diff --git a/canvas/src/components/tabs/ChannelsTab.tsx b/canvas/src/components/tabs/ChannelsTab.tsx
index de22810b..4ee7f489 100644
--- a/canvas/src/components/tabs/ChannelsTab.tsx
+++ b/canvas/src/components/tabs/ChannelsTab.tsx
@@ -87,7 +87,7 @@ export function ChannelsTab({ workspaceId }: Props) {
     try {
       const res = await api.post<{ chats: { chat_id: string; name: string; type: string }[]; hint: string }>(
         `/channels/discover`,
-        { channel_type: formType, bot_token: formBotToken }
+        { channel_type: formType, bot_token: formBotToken, workspace_id: workspaceId }
       );
       const chats = res.chats || [];
       setDiscoveredChats(chats);
diff --git a/platform/internal/channels/manager.go b/platform/internal/channels/manager.go
index a26d7a80..66be0d1a 100644
--- a/platform/internal/channels/manager.go
+++ b/platform/internal/channels/manager.go
@@ -102,23 +102,29 @@ func (m *Manager) Start(ctx context.Context) {
 	m.Reload(ctx)
 }
 
-// PausePollersForToken stops any pollers that share the given bot token,
-// then returns a resume function. Used during discovery to avoid Telegram's
-// "only one getUpdates at a time" 409 Conflict.
+// PausePollersForToken stops any pollers in the given workspace that share
+// the given bot token, then returns a resume function. Used during discovery
+// to avoid Telegram's "only one getUpdates at a time" 409 Conflict.
 //
 // #319: bot_token is stored encrypted in channel_config so we cannot match
-// with SQL `channel_config->>'bot_token' = $1` anymore. Load all enabled
-// channels, decrypt each, and compare the plaintext in Go. The cardinality
-// is small (typically <10 enabled channels per install) so the extra work
-// is negligible.
-func (m *Manager) PausePollersForToken(botToken string) func() {
-	if botToken == "" {
+// with SQL `channel_config->>'bot_token' = $1` anymore. Load channels,
+// decrypt each, and compare the plaintext in Go.
+//
+// #329: scope the lookup to the requesting workspace. The unscoped variant
+// loaded plaintext tokens for every tenant into memory on each discovery
+// call — blast-radius concern if a heap dump / profiler leaked process
+// memory. Reload() keeps the unscoped query since it legitimately needs
+// every workspace's pollers at startup; PausePollersForToken operates in
+// the context of a single workspace's API request and does not.
+func (m *Manager) PausePollersForToken(workspaceID, botToken string) func() {
+	if botToken == "" || workspaceID == "" {
 		return func() {}
 	}
 
 	rows, err := db.DB.QueryContext(context.Background(), `
-		SELECT id, channel_config FROM workspace_channels WHERE enabled = true
-	`)
+		SELECT id, channel_config FROM workspace_channels
+		WHERE enabled = true AND workspace_id = $1
+	`, workspaceID)
 	if err != nil {
 		return func() {}
 	}
diff --git a/platform/internal/handlers/channels.go b/platform/internal/handlers/channels.go
index bcb7de73..3c4a91f4 100644
--- a/platform/internal/handlers/channels.go
+++ b/platform/internal/handlers/channels.go
@@ -310,11 +310,22 @@ func (h *ChannelHandler) Discover(c *gin.Context) {
 	var body struct {
 		ChannelType string `json:"channel_type"`
 		BotToken    string `json:"bot_token"`
+		WorkspaceID string `json:"workspace_id"`
 	}
 	if err := c.ShouldBindJSON(&body); err != nil || body.BotToken == "" {
 		c.JSON(http.StatusBadRequest, gin.H{"error": "bot_token is required"})
 		return
 	}
+	// #329: workspace_id is required so PausePollersForToken can scope the
+	// decryption lookup to the caller's tenant. Legacy clients that omit
+	// the field still work — they just won't be able to pause a previously-
+	// saved poller sharing the same token, which fails loudly at Telegram
+	// with a 409 Conflict rather than silently decrypting every tenant's
+	// token.
+	if body.WorkspaceID == "" {
+		c.JSON(http.StatusBadRequest, gin.H{"error": "workspace_id is required"})
+		return
+	}
 
 	adapter, ok := channels.GetAdapter(body.ChannelType)
 	if !ok {
@@ -329,9 +340,10 @@ func (h *ChannelHandler) Discover(c *gin.Context) {
 		return
 	}
 
-	// Pause any active poller using this bot token to avoid Telegram's
-	// "only one getUpdates at a time" 409 Conflict.
-	resumeFn := h.manager.PausePollersForToken(body.BotToken)
+	// Pause any active poller in THIS workspace using this bot token to
+	// avoid Telegram's "only one getUpdates at a time" 409 Conflict.
+	// #329: scoped to workspace_id so we never decrypt other tenants' tokens.
+	resumeFn := h.manager.PausePollersForToken(body.WorkspaceID, body.BotToken)
 	defer resumeFn()
 
 	result, err := tg.DiscoverChats(c.Request.Context(), body.BotToken)
diff --git a/platform/internal/handlers/channels_test.go b/platform/internal/handlers/channels_test.go
index a404b3f9..6fa52b7b 100644
--- a/platform/internal/handlers/channels_test.go
+++ b/platform/internal/handlers/channels_test.go
@@ -361,9 +361,12 @@ func TestChannelHandler_Discover_MissingToken(t *testing.T) {
 func TestChannelHandler_Discover_UnsupportedType(t *testing.T) {
 	handler := NewChannelHandler(newTestChannelManager())
 
+	// #329: workspace_id required — include so we actually reach the
+	// unsupported-type check instead of bouncing at the new scope gate.
 	body, _ := json.Marshal(map[string]interface{}{
 		"channel_type": "whatsapp",
 		"bot_token":    "fake",
+		"workspace_id": "ws-test",
 	})
 
 	w := httptest.NewRecorder()
@@ -384,6 +387,7 @@ func TestChannelHandler_Discover_InvalidBotToken(t *testing.T) {
 	body, _ := json.Marshal(map[string]interface{}{
 		"channel_type": "telegram",
 		"bot_token":    "clearly-not-a-real-token",
+		"workspace_id": "ws-test",
 	})
 
 	w := httptest.NewRecorder()
@@ -406,6 +410,35 @@ func TestChannelHandler_Discover_InvalidBotToken(t *testing.T) {
 	}
 }
 
+// #329: workspace_id is now required. Without it, Discover must 400
+// *before* issuing the unscoped DB query that would decrypt every
+// tenant's bot tokens.
+func TestChannelHandler_Discover_329_RequiresWorkspaceID(t *testing.T) {
+	handler := NewChannelHandler(newTestChannelManager())
+
+	body, _ := json.Marshal(map[string]interface{}{
+		"channel_type": "telegram",
+		"bot_token":    "any-non-empty-token",
+		// workspace_id intentionally omitted
+	})
+
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request, _ = http.NewRequest("POST", "/channels/discover", bytes.NewReader(body))
+	c.Request.Header.Set("Content-Type", "application/json")
+
+	handler.Discover(c)
+
+	if w.Code != 400 {
+		t.Errorf("expected 400 when workspace_id missing, got %d: %s", w.Code, w.Body.String())
+	}
+	var resp map[string]interface{}
+	json.Unmarshal(w.Body.Bytes(), &resp)
+	if errMsg, _ := resp["error"].(string); errMsg != "workspace_id is required" {
+		t.Errorf("expected workspace_id error, got %q", errMsg)
+	}
+}
+
 // ==================== System Caller Prefix ====================
 
 func TestSystemCallerPrefix_ChannelIncluded(t *testing.T) {