fix(resume): cap history hydrate to the newest retained rows

src/tui/product-host.test.ts

@@ -19,6 +19,9 @@ import {
   type ProductHostConfig,
 } from "./product-host.js";
 import { buildModelsFirstCatalog, modelOptionId } from "./model-catalog.js";
+import { hydrateHistoryRows } from "./history-hydrate.js";
+import { MAX_RETAINED_STREAM_ROWS } from "./long-log.js";
+import { enterSubagentObserve } from "./shell/observe.js";
 
 function makeFakeSessionPort(): {
   readonly sends: string[];
@@ -136,6 +139,130 @@ describe("mountProductHost", () => {
     }
   });
 
+  test("history.hydrate caps oversized history at the newest retained rows (CL-9008)", async () => {
+    const { host, emitter } = await mountHeadless();
+    try {
+      const total = MAX_RETAINED_STREAM_ROWS + 200;
+      const blocks = Array.from({ length: total }, (_, i) => ({
+        type: "text",
+        content: `row-${i}`,
+      }));
+      emitter.emit("history.hydrate", blocks);
+      expect(host.shell.streamLog.length).toBe(MAX_RETAINED_STREAM_ROWS);
+      expect(host.shell.streamLog[0]).toEqual({
+        role: "assistant",
+        text: `row-${total - MAX_RETAINED_STREAM_ROWS}`,
+      });
+      expect(host.shell.streamLog[MAX_RETAINED_STREAM_ROWS - 1]).toEqual({
+        role: "assistant",
+        text: `row-${total - 1}`,
+      });
+      // Pre-sliced to the cap, so the append loop never trips retention
+      // eviction: no churn, no dropped-rows marker on resume.
+      expect(host.shell.streamLogBase).toBe(0);
+    } finally {
+      host.dispose();
+    }
+  });
+
+  test("history.hydrate below the cap paints every row (CL-9008)", async () => {
+    const { host, emitter } = await mountHeadless();
+    try {
+      const total = MAX_RETAINED_STREAM_ROWS - 100;
+      const blocks = Array.from({ length: total }, (_, i) => ({
+        type: "text",
+        content: `small-${i}`,
+      }));
+      emitter.emit("history.hydrate", blocks);
+      expect(host.shell.streamLog.length).toBe(total);
+      expect(host.shell.streamLog[0]).toEqual({
+        role: "assistant",
+        text: "small-0",
+      });
+      expect(host.shell.streamLog[total - 1]).toEqual({
+        role: "assistant",
+        text: `small-${total - 1}`,
+      });
+      expect(host.shell.streamLogBase).toBe(0);
+    } finally {
+      host.dispose();
+    }
+  });
+
+  test("history.hydrate keeps a tool pair merged atomically across the cap boundary (CL-9008)", async () => {
+    const { host, emitter } = await mountHeadless();
+    try {
+      const blocks = [
+        ...Array.from({ length: 605 }, (_, i) => ({
+          type: "text",
+          content: `row-${i}`,
+        })),
+        {
+          type: "tool_call",
+          name: "spawn_agent",
+          arguments: '{"description":"Fix CL-9008"}',
+          callId: "cut-1",
+        },
+        {
+          type: "tool_result",
+          name: "spawn_agent",
+          content: "done cut-1",
+          callId: "cut-1",
+        },
+      ];
+      emitter.emit("history.hydrate", blocks);
+      // Folding merges the pair inside hydration, so the row-level slice keeps
+      // and drops whole merged rows: 605 texts + 1 merged row → newest 600.
+      // A block-level slice would split the pair and paint 599 rows instead.
+      const expected = hydrateHistoryRows(blocks).slice(
+        -MAX_RETAINED_STREAM_ROWS,
+      );
+      expect(expected.length).toBe(MAX_RETAINED_STREAM_ROWS);
+      expect(host.shell.streamLog).toEqual(expected);
+      expect(host.shell.streamLog[0]).toEqual({
+        role: "assistant",
+        text: "row-6",
+      });
+      const last = host.shell.streamLog[host.shell.streamLog.length - 1];
+      expect(last?.pending).not.toBe(true);
+      expect(last?.text).toBe("done cut-1");
+      expect(host.shell.streamLogBase).toBe(0);
+    } finally {
+      host.dispose();
+    }
+  });
+
+  test("history.hydrate in observe mode lands the capped tail on the parent log (CL-9008)", async () => {
+    const { host, emitter } = await mountHeadless();
+    try {
+      enterSubagentObserve(host.shell, {
+        sessionId: "child-keeper",
+        agentId: "explorer",
+        description: "observe-mode hydrate keeper",
+        lines: [],
+      });
+      expect(host.shell.parentStreamLog).toEqual([]);
+      const visibleBefore = host.shell.streamLog.length;
+      const total = MAX_RETAINED_STREAM_ROWS + 200;
+      const blocks = Array.from({ length: total }, (_, i) => ({
+        type: "text",
+        content: `obs-row-${i}`,
+      }));
+      emitter.emit("history.hydrate", blocks);
+      // Observe routes hydrate rows to the parent snapshot only; the child
+      // view on screen is untouched.
+      expect(host.shell.streamLog.length).toBe(visibleBefore);
+      const expected = hydrateHistoryRows(blocks).slice(
+        -MAX_RETAINED_STREAM_ROWS,
+      );
+      expect(host.shell.parentStreamLog).toEqual(expected);
+      expect(host.shell.parentStreamLog?.length).toBe(MAX_RETAINED_STREAM_ROWS);
+      expect(host.shell.parentStreamLogBase).toBe(0);
+    } finally {
+      host.dispose();
+    }
+  });
+
   test("session.title updates the shell header", async () => {
     const { host, emitter } = await mountHeadless();
     try {

src/tui/product-host.ts

@@ -67,6 +67,7 @@ import {
 import { surfaceSystemNotice } from "./shell/prompt.js";
 import type { DeliverySettle, QueueKind } from "./delivery-queue.js";
 import { hydrateHistoryRows } from "./history-hydrate.js";
+import { MAX_RETAINED_STREAM_ROWS } from "./long-log.js";
 import type { StreamRow } from "./stream.js";
 
 import type { PendingImageAttachment } from "./image-attachments.js";
@@ -557,7 +558,9 @@ export async function mountProductHost(
 
   function onHistory(blocks: unknown): void {
     if (disposed) return;
-    for (const row of hydrateHistoryRows(blocks)) {
+    const rows = hydrateHistoryRows(blocks);
+    const tail = rows.slice(-MAX_RETAINED_STREAM_ROWS);
+    for (const row of tail) {
       appendStreamRow(shell, row);
     }
   }