diff --git a/open-sse/translator/request/claude-to-gemini.ts b/open-sse/translator/request/claude-to-gemini.ts
index 6696de2119..a69540e49d 100644
--- a/open-sse/translator/request/claude-to-gemini.ts
+++ b/open-sse/translator/request/claude-to-gemini.ts
@@ -6,12 +6,12 @@ import {
   cleanJSONSchemaForAntigravity,
 } from "../helpers/geminiHelper.ts";
 import { buildGeminiTools, sanitizeGeminiToolName } from "../helpers/geminiToolsSanitizer.ts";
-import { capMaxOutputTokens, capThinkingBudget } from "../../../src/lib/modelCapabilities.ts";
-import { getModelSpec } from "../../../src/shared/constants/modelSpecs.ts";
 import {
   buildGeminiThoughtSignatureKey,
   resolveGeminiThoughtSignature,
 } from "../../services/geminiThoughtSignatureStore.ts";
+import { capMaxOutputTokens, capThinkingBudget } from "../../../src/lib/modelCapabilities.ts";
+import { getModelSpec } from "../../../src/shared/constants/modelSpecs.ts";
 import { buildHistoricalToolResultContext } from "./openai-to-gemini/helpers.ts";
 
 /**
@@ -127,6 +127,9 @@ export function claudeToGeminiRequest(model, body, stream, credentials = null) {
 
   // ── Convert messages ───────────────────────────────────────────
   if (body.messages && Array.isArray(body.messages)) {
+    // Tool-ids whose functionCall was omitted (no stored thought_signature) so the
+    // matching tool_result becomes text instead of a Gemini-400'd functionResponse.
+    const omittedToolCallIds = new Set<string>();
     for (const msg of body.messages) {
       const parts = [];
       let shouldUseEmbeddedSignature = true;
diff --git a/tests/unit/vertex-functioncall-id-3440.test.ts b/tests/unit/vertex-functioncall-id-3440.test.ts
index 8609755c36..4fe5aebd92 100644
--- a/tests/unit/vertex-functioncall-id-3440.test.ts
+++ b/tests/unit/vertex-functioncall-id-3440.test.ts
@@ -11,15 +11,24 @@
 import test from "node:test";
 import assert from "node:assert/strict";
 
-const { openaiToGeminiRequest } = await import(
-  "../../open-sse/translator/request/openai-to-gemini.ts"
-);
-const { claudeToGeminiRequest } = await import(
-  "../../open-sse/translator/request/claude-to-gemini.ts"
-);
+const { openaiToGeminiRequest } =
+  await import("../../open-sse/translator/request/openai-to-gemini.ts");
+const { claudeToGeminiRequest } =
+  await import("../../open-sse/translator/request/claude-to-gemini.ts");
+const { buildGeminiThoughtSignatureKey, storeGeminiThoughtSignature } =
+  await import("../../open-sse/services/geminiThoughtSignatureStore.ts");
 
 type UnknownRecord = Record<string, unknown>;
 
+const CLAUDE_SIGNATURE_NAMESPACE = "regression-3440";
+
+function seedClaudeThoughtSignature() {
+  storeGeminiThoughtSignature(
+    buildGeminiThoughtSignatureKey(CLAUDE_SIGNATURE_NAMESPACE, "tu_weather_1"),
+    "SIG_3440"
+  );
+}
+
 function findFunctionCall(result: any): UnknownRecord | undefined {
   for (const content of result.contents ?? []) {
     for (const part of content.parts ?? []) {
@@ -29,6 +38,15 @@ function findFunctionCall(result: any): UnknownRecord | undefined {
   return undefined;
 }
 
+function findFunctionCallPart(result: any): UnknownRecord | undefined {
+  for (const content of result.contents ?? []) {
+    for (const part of content.parts ?? []) {
+      if (part?.functionCall) return part as UnknownRecord;
+    }
+  }
+  return undefined;
+}
+
 function findFunctionResponse(result: any): UnknownRecord | undefined {
   for (const content of result.contents ?? []) {
     for (const part of content.parts ?? []) {
@@ -126,18 +144,43 @@ test("#3440 OpenAI->Gemini: no provider hint PRESERVES id (default, non-vertex)"
 });
 
 test("#3440 Claude->Gemini: vertex provider omits id from functionCall and functionResponse", () => {
+  seedClaudeThoughtSignature();
   const result = claudeToGeminiRequest("gemini-2.5-pro", CLAUDE_TOOL_BODY, false, {
     _provider: "vertex",
+    _signatureNamespace: CLAUDE_SIGNATURE_NAMESPACE,
   });
-  assert.equal(findFunctionCall(result)?.id, undefined, "functionCall.id must be omitted for Vertex");
+  assert.equal(
+    findFunctionCall(result)?.id,
+    undefined,
+    "functionCall.id must be omitted for Vertex"
+  );
   assert.equal(
     findFunctionResponse(result)?.id,
     undefined,
     "functionResponse.id must be omitted for Vertex"
   );
+
+  const vertexPart = findFunctionCallPart(result);
+  assert.ok(vertexPart, "expected a functionCall part");
+  assert.equal(
+    vertexPart.thoughtSignature,
+    "SIG_3440",
+    "thoughtSignature must be replayed even for Vertex (only id is stripped)"
+  );
 });
 
 test("#3440 Claude->Gemini: no provider hint PRESERVES id (default, non-vertex)", () => {
-  const result = claudeToGeminiRequest("gemini-2.5-pro", CLAUDE_TOOL_BODY, false);
+  seedClaudeThoughtSignature();
+  const result = claudeToGeminiRequest("gemini-2.5-pro", CLAUDE_TOOL_BODY, false, {
+    _signatureNamespace: CLAUDE_SIGNATURE_NAMESPACE,
+  });
   assert.equal(findFunctionCall(result)?.id, "tu_weather_1");
+
+  const nonVertexPart = findFunctionCallPart(result);
+  assert.ok(nonVertexPart, "expected a functionCall part");
+  assert.equal(
+    nonVertexPart.thoughtSignature,
+    "SIG_3440",
+    "thoughtSignature must be replayed for direct Claude->Gemini path"
+  );
 });