diff --git a/apps/web/src/api/sessions.ts b/apps/web/src/api/sessions.ts
index 7bede1c..0bc13e8 100644
--- a/apps/web/src/api/sessions.ts
+++ b/apps/web/src/api/sessions.ts
@@ -54,13 +54,15 @@ export async function createInstance(
   branch?: string,
   newBranch?: string,
   configProfileId?: string,
-  sshKeyIds?: string[]
+  sshKeyIds?: string[],
+  workspaceId?: string
 ): Promise<ToolInstance> {
   const response = await apiClient.post(
     `/projects/${projectId}/repositories/${repoId}/instances`,
     {
       tool_type_id: toolTypeId,
       display_name: displayName,
+      workspace_id: workspaceId || undefined,
       clone_mode: cloneMode || "mount",
       branch: branch || undefined,
       new_branch: newBranch || undefined,
diff --git a/apps/web/src/pages/workspaces.tsx b/apps/web/src/pages/workspaces.tsx
index d055323..92f36a0 100644
--- a/apps/web/src/pages/workspaces.tsx
+++ b/apps/web/src/pages/workspaces.tsx
@@ -7,6 +7,7 @@ import { useWorkspaceActions } from "../hooks/use-workspace-actions";
 import { WorkspaceCard } from "../components/workspace-card";
 import { WorkspaceCreateForm } from "../components/workspace-create-form";
 import { StartToolModal } from "../components/start-tool-modal";
+import { createInstance, startInstance } from "../api/sessions";
 import type { Workspace } from "../types/workspace";
 
 export function WorkspacesPage() {
@@ -36,9 +37,25 @@ export function WorkspacesPage() {
 
   const handleStartTool = async (toolTypeId: string, configProfileId?: string) => {
     if (!startWorkspace) return;
-    // TODO: Call instance creation API with workspace_id
-    console.log("Start tool", { toolTypeId, configProfileId, workspace: startWorkspace.id });
-    setStartWorkspace(null);
+    try {
+      const instance = await createInstance(
+        projectId,
+        repoId,
+        toolTypeId,
+        `${startWorkspace.name} - ${toolTypeId}`,
+        undefined,
+        undefined,
+        undefined,
+        configProfileId,
+        [],
+        startWorkspace.id
+      );
+      await startInstance(projectId, repoId, instance.id, configProfileId);
+      setStartWorkspace(null);
+      await refresh();
+    } catch (err) {
+      alert(err instanceof Error ? err.message : "Failed to start tool");
+    }
   };
 
   return (