fix(harness): close a stream_event's open block before finish-step on error

packages/harness-runner/claude-code.test.ts

@@ -4,11 +4,13 @@ import {
   brokenStudioMcp,
   buildOptions,
   createDeltaCoalescer,
+  errorFinishChunks,
   isTransientProviderRejection,
   mcpServersFor,
   promptForRun,
   promptFromUserMessage,
 } from "./claude-code";
+import { UiChunkTranslator } from "./to-ui-chunks";
 
 /** A minimal valid wire input; each test overrides what it cares about. */
 function input(
@@ -500,3 +502,43 @@ describe("createDeltaCoalescer", () => {
     expect(c.push([odd])).toEqual([odd]);
   });
 });
+
+describe("errorFinishChunks", () => {
+  test("closes a block stream_event left open before finish-step", () => {
+    const translator = new UiChunkTranslator();
+    translator.translate({
+      type: "stream_event",
+      event: { type: "message_start" },
+    });
+    translator.translate({
+      type: "stream_event",
+      event: {
+        type: "content_block_start",
+        index: 0,
+        content_block: { type: "text", text: "" },
+      },
+    });
+    translator.translate({
+      type: "stream_event",
+      event: {
+        type: "content_block_delta",
+        index: 0,
+        delta: { type: "text_delta", text: "hi" },
+      },
+    });
+    // No `content_block_stop` — the SDK threw mid-block.
+    expect(errorFinishChunks(translator)).toEqual([
+      { type: "text-end", id: "stream-1" },
+      { type: "finish-step" },
+      { type: "finish", finishReason: "error" },
+    ]);
+  });
+
+  test("nothing left open: just finish-step and finish", () => {
+    const translator = new UiChunkTranslator();
+    expect(errorFinishChunks(translator)).toEqual([
+      { type: "finish-step" },
+      { type: "finish", finishReason: "error" },
+    ]);
+  });
+});

packages/harness-runner/claude-code.ts

@@ -830,11 +830,28 @@ export async function runClaudeCode(
     }
     startTurn(messageId ?? `msg_${Date.now()}`);
     emit({
-      chunks: [
-        { type: "finish-step" },
-        { type: "finish", finishReason: "error" },
-      ],
+      chunks: errorFinishChunks(translator),
       error,
     });
   }
 }
+
+/**
+ * Chunks for an aborted turn: close whatever `stream_event` left open, then
+ * finish the step and the run.
+ *
+ * Same orphan-end hazard the step boundary above guards against — the AI SDK
+ * reducer clears its open text/reasoning parts on `finish-step`, so an end
+ * emitted after it throws and drops the stream. An SDK throw mid-block (a
+ * network drop, a crash) reaches `fail()` with a block `stream_event` opened
+ * and never closed; without this, `finish-step` went out first and the part
+ * was left open forever instead of throwing — no crash, but a message that
+ * reads as still streaming after the run has already failed.
+ */
+export function errorFinishChunks(translator: UiChunkTranslator): unknown[] {
+  return [
+    ...translator.closeOpenStreamBlocks(),
+    { type: "finish-step" },
+    { type: "finish", finishReason: "error" },
+  ];
+}