diff --git a/crates/utopia-core/src/models.rs b/crates/utopia-core/src/models.rs
index dd1bc0969..f6f1b2e22 100644
--- a/crates/utopia-core/src/models.rs
+++ b/crates/utopia-core/src/models.rs
@@ -153,6 +153,8 @@ pub struct Source {
     pub sync_interval_minutes: Option<i32>,
     /// 标准 5 段 cron（服务器本地时区；与 sync_interval_minutes 互斥）
     pub sync_cron: Option<String>,
+    /// NULL = 文档不衰减；对象的形状见 `utopia_store::decay::DecaySettings`（RC-003）
+    pub decay: Option<serde_json::Value>,
     pub last_sync_at: Option<DateTime<Utc>>,
     /// never | queued | running | ok | failed
     pub last_sync_status: String,
@@ -197,6 +199,7 @@ mod source_extracts_tests {
             icon: None,
             sync_interval_minutes: None,
             sync_cron: None,
+            decay: None,
             last_sync_at: None,
             last_sync_status: "never".into(),
             last_sync_error: None,
@@ -353,6 +356,7 @@ pub struct SourceView {
     pub icon: Option<String>,
     pub sync_interval_minutes: Option<i32>,
     pub sync_cron: Option<String>,
+    pub decay: Option<serde_json::Value>,
     pub last_sync_at: Option<DateTime<Utc>>,
     pub last_sync_status: String,
     pub last_sync_error: Option<String>,
diff --git a/crates/utopia-server/src/api/sources_routes.rs b/crates/utopia-server/src/api/sources_routes.rs
index 7a4e62eb8..d98630a76 100644
--- a/crates/utopia-server/src/api/sources_routes.rs
+++ b/crates/utopia-server/src/api/sources_routes.rs
@@ -247,6 +247,18 @@ pub struct UpdateBody {
     /// 出现 schedule 字段即整体覆盖调度（interval 与 cron 互斥；两者皆 null = 关闭定时）
     #[serde(default)]
     pub schedule: Option<ScheduleBody>,
+    /// 缺省 = 不动；`null` = 关掉衰减；对象 = 打开（RC-003）
+    #[serde(default, deserialize_with = "present_even_when_null")]
+    pub decay: Option<Option<utopia_store::decay::DecaySettings>>,
+}
+
+/// 区分「没写这个字段」和「写成了 null」：前者外层是 `None`，后者是 `Some(None)`。
+fn present_even_when_null<'de, D, T>(deserializer: D) -> Result<Option<Option<T>>, D::Error>
+where
+    D: serde::Deserializer<'de>,
+    T: Deserialize<'de>,
+{
+    Option::<T>::deserialize(deserializer).map(Some)
 }
 
 #[derive(Deserialize)]
@@ -281,6 +293,7 @@ pub async fn update(
         body.icon.as_deref(),
         body.schedule
             .map(|s| (s.sync_interval_minutes, s.sync_cron)),
+        body.decay.as_ref().map(Option::as_ref),
     )
     .await?;
     state.emit_source(kb_id);
@@ -292,7 +305,11 @@ pub async fn update(
         "source.updated",
         "source",
         Some(source_id),
-        json!({ "name": body.name, "config_changed": config.is_some() }),
+        json!({
+            "name": body.name,
+            "config_changed": config.is_some(),
+            "decay_changed": body.decay.is_some()
+        }),
     )
     .await;
     Ok(Json(json!({ "source": mask_secrets(source) })))
@@ -893,9 +910,20 @@ mod statements_tests;
 
 #[cfg(test)]
 mod tests {
-    use super::{keep_secrets, validate_rss_config};
+    use super::{keep_secrets, validate_rss_config, UpdateBody};
     use serde_json::json;
 
+    #[test]
+    fn an_update_tells_a_missing_decay_from_a_null_one() {
+        let absent: UpdateBody = serde_json::from_value(json!({ "name": "n" })).unwrap();
+        let null: UpdateBody = serde_json::from_value(json!({ "decay": null })).unwrap();
+        let object: UpdateBody =
+            serde_json::from_value(json!({ "decay": { "max_items": 9 } })).unwrap();
+        assert_eq!(absent.decay, None);
+        assert_eq!(null.decay, Some(None));
+        assert_eq!(object.decay.unwrap().unwrap().max_items, 9);
+    }
+
     /// 认证判断写反是这类改动唯一值得担心的事（`ct_eq` 给的是 `Choice` 不是
     /// `bool`，判反一次就是全放行且一切看起来正常），所以两个方向都钉住
     #[test]
@@ -955,6 +983,7 @@ mod tests {
             icon: None,
             sync_interval_minutes: None,
             sync_cron: None,
+            decay: None,
             last_sync_at: None,
             last_sync_status: "never".into(),
             last_sync_error: None,
diff --git a/crates/utopia-store/src/decay.rs b/crates/utopia-store/src/decay.rs
index 9834b853f..e126ce01e 100644
--- a/crates/utopia-store/src/decay.rs
+++ b/crates/utopia-store/src/decay.rs
@@ -5,6 +5,7 @@
 
 use chrono::{DateTime, Utc};
 use serde::{Deserialize, Serialize};
+use utopia_core::AppError;
 
 pub const DEFAULT_HALF_LIFE_DAYS: f64 = 60.0;
 pub const MAX_HALF_LIFE_DAYS: f64 = 365.0;
@@ -37,6 +38,20 @@ impl Default for DecaySettings {
     }
 }
 
+impl DecaySettings {
+    /// 数据库不拦的范围：半衰期 1 到 365 天，容量和陈旧天数至少 1。
+    pub fn validate(&self) -> Result<(), AppError> {
+        let half_life_ok = (1..=MAX_HALF_LIFE_DAYS as u32).contains(&self.half_life_days);
+        if !half_life_ok || self.max_items == 0 || self.stale_after_days == 0 {
+            return Err(AppError::Validation(format!(
+                "decay needs max_items >= 1, stale_after_days >= 1 and half_life_days within 1 to {}",
+                MAX_HALF_LIFE_DAYS as u32
+            )));
+        }
+        Ok(())
+    }
+}
+
 /// 打分用到的一个文档：`key` 是文档名，`value` 是它的文字。
 pub struct Memory<'a> {
     pub key: &'a str,
@@ -212,6 +227,31 @@ mod tests {
         );
     }
 
+    #[test]
+    fn a_setting_out_of_range_is_refused() {
+        let ok = DecaySettings::default();
+        assert!(ok.validate().is_ok());
+        let bad = [
+            DecaySettings {
+                max_items: 0,
+                ..ok.clone()
+            },
+            DecaySettings {
+                half_life_days: 0,
+                ..ok.clone()
+            },
+            DecaySettings {
+                half_life_days: 366,
+                ..ok.clone()
+            },
+            DecaySettings {
+                stale_after_days: 0,
+                ..ok.clone()
+            },
+        ];
+        assert!(bad.iter().all(|settings| settings.validate().is_err()));
+    }
+
     #[test]
     fn a_misspelled_setting_is_refused() {
         let parsed = serde_json::from_str::<DecaySettings>(r#"{"max_item": 5}"#);
diff --git a/crates/utopia-store/src/sources.rs b/crates/utopia-store/src/sources.rs
index 7f596c6ec..6a9baf461 100644
--- a/crates/utopia-store/src/sources.rs
+++ b/crates/utopia-store/src/sources.rs
@@ -6,6 +6,8 @@ use utopia_core::models::{Role, Source, SourceKind, SourceView, SyncRun, SOURCE_
 use utopia_core::{secrets, AppError, AppResult};
 use uuid::Uuid;
 
+use crate::decay::DecaySettings;
+
 /// folder = 纯容器（上传/拖拽入内，无同步语义）；url/rss = 拉取型；api = 推送型。
 /// 本机目录监听（watch_folder）已否决——自部署用户看不到服务器磁盘；
 /// 对象存储 / WebDAV / Notion 是它的替代形态（0013）。
@@ -89,7 +91,7 @@ pub async fn list(pool: &PgPool, kb_id: Uuid) -> AppResult<Vec<SourceView>> {
     let rows: Vec<SourceView> = sqlx::query_as(&format!(
         "SELECT listed_source.id, listed_source.kind, listed_source.name,
                 listed_source.config - $2::text[] AS config, listed_source.icon,
-                listed_source.sync_interval_minutes, listed_source.sync_cron,
+                listed_source.sync_interval_minutes, listed_source.sync_cron, listed_source.decay,
                 listed_source.last_sync_at, listed_source.last_sync_status,
                 listed_source.last_sync_error, listed_source.last_sync_added,
                 (SELECT count(*) FROM documents d
@@ -249,7 +251,18 @@ pub async fn update(
     config: Option<&serde_json::Value>,
     icon: Option<&str>,
     schedule: Option<(Option<i32>, Option<String>)>,
+    decay: Option<Option<&DecaySettings>>,
 ) -> AppResult<Source> {
+    let decay = decay
+        .map(|settings| {
+            settings
+                .map(|settings| {
+                    settings.validate()?;
+                    serde_json::to_value(settings).map_err(|e| AppError::Other(e.into()))
+                })
+                .transpose()
+        })
+        .transpose()?;
     let schedule = match schedule {
         Some((interval, cron)) => {
             let cron_norm = cron.as_deref().map(validate_cron).transpose()?;
@@ -287,7 +300,8 @@ pub async fn update(
             config = COALESCE($3, config),
             icon = COALESCE($4, icon),
             sync_interval_minutes = CASE WHEN $5 THEN $6 ELSE sync_interval_minutes END,
-            sync_cron = CASE WHEN $5 THEN $7 ELSE sync_cron END
+            sync_cron = CASE WHEN $5 THEN $7 ELSE sync_cron END,
+            decay = CASE WHEN $8 THEN $9 ELSE decay END
          WHERE id = $1 RETURNING *",
     )
     .bind(id)
@@ -297,6 +311,8 @@ pub async fn update(
     .bind(schedule.is_some())
     .bind(schedule.as_ref().and_then(|(i, _)| *i))
     .bind(schedule.as_ref().and_then(|(_, c)| c.clone()))
+    .bind(decay.is_some())
+    .bind(decay.flatten())
     .fetch_one(&mut *tx)
     .await?;
 
diff --git a/crates/utopia-store/tests/store/a_source_decay_is_set_and_cleared_by_update.rs b/crates/utopia-store/tests/store/a_source_decay_is_set_and_cleared_by_update.rs
new file mode 100644
index 000000000..e98d18557
--- /dev/null
+++ b/crates/utopia-store/tests/store/a_source_decay_is_set_and_cleared_by_update.rs
@@ -0,0 +1,98 @@
+//! 改来源时设置、保留、关掉衰减（RC-003），打在真库上。
+//!
+//! 要钉住的行为：
+//!
+//! - **不写就不动**：更新别的字段，衰减原样
+//! - **写对象就打开**：存下完整的四个字段，之后默认值再变也不会动它
+//! - **写 null 就关掉**
+//! - **越界的值被拒绝**，库里的原值不变
+//! - **列表看得到**
+//!
+//! 没有 `UTOPIA_DATABASE_URL` 时跳过而不是失败。自建自拆，绝不碰已有的库。
+
+use sqlx::PgPool;
+use utopia_store::decay::DecaySettings;
+use utopia_store::sources;
+use uuid::Uuid;
+
+#[tokio::test]
+async fn a_source_decay_is_set_and_cleared_by_update() -> anyhow::Result<()> {
+    let Some(url) = utopia_store::test_db::url() else {
+        return Ok(());
+    };
+    let pool = PgPool::connect(&url).await?;
+    utopia_store::db::migrate(&pool).await?;
+
+    let (org, ws, kb, source) = (
+        Uuid::now_v7(),
+        Uuid::now_v7(),
+        Uuid::now_v7(),
+        Uuid::now_v7(),
+    );
+    sqlx::query("INSERT INTO organizations (id, name) VALUES ($1, 'source-decay-update-test')")
+        .bind(org)
+        .execute(&pool)
+        .await?;
+    sqlx::query(
+        "INSERT INTO workspaces (id, org_id, name) VALUES ($1, $2, 'source-decay-update-test')",
+    )
+    .bind(ws)
+    .bind(org)
+    .execute(&pool)
+    .await?;
+    sqlx::query(
+        "INSERT INTO knowledge_bases (id, workspace_id, name)
+         VALUES ($1, $2, 'source-decay-update-test')",
+    )
+    .bind(kb)
+    .bind(ws)
+    .execute(&pool)
+    .await?;
+    sqlx::query("INSERT INTO sources (id, kb_id, kind, name) VALUES ($1, $2, 'folder', 'f')")
+        .bind(source)
+        .bind(kb)
+        .execute(&pool)
+        .await?;
+
+    let run = async {
+        let renamed = sources::update(&pool, source, Some("g"), None, None, None, None).await?;
+        assert_eq!(renamed.decay, None, "a source starts without decay");
+
+        let settings = DecaySettings {
+            max_items: 3,
+            ..DecaySettings::default()
+        };
+        let on =
+            sources::update(&pool, source, None, None, None, None, Some(Some(&settings))).await?;
+        assert_eq!(
+            on.decay,
+            Some(serde_json::json!({
+                "max_items": 3, "half_life_days": 60, "stale_after_days": 30, "min_age_days": 30
+            }))
+        );
+
+        let kept = sources::update(&pool, source, Some("h"), None, None, None, None).await?;
+        assert_eq!(kept.decay, on.decay, "another field leaves decay alone");
+
+        let too_long = DecaySettings {
+            half_life_days: 400,
+            ..DecaySettings::default()
+        };
+        let refused =
+            sources::update(&pool, source, None, None, None, None, Some(Some(&too_long))).await;
+        assert!(refused.is_err());
+        let listed = sources::list(&pool, kb).await?;
+        assert_eq!(listed[0].decay, on.decay, "a refused value changes nothing");
+
+        let off = sources::update(&pool, source, None, None, None, None, Some(None)).await?;
+        assert_eq!(off.decay, None, "null turns decay off");
+        anyhow::Ok(())
+    }
+    .await;
+
+    sqlx::query("DELETE FROM organizations WHERE id = $1")
+        .bind(org)
+        .execute(&pool)
+        .await?;
+    run
+}
diff --git a/crates/utopia-store/tests/store/main.rs b/crates/utopia-store/tests/store/main.rs
index 7f446a50a..71ad210e5 100644
--- a/crates/utopia-store/tests/store/main.rs
+++ b/crates/utopia-store/tests/store/main.rs
@@ -95,6 +95,7 @@ mod a_signature_holds_on_every_path;
 mod a_signature_vector_lasts_while_its_text_does;
 mod a_similar_name_is_proposed_not_merged;
 mod a_single_name_candidate_is_not_counted;
+mod a_source_decay_is_set_and_cleared_by_update;
 mod a_source_decays_only_when_asked;
 mod a_source_kind_is_listed_once;
 mod a_source_reaches_only_where_it_was_granted;
diff --git a/docs/design/sources.md b/docs/design/sources.md
index d543cec15..1595178c9 100644
--- a/docs/design/sources.md
+++ b/docs/design/sources.md
@@ -125,6 +125,13 @@ indexed and never extracted (`sources.config.extract`) [0036 d7].
   HTML tables now are [0039].
 - Backup and restore (#712), a 100k-document benchmark (#713) [0016 E].
 
+## Decay
+
+A source may let its documents decay [RC-003](../decisions/re-cinq/RC-003-a-source-may-decay.md).
+`PATCH` a source with `decay` to set it: an object turns it on (a missing field takes Lore's value, so
+`{}` is Lore's behaviour), `null` turns it off, and leaving the field out changes nothing. A new
+source decays nothing.
+
 ## Open questions
 
 - Whether low engine confidence on an OCR or transcript segment should cap facts [0040].