feat: Add Antigravity provider integration + fix native mode startup

- Added Antigravity AI provider with Google OAuth authentication
- New integration client (antigravity.ts) with automatic endpoint fallback
- API routes for /api/antigravity/* (models, auth-status, test, chat)
- AntigravitySettings.tsx for Advanced Settings panel
- Updated session-api.ts and session-actions.ts for provider routing
- Updated opencode.jsonc with Antigravity plugin and 11 models:
  - Gemini 3 Pro Low/High, Gemini 3 Flash
  - Claude Sonnet 4.5 (+ thinking variants)
  - Claude Opus 4.5 (+ thinking variants)
  - GPT-OSS 120B Medium

- Fixed native mode startup error (was trying to launch __nomadarch_native__ as binary)
- Native mode workspaces now skip binary launch and are immediately ready

packages/ui/src/stores/session-actions.ts

@@ -1116,6 +1116,144 @@ async function streamZAIChat(
   })
 }
 
+async function streamAntigravityChat(
+  instanceId: string,
+  sessionId: string,
+  providerId: string,
+  modelId: string,
+  messages: ExternalChatMessage[],
+  messageId: string,
+  assistantMessageId: string,
+  assistantPartId: string,
+): Promise<void> {
+  const controller = new AbortController()
+  const timeoutId = setTimeout(() => controller.abort(), STREAM_TIMEOUT_MS)
+
+  // Get workspace path for tool execution
+  const instance = instances().get(instanceId)
+  const workspacePath = instance?.folder || ""
+
+  const response = await fetch("/api/antigravity/chat", {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    signal: controller.signal,
+    body: JSON.stringify({
+      model: modelId,
+      messages,
+      stream: true,
+      workspacePath,
+      enableTools: true,
+    }),
+  })
+
+  if (!response.ok) {
+    const errorText = await response.text().catch(() => "")
+    throw new Error(errorText || `Antigravity chat failed (${response.status})`)
+  }
+
+  const store = messageStoreBus.getOrCreate(instanceId)
+  store.beginStreamingUpdate()
+  let fullText = ""
+  let lastUpdateAt = 0
+
+  try {
+    await readSseStream(response, (data) => {
+      try {
+        const chunk = JSON.parse(data)
+        if (chunk?.error) throw new Error(chunk.error)
+
+        // Handle tool execution results (special events from backend)
+        if (chunk?.type === "tool_result") {
+          const toolResult = `\n\n✅ **Tool Executed:** ${chunk.content}\n\n`
+          fullText += toolResult
+          store.applyPartUpdate({
+            messageId: assistantMessageId,
+            part: { id: assistantPartId, type: "text", text: fullText } as any,
+          })
+
+          // Dispatch file change event to refresh sidebar
+          if (typeof window !== "undefined") {
+            console.log(`[EVENT] Dispatching FILE_CHANGE_EVENT for ${instanceId}`);
+            window.dispatchEvent(new CustomEvent(FILE_CHANGE_EVENT, { detail: { instanceId } }))
+          }
+
+          // Auto-trigger preview for HTML file writes
+          const content = chunk.content || ""
+          if (content.includes("Successfully wrote") &&
+            (content.includes(".html") || content.includes("index.") || content.includes(".htm"))) {
+            if (typeof window !== "undefined") {
+              const htmlMatch = content.match(/to\s+([^\s]+\.html?)/)
+              if (htmlMatch) {
+                const relativePath = htmlMatch[1]
+                const origin = typeof window !== "undefined" ? window.location.origin : "http://localhost:3000"
+                const apiOrigin = origin.replace(":3000", ":9898")
+                const previewUrl = `${apiOrigin}/api/workspaces/${instanceId}/serve/${relativePath}`
+
+                console.log(`[EVENT] Auto-preview triggered for ${previewUrl}`);
+                window.dispatchEvent(new CustomEvent(BUILD_PREVIEW_EVENT, {
+                  detail: { url: previewUrl, instanceId }
+                }))
+              }
+            }
+          }
+
+          return
+        }
+
+        const delta =
+          chunk?.choices?.[0]?.delta?.content ??
+          chunk?.choices?.[0]?.message?.content
+        if (typeof delta !== "string" || delta.length === 0) return
+        fullText += delta
+
+        const now = Date.now()
+        if (now - lastUpdateAt > 40) { // Limit to ~25 updates per second
+          lastUpdateAt = now
+          store.applyPartUpdate({
+            messageId: assistantMessageId,
+            part: { id: assistantPartId, type: "text", text: fullText } as any,
+          })
+        }
+      } catch (e) {
+        if (e instanceof Error) throw e
+      }
+    })
+
+    // Always apply final text update
+    store.applyPartUpdate({
+      messageId: assistantMessageId,
+      part: { id: assistantPartId, type: "text", text: fullText } as any,
+    })
+  } finally {
+    clearTimeout(timeoutId)
+    store.endStreamingUpdate()
+  }
+
+  store.upsertMessage({
+    id: assistantMessageId,
+    sessionId,
+    role: "assistant",
+    status: "complete",
+    updatedAt: Date.now(),
+    isEphemeral: false,
+  })
+  store.setMessageInfo(assistantMessageId, {
+    id: assistantMessageId,
+    role: "assistant",
+    providerID: providerId,
+    modelID: modelId,
+    time: { created: store.getMessageInfo(assistantMessageId)?.time?.created ?? Date.now(), completed: Date.now() },
+  } as any)
+  store.upsertMessage({
+    id: messageId,
+    sessionId,
+    role: "user",
+    status: "sent",
+    updatedAt: Date.now(),
+    isEphemeral: false,
+  })
+}
+
 async function sendMessage(
   instanceId: string,
   sessionId: string,
@@ -1264,7 +1402,7 @@ async function sendMessage(
     addDebugLog(`Merge System Instructions: ${Math.round(tPre2 - tPre1)}ms`, "warn")
   }
 
-  if (providerId === "ollama-cloud" || providerId === "qwen-oauth" || providerId === "opencode-zen" || providerId === "zai") {
+  if (providerId === "ollama-cloud" || providerId === "qwen-oauth" || providerId === "opencode-zen" || providerId === "zai" || providerId === "antigravity") {
     const store = messageStoreBus.getOrCreate(instanceId)
     const now = Date.now()
     const assistantMessageId = createId("msg")
@@ -1347,6 +1485,17 @@ async function sendMessage(
           assistantMessageId,
           assistantPartId,
         )
+      } else if (providerId === "antigravity") {
+        await streamAntigravityChat(
+          instanceId,
+          sessionId,
+          providerId,
+          effectiveModel.modelId,
+          externalMessages,
+          messageId,
+          assistantMessageId,
+          assistantPartId,
+        )
       } else {
         const qwenManager = new QwenOAuthManager()
         const token = await qwenManager.getValidToken()
@@ -1428,7 +1577,9 @@ async function sendMessage(
               ? "Z.AI request failed"
               : providerId === "opencode-zen"
                 ? "OpenCode Zen request failed"
-                : "Qwen request failed",
+                : providerId === "antigravity"
+                  ? "Antigravity request failed"
+                  : "Qwen request failed",
         message: error?.message || "Request failed",
         variant: "error",
         duration: 8000,

packages/ui/src/stores/session-api.ts

@@ -249,14 +249,83 @@ async function fetchZAIProvider(): Promise<Provider | null> {
   }
 }
 
+function getStoredAntigravityToken():
+  | { access_token: string; expires_in: number; created_at: number }
+  | null {
+  if (typeof window === "undefined") return null
+  try {
+    const raw = window.localStorage.getItem(getUserScopedKey("antigravity_oauth_token"))
+    if (!raw) return null
+    return JSON.parse(raw)
+  } catch {
+    return null
+  }
+}
+
+function isAntigravityTokenValid(token: { expires_in: number; created_at: number } | null): boolean {
+  if (!token) return false
+  const createdAt = token.created_at > 1e12 ? Math.floor(token.created_at / 1000) : token.created_at
+  const expiresAt = (createdAt + token.expires_in) * 1000 - 300000
+  return Date.now() < expiresAt
+}
+
+async function fetchAntigravityProvider(): Promise<Provider | null> {
+  // Check if user is authenticated with Antigravity (Google OAuth)
+  const token = getStoredAntigravityToken()
+  if (!isAntigravityTokenValid(token)) {
+    // Not authenticated - try to fetch models anyway (they show as available but require auth)
+    try {
+      const data = await fetchJson<{ models?: Array<{ id: string; name: string; limit?: Model["limit"] }> }>(
+        "/api/antigravity/models",
+      )
+      const models = Array.isArray(data?.models) ? data?.models ?? [] : []
+      if (models.length === 0) return null
+
+      return {
+        id: "antigravity",
+        name: "Antigravity (Google OAuth)",
+        models: models.map((model) => ({
+          id: model.id,
+          name: model.name,
+          providerId: "antigravity",
+          limit: model.limit,
+        })),
+        defaultModelId: "gemini-3-pro-high",
+      }
+    } catch {
+      return null
+    }
+  }
+
+  // User is authenticated - fetch full model list
+  const data = await fetchJson<{ models?: Array<{ id: string; name: string; limit?: Model["limit"] }> }>(
+    "/api/antigravity/models",
+  )
+  const models = Array.isArray(data?.models) ? data?.models ?? [] : []
+  if (models.length === 0) return null
+
+  return {
+    id: "antigravity",
+    name: "Antigravity (Google OAuth)",
+    models: models.map((model) => ({
+      id: model.id,
+      name: model.name,
+      providerId: "antigravity",
+      limit: model.limit,
+    })),
+    defaultModelId: "gemini-3-pro-high",
+  }
+}
+
 async function fetchExtraProviders(): Promise<Provider[]> {
-  const [ollama, zen, qwen, zai] = await Promise.all([
+  const [ollama, zen, qwen, zai, antigravity] = await Promise.all([
     fetchOllamaCloudProvider(),
     fetchOpenCodeZenProvider(),
     fetchQwenOAuthProvider(),
     fetchZAIProvider(),
+    fetchAntigravityProvider(),
   ])
-  return [ollama, zen, qwen, zai].filter((provider): provider is Provider => Boolean(provider))
+  return [ollama, zen, qwen, zai, antigravity].filter((provider): provider is Provider => Boolean(provider))
 }
 
 function removeDuplicateProviders(base: Provider[], extras: Provider[]): Provider[] {