fix(subagent): complete output selection contract

packages/subagent/subagent/tests/assistant-output.spec.ts

@@ -15,6 +15,22 @@ function reasoningDelta(text: string): SessionEvent {
   return { type: 'assistant/chunk', data: { chunk: { type: 'reasoning-delta', text } } } as SessionEvent
 }
 
+function toolResult(text: string): SessionEvent {
+  return {
+    type: 'tool/result',
+    data: {
+      message: {
+        content: [{
+          type: 'tool-result',
+          toolCallId: 'call-1',
+          content: [{ type: 'text', text }],
+          isError: false,
+        }],
+      },
+    },
+  } as SessionEvent
+}
+
 describe('finalAssistantOutput', () => {
   it('selects the last non-empty message past a later empty usage-only message', () => {
     const events = [
@@ -25,19 +41,30 @@ describe('finalAssistantOutput', () => {
     expect(finalAssistantOutput(events)).toEqual([{ type: 'text', text: 'step two' }])
   })
 
-  it('prefers a non-empty message over the streamed text', () => {
+  it('prefers a non-empty message over text streamed before and after it', () => {
     const events = [
-      textDelta('streamed '),
-      textDelta('text'),
+      textDelta('earlier partial'),
       message([{ type: 'text', text: 'complete answer' }]),
+      textDelta('later partial'),
+      message([]),
     ]
     expect(finalAssistantOutput(events)).toEqual([{ type: 'text', text: 'complete answer' }])
   })
 
-  it('falls back to accumulated text deltas when no non-empty message exists', () => {
+  it('treats textless assistant content as a non-empty message', () => {
+    const content: ContentBlock[] = [{ type: 'reasoning', text: 'complete reasoning' }]
+    expect(finalAssistantOutput([
+      textDelta('streamed text'),
+      message(content),
+      textDelta('later partial'),
+    ])).toEqual(content)
+  })
+
+  it('falls back to text deltas without including reasoning or tool-result content', () => {
     const events = [
       reasoningDelta('thinking'),
       textDelta('partial '),
+      toolResult('tool output'),
       textDelta('answer'),
       message([]),
     ]

packages/subagent/subagent/tests/continuation.spec.ts

@@ -1201,10 +1201,9 @@ describe('continuable review regressions', () => {
   })
 
   it('keeps the epoch\'s earlier text past a final empty usage-only message', async () => {
-    // Step 1 streams text plus a tool call; step 2 hits max-tokens having
-    // assembled only a tool-call block, so the loop appends an EMPTY
-    // assistant/message to host usage. The terminal edge reports the epoch's
-    // real answer text, not the internal usage marker.
+    // A tool-only max-tokens step records an empty assistant/message for
+    // usage. The terminal event retains the previous assistant content,
+    // including its tool call but not the intervening tool result.
     const { ctx, parent } = await setup([
       toolCallResponse('t1', 'noop', {}, 'partial one'),
       [

packages/subagent/subagent/tests/service.spec.ts

@@ -264,8 +264,8 @@ describe('SubagentService', () => {
       stopReason: 'completed',
     }))
 
-    // "No output" has ONE encoding on the end edge: the field is absent,
-    // never an empty array, matching the continuable epoch edge.
+    // The lifecycle event omits lastAssistantMessage when output is empty,
+    // matching the continuable epoch event.
     const silent = new StubProvider('silent', NO_CAPS, { output: [], stopReason: 'completed' })
     subagents.registerProvider(silent)
     const silentRun = await subagents.start('silent', baseRequest())