coder · ethanndickson · Jul 13, 2026 · Jul 8, 2026 · Jul 8, 2026 · Jul 8, 2026
diff --git a/coderd/exp_chats_test.go b/coderd/exp_chats_test.go
@@ -9085,6 +9085,43 @@ func TestRegenerateChatTitle(t *testing.T) {
 		require.Equal(t, "Test Chat", updated.Title)
 	})
 
+	t.Run("DoesNotBumpHistoryVersion", func(t *testing.T) {
+		t.Parallel()
+
+		ctx := testutil.Context(t, testutil.WaitLong)
+		client, db := newChatClientWithDatabase(t)
+		user := coderdtest.CreateFirstUser(t, client.Client)
+		modelConfig := createTitleGenerationModelConfig(t, client)
+
+		chat := dbgen.Chat(t, db, database.Chat{
+			OrganizationID:    user.OrganizationID,
+			OwnerID:           user.UserID,
+			LastModelConfigID: modelConfig.ID,
+			Title:             "history fence chat",
+		})
+		seedManualTitleSourceMessage(t, db, chat, modelConfig.ID)
+
+		// Leave history_version lagging snapshot_version, as when a
+		// generation task is in flight. A chat_messages write here would
+		// sync it and break that task's commit fence.
+		_, err := db.LockChatAndBumpSnapshotVersion(dbauthz.AsSystemRestricted(ctx), chat.ID)
+		require.NoError(t, err)
+
+		before, err := db.GetChatByID(dbauthz.AsSystemRestricted(ctx), chat.ID)
+		require.NoError(t, err)
+		require.NotEqual(t, before.SnapshotVersion, before.HistoryVersion,
+			"setup must leave history_version lagging snapshot_version")
+
+		updated, err := client.RegenerateChatTitle(ctx, chat.ID)
+		require.NoError(t, err)
+		require.Equal(t, "Test Chat", updated.Title)
+
+		after, err := db.GetChatByID(dbauthz.AsSystemRestricted(ctx), chat.ID)
+		require.NoError(t, err)
+		require.Equal(t, before.HistoryVersion, after.HistoryVersion,
+			"manual title regeneration must not touch chat_messages")
+	})
+
 	t.Run("NoDefaultModelConfig", func(t *testing.T) {
 		t.Parallel()
 
@@ -9256,6 +9293,41 @@ func TestProposeChatTitle(t *testing.T) {
 		require.True(t, persisted.UpdatedAt.Equal(before.UpdatedAt))
 	})
 
+	t.Run("DoesNotBumpHistoryVersion", func(t *testing.T) {
+		t.Parallel()
+
+		ctx := testutil.Context(t, testutil.WaitLong)
+		client, db := newChatClientWithDatabase(t)
+		user := coderdtest.CreateFirstUser(t, client.Client)
+		modelConfig := createTitleGenerationModelConfig(t, client)
+
+		chat := dbgen.Chat(t, db, database.Chat{
+			OrganizationID:    user.OrganizationID,
+			OwnerID:           user.UserID,
+			LastModelConfigID: modelConfig.ID,
+			Title:             "history fence chat",
+		})
+		seedManualTitleSourceMessage(t, db, chat, modelConfig.ID)
+
+		// See the matching TestRegenerateChatTitle subtest.
+		_, err := db.LockChatAndBumpSnapshotVersion(dbauthz.AsSystemRestricted(ctx), chat.ID)
+		require.NoError(t, err)
+
+		before, err := db.GetChatByID(dbauthz.AsSystemRestricted(ctx), chat.ID)
+		require.NoError(t, err)
+		require.NotEqual(t, before.SnapshotVersion, before.HistoryVersion,
+			"setup must leave history_version lagging snapshot_version")
+
+		resp, err := client.ProposeChatTitle(ctx, chat.ID)
+		require.NoError(t, err)
+		require.Equal(t, "Test Chat", resp.Title)
+
+		after, err := db.GetChatByID(dbauthz.AsSystemRestricted(ctx), chat.ID)
+		require.NoError(t, err)
+		require.Equal(t, before.HistoryVersion, after.HistoryVersion,
+			"title proposal must not touch chat_messages")
+	})
+
 	t.Run("NoDefaultModelConfig", func(t *testing.T) {
 		t.Parallel()
 

diff --git a/coderd/x/chatd/ARCHITECTURE.md b/coderd/x/chatd/ARCHITECTURE.md
@@ -259,7 +259,7 @@ Each row in `chat_messages` has a `revision` column. It stores the `chats.snapsh
 
 `chats.history_version` stores the latest `snapshot_version` in which chat message history changed. It starts at `0`, remains unchanged for non-history transitions, and is set to the current `snapshot_version` whenever a message is inserted or meaningfully updated. A newly created chat starts with `snapshot_version = 1`; because `Create` inserts initial history in that snapshot, the created chat's `history_version` becomes `1`. No-op message updates do not advance message `revision`, advance `history_version`, or reset `generation_attempt`. Whenever `history_version` changes, `generation_attempt` is reset to `0`; generation attempts are scoped to the current history version.
 
-Message revision triggers depend on the transition invariant that `snapshot_version` is allocated immediately after the chat row is locked and before any message mutation happens. Runtime code must not assign `chat_messages.revision` directly.
+Message revision triggers depend on the transition invariant that `snapshot_version` is allocated immediately after the chat row is locked and before any message mutation happens. Runtime code must not assign `chat_messages.revision` directly, and every `chat_messages` insert or update must go through a state machine transition: the triggers advance `history_version` on any write, so an out-of-band write (even of a hidden or soft-deleted row) moves `history_version` without a matching `snapshot_version` bump and breaks the fence of an in-flight generation task.
 
 A `BEFORE INSERT` trigger assigns the current chat `snapshot_version` to the inserted message row and records the same value as the chat's latest history version: