diff --git a/src/auth/antigravity/constants.ts b/src/auth/antigravity/constants.ts
index 6961549..0a71f49 100644
--- a/src/auth/antigravity/constants.ts
+++ b/src/auth/antigravity/constants.ts
@@ -57,7 +57,10 @@ export const ANTIGRAVITY_HEADERS = {
 } as const
 
 // Default Project ID (fallback when loadCodeAssist API fails)
-export const ANTIGRAVITY_DEFAULT_PROJECT_ID = ""
+// From opencode-antigravity-auth reference implementation
+export const ANTIGRAVITY_DEFAULT_PROJECT_ID = "rising-fact-p41fc"
+
+
 
 // Google OAuth endpoints
 export const GOOGLE_AUTH_URL = "https://accounts.google.com/o/oauth2/v2/auth"
@@ -66,3 +69,6 @@ export const GOOGLE_USERINFO_URL = "https://www.googleapis.com/oauth2/v1/userinf
 
 // Token refresh buffer (refresh 60 seconds before expiry)
 export const ANTIGRAVITY_TOKEN_REFRESH_BUFFER_MS = 60_000
+
+// Default thought signature to skip validation (CLIProxyAPI approach)
+export const SKIP_THOUGHT_SIGNATURE_VALIDATOR = "skip_thought_signature_validator"
diff --git a/src/auth/antigravity/fetch.ts b/src/auth/antigravity/fetch.ts
index 7d6a58c..29267ab 100644
--- a/src/auth/antigravity/fetch.ts
+++ b/src/auth/antigravity/fetch.ts
@@ -17,10 +17,11 @@
  * Debug logging available via ANTIGRAVITY_DEBUG=1 environment variable.
  */
 
-import { ANTIGRAVITY_ENDPOINT_FALLBACKS } from "./constants"
+import { ANTIGRAVITY_ENDPOINT_FALLBACKS, ANTIGRAVITY_DEFAULT_PROJECT_ID } from "./constants"
 import { fetchProjectContext, clearProjectContextCache } from "./project"
 import { isTokenExpired, refreshAccessToken, parseStoredToken, formatTokenForStorage } from "./token"
 import { transformRequest } from "./request"
+import { convertRequestBody, hasOpenAIMessages } from "./message-converter"
 import {
   transformResponse,
   transformStreamingResponse,
@@ -110,6 +111,16 @@ async function attemptFetch(
       }
     }
 
+    debugLog(`[BODY] Keys: ${Object.keys(parsedBody).join(", ")}`)
+    debugLog(`[BODY] Has contents: ${!!parsedBody.contents}, Has messages: ${!!parsedBody.messages}`)
+    if (parsedBody.contents) {
+      const contents = parsedBody.contents as Array<Record<string, unknown>>
+      debugLog(`[BODY] contents length: ${contents.length}`)
+      contents.forEach((c, i) => {
+        debugLog(`[BODY] contents[${i}].role: ${c.role}, parts: ${JSON.stringify(c.parts).substring(0, 200)}`)
+      })
+    }
+
     if (parsedBody.tools && Array.isArray(parsedBody.tools)) {
       const normalizedTools = normalizeToolsForGemini(parsedBody.tools as OpenAITool[])
       if (normalizedTools) {
@@ -117,6 +128,12 @@ async function attemptFetch(
       }
     }
 
+    if (hasOpenAIMessages(parsedBody)) {
+      debugLog(`[CONVERT] Converting OpenAI messages to Gemini contents`)
+      parsedBody = convertRequestBody(parsedBody, thoughtSignature)
+      debugLog(`[CONVERT] After conversion - Has contents: ${!!parsedBody.contents}`)
+    }
+
     const transformed = transformRequest({
       url,
       body: parsedBody,
@@ -208,20 +225,27 @@ async function transformResponseWithThinking(
 
   try {
     const text = await result.response.clone().text()
+    debugLog(`[TSIG][RESP] Response text length: ${text.length}`)
 
     if (streaming) {
       const signature = extractSignatureFromSsePayload(text)
+      debugLog(`[TSIG][RESP] SSE signature extracted: ${signature ? "yes" : "no"}`)
       if (signature) {
         setThoughtSignature(fetchInstanceId, signature)
-        debugLog(`[STREAMING] Stored thought signature for instance ${fetchInstanceId}`)
+        debugLog(`[TSIG][STORE] Stored signature for ${fetchInstanceId}: ${signature.substring(0, 30)}...`)
       }
     } else {
       const parsed = JSON.parse(text) as GeminiResponseBody
+      debugLog(`[TSIG][RESP] Parsed keys: ${Object.keys(parsed).join(", ")}`)
+      debugLog(`[TSIG][RESP] Has candidates: ${!!parsed.candidates}, count: ${parsed.candidates?.length ?? 0}`)
 
       const signature = extractSignatureFromResponse(parsed)
+      debugLog(`[TSIG][RESP] Signature extracted: ${signature ? signature.substring(0, 30) + "..." : "NONE"}`)
       if (signature) {
         setThoughtSignature(fetchInstanceId, signature)
-        debugLog(`Stored thought signature for instance ${fetchInstanceId}`)
+        debugLog(`[TSIG][STORE] Stored signature for ${fetchInstanceId}`)
+      } else {
+        debugLog(`[TSIG][WARN] No signature found in response!`)
       }
 
       if (shouldIncludeThinking(modelName)) {
@@ -349,14 +373,15 @@ export function createAntigravityFetch(
       }
     }
 
-    // Get project context
+    // Fetch project ID via loadCodeAssist (CLIProxyAPI approach)
     if (!cachedProjectId) {
       const projectContext = await fetchProjectContext(cachedTokens.access_token)
       cachedProjectId = projectContext.cloudaicompanionProject || ""
+      debugLog(`[PROJECT] Fetched project ID: "${cachedProjectId}"`)
     }
 
-    // Use project ID from refresh token if available, otherwise use fetched context
-    const projectId = refreshParts.projectId || cachedProjectId
+    const projectId = cachedProjectId
+    debugLog(`[PROJECT] Using project ID: "${projectId}"`)
 
     // Extract model name from request body
     let modelName: string | undefined
diff --git a/src/auth/antigravity/index.ts b/src/auth/antigravity/index.ts
index 8d2d4d3..147c4d5 100644
--- a/src/auth/antigravity/index.ts
+++ b/src/auth/antigravity/index.ts
@@ -8,5 +8,6 @@ export * from "./response"
 export * from "./tools"
 export * from "./thinking"
 export * from "./thought-signature-store"
+export * from "./message-converter"
 export * from "./fetch"
 export * from "./plugin"
diff --git a/src/auth/antigravity/message-converter.ts b/src/auth/antigravity/message-converter.ts
new file mode 100644
index 0000000..6a51a81
--- /dev/null
+++ b/src/auth/antigravity/message-converter.ts
@@ -0,0 +1,206 @@
+/**
+ * OpenAI → Gemini message format converter
+ * 
+ * Converts OpenAI-style messages to Gemini contents format,
+ * injecting thoughtSignature into functionCall parts.
+ */
+
+import { SKIP_THOUGHT_SIGNATURE_VALIDATOR } from "./constants"
+
+function debugLog(message: string): void {
+  if (process.env.ANTIGRAVITY_DEBUG === "1") {
+    console.log(`[antigravity-converter] ${message}`)
+  }
+}
+
+interface OpenAIMessage {
+  role: "system" | "user" | "assistant" | "tool"
+  content?: string | OpenAIContentPart[]
+  tool_calls?: OpenAIToolCall[]
+  tool_call_id?: string
+  name?: string
+}
+
+interface OpenAIContentPart {
+  type: string
+  text?: string
+  image_url?: { url: string }
+  [key: string]: unknown
+}
+
+interface OpenAIToolCall {
+  id: string
+  type: "function"
+  function: {
+    name: string
+    arguments: string
+  }
+}
+
+interface GeminiPart {
+  text?: string
+  functionCall?: {
+    name: string
+    args: Record<string, unknown>
+  }
+  functionResponse?: {
+    name: string
+    response: Record<string, unknown>
+  }
+  inlineData?: {
+    mimeType: string
+    data: string
+  }
+  thought_signature?: string
+  [key: string]: unknown
+}
+
+interface GeminiContent {
+  role: "user" | "model"
+  parts: GeminiPart[]
+}
+
+export function convertOpenAIToGemini(
+  messages: OpenAIMessage[],
+  thoughtSignature?: string
+): GeminiContent[] {
+  debugLog(`Converting ${messages.length} messages, signature: ${thoughtSignature ? "present" : "none"}`)
+  
+  const contents: GeminiContent[] = []
+
+  for (const msg of messages) {
+    if (msg.role === "system") {
+      contents.push({
+        role: "user",
+        parts: [{ text: typeof msg.content === "string" ? msg.content : "" }],
+      })
+      continue
+    }
+
+    if (msg.role === "user") {
+      const parts = convertContentToParts(msg.content)
+      contents.push({ role: "user", parts })
+      continue
+    }
+
+    if (msg.role === "assistant") {
+      const parts: GeminiPart[] = []
+
+      if (msg.content) {
+        parts.push(...convertContentToParts(msg.content))
+      }
+
+      if (msg.tool_calls && msg.tool_calls.length > 0) {
+        for (const toolCall of msg.tool_calls) {
+          let args: Record<string, unknown> = {}
+          try {
+            args = JSON.parse(toolCall.function.arguments)
+          } catch {
+            args = {}
+          }
+
+          const part: GeminiPart = {
+            functionCall: {
+              name: toolCall.function.name,
+              args,
+            },
+          }
+
+          // Always inject signature: use provided or default to skip validator (CLIProxyAPI approach)
+          part.thoughtSignature = thoughtSignature || SKIP_THOUGHT_SIGNATURE_VALIDATOR
+          debugLog(`Injected signature into functionCall: ${toolCall.function.name} (${thoughtSignature ? "provided" : "default"})`)
+
+          parts.push(part)
+        }
+      }
+
+      if (parts.length > 0) {
+        contents.push({ role: "model", parts })
+      }
+      continue
+    }
+
+    if (msg.role === "tool") {
+      let response: Record<string, unknown> = {}
+      try {
+        response = typeof msg.content === "string" 
+          ? JSON.parse(msg.content) 
+          : { result: msg.content }
+      } catch {
+        response = { result: msg.content }
+      }
+
+      const toolName = msg.name || "unknown"
+      
+      contents.push({
+        role: "user",
+        parts: [{
+          functionResponse: {
+            name: toolName,
+            response,
+          },
+        }],
+      })
+      continue
+    }
+  }
+
+  debugLog(`Converted to ${contents.length} content blocks`)
+  return contents
+}
+
+function convertContentToParts(content: string | OpenAIContentPart[] | undefined): GeminiPart[] {
+  if (!content) {
+    return [{ text: "" }]
+  }
+
+  if (typeof content === "string") {
+    return [{ text: content }]
+  }
+
+  const parts: GeminiPart[] = []
+  for (const part of content) {
+    if (part.type === "text" && part.text) {
+      parts.push({ text: part.text })
+    } else if (part.type === "image_url" && part.image_url?.url) {
+      const url = part.image_url.url
+      if (url.startsWith("data:")) {
+        const match = url.match(/^data:([^;]+);base64,(.+)$/)
+        if (match) {
+          parts.push({
+            inlineData: {
+              mimeType: match[1],
+              data: match[2],
+            },
+          })
+        }
+      }
+    }
+  }
+
+  return parts.length > 0 ? parts : [{ text: "" }]
+}
+
+export function hasOpenAIMessages(body: Record<string, unknown>): boolean {
+  return Array.isArray(body.messages) && body.messages.length > 0
+}
+
+export function convertRequestBody(
+  body: Record<string, unknown>,
+  thoughtSignature?: string
+): Record<string, unknown> {
+  if (!hasOpenAIMessages(body)) {
+    debugLog("No messages array found, returning body as-is")
+    return body
+  }
+
+  const messages = body.messages as OpenAIMessage[]
+  const contents = convertOpenAIToGemini(messages, thoughtSignature)
+
+  const converted = { ...body }
+  delete converted.messages
+  converted.contents = contents
+
+  debugLog(`Converted body: messages → contents (${contents.length} blocks)`)
+  return converted
+}
diff --git a/src/auth/antigravity/project.ts b/src/auth/antigravity/project.ts
index f4a74a7..4637fa5 100644
--- a/src/auth/antigravity/project.ts
+++ b/src/auth/antigravity/project.ts
@@ -116,7 +116,6 @@ async function callLoadCodeAssistAPI(
 /**
  * Fetch project context from Google's loadCodeAssist API.
  * Extracts the cloudaicompanionProject from the response.
- * Falls back to ANTIGRAVITY_DEFAULT_PROJECT_ID if API fails or returns empty.
  *
  * @param accessToken - Valid OAuth access token
  * @returns Project context with cloudaicompanionProject ID
@@ -124,26 +123,20 @@ async function callLoadCodeAssistAPI(
 export async function fetchProjectContext(
   accessToken: string
 ): Promise<AntigravityProjectContext> {
-  // Check cache first
   const cached = projectContextCache.get(accessToken)
   if (cached) {
     return cached
   }
 
-  // Call the API
   const response = await callLoadCodeAssistAPI(accessToken)
-
-  // Extract project ID from response
   const projectId = response
     ? extractProjectId(response.cloudaicompanionProject)
     : undefined
 
-  // Build result with fallback
   const result: AntigravityProjectContext = {
-    cloudaicompanionProject: projectId || ANTIGRAVITY_DEFAULT_PROJECT_ID,
+    cloudaicompanionProject: projectId || "",
   }
 
-  // Cache the result
   if (projectId) {
     projectContextCache.set(accessToken, result)
   }
diff --git a/src/auth/antigravity/request.ts b/src/auth/antigravity/request.ts
index dda38cd..d35eb04 100644
--- a/src/auth/antigravity/request.ts
+++ b/src/auth/antigravity/request.ts
@@ -8,6 +8,7 @@ import {
   ANTIGRAVITY_HEADERS,
   ANTIGRAVITY_ENDPOINT_FALLBACKS,
   ANTIGRAVITY_API_VERSION,
+  SKIP_THOUGHT_SIGNATURE_VALIDATOR,
 } from "./constants"
 import type { AntigravityRequestBody } from "./types"
 
@@ -175,18 +176,18 @@ export function injectThoughtSignatureIntoFunctionCalls(
   body: Record<string, unknown>,
   signature: string | undefined
 ): Record<string, unknown> {
-  debugLog(`[TSIG][INJECT] signature=${signature ? signature.substring(0, 20) + "..." : "none"}`)
-
-  if (!signature) {
-    return body
-  }
+  // Always use skip validator as fallback (CLIProxyAPI approach)
+  const effectiveSignature = signature || SKIP_THOUGHT_SIGNATURE_VALIDATOR
+  debugLog(`[TSIG][INJECT] signature=${effectiveSignature.substring(0, 30)}... (${signature ? "provided" : "default"})`)
+  debugLog(`[TSIG][INJECT] body keys: ${Object.keys(body).join(", ")}`)
 
   const contents = body.contents as ContentBlock[] | undefined
   if (!contents || !Array.isArray(contents)) {
-    debugLog(`[TSIG][INJECT] no contents array found`)
+    debugLog(`[TSIG][INJECT] No contents array! Has messages: ${!!body.messages}`)
     return body
   }
 
+  debugLog(`[TSIG][INJECT] Found ${contents.length} content blocks`)
   let injectedCount = 0
   const modifiedContents = contents.map((content) => {
     if (!content.parts || !Array.isArray(content.parts)) {
@@ -198,7 +199,7 @@ export function injectThoughtSignatureIntoFunctionCalls(
         injectedCount++
         return {
           ...part,
-          thoughtSignature: signature,
+          thoughtSignature: effectiveSignature,
         }
       }
       return part
diff --git a/src/auth/antigravity/tools.ts b/src/auth/antigravity/tools.ts
index f70c91f..5a10355 100644
--- a/src/auth/antigravity/tools.ts
+++ b/src/auth/antigravity/tools.ts
@@ -96,34 +96,31 @@ export function normalizeToolsForGemini(
   const functionDeclarations: GeminiFunctionDeclaration[] = []
 
   for (const tool of tools) {
-    // Handle function type tools
-    if (tool.type === "function" && tool.function) {
+    if (!tool || typeof tool !== "object") {
+      continue
+    }
+
+    const toolType = tool.type ?? "function"
+    if (toolType === "function" && tool.function) {
       const declaration: GeminiFunctionDeclaration = {
         name: tool.function.name,
       }
 
-      // Include description if present
       if (tool.function.description) {
         declaration.description = tool.function.description
       }
 
-      // Include parameters if present, default to empty object schema
       if (tool.function.parameters) {
         declaration.parameters = tool.function.parameters
       } else {
-        // Gemini requires parameters field, use empty object as default
         declaration.parameters = { type: "object", properties: {} }
       }
 
       functionDeclarations.push(declaration)
-    } else {
-      // Log warning for unsupported tool types (debug only)
-      if (process.env.ANTIGRAVITY_DEBUG === "1") {
-        console.warn(
-          `[antigravity-tools] Unsupported tool type: "${tool.type}". ` +
-            `Only "function" type tools are supported for Gemini. Tool will be skipped.`
-        )
-      }
+    } else if (toolType !== "function" && process.env.ANTIGRAVITY_DEBUG === "1") {
+      console.warn(
+        `[antigravity-tools] Unsupported tool type: "${toolType}". Tool will be skipped.`
+      )
     }
   }