diff --git a/open-sse/translator/response/openai-to-gemini-sse.ts b/open-sse/translator/response/openai-to-gemini-sse.ts
index 6613d061c90..4a39cf9175d 100644
--- a/open-sse/translator/response/openai-to-gemini-sse.ts
+++ b/open-sse/translator/response/openai-to-gemini-sse.ts
@@ -41,7 +41,14 @@ interface OpenAIToolCallDelta {
   index?: number;
   id?: string;
   type?: string;
-  function?: { name?: string; arguments?: string };
+  thoughtSignature?: string;
+  thought_signature?: string;
+  function?: {
+    name?: string;
+    arguments?: string;
+    thoughtSignature?: string;
+    thought_signature?: string;
+  };
 }
 
 interface OpenAIChoiceDelta {
@@ -84,6 +91,7 @@ interface GeminiFunctionResponse {
 interface GeminiPart {
   text?: string;
   thought?: boolean;
+  thoughtSignature?: string;
   functionCall?: GeminiFunctionCall;
   functionResponse?: GeminiFunctionResponse;
 }
@@ -95,7 +103,10 @@ interface GeminiPart {
  * `functionCall` part once `finish_reason` arrives.
  */
 export interface GeminiToolCallState {
-  toolCallAccum?: Record<number, { id: string; name: string; arguments: string }>;
+  toolCallAccum?: Record<
+    number,
+    { id: string; name: string; arguments: string; thoughtSignature?: string }
+  >;
 }
 
 interface GeminiCandidate {
@@ -154,6 +165,15 @@ export function openAIChunkToGeminiChunk(
       if (tc.id) entry.id = tc.id;
       if (tc.function?.name) entry.name += tc.function.name;
       if (tc.function?.arguments) entry.arguments += tc.function.arguments;
+      // Preserve a genuine Gemini thought signature when the upstream carries it
+      // (top-level `thoughtSignature`/`thought_signature` or nested on `function`),
+      // so the flushed functionCall can be replayed on the next tool turn (#400).
+      const sig =
+        tc.thoughtSignature ??
+        tc.thought_signature ??
+        tc.function?.thoughtSignature ??
+        tc.function?.thought_signature;
+      if (typeof sig === "string" && sig.length > 0) entry.thoughtSignature = sig;
     }
   }
 
@@ -168,7 +188,10 @@ export function openAIChunkToGeminiChunk(
       } catch {
         args = {};
       }
-      parts.push({ functionCall: { name: entry.name, args } });
+      parts.push({
+        ...(entry.thoughtSignature ? { thoughtSignature: entry.thoughtSignature } : {}),
+        functionCall: { name: entry.name, args },
+      });
     }
   }
 
@@ -294,7 +317,14 @@ export function transformOpenAISSEToGeminiSSE(upstreamResponse: Response, model:
 interface OpenAIToolCall {
   id?: string;
   type?: string;
-  function?: { name?: string; arguments?: string };
+  thoughtSignature?: string;
+  thought_signature?: string;
+  function?: {
+    name?: string;
+    arguments?: string;
+    thoughtSignature?: string;
+    thought_signature?: string;
+  };
 }
 
 interface OpenAIMessage {
@@ -395,7 +425,15 @@ export async function convertOpenAIResponseToGemini(
     } catch {
       args = {};
     }
-    parts.push({ functionCall: { name: tc.function?.name || "", args } });
+    const sig =
+      tc.thoughtSignature ??
+      tc.thought_signature ??
+      tc.function?.thoughtSignature ??
+      tc.function?.thought_signature;
+    parts.push({
+      ...(typeof sig === "string" && sig.length > 0 ? { thoughtSignature: sig } : {}),
+      functionCall: { name: tc.function?.name || "", args },
+    });
   }
 
   const finishReason = OPENAI_TO_GEMINI_FINISH_REASON[finish_reason ?? "stop"] ?? "STOP";
diff --git a/tests/unit/translator-openai-to-gemini-sse.test.ts b/tests/unit/translator-openai-to-gemini-sse.test.ts
index 96a0b222e1c..3d0f91877c3 100644
--- a/tests/unit/translator-openai-to-gemini-sse.test.ts
+++ b/tests/unit/translator-openai-to-gemini-sse.test.ts
@@ -226,3 +226,106 @@ test("convertOpenAIResponseToGemini: surfaces upstream error bodies untouched",
   const body = (await out.json()) as { error: { code: number } };
   assert.equal(body.error.code, 429);
 });
+
+test("transformOpenAISSEToGeminiSSE: preserves genuine thoughtSignature across streamed tool_calls deltas", async () => {
+  const upstream = makeOpenAISSEResponse([
+    'data: {"choices":[{"delta":{"tool_calls":[{"index":0,"id":"call_sig123","type":"function","thoughtSignature":"sig_stream_123","function":{"name":"get_weather","arguments":"{\\"city\\":"}}]},"finish_reason":null}]}',
+    'data: {"choices":[{"delta":{"tool_calls":[{"index":0,"function":{"arguments":"\\"Tokyo\\"}"}}]},"finish_reason":null}]}',
+    'data: {"choices":[{"delta":{},"finish_reason":"tool_calls"}]}',
+  ]);
+
+  const out = transformOpenAISSEToGeminiSSE(upstream, "gemini/gemini-pro");
+  const events = await readGeminiSSE(out);
+  assert.equal(events.length, 1);
+
+  const candidate = events[0].candidates[0];
+  assert.equal(candidate.finishReason, "STOP");
+
+  const fcPart = candidate.content.parts.find((p) => "functionCall" in p) as
+    | { thoughtSignature?: string; functionCall: { name: string; args: Record<string, unknown> } }
+    | undefined;
+
+  assert.ok(fcPart);
+  assert.equal(fcPart.functionCall.name, "get_weather");
+  assert.deepEqual(fcPart.functionCall.args, { city: "Tokyo" });
+  assert.equal(fcPart.thoughtSignature, "sig_stream_123");
+});
+
+test("convertOpenAIResponseToGemini: preserves genuine thoughtSignature in non-stream tool calls", async () => {
+  const upstream = Response.json({
+    choices: [
+      {
+        message: {
+          role: "assistant",
+          content: null,
+          tool_calls: [
+            {
+              id: "call_sig456",
+              type: "function",
+              thoughtSignature: "sig_nonstream_456",
+              function: { name: "search", arguments: '{"q":"gemini"}' },
+            },
+          ],
+        },
+        finish_reason: "tool_calls",
+      },
+    ],
+    model: "gemini-3.5-flash-lite",
+  });
+
+  const out = await convertOpenAIResponseToGemini(upstream, "fallback");
+  const body = (await out.json()) as {
+    candidates: Array<{
+      content: {
+        parts: Array<{
+          thoughtSignature?: string;
+          functionCall?: { name: string; args: Record<string, unknown> };
+        }>;
+      };
+    }>;
+  };
+
+  const fcPart = body.candidates[0].content.parts.find((p) => p.functionCall);
+  assert.ok(fcPart?.functionCall);
+  assert.equal(fcPart.functionCall.name, "search");
+  assert.deepEqual(fcPart.functionCall.args, { q: "gemini" });
+  assert.equal(fcPart.thoughtSignature, "sig_nonstream_456");
+});
+
+test("convertOpenAIResponseToGemini: omits thoughtSignature when none was provided by upstream", async () => {
+  const upstream = Response.json({
+    choices: [
+      {
+        message: {
+          role: "assistant",
+          content: null,
+          tool_calls: [
+            {
+              id: "call_plain",
+              type: "function",
+              function: { name: "ping", arguments: "{}" },
+            },
+          ],
+        },
+        finish_reason: "tool_calls",
+      },
+    ],
+  });
+
+  const out = await convertOpenAIResponseToGemini(upstream, "fallback");
+  const body = (await out.json()) as {
+    candidates: Array<{
+      content: {
+        parts: Array<{
+          thoughtSignature?: string;
+          functionCall?: { name: string; args: Record<string, unknown> };
+        }>;
+      };
+    }>;
+  };
+
+  const fcPart = body.candidates[0].content.parts.find((p) => p.functionCall);
+  assert.ok(fcPart?.functionCall);
+  assert.equal(fcPart.functionCall.name, "ping");
+  assert.equal("thoughtSignature" in fcPart, false);
+});