diff --git a/_primitives/_rust/kei-chat-store/src/schema.rs b/_primitives/_rust/kei-chat-store/src/schema.rs
index 2291c94..56d58d7 100644
--- a/_primitives/_rust/kei-chat-store/src/schema.rs
+++ b/_primitives/_rust/kei-chat-store/src/schema.rs
@@ -1,22 +1,26 @@
 //! kei-chat-store EntitySchema — declarative spec consumed by
 //! `kei_entity_store::Store` and its verb templates.
 //!
-//! Shape (Layer-A convergence, 2026-04-23):
+//! Shape (Layer-A convergence, 2026-04-23; cost-column re-migration
+//! 2026-04-23 wave 8):
 //!
 //! - Primary entity = `chat_messages` (INTEGER PK; required by engine).
 //!   Engine owns: create / get / list / search verbs + FTS reindex.
-//! - Bespoke: `chat_sessions` has a TEXT UUID primary key that the
-//!   engine's `FieldKind::IntegerPk` cannot represent, so its DDL rides
-//!   the engine's `custom_migrations` slot and its CRUD stays in
-//!   `sessions.rs` (analogous to kei-task's milestones / deps / graph).
-//! - Archive: sessions use a TEXT `status` enum ('active' | 'archived')
-//!   rather than an INTEGER flag, so the engine `archive` verb is NOT
-//!   enabled. Session archival stays bespoke.
-//! - Per-message `cost` (REAL in legacy) is dropped from `chat_messages`:
-//!   the engine has no REAL FieldKind and the only consumer is the
-//!   session-level aggregate `chat_sessions.total_cost`, updated
-//!   bespoke in `save_message`. No caller reads per-message cost in
-//!   current tests or the CLI surface.
+//! - Bespoke: `chat_sessions` has a TEXT UUID primary key. The engine
+//!   gained `FieldKind::TextPk` in wave 8 but Store::open currently
+//!   takes a SINGLE EntitySchema, so a second managed schema for
+//!   sessions would require engine multi-schema support. Until that
+//!   lands, the session DDL rides `custom_migrations` and its CRUD
+//!   stays in `sessions.rs` (analogous to kei-task's milestones / deps
+//!   / graph). **Known open:** promote chat_sessions to a second
+//!   EntitySchema (`TextPk` + `TextArchiveEnum`) once engine gains
+//!   multi-schema support.
+//! - Archive: sessions use a TEXT `status` enum ('active' | 'archived').
+//!   Session archival stays bespoke (same reason as above).
+//! - Per-message `cost` (REAL) is restored via
+//!   `FieldKind::RealDefault(0.0)` — wave-8 addition. Engine-managed
+//!   INSERT/SELECT, no bespoke SQL needed. Previously dropped when
+//!   the engine had no REAL kind; re-instated 2026-04-23.
 //!
 //! FTS column-name change vs pre-convergence shape:
 //!   legacy  fts_chat(message_id, session_id UNINDEXED, content)
@@ -34,6 +38,7 @@ static FIELDS: &[FieldDef] = &[
     FieldDef::text_nn("content"),
     FieldDef::integer("tokens_in"),
     FieldDef::integer("tokens_out"),
+    FieldDef::real_default("cost", 0.0),
     FieldDef::created_at(),
 ];
 
diff --git a/_primitives/_rust/kei-chat-store/src/search.rs b/_primitives/_rust/kei-chat-store/src/search.rs
index 591ef11..198b7e8 100644
--- a/_primitives/_rust/kei-chat-store/src/search.rs
+++ b/_primitives/_rust/kei-chat-store/src/search.rs
@@ -4,8 +4,8 @@
 //! `kei_entity_store::verbs::search` using `CHAT_SCHEMA`. The engine
 //! handles FTS5 JOIN + rank ordering; this module maps the generic
 //! JSON result back to typed `ChatMessage` rows for legacy callers.
-//! Per-message `cost` is not persisted after the convergence (see
-//! `schema.rs` note); `cost` is populated as 0.0 on every hit.
+//! Per-message `cost` is persisted (engine `RealDefault` field);
+//! `row_to_message` reads it back as f64.
 
 use crate::schema::CHAT_SCHEMA;
 use crate::sessions::ChatMessage;
@@ -32,7 +32,7 @@ fn row_to_message(r: &Value) -> Result<ChatMessage> {
         content: r["content"].as_str().unwrap_or("").into(),
         tokens_in: r["tokens_in"].as_i64().unwrap_or(0),
         tokens_out: r["tokens_out"].as_i64().unwrap_or(0),
-        cost: 0.0,
+        cost: r["cost"].as_f64().unwrap_or(0.0),
         created_at: r["created_at"].as_i64().unwrap_or(0),
     })
 }
diff --git a/_primitives/_rust/kei-chat-store/src/sessions.rs b/_primitives/_rust/kei-chat-store/src/sessions.rs
index a2e4b96..39cd56e 100644
--- a/_primitives/_rust/kei-chat-store/src/sessions.rs
+++ b/_primitives/_rust/kei-chat-store/src/sessions.rs
@@ -61,6 +61,7 @@ pub fn save_message(store: &Store, msg: &ChatMessage) -> Result<i64> {
         "content": msg.content,
         "tokens_in": msg.tokens_in,
         "tokens_out": msg.tokens_out,
+        "cost": msg.cost,
         "created_at": msg.created_at,
     });
     let v = v_create::run(store.conn(), &CHAT_SCHEMA, payload)
diff --git a/_primitives/_rust/kei-chat-store/tests/integration.rs b/_primitives/_rust/kei-chat-store/tests/integration.rs
index 9385315..0d424e3 100644
--- a/_primitives/_rust/kei-chat-store/tests/integration.rs
+++ b/_primitives/_rust/kei-chat-store/tests/integration.rs
@@ -62,6 +62,26 @@ fn engine_migration_parity_smoke() {
     assert_eq!(sess.message_count, 1);
 }
 
+#[test]
+fn cost_roundtrips_via_search() {
+    // Wave-8 re-migration: cost is re-instated as engine-managed
+    // RealDefault column. The value written via save_message must be
+    // visible on the ChatMessage returned from search (no longer 0.0).
+    let s = mk();
+    let sid = start_session(&s, "demo", "", "").unwrap();
+    save_message(&s, &ChatMessage {
+        session_id: sid, role: "user".into(),
+        content: "rust async tokio bench cost-marker".into(),
+        tokens_in: 1, tokens_out: 1, cost: 0.00777,
+        ..Default::default()
+    }).unwrap();
+    let hits = search(&s, "cost-marker", 10).unwrap();
+    assert_eq!(hits.len(), 1);
+    assert!((hits[0].cost - 0.00777).abs() < 1e-9,
+        "cost should round-trip via engine RealDefault column, got {}",
+        hits[0].cost);
+}
+
 #[test]
 fn stats_aggregates() {
     let s = mk();