diff --git a/src/gateway/http.rs b/src/gateway/http.rs
index 6f1b380..8525e6e 100644
--- a/src/gateway/http.rs
+++ b/src/gateway/http.rs
@@ -1217,3 +1217,154 @@ pub async fn session_selected_model(
         .unwrap_or((None, None));
     Json(SessionSelectedModelResponse { provider, model })
 }
+
+#[derive(Deserialize)]
+pub struct SelectTopicModelRequest {
+    pub session_id: String,
+    pub topic_id: String,
+    pub provider: Option<String>,
+    pub model: Option<String>,
+}
+
+/// POST /api/topic/select-model — 设置（或清除）话题级用户模型覆盖。
+///
+/// 双写语义（session store 的 key 一律取 topic 行自带的 session_id，不信任请求体）：
+/// - 设置：写 topics 行（话题记忆，物化）+ 内存缓存 + session store（成为新话题默认）
+/// - 清除（provider/model 均空）：移除话题级选择，并同步清除 session 级选择
+///   （否则 agent 会在下一条消息把 session 级重新物化回 topic 行，重置永远无效）
+pub async fn topic_select_model(
+    State(state): State<Arc<GatewayState>>,
+    Json(req): Json<SelectTopicModelRequest>,
+) -> (StatusCode, Json<SelectModelResponse>) {
+    // 规范化：trim 后空字符串视为 None（与 frontmatter 解析逻辑一致）
+    let provider = req
+        .provider
+        .map(|s| s.trim().to_string())
+        .filter(|s| !s.is_empty());
+    let model = req
+        .model
+        .map(|s| s.trim().to_string())
+        .filter(|s| !s.is_empty());
+
+    // 话题必须存在（防止对已删除话题静默写空）。
+    // 同时取 topic 行自带的 session_id 作为双写目标——不信任请求体中的 session_id，
+    // 防止客户端误传导致污染其他 session 的默认模型。
+    let store = state.session_manager.store();
+    let topic_session_id = match store.get_topic(&req.topic_id) {
+        Ok(Some(topic)) => topic.session_id,
+        Ok(None) => {
+            return (
+                StatusCode::NOT_FOUND,
+                Json(SelectModelResponse {
+                    success: false,
+                    error: Some(format!("topic '{}' not found", req.topic_id)),
+                }),
+            );
+        }
+        Err(e) => {
+            return (
+                StatusCode::INTERNAL_SERVER_ERROR,
+                Json(SelectModelResponse {
+                    success: false,
+                    error: Some(format!("failed to load topic: {}", e)),
+                }),
+            );
+        }
+    };
+
+    // 校验：provider/model 名必须在 config 的 providers/models 表中存在
+    let config = state.config.read().await;
+    if let Some(name) = provider.as_ref() {
+        if !config.providers.contains_key(name) {
+            return (
+                StatusCode::BAD_REQUEST,
+                Json(SelectModelResponse {
+                    success: false,
+                    error: Some(format!("provider '{}' not found in config", name)),
+                }),
+            );
+        }
+    }
+    if let Some(name) = model.as_ref() {
+        if !config.models.contains_key(name) {
+            return (
+                StatusCode::BAD_REQUEST,
+                Json(SelectModelResponse {
+                    success: false,
+                    error: Some(format!("model '{}' not found in config", name)),
+                }),
+            );
+        }
+    }
+    drop(config);
+
+    let is_clear = provider.is_none() && model.is_none();
+    if let Err(e) = store.update_topic_model(&req.topic_id, provider.as_deref(), model.as_deref()) {
+        return (
+            StatusCode::INTERNAL_SERVER_ERROR,
+            Json(SelectModelResponse {
+                success: false,
+                error: Some(format!("failed to persist topic model: {}", e)),
+            }),
+        );
+    }
+
+    state
+        .topic_model_selections
+        .set(&req.topic_id, provider.clone(), model.clone());
+
+    // 双写 session store（key 一律取 topic 行的 session_id）：
+    // - 显式设置：成为新话题的初始默认（"最近使用"）
+    // - 清除：同步移除 session 级选择。否则"重置为默认"会陷入死循环——
+    //   topic 行清空后 agent 命中 session 级又重新物化回 topic 行，重置形同虚设
+    if is_clear {
+        state.model_selections.set(&topic_session_id, None, None);
+    } else {
+        state.model_selections.set(&topic_session_id, provider, model);
+    }
+
+    (
+        StatusCode::OK,
+        Json(SelectModelResponse {
+            success: true,
+            error: None,
+        }),
+    )
+}
+
+/// GET /api/topic/selected-model?topic_id=... — 返回话题生效的用户模型选择。
+///
+/// 语义与 session 端点一致：只反映用户选择（topic 级优先，miss 回退 session 级），
+/// 不解析 expert/config 默认。
+#[derive(Deserialize)]
+pub struct TopicSelectedModelQuery {
+    pub topic_id: String,
+}
+
+pub async fn topic_selected_model(
+    State(state): State<Arc<GatewayState>>,
+    Query(q): Query<TopicSelectedModelQuery>,
+) -> Json<SessionSelectedModelResponse> {
+    // topic 级命中直接返回；miss 时按 topic 行的 session_id 回退 session 级
+    let (provider, model) = match state.topic_model_selections.get(&q.topic_id) {
+        Some(selection) => selection,
+        None => match state.session_manager.store().get_topic(&q.topic_id) {
+            Ok(Some(topic)) => {
+                // 双保险：SQLite 有物化值但缓存 miss（理论上不会发生）时回填缓存
+                if topic.provider.is_some() || topic.model.is_some() {
+                    state.topic_model_selections.set(
+                        &q.topic_id,
+                        topic.provider.clone(),
+                        topic.model.clone(),
+                    );
+                }
+                state
+                    .model_selections
+                    .get(&topic.session_id)
+                    .unwrap_or((None, None))
+            }
+            _ => (None, None),
+        },
+    };
+    Json(SessionSelectedModelResponse { provider, model })
+}
diff --git a/src/gateway/mod.rs b/src/gateway/mod.rs
index 3d912c8..5773ddf 100644
--- a/src/gateway/mod.rs
+++ b/src/gateway/mod.rs
@@ -75,6 +75,8 @@ pub struct GatewayState {
     pub subagent_executor: Option<Arc<dyn crate::tools::SubAgentRuntime>>,
     /// per-session 的用户模型选择（覆盖专家配置）
     pub model_selections: Arc<model_selection::ModelSelectionStore>,
+    /// per-topic 的用户模型选择（最高优先级；物化保证话题模型不随 session 选择漂移）
+    pub topic_model_selections: Arc<model_selection::ModelSelectionStore>,
     /// Prometheus metrics handle（/metrics 端点渲染用）。
     /// None 表示 recorder 安装失败；热重启时从 OnceLock 缓存复用，不会因重复安装而变为 None。
     pub prometheus_handle: Option<metrics_exporter_prometheus::PrometheusHandle>,
@@ -108,7 +110,7 @@ impl GatewayState {
             mcp_servers: config.mcp_servers.clone(),
         };
 
-        let (session_manager, task_repository, mcp_manager, subagent_runtime, model_selections, subagent_executor) =
+        let (session_manager, task_repository, mcp_manager, subagent_runtime, model_selections, topic_model_selections, subagent_executor) =
             build_session_manager_with_sender(
                 agent_prompt_reinject_every,
                 show_tool_results,
@@ -155,6 +157,7 @@ impl GatewayState {
             subagent_runtime,
             subagent_executor,
             model_selections,
+            topic_model_selections,
             prometheus_handle,
         })
     }
@@ -369,6 +372,14 @@ pub async fn run(
             "/api/session/selected-model",
             routing::get(http::session_selected_model),
         )
+        .route(
+            "/api/topic/select-model",
+            routing::post(http::topic_select_model),
+        )
+        .route(
+            "/api/topic/selected-model",
+            routing::get(http::topic_selected_model),
+        )
         .route("/ws", routing::get(ws::ws_handler))
         .route("/metrics", routing::get(http::metrics_handler));
 
diff --git a/src/gateway/runtime.rs b/src/gateway/runtime.rs
index 0b1ee18..cf5203b 100644
--- a/src/gateway/runtime.rs
+++ b/src/gateway/runtime.rs
@@ -67,6 +67,7 @@ pub(crate) fn build_session_manager(
         Option<Arc<McpClientManager>>,
         Arc<SubagentRuntime>,
         Arc<ModelSelectionStore>,
+        Arc<ModelSelectionStore>,
         Option<Arc<dyn SubAgentRuntime>>,
     ),
     AgentError,
@@ -120,6 +121,7 @@ pub(crate) fn build_session_manager_with_sender(
         Option<Arc<McpClientManager>>,
         Arc<SubagentRuntime>,
         Arc<ModelSelectionStore>,
+        Arc<ModelSelectionStore>,
         Option<Arc<dyn SubAgentRuntime>>,
     ),
     AgentError,
@@ -128,6 +130,22 @@ pub(crate) fn build_session_manager_with_sender(
         SessionStore::new()
             .map_err(|err| AgentError::Other(format!("session store init error: {}", err)))?,
     );
+    // 模型选择内存缓存：session 级 + topic 级（topic 级启动时从 topics 表预热）
+    let model_selections = Arc::new(ModelSelectionStore::new());
+    let topic_model_selections = Arc::new(ModelSelectionStore::new());
+    match store.list_topic_model_selections() {
+        Ok(entries) => {
+            for (topic_id, provider, model) in entries {
+                topic_model_selections.set(&topic_id, provider, model);
+            }
+        }
+        Err(err) => {
+            tracing::warn!(
+                error = %err,
+                "build_session_manager: failed to preload topic model selections"
+            );
+        }
+    }
     let known_agents = provider_configs.keys().cloned().collect::<HashSet<_>>();
     let provider_configs = ProviderConfigService::new(
         provider_config.clone(),
@@ -254,6 +272,8 @@ pub(crate) fn build_session_manager_with_sender(
             bus.clone(),
             store.clone(),
             skills.clone(),
+            Some(model_selections.clone()),
+            Some(topic_model_selections.clone()),
         ));
 
         // 注册 task 工具到子代理工具集（需在 runtime 创建之后，打破循环依赖）
@@ -320,7 +340,6 @@ pub(crate) fn build_session_manager_with_sender(
     );
 
     let prompt_repository: Arc<dyn PromptInjectionRepository> = store.clone();
-    let model_selections = Arc::new(ModelSelectionStore::new());
     let observer: Arc<dyn crate::observability::Observer> =
         crate::observability::metrics::default_observer();
     let agent_factory = AgentFactory::new(
@@ -332,6 +351,8 @@ pub(crate) fn build_session_manager_with_sender(
         prompt_repository.clone(),
         model_resolver.clone(),
         model_selections.clone(),
+        topic_model_selections.clone(),
+        store.clone(),
         compaction_config,
         Some(observer),
     );
@@ -376,6 +397,7 @@ pub(crate) fn build_session_manager_with_sender(
         mcp_manager,
         subagent_runtime,
         model_selections,
+        topic_model_selections,
         subagent_executor,
     ))
 }
diff --git a/src/gateway/session.rs b/src/gateway/session.rs
index 4e3d859..8392cb5 100644
--- a/src/gateway/session.rs
+++ b/src/gateway/session.rs
@@ -309,6 +309,8 @@ impl Session {
             prompt_repository.clone(),
             model_resolver,
             Arc::new(super::model_selection::ModelSelectionStore::new()),
+            Arc::new(super::model_selection::ModelSelectionStore::new()),
+            store.clone(),
             crate::config::CompactionConfig::default(),
             None,
         );
@@ -994,7 +996,7 @@ impl SessionManager {
             model_resolver,
             crate::config::CompactionConfig::default(),
         )
-        .map(|(session_manager, _, _, _, _, _)| session_manager)
+        .map(|(session_manager, _, _, _, _, _, _)| session_manager)
     }
 
     pub fn tools(&self) -> Arc<ToolRegistry> {
diff --git a/src/storage/migrations.rs b/src/storage/migrations.rs
index 544baae..52af32c 100644
--- a/src/storage/migrations.rs
+++ b/src/storage/migrations.rs
@@ -93,6 +93,18 @@ pub(super) fn ensure_messages_schema(conn: &Connection) -> Result<(), StorageErr
     Ok(())
 }
 
+/// topics 表：话题级模型选择列（用户在特定话题内显式选择的 provider/model）。
+/// NULL 表示该话题无显式选择（运行时按 session 级 → expert → config 链解析）。
+pub(super) fn ensure_topics_schema(conn: &Connection) -> Result<(), StorageError> {
+    if !has_column(conn, "topics", "provider")? {
+        add_column_if_missing(conn, "ALTER TABLE topics ADD COLUMN provider TEXT")?;
+    }
+    if !has_column(conn, "topics", "model")? {
+        add_column_if_missing(conn, "ALTER TABLE topics ADD COLUMN model TEXT")?;
+    }
+    Ok(())
+}
+
 pub(super) fn ensure_scheduler_schema(conn: &Connection) -> Result<(), StorageError> {
     if !has_column(conn, "scheduler_jobs", "schedule_json")? {
         conn.execute(
diff --git a/src/storage/mod.rs b/src/storage/mod.rs
index ecba5e7..1d1c1be 100644
--- a/src/storage/mod.rs
+++ b/src/storage/mod.rs
@@ -231,6 +231,7 @@ impl SessionStore {
 
         ensure_sessions_schema(&conn)?;
         ensure_messages_schema(&conn)?;
+        ensure_topics_schema(&conn)?;
         ensure_scheduler_schema(&conn)?;
         ensure_memory_scope_key_migration(&conn)?;
         ensure_todos_schema(&conn)?;
@@ -468,7 +469,7 @@ impl SessionStore {
     pub fn get_topic(&self, topic_id: &str) -> Result<Option<TopicRecord>, StorageError> {
         let conn = self.pool.get()?;
         let mut stmt = conn.prepare(
-            "SELECT id, session_id, title, description, created_at, updated_at, last_active_at, message_count FROM topics WHERE id = ?1",
+            "SELECT id, session_id, title, description, created_at, updated_at, last_active_at, message_count, provider, model FROM topics WHERE id = ?1",
         )?;
 
         stmt.query_row(params![topic_id], |row| {
@@ -481,6 +482,8 @@ impl SessionStore {
                 updated_at: row.get(5)?,
                 last_active_at: row.get(6)?,
                 message_count: row.get(7)?,
+                provider: row.get(8)?,
+                model: row.get(9)?,
             })
         })
         .optional()
@@ -490,7 +493,7 @@ impl SessionStore {
     pub fn list_topics(&self, session_id: &str) -> Result<Vec<TopicRecord>, StorageError> {
         let conn = self.pool.get()?;
         let mut stmt = conn.prepare(
-            "SELECT id, session_id, title, description, created_at, updated_at, last_active_at, message_count FROM topics WHERE session_id = ?1 ORDER BY last_active_at DESC"
+            "SELECT id, session_id, title, description, created_at, updated_at, last_active_at, message_count, provider, model FROM topics WHERE session_id = ?1 ORDER BY last_active_at DESC"
         )?;
 
         let rows = stmt.query_map(params![session_id], |row| {
@@ -503,6 +506,8 @@ impl SessionStore {
                 updated_at: row.get(5)?,
                 last_active_at: row.get(6)?,
                 message_count: row.get(7)?,
+                provider: row.get(8)?,
+                model: row.get(9)?,
             })
         })?;
 
@@ -544,6 +549,40 @@ impl SessionStore {
         Ok(())
     }
 
+    /// 设置/清除话题级模型选择。provider 与 model 均为 None 时清除（恢复继承）。
+    pub fn update_topic_model(
+        &self,
+        topic_id: &str,
+        provider: Option<&str>,
+        model: Option<&str>,
+    ) -> Result<(), StorageError> {
+        let now = current_timestamp();
+        let conn = self.pool.get()?;
+        conn.execute(
+            "UPDATE topics SET provider = ?2, model = ?3, updated_at = ?4 WHERE id = ?1",
+            params![topic_id, provider, model, now],
+        )?;
+        Ok(())
+    }
+
+    /// 全量读出话题级模型选择，供启动时预热内存缓存。
+    pub fn list_topic_model_selections(
+        &self,
+    ) -> Result<Vec<(String, Option<String>, Option<String>)>, StorageError> {
+        let conn = self.pool.get()?;
+        let mut stmt = conn.prepare(
+            "SELECT id, provider, model FROM topics WHERE provider IS NOT NULL OR model IS NOT NULL",
+        )?;
+        let rows = stmt.query_map([], |row| {
+            Ok((row.get(0)?, row.get(1)?, row.get(2)?))
+        })?;
+        let mut result = Vec::new();
+        for row in rows {
+            result.push(row?);
+        }
+        Ok(result)
+    }
+
     pub fn touch_topic(&self, topic_id: &str) -> Result<(), StorageError> {
         let now = current_timestamp();
         let conn = self.pool.get()?;
diff --git a/src/storage/records.rs b/src/storage/records.rs
index 5085f9e..6c52c76 100644
--- a/src/storage/records.rs
+++ b/src/storage/records.rs
@@ -109,6 +109,9 @@ pub struct TopicRecord {
     pub updated_at: i64,
     pub last_active_at: i64,
     pub message_count: i64,
+    /// 话题级用户模型选择（NULL 表示无显式选择，运行时按继承链解析）
+    pub provider: Option<String>,
+    pub model: Option<String>,
 }
 
 /// pending_subagents 表的记录，跟踪异步子代理执行状态。