feat(antigravity): add thought signature support for multi-turn conversations

Gemini 3 Pro requires thoughtSignature on function call blocks in
subsequent requests. This commit:

- Add thought-signature-store for session-based signature storage
- Extract signature from both streaming (SSE) and non-streaming responses
- Inject signature into functionCall parts on subsequent requests
- Maintain consistent sessionId per fetch instance

Debug logging available via ANTIGRAVITY_DEBUG=1

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/auth/antigravity/fetch.ts

@@ -21,9 +21,19 @@ import { ANTIGRAVITY_ENDPOINT_FALLBACKS } from "./constants"
 import { fetchProjectContext, clearProjectContextCache } from "./project"
 import { isTokenExpired, refreshAccessToken, parseStoredToken, formatTokenForStorage } from "./token"
 import { transformRequest } from "./request"
-import { transformResponse, transformStreamingResponse, isStreamingResponse } from "./response"
+import {
+  transformResponse,
+  transformStreamingResponse,
+  isStreamingResponse,
+  extractSignatureFromSsePayload,
+} from "./response"
 import { normalizeToolsForGemini, type OpenAITool } from "./tools"
 import { extractThinkingBlocks, shouldIncludeThinking, transformResponseThinking } from "./thinking"
+import {
+  getThoughtSignature,
+  setThoughtSignature,
+  getOrCreateSessionId,
+} from "./thought-signature-store"
 import type { AntigravityTokens } from "./types"
 
 /**
@@ -65,14 +75,22 @@ function isRetryableError(status: number): boolean {
   return false
 }
 
-async function attemptFetch(
+interface AttemptFetchOptions {
-  endpoint: string,
+  endpoint: string
-  url: string,
+  url: string
-  init: RequestInit,
+  init: RequestInit
-  accessToken: string,
+  accessToken: string
-  projectId: string,
+  projectId: string
+  sessionId: string
   modelName?: string
+  thoughtSignature?: string
+}
+
+async function attemptFetch(
+  options: AttemptFetchOptions
 ): Promise<Response | null | "pass-through"> {
+  const { endpoint, url, init, accessToken, projectId, sessionId, modelName, thoughtSignature } =
+    options
   debugLog(`Trying endpoint: ${endpoint}`)
 
   try {
@@ -99,14 +117,16 @@ async function attemptFetch(
       }
     }
 
-    const transformed = transformRequest(
+    const transformed = transformRequest({
       url,
-      parsedBody,
+      body: parsedBody,
       accessToken,
       projectId,
+      sessionId,
       modelName,
-      endpoint
+      endpointOverride: endpoint,
-    )
+      thoughtSignature,
+    })
 
     const response = await fetch(transformed.url, {
       method: init.method || "POST",
@@ -129,16 +149,56 @@ async function attemptFetch(
   }
 }
 
-/**
+interface GeminiResponsePart {
- * Transform response with thinking extraction if applicable
+  thoughtSignature?: string
- */
+  thought_signature?: string
+  functionCall?: Record<string, unknown>
+  text?: string
+  [key: string]: unknown
+}
+
+interface GeminiResponseCandidate {
+  content?: {
+    parts?: GeminiResponsePart[]
+    [key: string]: unknown
+  }
+  [key: string]: unknown
+}
+
+interface GeminiResponseBody {
+  candidates?: GeminiResponseCandidate[]
+  [key: string]: unknown
+}
+
+function extractSignatureFromResponse(parsed: GeminiResponseBody): string | undefined {
+  if (!parsed.candidates || !Array.isArray(parsed.candidates)) {
+    return undefined
+  }
+
+  for (const candidate of parsed.candidates) {
+    const parts = candidate.content?.parts
+    if (!parts || !Array.isArray(parts)) {
+      continue
+    }
+
+    for (const part of parts) {
+      const sig = part.thoughtSignature || part.thought_signature
+      if (sig && typeof sig === "string") {
+        return sig
+      }
+    }
+  }
+
+  return undefined
+}
+
 async function transformResponseWithThinking(
   response: Response,
-  modelName: string
+  modelName: string,
+  fetchInstanceId: string
 ): Promise<Response> {
   const streaming = isStreamingResponse(response)
 
-  // Transform response based on streaming mode
   let result
   if (streaming) {
     result = await transformStreamingResponse(response)
@@ -146,13 +206,25 @@ async function transformResponseWithThinking(
     result = await transformResponse(response)
   }
 
-  // Apply thinking extraction for high-thinking models
-  if (!streaming && shouldIncludeThinking(modelName)) {
   try {
     const text = await result.response.clone().text()
-      const parsed = JSON.parse(text) as Record<string, unknown>
 
-      // Extract and transform thinking blocks
+    if (streaming) {
+      const signature = extractSignatureFromSsePayload(text)
+      if (signature) {
+        setThoughtSignature(fetchInstanceId, signature)
+        debugLog(`[STREAMING] Stored thought signature for instance ${fetchInstanceId}`)
+      }
+    } else {
+      const parsed = JSON.parse(text) as GeminiResponseBody
+
+      const signature = extractSignatureFromResponse(parsed)
+      if (signature) {
+        setThoughtSignature(fetchInstanceId, signature)
+        debugLog(`Stored thought signature for instance ${fetchInstanceId}`)
+      }
+
+      if (shouldIncludeThinking(modelName)) {
         const thinkingResult = extractThinkingBlocks(parsed)
         if (thinkingResult.hasThinking) {
           const transformed = transformResponseThinking(parsed)
@@ -162,10 +234,9 @@ async function transformResponseWithThinking(
             headers: result.response.headers,
           })
         }
-    } catch {
-      // If thinking extraction fails, return original transformed response
       }
     }
+  } catch {}
 
   return result.response
 }
@@ -207,9 +278,9 @@ export function createAntigravityFetch(
   clientId?: string,
   clientSecret?: string
 ): (url: string, init?: RequestInit) => Promise<Response> {
-  // Cache for current token state
   let cachedTokens: AntigravityTokens | null = null
   let cachedProjectId: string | null = null
+  const fetchInstanceId = crypto.randomUUID()
 
   return async (url: string, init: RequestInit = {}): Promise<Response> => {
     debugLog(`Intercepting request to: ${url}`)
@@ -304,18 +375,23 @@ export function createAntigravityFetch(
     }
 
     const maxEndpoints = Math.min(ANTIGRAVITY_ENDPOINT_FALLBACKS.length, 3)
+    const sessionId = getOrCreateSessionId(fetchInstanceId)
+    const thoughtSignature = getThoughtSignature(fetchInstanceId)
+    debugLog(`[TSIG][GET] sessionId=${sessionId}, signature=${thoughtSignature ? thoughtSignature.substring(0, 20) + "..." : "none"}`)
 
     for (let i = 0; i < maxEndpoints; i++) {
       const endpoint = ANTIGRAVITY_ENDPOINT_FALLBACKS[i]
 
-      const response = await attemptFetch(
+      const response = await attemptFetch({
         endpoint,
         url,
         init,
-        cachedTokens.access_token,
+        accessToken: cachedTokens.access_token,
         projectId,
-        modelName
+        sessionId,
-      )
+        modelName,
+        thoughtSignature,
+      })
 
       if (response === "pass-through") {
         debugLog("Non-string body detected, passing through with auth headers")
@@ -328,7 +404,12 @@ export function createAntigravityFetch(
 
       if (response) {
         debugLog(`Success with endpoint: ${endpoint}`)
-        return transformResponseWithThinking(response, modelName || "")
+        const transformedResponse = await transformResponseWithThinking(
+          response,
+          modelName || "",
+          fetchInstanceId
+        )
+        return transformedResponse
       }
     }
 

src/auth/antigravity/index.ts

@@ -1,5 +1,3 @@
-// Antigravity auth module barrel export
-
 export * from "./types"
 export * from "./constants"
 export * from "./oauth"
@@ -9,5 +7,6 @@ export * from "./request"
 export * from "./response"
 export * from "./tools"
 export * from "./thinking"
+export * from "./thought-signature-store"
 export * from "./fetch"
 export * from "./plugin"

src/auth/antigravity/request.ts

@@ -132,15 +132,11 @@ function generateRequestId(): string {
   return `agent-${crypto.randomUUID()}`
 }
 
-function generateSessionId(): string {
-  const n = Math.floor(Math.random() * 9_000_000_000_000_000_000)
-  return `-${n}`
-}
-
 export function wrapRequestBody(
   body: Record<string, unknown>,
   projectId: string,
-  modelName: string
+  modelName: string,
+  sessionId: string
 ): AntigravityRequestBody {
   const requestPayload = { ...body }
   delete requestPayload.model
@@ -152,11 +148,69 @@ export function wrapRequestBody(
     requestId: generateRequestId(),
     request: {
       ...requestPayload,
-      sessionId: generateSessionId(),
+      sessionId,
     },
   }
 }
 
+interface ContentPart {
+  functionCall?: Record<string, unknown>
+  thoughtSignature?: string
+  [key: string]: unknown
+}
+
+interface ContentBlock {
+  role?: string
+  parts?: ContentPart[]
+  [key: string]: unknown
+}
+
+function debugLog(message: string): void {
+  if (process.env.ANTIGRAVITY_DEBUG === "1") {
+    console.log(`[antigravity-request] ${message}`)
+  }
+}
+
+export function injectThoughtSignatureIntoFunctionCalls(
+  body: Record<string, unknown>,
+  signature: string | undefined
+): Record<string, unknown> {
+  debugLog(`[TSIG][INJECT] signature=${signature ? signature.substring(0, 20) + "..." : "none"}`)
+
+  if (!signature) {
+    return body
+  }
+
+  const contents = body.contents as ContentBlock[] | undefined
+  if (!contents || !Array.isArray(contents)) {
+    debugLog(`[TSIG][INJECT] no contents array found`)
+    return body
+  }
+
+  let injectedCount = 0
+  const modifiedContents = contents.map((content) => {
+    if (!content.parts || !Array.isArray(content.parts)) {
+      return content
+    }
+
+    const modifiedParts = content.parts.map((part) => {
+      if (part.functionCall && !part.thoughtSignature) {
+        injectedCount++
+        return {
+          ...part,
+          thoughtSignature: signature,
+        }
+      }
+      return part
+    })
+
+    return { ...content, parts: modifiedParts }
+  })
+
+  debugLog(`[TSIG][INJECT] injected signature into ${injectedCount} functionCall(s)`)
+  return { ...body, contents: modifiedContents }
+}
+
 /**
  * Detect if request is for streaming.
  * Checks both action name and request body for stream flag.
@@ -183,48 +237,45 @@ export function isStreamingRequest(
   return false
 }
 
-/**
+export interface TransformRequestOptions {
- * Transform an OpenAI-format request to Antigravity format.
+  url: string
- * This is the main transformation function used by the fetch interceptor.
+  body: Record<string, unknown>
- *
+  accessToken: string
- * @param url - Original request URL
+  projectId: string
- * @param body - Original request body (OpenAI format)
+  sessionId: string
- * @param accessToken - OAuth access token for Authorization
+  modelName?: string
- * @param projectId - GCP project ID for wrapper
- * @param modelName - Model name to use (overrides body.model if provided)
- * @param endpointOverride - Optional endpoint override (uses first fallback if not provided)
- * @returns Transformed request with URL, headers, body, and streaming flag
- */
-export function transformRequest(
-  url: string,
-  body: Record<string, unknown>,
-  accessToken: string,
-  projectId: string,
-  modelName?: string,
   endpointOverride?: string
-): TransformedRequest {
+  thoughtSignature?: string
-  // Determine model name (parameter override > body > URL)
+}
+
+export function transformRequest(options: TransformRequestOptions): TransformedRequest {
+  const {
+    url,
+    body,
+    accessToken,
+    projectId,
+    sessionId,
+    modelName,
+    endpointOverride,
+    thoughtSignature,
+  } = options
+
   const effectiveModel =
     modelName || extractModelFromBody(body) || extractModelFromUrl(url) || "gemini-3-pro-preview"
 
-  // Determine if streaming
   const streaming = isStreamingRequest(url, body)
-
-  // Determine action (default to appropriate generate action)
   const action = streaming ? "streamGenerateContent" : "generateContent"
 
-  // Build URL
   const endpoint = endpointOverride || getDefaultEndpoint()
   const transformedUrl = buildAntigravityUrl(endpoint, action, streaming)
 
-  // Build headers
   const headers = buildRequestHeaders(accessToken)
   if (streaming) {
     headers["Accept"] = "text/event-stream"
   }
 
-  // Wrap body in Antigravity format
+  const bodyWithSignature = injectThoughtSignatureIntoFunctionCalls(body, thoughtSignature)
-  const wrappedBody = wrapRequestBody(body, projectId, effectiveModel)
+  const wrappedBody = wrapRequestBody(bodyWithSignature, projectId, effectiveModel, sessionId)
 
   return {
     url: transformedUrl,

src/auth/antigravity/response.ts

@@ -465,6 +465,61 @@ export function isStreamingResponse(response: Response): boolean {
   return contentType.includes("text/event-stream")
 }
 
+/**
+ * Extract thought signature from SSE payload text
+ *
+ * Looks for thoughtSignature in SSE events:
+ * data: { "response": { "candidates": [{ "content": { "parts": [{ "thoughtSignature": "..." }] } }] } }
+ *
+ * Returns the last found signature (most recent in the stream).
+ *
+ * @param payload - SSE payload text
+ * @returns Last thought signature if found
+ */
+export function extractSignatureFromSsePayload(payload: string): string | undefined {
+  const lines = payload.split("\n")
+  let lastSignature: string | undefined
+
+  for (const line of lines) {
+    if (!line.startsWith("data:")) {
+      continue
+    }
+
+    const json = line.slice(5).trim()
+    if (!json || json === "[DONE]") {
+      continue
+    }
+
+    try {
+      const parsed = JSON.parse(json) as Record<string, unknown>
+
+      // Check in response wrapper (Antigravity format)
+      const response = (parsed.response || parsed) as Record<string, unknown>
+      const candidates = response.candidates as Array<Record<string, unknown>> | undefined
+
+      if (candidates && Array.isArray(candidates)) {
+        for (const candidate of candidates) {
+          const content = candidate.content as Record<string, unknown> | undefined
+          const parts = content?.parts as Array<Record<string, unknown>> | undefined
+
+          if (parts && Array.isArray(parts)) {
+            for (const part of parts) {
+              const sig = (part.thoughtSignature || part.thought_signature) as string | undefined
+              if (sig && typeof sig === "string") {
+                lastSignature = sig
+              }
+            }
+          }
+        }
+      }
+    } catch {
+      // Continue to next line if parsing fails
+    }
+  }
+
+  return lastSignature
+}
+
 /**
  * Extract usage from SSE payload text
  *

src/auth/antigravity/thought-signature-store.ts

@@ -0,0 +1,97 @@
+/**
+ * Thought Signature Store
+ *
+ * Stores and retrieves thought signatures for multi-turn conversations.
+ * Gemini 3 Pro requires thought_signature on function call content blocks
+ * in subsequent requests to maintain reasoning continuity.
+ *
+ * Key responsibilities:
+ * - Store the latest thought signature per session
+ * - Provide signature for injection into function call requests
+ * - Clear signatures when sessions end
+ */
+
+/**
+ * In-memory store for thought signatures indexed by session ID
+ */
+const signatureStore = new Map<string, string>()
+
+/**
+ * In-memory store for session IDs per fetch instance
+ * Used to maintain consistent sessionId across multi-turn conversations
+ */
+const sessionIdStore = new Map<string, string>()
+
+/**
+ * Store a thought signature for a session
+ *
+ * @param sessionKey - Unique session identifier (typically fetch instance ID)
+ * @param signature - The thought signature from model response
+ */
+export function setThoughtSignature(sessionKey: string, signature: string): void {
+  if (sessionKey && signature) {
+    signatureStore.set(sessionKey, signature)
+  }
+}
+
+/**
+ * Retrieve the stored thought signature for a session
+ *
+ * @param sessionKey - Unique session identifier
+ * @returns The stored signature or undefined if not found
+ */
+export function getThoughtSignature(sessionKey: string): string | undefined {
+  return signatureStore.get(sessionKey)
+}
+
+/**
+ * Clear the thought signature for a session
+ *
+ * @param sessionKey - Unique session identifier
+ */
+export function clearThoughtSignature(sessionKey: string): void {
+  signatureStore.delete(sessionKey)
+}
+
+/**
+ * Store or retrieve a persistent session ID for a fetch instance
+ *
+ * @param fetchInstanceId - Unique identifier for the fetch instance
+ * @param sessionId - Optional session ID to store (if not provided, returns existing or generates new)
+ * @returns The session ID for this fetch instance
+ */
+export function getOrCreateSessionId(fetchInstanceId: string, sessionId?: string): string {
+  if (sessionId) {
+    sessionIdStore.set(fetchInstanceId, sessionId)
+    return sessionId
+  }
+
+  const existing = sessionIdStore.get(fetchInstanceId)
+  if (existing) {
+    return existing
+  }
+
+  const n = Math.floor(Math.random() * Number.MAX_SAFE_INTEGER)
+  const newSessionId = `-${n}`
+  sessionIdStore.set(fetchInstanceId, newSessionId)
+  return newSessionId
+}
+
+/**
+ * Clear the session ID for a fetch instance
+ *
+ * @param fetchInstanceId - Unique identifier for the fetch instance
+ */
+export function clearSessionId(fetchInstanceId: string): void {
+  sessionIdStore.delete(fetchInstanceId)
+}
+
+/**
+ * Clear all stored data for a fetch instance (signature + session ID)
+ *
+ * @param fetchInstanceId - Unique identifier for the fetch instance
+ */
+export function clearFetchInstanceData(fetchInstanceId: string): void {
+  signatureStore.delete(fetchInstanceId)
+  sessionIdStore.delete(fetchInstanceId)
+}