diff --git a/src/agents/sessions/tools/find.ts b/src/agents/sessions/tools/find.ts
index 1c926f457f2..0c2e67742a6 100644
--- a/src/agents/sessions/tools/find.ts
+++ b/src/agents/sessions/tools/find.ts
@@ -8,7 +8,7 @@ import { keyHint } from "../../modes/interactive/components/keybinding-hints.js"
 import type { AgentTool } from "../../runtime/index.js";
 import { ensureTool } from "../../utils/tools-manager.js";
 import type { ToolDefinition, ToolRenderResultOptions } from "../extensions/types.js";
-import { normalizePositiveLimit } from "./limits.js";
+import { appendBoundedTextTail, normalizePositiveLimit } from "./limits.js";
 import { resolveToCwd } from "./path-utils.js";
 import { getTextOutput, invalidArgText, shortenPath, str } from "./render-utils.js";
 import type { FindToolDetails } from "./tool-contracts.js";
@@ -277,7 +277,7 @@ export function createFindToolDefinition(
             };
 
             child.stderr?.on("data", (chunk) => {
-              stderr += chunk.toString();
+              stderr = appendBoundedTextTail(stderr, chunk);
             });
 
             rl.on("line", (line) => {
diff --git a/src/agents/sessions/tools/grep.ts b/src/agents/sessions/tools/grep.ts
index ddc04ab20c3..2848d33bb37 100644
--- a/src/agents/sessions/tools/grep.ts
+++ b/src/agents/sessions/tools/grep.ts
@@ -8,7 +8,7 @@ import { keyHint } from "../../modes/interactive/components/keybinding-hints.js"
 import type { AgentTool } from "../../runtime/index.js";
 import { ensureTool } from "../../utils/tools-manager.js";
 import type { ToolDefinition, ToolRenderResultOptions } from "../extensions/types.js";
-import { normalizePositiveLimit } from "./limits.js";
+import { appendBoundedTextTail, normalizePositiveLimit } from "./limits.js";
 import { resolveToCwd } from "./path-utils.js";
 import { getTextOutput, invalidArgText, shortenPath, str } from "./render-utils.js";
 import type { GrepToolDetails } from "./tool-contracts.js";
@@ -270,7 +270,7 @@ export function createGrepToolDefinition(
             };
             signal?.addEventListener("abort", onAbort, { once: true });
             child.stderr?.on("data", (chunk) => {
-              stderr += chunk.toString();
+              stderr = appendBoundedTextTail(stderr, chunk);
             });
 
             const formatBlock = async (filePath: string, lineNumber: number): Promise<string[]> => {
diff --git a/src/agents/sessions/tools/limits.test.ts b/src/agents/sessions/tools/limits.test.ts
index aff3b858b78..a28245c19cb 100644
--- a/src/agents/sessions/tools/limits.test.ts
+++ b/src/agents/sessions/tools/limits.test.ts
@@ -1,5 +1,9 @@
 import { describe, expect, it } from "vitest";
-import { normalizePositiveLimit } from "./limits.js";
+import {
+  appendBoundedTextTail,
+  normalizePositiveLimit,
+  SESSION_TOOL_STDERR_TAIL_BYTES,
+} from "./limits.js";
 
 describe("session tool limits", () => {
   it.each([
@@ -13,4 +17,25 @@ describe("session tool limits", () => {
   ])("normalizes %s to %s", (input, expected) => {
     expect(normalizePositiveLimit(input, 500)).toBe(expected);
   });
+
+  it("keeps a bounded tail of accumulated child output", () => {
+    let output = appendBoundedTextTail("old-", "middle-", 12);
+    output = appendBoundedTextTail(output, "recent", 12);
+
+    expect(output).toBe("iddle-recent");
+    expect(Buffer.byteLength(output, "utf8")).toBeLessThanOrEqual(12);
+  });
+
+  it("clips oversized chunks to the configured tail bytes", () => {
+    const output = appendBoundedTextTail("ignored", Buffer.from("x".repeat(128)), 16);
+
+    expect(output).toBe("x".repeat(16));
+    expect(Buffer.byteLength(output, "utf8")).toBe(16);
+  });
+
+  it("uses the session stderr tail limit by default", () => {
+    const output = appendBoundedTextTail("", "x".repeat(SESSION_TOOL_STDERR_TAIL_BYTES + 1));
+
+    expect(Buffer.byteLength(output, "utf8")).toBe(SESSION_TOOL_STDERR_TAIL_BYTES);
+  });
 });
diff --git a/src/agents/sessions/tools/limits.ts b/src/agents/sessions/tools/limits.ts
index 128a56e56c5..2f420c62bdf 100644
--- a/src/agents/sessions/tools/limits.ts
+++ b/src/agents/sessions/tools/limits.ts
@@ -4,3 +4,27 @@ export function normalizePositiveLimit(value: number | undefined, fallback: numb
   }
   return Math.max(1, Math.floor(value));
 }
+
+export const SESSION_TOOL_STDERR_TAIL_BYTES = 64 * 1024;
+
+export function appendBoundedTextTail(
+  current: string,
+  chunk: Buffer | string,
+  maxBytes = SESSION_TOOL_STDERR_TAIL_BYTES,
+): string {
+  const effectiveMaxBytes = normalizePositiveLimit(maxBytes, SESSION_TOOL_STDERR_TAIL_BYTES);
+  const chunkBuffer = Buffer.isBuffer(chunk) ? chunk : Buffer.from(chunk);
+  if (chunkBuffer.byteLength >= effectiveMaxBytes) {
+    return chunkBuffer.subarray(chunkBuffer.byteLength - effectiveMaxBytes).toString("utf8");
+  }
+
+  const currentBuffer = Buffer.from(current);
+  const nextBytes = currentBuffer.byteLength + chunkBuffer.byteLength;
+  if (nextBytes <= effectiveMaxBytes) {
+    return `${current}${chunkBuffer.toString("utf8")}`;
+  }
+
+  const currentTailBytes = Math.max(0, effectiveMaxBytes - chunkBuffer.byteLength);
+  const currentTail = currentBuffer.subarray(currentBuffer.byteLength - currentTailBytes);
+  return Buffer.concat([currentTail, chunkBuffer], effectiveMaxBytes).toString("utf8");
+}