From 825193400773cceab9b92f6bc63247d6dde3d580 Mon Sep 17 00:00:00 2001
From: Dax Raad <d@ironbay.co>
Date: Sat, 15 Aug 2026 19:26:16 -0400
Subject: [PATCH] fix(core): batch initial streamed delta

---
 packages/core/src/session/runner/publish-llm-event.ts |  4 ++++
 packages/core/test/session-runner-tool-events.test.ts | 11 ++++-------
 packages/core/test/session-runner.test.ts             |  4 ++--
 3 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/packages/core/src/session/runner/publish-llm-event.ts b/packages/core/src/session/runner/publish-llm-event.ts
index 1527a0f961e..7b17266ac1a 100644
--- a/packages/core/src/session/runner/publish-llm-event.ts
+++ b/packages/core/src/session/runner/publish-llm-event.ts
@@ -150,6 +150,10 @@ export const createLLMEventPublisher = (bus: Pick<Bus.Interface, "publish">, inp
       if (!current) return yield* Effect.die(new Error(`${name} delta before start: ${id}`))
       if (!current.pending) return undefined
       const now = yield* Clock.currentTimeMillis
+      if (!force && current.publishedAt === undefined) {
+        current.publishedAt = now
+        return undefined
+      }
       if (!force && current.publishedAt !== undefined && now - current.publishedAt < deltaBatchInterval)
         return undefined
       yield* delta(id, current.pending, current.ordinal)
diff --git a/packages/core/test/session-runner-tool-events.test.ts b/packages/core/test/session-runner-tool-events.test.ts
index ed6146465e7..0f36123698b 100644
--- a/packages/core/test/session-runner-tool-events.test.ts
+++ b/packages/core/test/session-runner-tool-events.test.ts
@@ -217,16 +217,13 @@ it.effect("batches text deltas and flushes pending text before the terminal even
       { discard: true },
     )
 
-    expect(published.filter((event) => event.type === "session.text.delta").map((event) => event.data)).toMatchObject([
-      { delta: "one" },
-    ])
+    expect(published.filter((event) => event.type === "session.text.delta")).toHaveLength(0)
     yield* TestClock.adjust("99 millis")
-    expect(published.filter((event) => event.type === "session.text.delta")).toHaveLength(1)
+    expect(published.filter((event) => event.type === "session.text.delta")).toHaveLength(0)
     yield* TestClock.adjust("1 millis")
     yield* publisher.publish(LLMEvent.textDelta({ id: "text", text: " four" }))
     expect(published.filter((event) => event.type === "session.text.delta").map((event) => event.data)).toMatchObject([
-      { delta: "one" },
-      { delta: " two three four" },
+      { delta: "one two three four" },
     ])
 
     yield* publisher.publish(LLMEvent.textDelta({ id: "text", text: " five" }))
@@ -253,7 +250,7 @@ it.effect("batches reasoning deltas and flushes pending reasoning before the ter
 
     expect(
       published.filter((event) => event.type === "session.reasoning.delta").map((event) => event.data),
-    ).toMatchObject([{ delta: "one" }, { delta: " two three" }])
+    ).toMatchObject([{ delta: "one two three" }])
     expect(published.slice(-2).map((event) => event.type)).toEqual([
       "session.reasoning.delta",
       "session.reasoning.ended.1",
diff --git a/packages/core/test/session-runner.test.ts b/packages/core/test/session-runner.test.ts
index c718b414b96..e55f1d8e278 100644
--- a/packages/core/test/session-runner.test.ts
+++ b/packages/core/test/session-runner.test.ts
@@ -767,7 +767,7 @@ const verifyEphemeralDeltas = (kind: FragmentKind) =>
     yield* admit(session, prompt)
     const bus = yield* Bus.Service
     const live = fixture.delta
-      ? yield* bus.subscribe(fixture.delta).pipe(Stream.take(2), Stream.runCollect, Effect.forkScoped)
+      ? yield* bus.subscribe(fixture.delta).pipe(Stream.take(1), Stream.runCollect, Effect.forkScoped)
       : undefined
     yield* Effect.yieldNow
     yield* TestLLM.push(fixture.completeEvents)
@@ -785,7 +785,7 @@ const verifyEphemeralDeltas = (kind: FragmentKind) =>
       : []
     if (live) {
       const streamed = Array.from(yield* Fiber.join(live))
-      expect(streamed).toHaveLength(2)
+      expect(streamed).toHaveLength(1)
       expect(
         streamed
           .map((event) => {