diff --git a/src/api/transform/__tests__/gemini-format.spec.ts b/src/api/transform/__tests__/gemini-format.spec.ts
index 14ab6f8d8f..cb62729391 100644
--- a/src/api/transform/__tests__/gemini-format.spec.ts
+++ b/src/api/transform/__tests__/gemini-format.spec.ts
@@ -107,7 +107,7 @@ describe("convertAnthropicMessageToGemini", () => {
 		expect(() => convertAnthropicMessageToGemini(anthropicMessage)).toThrow("Unsupported image source type")
 	})
 
-	it("should convert a message with tool use", () => {
+	it("should convert a message with tool use without thoughtSignature when none exists", () => {
 		const anthropicMessage: Anthropic.Messages.MessageParam = {
 			role: "assistant",
 			content: [
@@ -133,7 +133,6 @@ describe("convertAnthropicMessageToGemini", () => {
 							name: "calculator",
 							args: { operation: "add", numbers: [2, 3] },
 						},
-						thoughtSignature: "skip_thought_signature_validator",
 					},
 				],
 			},
diff --git a/src/api/transform/gemini-format.ts b/src/api/transform/gemini-format.ts
index bc4dc4aa54..a123226012 100644
--- a/src/api/transform/gemini-format.ts
+++ b/src/api/transform/gemini-format.ts
@@ -42,10 +42,11 @@ export function convertAnthropicContentToGemini(
 	// Determine the signature to attach to function calls.
 	// If we're in a mode that expects signatures (includeThoughtSignatures is true):
 	// 1. Use the actual signature if we found one in the history/content.
-	// 2. Fallback to "skip_thought_signature_validator" if missing (e.g. cross-model history).
+	// 2. Don't use a fallback - only include thoughtSignature if we have a real one.
+	//    The fallback "skip_thought_signature_validator" causes 400 errors with Gemini 3 models.
 	let functionCallSignature: string | undefined
-	if (includeThoughtSignatures) {
-		functionCallSignature = activeThoughtSignature || "skip_thought_signature_validator"
+	if (includeThoughtSignatures && activeThoughtSignature) {
+		functionCallSignature = activeThoughtSignature
 	}
 
 	if (typeof content === "string") {