diff --git a/src/agents/gigachat-stream.tool-calls.test.ts b/src/agents/gigachat-stream.tool-calls.test.ts
index 9a78e1d1d31..745c2cbc59a 100644
--- a/src/agents/gigachat-stream.tool-calls.test.ts
+++ b/src/agents/gigachat-stream.tool-calls.test.ts
@@ -1,5 +1,5 @@
 import { Readable } from "node:stream";
-import { describe, expect, it, vi } from "vitest";
+import { beforeEach, describe, expect, it, vi } from "vitest";
 
 const updateToken = vi.fn(async () => {});
 const request = vi.fn();
@@ -22,6 +22,10 @@ function createSseStream(lines: string[]): Readable {
 }
 
 describe("createGigachatStreamFn tool calling", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
   it("round-trips sanitized tool names for streamed function calls", async () => {
     request.mockResolvedValueOnce({
       status: 200,
@@ -156,7 +160,6 @@ describe("createGigachatStreamFn tool calling", () => {
       status: 200,
       data: createSseStream(['data: {"choices":[{"delta":{"content":"done"}}]}', "data: [DONE]"]),
     });
-
     const streamFn = createGigachatStreamFn({
       baseUrl: "https://gigachat.devices.sberbank.ru/api/v1",
       authMode: "oauth",
@@ -233,4 +236,41 @@ describe("createGigachatStreamFn tool calling", () => {
     );
     expect(event.content).toEqual([{ type: "text", text: "done" }]);
   });
+
+  it("rejects tool-name sanitization collisions before sending the request", async () => {
+    const streamFn = createGigachatStreamFn({
+      baseUrl: "https://gigachat.devices.sberbank.ru/api/v1",
+      authMode: "oauth",
+    });
+
+    const stream = streamFn(
+      { api: "gigachat", provider: "gigachat", id: "GigaChat-2-Max" } as never,
+      {
+        messages: [],
+        tools: [
+          {
+            name: "llm-task",
+            description: "Run a task",
+            parameters: { type: "object", properties: {} },
+          },
+          {
+            name: "llm_task",
+            description: "Run another task",
+            parameters: { type: "object", properties: {} },
+          },
+        ],
+      } as never,
+      { apiKey: "token" } as never,
+    );
+
+    const event = await stream.result();
+
+    expect(event.stopReason).toBe("error");
+    expect(event.errorMessage).toBe(
+      'GigaChat tool name collision after sanitization: "llm_task" and "llm-task" both map to "llm_task"',
+    );
+    expect(event.content).toEqual([]);
+    expect(updateToken).not.toHaveBeenCalled();
+    expect(request).not.toHaveBeenCalled();
+  });
 });
diff --git a/src/agents/gigachat-stream.ts b/src/agents/gigachat-stream.ts
index caf67c8ddeb..a192700daad 100644
--- a/src/agents/gigachat-stream.ts
+++ b/src/agents/gigachat-stream.ts
@@ -105,12 +105,13 @@ function rememberToolNameMapping(
   originalName: string,
 ): string {
   const gigaName = toGigaChatToolName(originalName);
+  const existingOriginalName = reverse.get(gigaName);
   forward.set(originalName, gigaName);
-  if (!reverse.has(gigaName)) {
+  if (!existingOriginalName) {
     reverse.set(gigaName, originalName);
-  } else if (reverse.get(gigaName) !== originalName) {
-    log.warn(
-      `GigaChat: tool name collision after sanitization: "${originalName}" and "${reverse.get(gigaName)}" both map to "${gigaName}"`,
+  } else if (existingOriginalName !== originalName) {
+    throw new Error(
+      `GigaChat tool name collision after sanitization: "${originalName}" and "${existingOriginalName}" both map to "${gigaName}"`,
     );
   }
   return gigaName;