diff --git a/coderd/chatd/chatd.go b/coderd/chatd/chatd.go
index 31be73030d..1e18ab8322 100644
--- a/coderd/chatd/chatd.go
+++ b/coderd/chatd/chatd.go
@@ -988,6 +988,7 @@ func (p *Server) Subscribe(
 	ctx context.Context,
 	chatID uuid.UUID,
 	requestHeader http.Header,
+	afterMessageID int64,
 ) (
 	[]codersdk.ChatStreamEvent,
 	<-chan codersdk.ChatStreamEvent,
@@ -1013,8 +1014,14 @@ func (p *Server) Subscribe(
 		}
 	}
 
-	// Load initial messages from DB
-	messages, err := p.db.GetChatMessagesByChatID(ctx, chatID)
+	// Load initial messages from DB. When afterMessageID > 0 the
+	// caller already has messages up to that ID (e.g. from the REST
+	// endpoint), so we only fetch newer ones to avoid sending
+	// duplicate data.
+	messages, err := p.db.GetChatMessagesByChatID(ctx, database.GetChatMessagesByChatIDParams{
+		ChatID:  chatID,
+		AfterID: afterMessageID,
+	})
 	if err == nil {
 		for _, msg := range messages {
 			sdkMsg := db2sdk.ChatMessage(msg)
@@ -1191,23 +1198,24 @@ func (p *Server) Subscribe(
 				case notify := <-notifications:
 					// Handle different notification types
 					if notify.AfterMessageID > 0 {
-						// Read new messages from DB
-						messages, err := p.db.GetChatMessagesByChatID(mergedCtx, chatID)
+						// Read only new messages from DB.
+						messages, err := p.db.GetChatMessagesByChatID(mergedCtx, database.GetChatMessagesByChatIDParams{
+							ChatID:  chatID,
+							AfterID: lastMessageID,
+						})
 						if err == nil {
 							for _, msg := range messages {
-								if msg.ID > lastMessageID {
-									sdkMsg := db2sdk.ChatMessage(msg)
-									select {
-									case <-mergedCtx.Done():
-										return
-									case mergedEvents <- codersdk.ChatStreamEvent{
-										Type:    codersdk.ChatStreamEventTypeMessage,
-										ChatID:  chatID,
-										Message: &sdkMsg,
-									}:
-									}
-									lastMessageID = msg.ID
+								sdkMsg := db2sdk.ChatMessage(msg)
+								select {
+								case <-mergedCtx.Done():
+									return
+								case mergedEvents <- codersdk.ChatStreamEvent{
+									Type:    codersdk.ChatStreamEventTypeMessage,
+									ChatID:  chatID,
+									Message: &sdkMsg,
+								}:
 								}
+								lastMessageID = msg.ID
 							}
 						}
 					}
diff --git a/coderd/chatd/chatd_test.go b/coderd/chatd/chatd_test.go
index cfab2da299..1c494ed75d 100644
--- a/coderd/chatd/chatd_test.go
+++ b/coderd/chatd/chatd_test.go
@@ -27,6 +27,7 @@ import (
 	"github.com/coder/coder/v2/coderd/database/dbgen"
 	"github.com/coder/coder/v2/coderd/database/dbtestutil"
 	dbpubsub "github.com/coder/coder/v2/coderd/database/pubsub"
+	"github.com/coder/coder/v2/coderd/util/slice"
 	"github.com/coder/coder/v2/codersdk"
 	"github.com/coder/coder/v2/provisioner/echo"
 	"github.com/coder/coder/v2/testutil"
@@ -60,7 +61,7 @@ func TestInterruptChatBroadcastsStatusAcrossInstances(t *testing.T) {
 	})
 	require.NoError(t, err)
 
-	_, events, cancel, ok := replicaB.Subscribe(ctx, chat.ID, nil)
+	_, events, cancel, ok := replicaB.Subscribe(ctx, chat.ID, nil, 0)
 	require.True(t, ok)
 	t.Cleanup(cancel)
 
@@ -202,7 +203,10 @@ func TestSendMessageQueueBehaviorQueuesWhenBusy(t *testing.T) {
 	require.NoError(t, err)
 	require.Len(t, queued, 1)
 
-	messages, err := db.GetChatMessagesByChatID(ctx, chat.ID)
+	messages, err := db.GetChatMessagesByChatID(ctx, database.GetChatMessagesByChatIDParams{
+		ChatID:  chat.ID,
+		AfterID: 0,
+	})
 	require.NoError(t, err)
 	require.Len(t, messages, 1)
 }
@@ -252,7 +256,10 @@ func TestSendMessageInterruptBehaviorSendsImmediatelyWhenBusy(t *testing.T) {
 	require.NoError(t, err)
 	require.Len(t, queued, 0)
 
-	messages, err := db.GetChatMessagesByChatID(ctx, chat.ID)
+	messages, err := db.GetChatMessagesByChatID(ctx, database.GetChatMessagesByChatIDParams{
+		ChatID:  chat.ID,
+		AfterID: 0,
+	})
 	require.NoError(t, err)
 	require.Len(t, messages, 2)
 	require.Equal(t, messages[len(messages)-1].ID, result.Message.ID)
@@ -275,7 +282,10 @@ func TestEditMessageUpdatesAndTruncatesAndClearsQueue(t *testing.T) {
 	})
 	require.NoError(t, err)
 
-	initialMessages, err := db.GetChatMessagesByChatID(ctx, chat.ID)
+	initialMessages, err := db.GetChatMessagesByChatID(ctx, database.GetChatMessagesByChatIDParams{
+		ChatID:  chat.ID,
+		AfterID: 0,
+	})
 	require.NoError(t, err)
 	require.Len(t, initialMessages, 1)
 	editedMessageID := initialMessages[0].ID
@@ -322,7 +332,10 @@ func TestEditMessageUpdatesAndTruncatesAndClearsQueue(t *testing.T) {
 	require.Len(t, editedSDK.Content, 1)
 	require.Equal(t, "edited", editedSDK.Content[0].Text)
 
-	messages, err := db.GetChatMessagesByChatID(ctx, chat.ID)
+	messages, err := db.GetChatMessagesByChatID(ctx, database.GetChatMessagesByChatIDParams{
+		ChatID:  chat.ID,
+		AfterID: 0,
+	})
 	require.NoError(t, err)
 	require.Len(t, messages, 1)
 	require.Equal(t, editedMessageID, messages[0].ID)
@@ -657,7 +670,7 @@ func TestSubscribeSnapshotIncludesStatusEvent(t *testing.T) {
 	})
 	require.NoError(t, err)
 
-	snapshot, _, cancel, ok := replica.Subscribe(ctx, chat.ID, nil)
+	snapshot, _, cancel, ok := replica.Subscribe(ctx, chat.ID, nil, 0)
 	require.True(t, ok)
 	t.Cleanup(cancel)
 
@@ -686,7 +699,7 @@ func TestSubscribeNoPubsubNoDuplicateMessageParts(t *testing.T) {
 	})
 	require.NoError(t, err)
 
-	snapshot, events, cancel, ok := replica.Subscribe(ctx, chat.ID, nil)
+	snapshot, events, cancel, ok := replica.Subscribe(ctx, chat.ID, nil, 0)
 	require.True(t, ok)
 	t.Cleanup(cancel)
 
@@ -708,6 +721,87 @@ func TestSubscribeNoPubsubNoDuplicateMessageParts(t *testing.T) {
 	}
 }
 
+func TestSubscribeAfterMessageID(t *testing.T) {
+	t.Parallel()
+
+	db, ps := dbtestutil.NewDB(t)
+	replica := newTestServer(t, db, ps, uuid.New())
+
+	ctx := testutil.Context(t, testutil.WaitLong)
+	user, model := seedChatDependencies(ctx, t, db)
+
+	// Create a chat — this inserts one initial "user" message.
+	chat, err := replica.CreateChat(ctx, chatd.CreateOptions{
+		OwnerID:            user.ID,
+		Title:              "after-id-test",
+		ModelConfigID:      model.ID,
+		InitialUserContent: []fantasy.Content{fantasy.TextContent{Text: "first"}},
+	})
+	require.NoError(t, err)
+
+	// Insert two more messages so we have three total visible
+	// messages (the initial user message plus these two).
+	msg2, err := db.InsertChatMessage(ctx, database.InsertChatMessageParams{
+		ChatID:              chat.ID,
+		ModelConfigID:       uuid.NullUUID{UUID: model.ID, Valid: true},
+		Role:                "assistant",
+		Content:             pqtype.NullRawMessage{RawMessage: json.RawMessage(`"second"`), Valid: true},
+		Visibility:          database.ChatMessageVisibilityBoth,
+		InputTokens:         sql.NullInt64{},
+		OutputTokens:        sql.NullInt64{},
+		TotalTokens:         sql.NullInt64{},
+		ReasoningTokens:     sql.NullInt64{},
+		CacheCreationTokens: sql.NullInt64{},
+		CacheReadTokens:     sql.NullInt64{},
+		ContextLimit:        sql.NullInt64{},
+		Compressed:          sql.NullBool{},
+	})
+	require.NoError(t, err)
+
+	_, err = db.InsertChatMessage(ctx, database.InsertChatMessageParams{
+		ChatID:              chat.ID,
+		ModelConfigID:       uuid.NullUUID{UUID: model.ID, Valid: true},
+		Role:                "user",
+		Content:             pqtype.NullRawMessage{RawMessage: json.RawMessage(`"third"`), Valid: true},
+		Visibility:          database.ChatMessageVisibilityBoth,
+		InputTokens:         sql.NullInt64{},
+		OutputTokens:        sql.NullInt64{},
+		TotalTokens:         sql.NullInt64{},
+		ReasoningTokens:     sql.NullInt64{},
+		CacheCreationTokens: sql.NullInt64{},
+		CacheReadTokens:     sql.NullInt64{},
+		ContextLimit:        sql.NullInt64{},
+		Compressed:          sql.NullBool{},
+	})
+	require.NoError(t, err)
+
+	// Control: Subscribe with afterMessageID=0 returns ALL messages.
+	allSnapshot, _, cancelAll, ok := replica.Subscribe(ctx, chat.ID, nil, 0)
+	require.True(t, ok)
+	cancelAll()
+
+	allMessages := filterMessageEvents(allSnapshot)
+	require.Len(t, allMessages, 3, "afterMessageID=0 should return all three messages")
+
+	// Subscribe with afterMessageID set to the second message's ID.
+	// Only the third message (inserted after msg2) should appear.
+	partialSnapshot, _, cancelPartial, ok := replica.Subscribe(ctx, chat.ID, nil, msg2.ID)
+	require.True(t, ok)
+	cancelPartial()
+
+	partialMessages := filterMessageEvents(partialSnapshot)
+	require.Len(t, partialMessages, 1, "afterMessageID=msg2.ID should return only messages after msg2")
+	require.Equal(t, "user", partialMessages[0].Message.Role)
+}
+
+// filterMessageEvents returns only the Message-type events from a
+// snapshot slice, which is useful for ignoring status / queue events.
+func filterMessageEvents(events []codersdk.ChatStreamEvent) []codersdk.ChatStreamEvent {
+	return slice.Filter(events, func(e codersdk.ChatStreamEvent) bool {
+		return e.Type == codersdk.ChatStreamEventTypeMessage
+	})
+}
+
 func TestCreateWorkspaceTool_EndToEnd(t *testing.T) {
 	t.Parallel()
 
diff --git a/coderd/chatd/subagent.go b/coderd/chatd/subagent.go
index ad2991fd09..9bafd360e3 100644
--- a/coderd/chatd/subagent.go
+++ b/coderd/chatd/subagent.go
@@ -397,7 +397,10 @@ func latestSubagentAssistantMessage(
 	store database.Store,
 	chatID uuid.UUID,
 ) (string, error) {
-	messages, err := store.GetChatMessagesByChatID(ctx, chatID)
+	messages, err := store.GetChatMessagesByChatID(ctx, database.GetChatMessagesByChatIDParams{
+		ChatID:  chatID,
+		AfterID: 0,
+	})
 	if err != nil {
 		return "", xerrors.Errorf("get chat messages: %w", err)
 	}
diff --git a/coderd/chats.go b/coderd/chats.go
index c706ae2db1..32539589c2 100644
--- a/coderd/chats.go
+++ b/coderd/chats.go
@@ -368,7 +368,10 @@ func (api *API) getChat(rw http.ResponseWriter, r *http.Request) {
 	chat := httpmw.ChatParam(r)
 	chatID := chat.ID
 
-	messages, err := api.Database.GetChatMessagesByChatID(ctx, chatID)
+	messages, err := api.Database.GetChatMessagesByChatID(ctx, database.GetChatMessagesByChatIDParams{
+		ChatID:  chatID,
+		AfterID: 0,
+	})
 	if err != nil {
 		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
 			Message: "Failed to get chat messages.",
@@ -681,7 +684,20 @@ func (api *API) streamChat(rw http.ResponseWriter, r *http.Request) {
 		<-senderClosed
 	}()
 
-	snapshot, events, cancel, ok := api.chatDaemon.Subscribe(ctx, chatID, r.Header)
+	var afterMessageID int64
+	if v := r.URL.Query().Get("after_id"); v != "" {
+		var err error
+		afterMessageID, err = strconv.ParseInt(v, 10, 64)
+		if err != nil {
+			httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+				Message: "Invalid after_id parameter.",
+				Detail:  err.Error(),
+			})
+			return
+		}
+	}
+
+	snapshot, events, cancel, ok := api.chatDaemon.Subscribe(ctx, chatID, r.Header, afterMessageID)
 	if !ok {
 		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
 			Message: "Chat streaming is not available.",
diff --git a/coderd/database/dbauthz/dbauthz.go b/coderd/database/dbauthz/dbauthz.go
index 6ba7e7710b..28878df18e 100644
--- a/coderd/database/dbauthz/dbauthz.go
+++ b/coderd/database/dbauthz/dbauthz.go
@@ -2465,13 +2465,13 @@ func (q *querier) GetChatMessageByID(ctx context.Context, id int64) (database.Ch
 	return msg, nil
 }
 
-func (q *querier) GetChatMessagesByChatID(ctx context.Context, chatID uuid.UUID) ([]database.ChatMessage, error) {
+func (q *querier) GetChatMessagesByChatID(ctx context.Context, arg database.GetChatMessagesByChatIDParams) ([]database.ChatMessage, error) {
 	// Authorize read on the parent chat.
-	_, err := q.GetChatByID(ctx, chatID)
+	_, err := q.GetChatByID(ctx, arg.ChatID)
 	if err != nil {
 		return nil, err
 	}
-	return q.db.GetChatMessagesByChatID(ctx, chatID)
+	return q.db.GetChatMessagesByChatID(ctx, arg)
 }
 
 func (q *querier) GetChatMessagesForPromptByChatID(ctx context.Context, chatID uuid.UUID) ([]database.ChatMessage, error) {
diff --git a/coderd/database/dbauthz/dbauthz_test.go b/coderd/database/dbauthz/dbauthz_test.go
index b1114c93fc..9787144913 100644
--- a/coderd/database/dbauthz/dbauthz_test.go
+++ b/coderd/database/dbauthz/dbauthz_test.go
@@ -473,9 +473,10 @@ func (s *MethodTestSuite) TestChats() {
 	s.Run("GetChatMessagesByChatID", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
 		chat := testutil.Fake(s.T(), faker, database.Chat{})
 		msgs := []database.ChatMessage{testutil.Fake(s.T(), faker, database.ChatMessage{ChatID: chat.ID})}
+		arg := database.GetChatMessagesByChatIDParams{ChatID: chat.ID, AfterID: 0}
 		dbm.EXPECT().GetChatByID(gomock.Any(), chat.ID).Return(chat, nil).AnyTimes()
-		dbm.EXPECT().GetChatMessagesByChatID(gomock.Any(), chat.ID).Return(msgs, nil).AnyTimes()
-		check.Args(chat.ID).Asserts(chat, policy.ActionRead).Returns(msgs)
+		dbm.EXPECT().GetChatMessagesByChatID(gomock.Any(), arg).Return(msgs, nil).AnyTimes()
+		check.Args(arg).Asserts(chat, policy.ActionRead).Returns(msgs)
 	}))
 	s.Run("GetChatMessagesForPromptByChatID", s.Mocked(func(dbm *dbmock.MockStore, faker *gofakeit.Faker, check *expects) {
 		chat := testutil.Fake(s.T(), faker, database.Chat{})
diff --git a/coderd/database/dbmetrics/querymetrics.go b/coderd/database/dbmetrics/querymetrics.go
index e7bc3562e5..e1b8bebf1c 100644
--- a/coderd/database/dbmetrics/querymetrics.go
+++ b/coderd/database/dbmetrics/querymetrics.go
@@ -1007,7 +1007,7 @@ func (m queryMetricsStore) GetChatMessageByID(ctx context.Context, id int64) (da
 	return r0, r1
 }
 
-func (m queryMetricsStore) GetChatMessagesByChatID(ctx context.Context, chatID uuid.UUID) ([]database.ChatMessage, error) {
+func (m queryMetricsStore) GetChatMessagesByChatID(ctx context.Context, chatID database.GetChatMessagesByChatIDParams) ([]database.ChatMessage, error) {
 	start := time.Now()
 	r0, r1 := m.s.GetChatMessagesByChatID(ctx, chatID)
 	m.queryLatencies.WithLabelValues("GetChatMessagesByChatID").Observe(time.Since(start).Seconds())
diff --git a/coderd/database/dbmock/dbmock.go b/coderd/database/dbmock/dbmock.go
index 6e1476a3d0..3d767d025c 100644
--- a/coderd/database/dbmock/dbmock.go
+++ b/coderd/database/dbmock/dbmock.go
@@ -1838,18 +1838,18 @@ func (mr *MockStoreMockRecorder) GetChatMessageByID(ctx, id any) *gomock.Call {
 }
 
 // GetChatMessagesByChatID mocks base method.
-func (m *MockStore) GetChatMessagesByChatID(ctx context.Context, chatID uuid.UUID) ([]database.ChatMessage, error) {
+func (m *MockStore) GetChatMessagesByChatID(ctx context.Context, arg database.GetChatMessagesByChatIDParams) ([]database.ChatMessage, error) {
 	m.ctrl.T.Helper()
-	ret := m.ctrl.Call(m, "GetChatMessagesByChatID", ctx, chatID)
+	ret := m.ctrl.Call(m, "GetChatMessagesByChatID", ctx, arg)
 	ret0, _ := ret[0].([]database.ChatMessage)
 	ret1, _ := ret[1].(error)
 	return ret0, ret1
 }
 
 // GetChatMessagesByChatID indicates an expected call of GetChatMessagesByChatID.
-func (mr *MockStoreMockRecorder) GetChatMessagesByChatID(ctx, chatID any) *gomock.Call {
+func (mr *MockStoreMockRecorder) GetChatMessagesByChatID(ctx, arg any) *gomock.Call {
 	mr.mock.ctrl.T.Helper()
-	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatMessagesByChatID", reflect.TypeOf((*MockStore)(nil).GetChatMessagesByChatID), ctx, chatID)
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatMessagesByChatID", reflect.TypeOf((*MockStore)(nil).GetChatMessagesByChatID), ctx, arg)
 }
 
 // GetChatMessagesForPromptByChatID mocks base method.
diff --git a/coderd/database/querier.go b/coderd/database/querier.go
index bc207cd6cf..03a3660466 100644
--- a/coderd/database/querier.go
+++ b/coderd/database/querier.go
@@ -214,7 +214,7 @@ type sqlcQuerier interface {
 	GetChatDiffStatusByChatID(ctx context.Context, chatID uuid.UUID) (ChatDiffStatus, error)
 	GetChatDiffStatusesByChatIDs(ctx context.Context, chatIds []uuid.UUID) ([]ChatDiffStatus, error)
 	GetChatMessageByID(ctx context.Context, id int64) (ChatMessage, error)
-	GetChatMessagesByChatID(ctx context.Context, chatID uuid.UUID) ([]ChatMessage, error)
+	GetChatMessagesByChatID(ctx context.Context, arg GetChatMessagesByChatIDParams) ([]ChatMessage, error)
 	GetChatMessagesForPromptByChatID(ctx context.Context, chatID uuid.UUID) ([]ChatMessage, error)
 	GetChatModelConfigByID(ctx context.Context, id uuid.UUID) (ChatModelConfig, error)
 	GetChatModelConfigByProviderAndModel(ctx context.Context, arg GetChatModelConfigByProviderAndModelParams) (ChatModelConfig, error)
diff --git a/coderd/database/queries.sql.go b/coderd/database/queries.sql.go
index 3a3ef330a2..e8e37d3069 100644
--- a/coderd/database/queries.sql.go
+++ b/coderd/database/queries.sql.go
@@ -3112,13 +3112,19 @@ FROM
     chat_messages
 WHERE
     chat_id = $1::uuid
+    AND id > $2::bigint
     AND visibility IN ('user', 'both')
 ORDER BY
     created_at ASC
 `
 
-func (q *sqlQuerier) GetChatMessagesByChatID(ctx context.Context, chatID uuid.UUID) ([]ChatMessage, error) {
-	rows, err := q.db.QueryContext(ctx, getChatMessagesByChatID, chatID)
+type GetChatMessagesByChatIDParams struct {
+	ChatID  uuid.UUID `db:"chat_id" json:"chat_id"`
+	AfterID int64     `db:"after_id" json:"after_id"`
+}
+
+func (q *sqlQuerier) GetChatMessagesByChatID(ctx context.Context, arg GetChatMessagesByChatIDParams) ([]ChatMessage, error) {
+	rows, err := q.db.QueryContext(ctx, getChatMessagesByChatID, arg.ChatID, arg.AfterID)
 	if err != nil {
 		return nil, err
 	}
diff --git a/coderd/database/queries/chats.sql b/coderd/database/queries/chats.sql
index bf75a12e62..71ca871544 100644
--- a/coderd/database/queries/chats.sql
+++ b/coderd/database/queries/chats.sql
@@ -41,6 +41,7 @@ FROM
     chat_messages
 WHERE
     chat_id = @chat_id::uuid
+    AND id > @after_id::bigint
     AND visibility IN ('user', 'both')
 ORDER BY
     created_at ASC;
diff --git a/site/src/api/api.ts b/site/src/api/api.ts
index 0b17e2fc2e..c1742cff45 100644
--- a/site/src/api/api.ts
+++ b/site/src/api/api.ts
@@ -140,9 +140,16 @@ export const watchWorkspace = (
 
 export const watchChat = (
 	chatId: string,
+	afterMessageId?: number,
 ): OneWayWebSocket<TypesGen.ServerSentEvent> => {
+	const params = new URLSearchParams();
+	if (afterMessageId !== undefined && afterMessageId > 0) {
+		params.set("after_id", afterMessageId.toString());
+	}
+	const query = params.toString();
+	const route = `/api/experimental/chats/${chatId}/stream${query ? `?${query}` : ""}`;
 	return new OneWayWebSocket({
-		apiRoute: `/api/experimental/chats/${chatId}/stream`,
+		apiRoute: route,
 	});
 };
 
diff --git a/site/src/pages/AgentsPage/AgentDetail/ChatContext.test.tsx b/site/src/pages/AgentsPage/AgentDetail/ChatContext.test.tsx
index dd7eb4bb16..28306a7545 100644
--- a/site/src/pages/AgentsPage/AgentDetail/ChatContext.test.tsx
+++ b/site/src/pages/AgentsPage/AgentDetail/ChatContext.test.tsx
@@ -202,7 +202,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, 1);
 		});
 
 		act(() => {
@@ -283,7 +283,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, 1);
 		});
 
 		act(() => {
@@ -358,7 +358,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, 1);
 		});
 
 		act(() => {
@@ -460,7 +460,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, 1);
 		});
 
 		const streamBaseline = streamRenderCount;
@@ -526,7 +526,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, 1);
 		});
 
 		act(() => {
@@ -601,7 +601,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, 1);
 		});
 
 		act(() => {
@@ -696,7 +696,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, 1);
 		});
 		expect(result.current.queuedMessages.map((message) => message.id)).toEqual([
 			queuedMessage.id,
@@ -781,7 +781,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, 1);
 		});
 
 		act(() => {
@@ -852,7 +852,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID1);
+			expect(watchChat).toHaveBeenCalledWith(chatID1, 1);
 		});
 
 		act(() => {
@@ -888,7 +888,7 @@ describe("useChatStore", () => {
 		});
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID2);
+			expect(watchChat).toHaveBeenCalledWith(chatID2, 10);
 		});
 
 		// The old WebSocket was closed during effect cleanup.
@@ -935,7 +935,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, 1);
 		});
 
 		act(() => {
@@ -991,7 +991,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, 1);
 		});
 
 		// Build up stream state so we can observe whether it gets cleared.
@@ -1093,7 +1093,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, 1);
 		});
 
 		// Build up stream state first.
@@ -1193,7 +1193,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID1);
+			expect(watchChat).toHaveBeenCalledWith(chatID1, 1);
 		});
 
 		act(() => {
@@ -1229,7 +1229,7 @@ describe("useChatStore", () => {
 		});
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID2);
+			expect(watchChat).toHaveBeenCalledWith(chatID2, 10);
 		});
 
 		expect(result.current.streamState).toBeNull();
@@ -1284,7 +1284,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID1);
+			expect(watchChat).toHaveBeenCalledWith(chatID1, 1);
 		});
 
 		// Verify queued messages from chat-1 are present.
@@ -1310,7 +1310,7 @@ describe("useChatStore", () => {
 		// After the switch, queued messages from chat-1 should NOT be
 		// visible — the store resets them on chatID change.
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID2);
+			expect(watchChat).toHaveBeenCalledWith(chatID2, undefined);
 		});
 		expect(result.current.queuedMessages).toEqual([]);
 	});
@@ -1352,7 +1352,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, undefined);
 		});
 
 		// Emit a batch with message_parts followed by a status change
@@ -1424,7 +1424,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, undefined);
 		});
 
 		act(() => {
@@ -1483,7 +1483,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, undefined);
 		});
 
 		act(() => {
@@ -1536,7 +1536,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, undefined);
 		});
 
 		act(() => {
@@ -1598,7 +1598,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, undefined);
 		});
 
 		// Set retry state first.
@@ -1676,7 +1676,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, undefined);
 		});
 
 		act(() => {
@@ -1734,7 +1734,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, undefined);
 		});
 
 		act(() => {
@@ -1783,7 +1783,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, undefined);
 		});
 
 		// Set an error via an error stream event first.
@@ -1847,7 +1847,7 @@ describe("useChatStore", () => {
 		);
 
 		await waitFor(() => {
-			expect(watchChat).toHaveBeenCalledWith(chatID);
+			expect(watchChat).toHaveBeenCalledWith(chatID, undefined);
 		});
 
 		// Transition to running — should call clearChatErrorReason.
diff --git a/site/src/pages/AgentsPage/AgentDetail/ChatContext.ts b/site/src/pages/AgentsPage/AgentDetail/ChatContext.ts
index 10f2186ed5..9cd93d88dc 100644
--- a/site/src/pages/AgentsPage/AgentDetail/ChatContext.ts
+++ b/site/src/pages/AgentsPage/AgentDetail/ChatContext.ts
@@ -435,6 +435,17 @@ export const useChatStore = (
 
 	const store = storeRef.current;
 
+	// Compute the last REST-fetched message ID so the stream can
+	// skip messages the client already has. We use a ref so the
+	// socket effect can read the latest value without including
+	// chatMessages in its dependency array (which would cause
+	// unnecessary reconnections).
+	const lastMessageIdRef = useRef<number | undefined>(undefined);
+	lastMessageIdRef.current =
+		chatMessages && chatMessages.length > 0
+			? chatMessages[chatMessages.length - 1].id
+			: undefined;
+
 	const updateSidebarChat = useCallback(
 		(updater: (chat: TypesGen.Chat) => TypesGen.Chat) => {
 			if (!chatID) {
@@ -550,7 +561,9 @@ export const useChatStore = (
 			return;
 		}
 
-		const socket = watchChat(chatID);
+		// Pass the last REST-fetched message ID so the stream
+		// only sends newer messages.
+		const socket = watchChat(chatID, lastMessageIdRef.current);
 		const handleMessage = (
 			payload: OneWayMessageEvent<TypesGen.ServerSentEvent>,
 		) => {