From 1895c2feafc07a22bf69af4325c2841cdf8659d0 Mon Sep 17 00:00:00 2001
From: qer <qer@msh.team>
Date: Mon, 15 Jun 2026 18:38:20 +0800
Subject: [PATCH] fix(read): report maxLinesReached in tail mode

When reading from the tail with line_offset=-MAX_LINES, the Read tool
never reported maxLinesReached even though it hit the cap. This made
tail mode inconsistent with forward mode.

- Set maxLinesReached in readTail when effectiveLimit >= MAX_LINES and
  entries.length >= effectiveLimit.
- Add regression test ensuring tail mode reports the cap.
---
 .../agent-core/src/tools/builtin/file/read.ts    |  2 +-
 packages/agent-core/test/tools/read.test.ts      | 16 ++++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/packages/agent-core/src/tools/builtin/file/read.ts b/packages/agent-core/src/tools/builtin/file/read.ts
index 252eef3af..7daf07c4e 100644
--- a/packages/agent-core/src/tools/builtin/file/read.ts
+++ b/packages/agent-core/src/tools/builtin/file/read.ts
@@ -412,7 +412,7 @@ export class ReadTool implements BuiltinTool<ReadInput> {
     return this.finishReadResult({
       renderedLines,
       truncatedLineNumbers,
-      maxLinesReached: false,
+      maxLinesReached: effectiveLimit >= MAX_LINES && entries.length >= effectiveLimit,
       maxBytesReached,
       lineEndingStyle,
       startLine: renderedCandidates[0]?.entry.lineNo ?? 0,
diff --git a/packages/agent-core/test/tools/read.test.ts b/packages/agent-core/test/tools/read.test.ts
index 4ab97c1bb..34826b9a6 100644
--- a/packages/agent-core/test/tools/read.test.ts
+++ b/packages/agent-core/test/tools/read.test.ts
@@ -622,6 +622,22 @@ describe('ReadTool', () => {
     expect(result.output).not.toContain(`${String(MAX_LINES + 1)}\tline ${String(MAX_LINES + 1)}`);
   });
 
+  it('caps tail reads at MAX_LINES', async () => {
+    const content = Array.from({ length: MAX_LINES + 5 }, (_, i) => `line ${String(i + 1)}`).join(
+      '\n',
+    );
+    const tool = toolWithContent(content);
+
+    const result = await executeTool(
+      tool,
+      context({ path: '/tmp/tail-big.txt', line_offset: -MAX_LINES }),
+    );
+
+    expect(result.output).toContain(`Max ${String(MAX_LINES)} lines reached.`);
+    expect(result.output).toContain(`${String(MAX_LINES + 5)}\tline ${String(MAX_LINES + 5)}`);
+    expect(result.output).toMatch(/^6\tline 6/);
+  });
+
   it('tail byte truncation keeps the newest lines closest to EOF', async () => {
     const numLines = Math.floor(MAX_BYTES / 1001) + 20;
     const content = Array.from({ length: numLines }, (_, i) => {