fix(mode): stabilize plan-mode model experience

packages/llm/llm-deepseek/tests/serialize.spec.ts

@@ -187,22 +187,12 @@ describe('serializeRequest', () => {
   })
 })
 
-describe('review fixes: assistant content shapes', () => {
-  it('serializes a content-less, tool-call-less assistant message as "" content, never null', () => {
-    // Aborted/empty assistant turns: no text, no calls → "". The earlier
-    // null shape was live-falsified: the API 400s a null-content assistant
-    // message without tool_calls ("content or tool_calls must be set").
+describe('assistant empty and tool-call content shapes', () => {
+  it('serializes a content-less, tool-call-less assistant message as null content', () => {
+    // Aborted/empty assistant turns: no text, no calls → null (the wire
+    // accepts it; "" is reserved for tool-call turns per the samples).
     const wire = serializeMessages([{ role: 'assistant', content: [] }])
-    expect(wire).toEqual([{ role: 'assistant', content: '' }])
-  })
-
-  it('serializes a reasoning-ONLY assistant message as "" content with the reasoning dropped', () => {
-    // The model can answer entirely in the reasoning channel (a v4-flash
-    // greeting did, live). The passback rule keeps reasoning_content off
-    // plain turns, and content must still be SET — a null here poisoned the
-    // session log and bricked every later turn of that session.
-    const wire = serializeMessages([{ role: 'assistant', content: [{ type: 'reasoning', text: '你好！有什么我可以帮你的吗？' }] }])
-    expect(wire).toEqual([{ role: 'assistant', content: '' }])
+    expect(wire).toEqual([{ role: 'assistant', content: null }])
   })
 
   it('serializes tool-call turns with empty string content, not null', () => {