diff --git a/packages/ai/src/providers/anthropic.ts b/packages/ai/src/providers/anthropic.ts
index 2fe83370..d40224f2 100644
--- a/packages/ai/src/providers/anthropic.ts
+++ b/packages/ai/src/providers/anthropic.ts
@@ -26,7 +26,7 @@ import { AssistantMessageEventStream } from "../utils/event-stream.js";
 import { parseStreamingJson } from "../utils/json-parse.js";
 import { sanitizeSurrogates } from "../utils/sanitize-unicode.js";
 
-import { transformMessages } from "./transorm-messages.js";
+import { transformMessages } from "./transform-messages.js";
 
 // Stealth mode: Mimic Claude Code's tool naming exactly
 const claudeCodeVersion = "2.1.2";
diff --git a/packages/ai/src/providers/google-shared.ts b/packages/ai/src/providers/google-shared.ts
index 60d8b058..9c2381a7 100644
--- a/packages/ai/src/providers/google-shared.ts
+++ b/packages/ai/src/providers/google-shared.ts
@@ -5,7 +5,7 @@
 import { type Content, FinishReason, FunctionCallingConfigMode, type Part, type Schema } from "@google/genai";
 import type { Context, ImageContent, Model, StopReason, TextContent, Tool } from "../types.js";
 import { sanitizeSurrogates } from "../utils/sanitize-unicode.js";
-import { transformMessages } from "./transorm-messages.js";
+import { transformMessages } from "./transform-messages.js";
 
 type GoogleApiType = "google-generative-ai" | "google-gemini-cli" | "google-vertex";
 
@@ -42,6 +42,22 @@ export function retainThoughtSignature(existing: string | undefined, incoming: s
 	return existing;
 }
 
+// Thought signatures must be base64 for Google APIs (TYPE_BYTES).
+const base64SignaturePattern = /^[A-Za-z0-9+/]+={0,2}$/;
+
+function isValidThoughtSignature(signature: string | undefined): boolean {
+	if (!signature) return false;
+	if (signature.length % 4 !== 0) return false;
+	return base64SignaturePattern.test(signature);
+}
+
+/**
+ * Only keep signatures from the same provider/model and with valid base64.
+ */
+function resolveThoughtSignature(isSameProviderAndModel: boolean, signature: string | undefined): string | undefined {
+	return isSameProviderAndModel && isValidThoughtSignature(signature) ? signature : undefined;
+}
+
 /**
  * Convert internal messages to Gemini Content[] format.
  */
@@ -85,9 +101,10 @@ export function convertMessages<T extends GoogleApiType>(model: Model<T>, contex
 				if (block.type === "text") {
 					// Skip empty text blocks - they can cause issues with some models (e.g. Claude via Antigravity)
 					if (!block.text || block.text.trim() === "") continue;
+					const thoughtSignature = resolveThoughtSignature(isSameProviderAndModel, block.textSignature);
 					parts.push({
 						text: sanitizeSurrogates(block.text),
-						...(block.textSignature && { thoughtSignature: block.textSignature }),
+						...(thoughtSignature && { thoughtSignature }),
 					});
 				} else if (block.type === "thinking") {
 					// Skip empty thinking blocks
@@ -95,10 +112,11 @@ export function convertMessages<T extends GoogleApiType>(model: Model<T>, contex
 					// Only keep as thinking block if same provider AND same model
 					// Otherwise convert to plain text (no tags to avoid model mimicking them)
 					if (isSameProviderAndModel) {
+						const thoughtSignature = resolveThoughtSignature(isSameProviderAndModel, block.thinkingSignature);
 						parts.push({
 							thought: true,
 							text: sanitizeSurrogates(block.thinking),
-							...(block.thinkingSignature && { thoughtSignature: block.thinkingSignature }),
+							...(thoughtSignature && { thoughtSignature }),
 						});
 					} else {
 						parts.push({
@@ -112,8 +130,9 @@ export function convertMessages<T extends GoogleApiType>(model: Model<T>, contex
 							args: block.arguments,
 						},
 					};
-					if (block.thoughtSignature) {
-						part.thoughtSignature = block.thoughtSignature;
+					const thoughtSignature = resolveThoughtSignature(isSameProviderAndModel, block.thoughtSignature);
+					if (thoughtSignature) {
+						part.thoughtSignature = thoughtSignature;
 					}
 					parts.push(part);
 				}
diff --git a/packages/ai/src/providers/openai-codex-responses.ts b/packages/ai/src/providers/openai-codex-responses.ts
index cbb73dbc..4bbda74a 100644
--- a/packages/ai/src/providers/openai-codex-responses.ts
+++ b/packages/ai/src/providers/openai-codex-responses.ts
@@ -42,7 +42,7 @@ import {
 	transformRequestBody,
 } from "./openai-codex/request-transformer.js";
 import { parseCodexError, parseCodexSseStream } from "./openai-codex/response-handler.js";
-import { transformMessages } from "./transorm-messages.js";
+import { transformMessages } from "./transform-messages.js";
 
 export interface OpenAICodexResponsesOptions extends StreamOptions {
 	reasoningEffort?: "none" | "minimal" | "low" | "medium" | "high" | "xhigh";
diff --git a/packages/ai/src/providers/openai-completions.ts b/packages/ai/src/providers/openai-completions.ts
index 091996ff..37c2dd13 100644
--- a/packages/ai/src/providers/openai-completions.ts
+++ b/packages/ai/src/providers/openai-completions.ts
@@ -27,7 +27,7 @@ import type {
 import { AssistantMessageEventStream } from "../utils/event-stream.js";
 import { parseStreamingJson } from "../utils/json-parse.js";
 import { sanitizeSurrogates } from "../utils/sanitize-unicode.js";
-import { transformMessages } from "./transorm-messages.js";
+import { transformMessages } from "./transform-messages.js";
 
 /**
  * Normalize tool call ID for Mistral.
diff --git a/packages/ai/src/providers/openai-responses.ts b/packages/ai/src/providers/openai-responses.ts
index bd760440..c10dd68e 100644
--- a/packages/ai/src/providers/openai-responses.ts
+++ b/packages/ai/src/providers/openai-responses.ts
@@ -28,7 +28,7 @@ import type {
 import { AssistantMessageEventStream } from "../utils/event-stream.js";
 import { parseStreamingJson } from "../utils/json-parse.js";
 import { sanitizeSurrogates } from "../utils/sanitize-unicode.js";
-import { transformMessages } from "./transorm-messages.js";
+import { transformMessages } from "./transform-messages.js";
 
 /** Fast deterministic hash to shorten long strings */
 function shortHash(str: string): string {
diff --git a/packages/ai/src/providers/transorm-messages.ts b/packages/ai/src/providers/transform-messages.ts
similarity index 100%
rename from packages/ai/src/providers/transorm-messages.ts
rename to packages/ai/src/providers/transform-messages.ts