fix: stop newline block streaming from sending per paragraph

src/agents/pi-embedded-block-chunker.test.ts

@@ -11,20 +11,12 @@ function createFlushOnParagraphChunker(params: { minChars: number; maxChars: num
   });
 }
 
-function drainChunks(chunker: EmbeddedBlockChunker) {
+function drainChunks(chunker: EmbeddedBlockChunker, force = false) {
   const chunks: string[] = [];
-  chunker.drain({ force: false, emit: (chunk) => chunks.push(chunk) });
+  chunker.drain({ force, emit: (chunk) => chunks.push(chunk) });
   return chunks;
 }
 
-function expectFlushAtFirstParagraphBreak(text: string) {
-  const chunker = createFlushOnParagraphChunker({ minChars: 100, maxChars: 200 });
-  chunker.append(text);
-  const chunks = drainChunks(chunker);
-  expect(chunks).toEqual(["First paragraph."]);
-  expect(chunker.bufferedText).toBe("Second paragraph.");
-}
-
 describe("EmbeddedBlockChunker", () => {
   it("breaks at paragraph boundary right after fence close", () => {
     const chunker = new EmbeddedBlockChunker({
@@ -54,12 +46,25 @@ describe("EmbeddedBlockChunker", () => {
     expect(chunker.bufferedText).toMatch(/^After/);
   });
 
-  it("flushes paragraph boundaries before minChars when flushOnParagraph is set", () => {
-    expectFlushAtFirstParagraphBreak("First paragraph.\n\nSecond paragraph.");
+  it("waits until minChars before flushing paragraph boundaries when flushOnParagraph is set", () => {
+    const chunker = createFlushOnParagraphChunker({ minChars: 30, maxChars: 200 });
+
+    chunker.append("First paragraph.\n\nSecond paragraph.\n\nThird paragraph.");
+
+    const chunks = drainChunks(chunker);
+
+    expect(chunks).toEqual(["First paragraph.\n\nSecond paragraph."]);
+    expect(chunker.bufferedText).toBe("Third paragraph.");
   });
 
-  it("treats blank lines with whitespace as paragraph boundaries when flushOnParagraph is set", () => {
-    expectFlushAtFirstParagraphBreak("First paragraph.\n \nSecond paragraph.");
+  it("still force flushes buffered paragraphs below minChars at the end", () => {
+    const chunker = createFlushOnParagraphChunker({ minChars: 100, maxChars: 200 });
+
+    chunker.append("First paragraph.\n \nSecond paragraph.");
+
+    expect(drainChunks(chunker)).toEqual([]);
+    expect(drainChunks(chunker, true)).toEqual(["First paragraph.\n \nSecond paragraph."]);
+    expect(chunker.bufferedText).toBe("");
   });
 
   it("falls back to maxChars when flushOnParagraph is set and no paragraph break exists", () => {
@@ -97,7 +102,7 @@ describe("EmbeddedBlockChunker", () => {
 
   it("ignores paragraph breaks inside fences when flushOnParagraph is set", () => {
     const chunker = new EmbeddedBlockChunker({
-      minChars: 100,
+      minChars: 10,
       maxChars: 200,
       breakPreference: "paragraph",
       flushOnParagraph: true,

src/agents/pi-embedded-block-chunker.ts

@@ -5,7 +5,7 @@ export type BlockReplyChunking = {
   minChars: number;
   maxChars: number;
   breakPreference?: "paragraph" | "newline" | "sentence";
-  /** When true, flush eagerly on \n\n paragraph boundaries regardless of minChars. */
+  /** When true, prefer \n\n paragraph boundaries once minChars has been satisfied. */
   flushOnParagraph?: boolean;
 };
 
@@ -129,7 +129,7 @@ export class EmbeddedBlockChunker {
     const minChars = Math.max(1, Math.floor(this.#chunking.minChars));
     const maxChars = Math.max(minChars, Math.floor(this.#chunking.maxChars));
 
-    if (this.#buffer.length < minChars && !force && !this.#chunking.flushOnParagraph) {
+    if (this.#buffer.length < minChars && !force) {
       return;
     }
 
@@ -150,12 +150,12 @@ export class EmbeddedBlockChunker {
       const reopenPrefix = reopenFence ? `${reopenFence.openLine}\n` : "";
       const remainingLength = reopenPrefix.length + (source.length - start);
 
-      if (!force && !this.#chunking.flushOnParagraph && remainingLength < minChars) {
+      if (!force && remainingLength < minChars) {
         break;
       }
 
       if (this.#chunking.flushOnParagraph && !force) {
-        const paragraphBreak = findNextParagraphBreak(source, fenceSpans, start);
+        const paragraphBreak = findNextParagraphBreak(source, fenceSpans, start, minChars);
         const paragraphLimit = Math.max(1, maxChars - reopenPrefix.length);
         if (paragraphBreak && paragraphBreak.index - start <= paragraphLimit) {
           const chunk = `${reopenPrefix}${source.slice(start, paragraphBreak.index)}`;
@@ -175,12 +175,7 @@ export class EmbeddedBlockChunker {
       const breakResult =
         force && remainingLength <= maxChars
           ? this.#pickSoftBreakIndex(view, fenceSpans, 1, start)
-          : this.#pickBreakIndex(
-              view,
-              fenceSpans,
-              force || this.#chunking.flushOnParagraph ? 1 : undefined,
-              start,
-            );
+          : this.#pickBreakIndex(view, fenceSpans, force ? 1 : undefined, start);
       if (breakResult.index <= 0) {
         if (force) {
           emit(`${reopenPrefix}${source.slice(start)}`);
@@ -205,7 +200,7 @@ export class EmbeddedBlockChunker {
 
       const nextLength =
         (reopenFence ? `${reopenFence.openLine}\n`.length : 0) + (source.length - start);
-      if (nextLength < minChars && !force && !this.#chunking.flushOnParagraph) {
+      if (nextLength < minChars && !force) {
         break;
       }
       if (nextLength < maxChars && !force && !this.#chunking.flushOnParagraph) {
@@ -401,6 +396,7 @@ function findNextParagraphBreak(
   buffer: string,
   fenceSpans: FenceSpan[],
   startIndex = 0,
+  minCharsFromStart = 1,
 ): ParagraphBreak | null {
   if (startIndex < 0) {
     return null;
@@ -413,6 +409,9 @@ function findNextParagraphBreak(
     if (index < 0) {
       continue;
     }
+    if (index - startIndex < minCharsFromStart) {
+      continue;
+    }
     if (!isSafeFenceBreak(fenceSpans, index)) {
       continue;
     }

src/auto-reply/reply/block-reply-coalescer.ts

@@ -89,8 +89,8 @@ export function createBlockReplyCoalescer(params: {
       return;
     }
 
-    // When flushOnEnqueue is set (chunkMode="newline"), each enqueued payload is treated
-    // as a separate paragraph and flushed immediately so delivery matches streaming boundaries.
+    // When flushOnEnqueue is set, treat each enqueued payload as its own outbound block
+    // and flush immediately instead of waiting for coalescing thresholds.
     if (flushOnEnqueue) {
       if (bufferText) {
         void flush({ force: true });

src/auto-reply/reply/block-streaming.test.ts

@@ -68,7 +68,7 @@ describe("resolveEffectiveBlockStreamingConfig", () => {
     });
 
     expect(resolved.chunking.flushOnParagraph).toBe(true);
-    expect(resolved.coalescing.flushOnEnqueue).toBe(true);
+    expect(resolved.coalescing.flushOnEnqueue).toBeUndefined();
     expect(resolved.coalescing.joiner).toBe("\n\n");
   });
 

src/auto-reply/reply/block-streaming.ts

@@ -66,7 +66,7 @@ export type BlockStreamingCoalescing = {
   maxChars: number;
   idleMs: number;
   joiner: string;
-  /** When true, the coalescer flushes the buffer on each enqueue (paragraph-boundary flush). */
+  /** Internal escape hatch for transports that truly need per-enqueue flushing. */
   flushOnEnqueue?: boolean;
 };
 
@@ -147,7 +147,7 @@ export function resolveEffectiveBlockStreamingConfig(params: {
         : chunking.breakPreference === "newline"
           ? "\n"
           : "\n\n"),
-    flushOnEnqueue: coalescingDefaults?.flushOnEnqueue ?? chunking.flushOnParagraph === true,
+    ...(coalescingDefaults?.flushOnEnqueue === true ? { flushOnEnqueue: true } : {}),
   };
 
   return { chunking, coalescing };
@@ -161,9 +161,9 @@ export function resolveBlockStreamingChunking(
   const { providerKey, textLimit } = resolveProviderChunkContext(cfg, provider, accountId);
   const chunkCfg = cfg?.agents?.defaults?.blockStreamingChunk;
 
-  // When chunkMode="newline", the outbound delivery splits on paragraph boundaries.
-  // The block chunker should flush eagerly on \n\n boundaries during streaming,
-  // regardless of minChars, so each paragraph is sent as its own message.
+  // When chunkMode="newline", outbound delivery prefers paragraph boundaries.
+  // Keep the chunker paragraph-aware during streaming, but still let minChars
+  // control when a buffered paragraph is ready to flush.
   const chunkMode = resolveChunkMode(cfg, providerKey, accountId);
 
   const maxRequested = Math.max(1, Math.floor(chunkCfg?.maxChars ?? DEFAULT_BLOCK_STREAM_MAX));
@@ -192,7 +192,6 @@ export function resolveBlockStreamingCoalescing(
     maxChars: number;
     breakPreference: "paragraph" | "newline" | "sentence";
   },
-  opts?: { chunkMode?: "length" | "newline" },
 ): BlockStreamingCoalescing | undefined {
   const { providerKey, providerId, textLimit } = resolveProviderChunkContext(
     cfg,
@@ -200,9 +199,6 @@ export function resolveBlockStreamingCoalescing(
     accountId,
   );
 
-  // Resolve the outbound chunkMode so the coalescer can flush on paragraph boundaries
-  // when chunkMode="newline", matching the delivery-time splitting behavior.
-  const chunkMode = opts?.chunkMode ?? resolveChunkMode(cfg, providerKey, accountId);
   const providerDefaults = providerId
     ? getChannelPlugin(providerId)?.streaming?.blockStreamingCoalesceDefaults
     : undefined;
@@ -237,6 +233,5 @@ export function resolveBlockStreamingCoalescing(
     maxChars,
     idleMs,
     joiner,
-    flushOnEnqueue: chunkMode === "newline",
   };
 }

src/auto-reply/reply/reply-utils.test.ts

@@ -675,6 +675,39 @@ describe("block reply coalescer", () => {
     coalescer.stop();
   });
 
+  it("keeps buffering newline-style chunks until minChars is reached", async () => {
+    vi.useFakeTimers();
+    const { flushes, coalescer } = createBlockCoalescerHarness({
+      minChars: 25,
+      maxChars: 2000,
+      idleMs: 50,
+      joiner: "\n\n",
+    });
+
+    coalescer.enqueue({ text: "First paragraph" });
+    coalescer.enqueue({ text: "Second paragraph" });
+
+    await vi.advanceTimersByTimeAsync(50);
+    expect(flushes).toEqual(["First paragraph\n\nSecond paragraph"]);
+    coalescer.stop();
+  });
+
+  it("force flushes buffered newline-style chunks even below minChars", async () => {
+    const { flushes, coalescer } = createBlockCoalescerHarness({
+      minChars: 100,
+      maxChars: 2000,
+      idleMs: 50,
+      joiner: "\n\n",
+    });
+
+    coalescer.enqueue({ text: "First paragraph" });
+    coalescer.enqueue({ text: "Second paragraph" });
+    await coalescer.flush({ force: true });
+
+    expect(flushes).toEqual(["First paragraph\n\nSecond paragraph"]);
+    coalescer.stop();
+  });
+
   it("flushes immediately per enqueue when flushOnEnqueue is set", async () => {
     const cases = [
       {