diff --git a/src/domains/chat/diagram.test.ts b/src/domains/chat/diagram.test.ts
index e2e6d50..b671b0a 100644
--- a/src/domains/chat/diagram.test.ts
+++ b/src/domains/chat/diagram.test.ts
@@ -105,7 +105,7 @@ describe("generateChatDiagramSpec", () => {
     })
 
     expect(generateObject).toHaveBeenCalledWith({
-      model: "google/gemini-3-flash",
+      model: "deepseek/deepseek-v4.1-flash",
       schema: expect.any(Object),
       prompt: expect.stringContaining("Cloud revenue was 42"),
     })
@@ -144,7 +144,7 @@ describe("generateChatDiagramSpec", () => {
 
     expect(generateObject).toHaveBeenCalledTimes(2)
     expect(vi.mocked(generateObject).mock.calls[1]?.[0]).toEqual({
-      model: "google/gemini-3-flash",
+      model: "deepseek/deepseek-v4.1-flash",
       schema: expect.any(Object),
       prompt: expect.stringContaining("The previous diagram-generation output"),
     })
diff --git a/src/domains/chat/route-service.test.ts b/src/domains/chat/route-service.test.ts
index d27b32d..80d4f7b 100644
--- a/src/domains/chat/route-service.test.ts
+++ b/src/domains/chat/route-service.test.ts
@@ -373,7 +373,7 @@ describe("chat route services", () => {
     })
     const generateInput = mocks.generateObject.mock.calls[0]?.[0]
     expect(generateInput).toMatchObject({
-      model: "google/gemini-3-flash",
+      model: "deepseek/deepseek-v4.1-flash",
     })
     expect(JSON.stringify(generateInput)).not.toContain(
       "knowhere-storage.example",
diff --git a/src/lib/ai.ts b/src/lib/ai.ts
index b1313ff..0a4ec1a 100644
--- a/src/lib/ai.ts
+++ b/src/lib/ai.ts
@@ -9,4 +9,4 @@
  * Change CHAT_MODEL here (or via env) when we want to try a different model.
  */
 
-export const CHAT_MODEL = process.env.CHAT_MODEL ?? "google/gemini-3-flash"
+export const CHAT_MODEL = process.env.CHAT_MODEL ?? "deepseek/deepseek-v4.1-flash"