diff --git a/mateclaw-server/src/main/java/vip/mate/channel/ChannelMessageRouter.java b/mateclaw-server/src/main/java/vip/mate/channel/ChannelMessageRouter.java
index d0ecc33c..17adde6b 100644
--- a/mateclaw-server/src/main/java/vip/mate/channel/ChannelMessageRouter.java
+++ b/mateclaw-server/src/main/java/vip/mate/channel/ChannelMessageRouter.java
@@ -1310,6 +1310,15 @@ public class ChannelMessageRouter {
         return message.getChannelType() + ":" + identifier;
     }
 
+    /**
+     * Read-only existence check for a conversation by its logical id. Used by
+     * adapters that need to alias a legacy conversationId scheme to a new one
+     * without rewriting stored rows (e.g. Feishu group session-id migration).
+     */
+    public boolean conversationExists(String conversationId) {
+        return conversationService.findByConversationId(conversationId) != null;
+    }
+
     /**
      * Build a sender-attribution tag for group messages. Returns
      * {@code [@senderName]} when the message is from a multi-user channel
diff --git a/mateclaw-server/src/main/java/vip/mate/channel/feishu/FeishuChannelAdapter.java b/mateclaw-server/src/main/java/vip/mate/channel/feishu/FeishuChannelAdapter.java
index 0fc583dc..0bea5a1b 100644
--- a/mateclaw-server/src/main/java/vip/mate/channel/feishu/FeishuChannelAdapter.java
+++ b/mateclaw-server/src/main/java/vip/mate/channel/feishu/FeishuChannelAdapter.java
@@ -1116,6 +1116,11 @@ public class FeishuChannelAdapter extends AbstractChannelAdapter implements Stre
         // prompt only exposes the file name (not its path) to the model — so if this id does
         // not match, ReadFileTool / DocumentExtractTool cannot find the cached file.
         String shortSuffix = generateShortSessionSuffix(chatId, senderOpenId, isGroup);
+        // 群会话改用完整 chatId，但存量旧会话仍在 legacy 后缀下：读时别名回退，
+        // 让升级前已存在的群沿用旧 conversationId 延续，不重写存量行。
+        if (isGroup && chatId != null) {
+            shortSuffix = resolveGroupSessionSuffix(chatId);
+        }
         String conversationId = buildConversationId(shortSuffix, senderOpenId, isGroup);
 
         String stagedUploadPath = null;
@@ -1556,19 +1561,22 @@ public class FeishuChannelAdapter extends AbstractChannelAdapter implements Stre
         return null;
     }
 
-    // ==================== 会话 ID 优化 ====================
+    // ==================== 会话 ID ====================
 
     /**
-     * 生成更短的会话标识后缀
-     * - 群聊：app_id 后 4 位 + "_" + chat_id 后 8 位
-     * - 私聊：open_id 后 12 位
+     * 生成会话标识后缀。
+     * <ul>
+     *   <li>群聊：直接使用完整 {@code chatId}（全局唯一）。旧实现用 {@code {appId后4}_{chatId后8}}
+     *       截断后缀，不同群的 {@code chatId} 后 8 位可能相同 → 会话串台。改用完整 chatId 消除碰撞。
+     *       存量旧会话不重写，由 {@link #resolveGroupSessionSuffix} 做读时别名回退。</li>
+     *   <li>私聊：保持原状（取 {@code openId} 后 12 位）。注意私聊路径下该后缀实际不参与
+     *       conversationId——{@link #buildConversationId} 对 DM 直接用完整 {@code senderOpenId}，
+     *       故私聊会话 ID 不受本次改动影响。</li>
+     * </ul>
      */
     private String generateShortSessionSuffix(String chatId, String openId, boolean isGroup) {
         if (isGroup && chatId != null) {
-            String appId = getConfigString("app_id", "");
-            String appSuffix = appId.length() >= 4 ? appId.substring(appId.length() - 4) : appId;
-            String chatSuffix = chatId.length() >= 8 ? chatId.substring(chatId.length() - 8) : chatId;
-            return appSuffix + "_" + chatSuffix;
+            return chatId;
         }
         if (openId != null) {
             return openId.length() >= 12 ? openId.substring(openId.length() - 12) : openId;
@@ -1579,6 +1587,49 @@ public class FeishuChannelAdapter extends AbstractChannelAdapter implements Stre
         return null;
     }
 
+    /**
+     * 旧群会话后缀算法：{@code {appId后4}_{chatId后8}}。仅用于读时别名回退——
+     * 在不重写存量行的前提下，让升级前已存在的群会话沿用旧 conversationId 无缝延续。
+     */
+    // Package-private for testing.
+    String legacyGroupSuffix(String chatId) {
+        if (chatId == null) return null;
+        String appId = getConfigString("app_id", "");
+        String appSuffix = appId.length() >= 4 ? appId.substring(appId.length() - 4) : appId;
+        String chatSuffix = chatId.length() >= 8 ? chatId.substring(chatId.length() - 8) : chatId;
+        return appSuffix + "_" + chatSuffix;
+    }
+
+    /**
+     * 群会话后缀的读时别名回退（不重写存量）：
+     * <ul>
+     *   <li>新群（两个 key 都无会话）→ 用完整 chatId 的 canonical key；</li>
+     *   <li>已迁移群（canonical key 已有会话）→ 用 canonical；</li>
+     *   <li>存量群（canonical 无、legacy 有）→ 沿用 legacy key，历史无缝延续。</li>
+     * </ul>
+     */
+    // Package-private for testing.
+    String resolveGroupSessionSuffix(String chatId) {
+        String canonical = chatId;
+        String legacy = legacyGroupSuffix(chatId);
+        if (legacy == null || legacy.equals(canonical)) return canonical;
+        boolean canonicalExists = messageRouter.conversationExists(CHANNEL_TYPE + ":" + canonical);
+        boolean legacyExists = !canonicalExists
+                && messageRouter.conversationExists(CHANNEL_TYPE + ":" + legacy);
+        String picked = pickGroupSessionSuffix(canonical, legacy, canonicalExists, legacyExists);
+        if (picked.equals(legacy)) {
+            log.info("[feishu] Reusing legacy group session id for chat={} (read-time alias, no migration write)", chatId);
+        }
+        return picked;
+    }
+
+    /** Package-private for testing: 纯选择逻辑——存量 legacy 会话存在且尚未迁移时沿用 legacy，否则用 canonical。 */
+    static String pickGroupSessionSuffix(String canonical, String legacy,
+                                         boolean canonicalExists, boolean legacyExists) {
+        if (!canonicalExists && legacyExists) return legacy;
+        return canonical;
+    }
+
     /**
      * Compute the conversationId that {@link ChannelMessageRouter} would
      * derive for this chat, so we can save inbound files to the matching
diff --git a/mateclaw-server/src/test/java/vip/mate/channel/feishu/FeishuSessionIdTest.java b/mateclaw-server/src/test/java/vip/mate/channel/feishu/FeishuSessionIdTest.java
new file mode 100644
index 00000000..b09fcd18
--- /dev/null
+++ b/mateclaw-server/src/test/java/vip/mate/channel/feishu/FeishuSessionIdTest.java
@@ -0,0 +1,99 @@
+package vip.mate.channel.feishu;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.junit.jupiter.api.Test;
+import vip.mate.channel.ChannelMessageRouter;
+import vip.mate.channel.model.ChannelEntity;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+/**
+ * #299：群会话 ID 改用完整 chatId 避免后缀碰撞，存量旧会话用读时别名回退（不重写）。
+ */
+class FeishuSessionIdTest {
+
+    private static final String CHANNEL = FeishuChannelAdapter.CHANNEL_TYPE; // "feishu"
+    private static final String APP_ID = "cli_a1b2c3d4e5f6";
+
+    // ==================== legacyGroupSuffix：旧后缀算法 + 碰撞演示 ====================
+
+    @Test
+    void legacySuffix_format_appLast4UnderscoreChatLast8() {
+        FeishuChannelAdapter adapter = newAdapter(mock(ChannelMessageRouter.class));
+        // appId 后 4 = "e5f6"; chatId 后 8 = "11112222"
+        assertEquals("e5f6_11112222", adapter.legacyGroupSuffix("oc_aaaa11112222"));
+    }
+
+    @Test
+    void legacySuffix_collides_whileFullChatIdDoesNot() {
+        FeishuChannelAdapter adapter = newAdapter(mock(ChannelMessageRouter.class));
+        String chatA = "oc_AAAA_11112222";
+        String chatB = "oc_BBBB_11112222"; // 不同群，但 chatId 后 8 位相同
+        // 旧后缀碰撞（这正是 #299 要修的 bug）……
+        assertEquals(adapter.legacyGroupSuffix(chatA), adapter.legacyGroupSuffix(chatB));
+        // ……而完整 chatId 不碰撞。
+        assertNotEquals(chatA, chatB);
+    }
+
+    // ==================== pickGroupSessionSuffix：纯选择逻辑 ====================
+
+    @Test
+    void pick_newGroup_neitherExists_usesCanonical() {
+        assertEquals("oc_full", FeishuChannelAdapter.pickGroupSessionSuffix(
+                "oc_full", "e5f6_ocfull99", false, false));
+    }
+
+    @Test
+    void pick_canonicalAlreadyExists_usesCanonical() {
+        assertEquals("oc_full", FeishuChannelAdapter.pickGroupSessionSuffix(
+                "oc_full", "e5f6_ocfull99", true, false));
+    }
+
+    @Test
+    void pick_onlyLegacyExists_reusesLegacy() {
+        assertEquals("e5f6_ocfull99", FeishuChannelAdapter.pickGroupSessionSuffix(
+                "oc_full", "e5f6_ocfull99", false, true));
+    }
+
+    // ==================== resolveGroupSessionSuffix：读时别名回退（含路由查找） ====================
+
+    @Test
+    void resolve_newGroup_returnsFullChatId() {
+        ChannelMessageRouter router = mock(ChannelMessageRouter.class);
+        when(router.conversationExists(org.mockito.ArgumentMatchers.anyString())).thenReturn(false);
+        FeishuChannelAdapter adapter = newAdapter(router);
+        assertEquals("oc_aaaa11112222", adapter.resolveGroupSessionSuffix("oc_aaaa11112222"));
+    }
+
+    @Test
+    void resolve_canonicalExists_returnsFullChatId() {
+        ChannelMessageRouter router = mock(ChannelMessageRouter.class);
+        when(router.conversationExists(CHANNEL + ":oc_aaaa11112222")).thenReturn(true);
+        FeishuChannelAdapter adapter = newAdapter(router);
+        assertEquals("oc_aaaa11112222", adapter.resolveGroupSessionSuffix("oc_aaaa11112222"));
+    }
+
+    @Test
+    void resolve_legacyConversationExists_reusesLegacyId() {
+        ChannelMessageRouter router = mock(ChannelMessageRouter.class);
+        when(router.conversationExists(CHANNEL + ":oc_aaaa11112222")).thenReturn(false);
+        when(router.conversationExists(CHANNEL + ":e5f6_11112222")).thenReturn(true);
+        FeishuChannelAdapter adapter = newAdapter(router);
+        // 存量群：canonical 无、legacy 有 → 沿用 legacy，历史无缝延续。
+        assertEquals("e5f6_11112222", adapter.resolveGroupSessionSuffix("oc_aaaa11112222"));
+    }
+
+    // ==================== helpers ====================
+
+    private static FeishuChannelAdapter newAdapter(ChannelMessageRouter router) {
+        ChannelEntity e = new ChannelEntity();
+        e.setId(1L);
+        e.setChannelType("feishu");
+        e.setConfigJson("{\"app_id\":\"" + APP_ID + "\",\"app_secret\":\"y\"}");
+        return new FeishuChannelAdapter(
+                e, router, new ObjectMapper(),
+                null, null, null, null, null, null, null);
+    }
+}