fix(core): settle interrupted assistant steps (#33266)

packages/core/test/session-runner-message.test.ts

@@ -14,6 +14,39 @@ const id = (value: string) => SessionMessage.ID.make(`msg_${value}`)
 const model = Model.make({ id: "model", provider: "provider", route: OpenAIChat.route })
 
 describe("toLLMMessages", () => {
+  test("omits empty assistant turns", () => {
+    const assistant = (value: string, content: SessionMessage.Assistant["content"]) =>
+      new SessionMessage.Assistant({
+        id: id(value),
+        type: "assistant",
+        agent: "build",
+        model: { id: ModelV2.ID.make("model"), providerID: ProviderV2.ID.make("provider") },
+        content,
+        time: { created, completed: created },
+      })
+    const messages = toLLMMessages(
+      [
+        assistant("empty", []),
+        assistant("empty-text", [new SessionMessage.AssistantText({ type: "text", id: "empty", text: "" })]),
+        assistant("empty-reasoning", [
+          new SessionMessage.AssistantReasoning({ type: "reasoning", id: "empty-reasoning", text: "" }),
+        ]),
+        assistant("text", [new SessionMessage.AssistantText({ type: "text", id: "text", text: "Partial" })]),
+        assistant("reasoning", [
+          new SessionMessage.AssistantReasoning({
+            type: "reasoning",
+            id: "reasoning",
+            text: "",
+            providerMetadata: { anthropic: { signature: "sig_1" } },
+          }),
+        ]),
+      ],
+      model,
+    )
+
+    expect(messages.map((message) => message.id)).toEqual([id("text"), id("reasoning")])
+  })
+
   test("maps every top-level V2 Session message type", () => {
     const file = new FileAttachment({ uri: "data:image/png;base64,aGVsbG8=", mime: "image/png", name: "hello.png" })
     const messages = toLLMMessages(

packages/core/test/session-runner.test.ts

@@ -547,6 +547,8 @@ const verifyPartialFlushOnInterruption = (kind: FragmentKind) =>
       { type: "user", text: prompt },
       {
         type: "assistant",
+        finish: "error",
+        error: { type: "unknown", message: "Provider turn interrupted" },
         content: [
           kind === "tool input"
             ? { type: "tool", id: fragmentID(kind, "interrupted"), state: { status: "error" } }