<&Value>) -> Option<String> {
 pub(crate) fn derive_conversation_anchor(
     request: &CodexResponsesRequest,
 ) -> Option<(&'static str, String)> {
+    thread_spawn_conversation_anchor(request).or_else(|| {
+        request
+            .client_session_id
+            .as_deref()
+            .map(|value| ("session", value.to_owned()))
+            .or_else(|| {
+                request
+                    .client_conversation_id
+                    .as_deref()
+                    .map(|value| ("conversation", value.to_owned()))
+            })
+            .or_else(|| {
+                request
+                    .client_thread_id
+                    .as_deref()
+                    .map(|value| ("thread", value.to_owned()))
+            })
+            .or_else(|| {
+                request
+                    .prompt_cache_key()
+                    .map(|value| ("prompt-cache", value.to_owned()))
+            })
+            .or_else(|| derive_stable_conversation_key(request).map(|value| ("request", value)))
+    })
+}
+
+/// A `thread_spawn` shares its parent session but is a separate child execution.
+/// Its child thread must therefore own the affinity and WebSocket pool identity.
+fn thread_spawn_conversation_anchor(
+    request: &CodexResponsesRequest,
+) -> Option<(&'static str, String)> {
+    if request.subagent_kind().as_deref() != Some(THREAD_SPAWN_SUBAGENT_KIND) {
+        return None;
+    }
+
     request
-        .client_session_id
+        .client_thread_id
         .as_deref()
-        .map(|value| ("session", value.to_owned()))
-        .or_else(|| {
-            request
-                .client_conversation_id
-                .as_deref()
-                .map(|value| ("conversation", value.to_owned()))
+        .map(str::trim)
+        .filter(|thread_id| !thread_id.is_empty())
+        .map(|thread_id| {
+            (
+                "subagent-thread",
+                format!("{THREAD_SPAWN_CONVERSATION_PREFIX}{thread_id}"),
+            )
         })
-        .or_else(|| {
-            request
-                .client_thread_id
-                .as_deref()
-                .map(|value| ("thread", value.to_owned()))
-        })
-        .or_else(|| {
-            request
-                .prompt_cache_key()
-                .map(|value| ("prompt-cache", value.to_owned()))
-        })
-        .or_else(|| derive_stable_conversation_key(request).map(|value| ("request", value)))
 }
 
 const LEADING_SYSTEM_REMINDER_OPEN: &str = "<system-reminder>";
diff --git a/backend/crates/providers/openai/tests/provider/contract.rs b/backend/crates/providers/openai/tests/provider/contract.rs
index 40e96bd00..586020f41 100644
--- a/backend/crates/providers/openai/tests/provider/contract.rs
+++ b/backend/crates/providers/openai/tests/provider/contract.rs
@@ -212,6 +212,27 @@ fn generate_operation() -> Operation {
     ))
 }
 
+fn generate_with_session_context(
+    session_id: &str,
+    thread_id: Option<&str>,
+    turn_metadata: Option<&str>,
+) -> GenerateRequest {
+    let mut body = Map::from_iter([
+        ("model".to_owned(), json!("gpt-5.4")),
+        ("input".to_owned(), json!("hello")),
+        ("session_id".to_owned(), json!(session_id)),
+    ]);
+    if let Some(thread_id) = thread_id {
+        body.insert("thread_id".to_owned(), json!(thread_id));
+    }
+    if let Some(turn_metadata) = turn_metadata {
+        body.insert("turnMetadata".to_owned(), json!(turn_metadata));
+    }
+    GenerateRequest::from_protocol_payload(
+        ProtocolPayload::json_object("openai", body).expect("OpenAI payload"),
+    )
+}
+
 fn http_generate_operation() -> Operation {
     let payload = ProtocolPayload::json_object(
         "openai",
@@ -1171,6 +1192,58 @@ async fn explicit_session_id_should_override_turn_specific_prompt_cache_keys_for
     assert_eq!(keys[0], keys[1]);
 }
 
+#[tokio::test]
+async fn thread_spawn_children_should_have_independent_session_affinity_keys() {
+    let store = Arc::new(MemoryAccountStore::default());
+    create_account(&store, "acct_thread_spawn_affinity").await;
+    let affinity = Arc::new(MemorySessionAffinity::default());
+    let provider = provider_with_affinity(&store, Arc::clone(&affinity));
+    let thread_spawn = r#"{"subagent_kind":"thread_spawn"}"#;
+
+    for (request_id, thread_id, turn_metadata) in [
+        (
+            "req_thread_spawn_first",
+            Some("child-one"),
+            Some(thread_spawn),
+        ),
+        (
+            "req_thread_spawn_second",
+            Some("child-two"),
+            Some(thread_spawn),
+        ),
+        (
+            "req_thread_spawn_repeat",
+            Some("child-one"),
+            Some(thread_spawn),
+        ),
+        ("req_thread_spawn_fallback", None, Some(thread_spawn)),
+        ("req_thread_spawn_parent", None, None),
+    ] {
+        let stream = provider
+            .execute(
+                planned_request(
+                    "openai",
+                    Operation::Generate(generate_with_session_context(
+                        "parent-session",
+                        thread_id,
+                        turn_metadata,
+                    )),
+                ),
+                context(request_id, CancellationToken::new()),
+            )
+            .await
+            .expect("prepare provider stream");
+        drop(stream);
+    }
+
+    let keys = affinity.lookup_keys();
+    assert_eq!(keys.len(), 5);
+    assert_ne!(keys[0], keys[1]);
+    assert_eq!(keys[0], keys[2]);
+    assert_ne!(keys[3], keys[4]);
+    assert_ne!(keys[0], keys[3]);
+}
+
 #[tokio::test]
 async fn invalid_local_conversation_id_should_fall_back_to_an_opaque_affinity_key() {
     let store = Arc::new(MemoryAccountStore::default());