From 2ba92c168147f133e0249231c30f31e4704afed1 Mon Sep 17 00:00:00 2001
From: breken-ai <312387581+breken-ai@users.noreply.github.com>
Date: Fri, 25 Sep 2026 03:37:38 -0700
Subject: [PATCH] fix(compiler): leave tail -n +N to the VFS shell instead of
 printing the last N lines

tail -n +N prints from line N onward. The compiled fast path read +N as N and
returned the last N lines, so an agent paging a memory file with tail -n +3
got the wrong part of it. Decline the +N form so the VFS shell, whose tail
implements it, answers instead.
---
 src/hooks/bash-command-compiler.ts             |  3 +++
 .../claude-code/bash-command-compiler.test.ts  | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/src/hooks/bash-command-compiler.ts b/src/hooks/bash-command-compiler.ts
index 68e153423..12c2f4f75 100644
--- a/src/hooks/bash-command-compiler.ts
+++ b/src/hooks/bash-command-compiler.ts
@@ -177,6 +177,9 @@ function parseHeadTailStage(stage: string): { lineLimit: number; fromEnd: boolea
     return { lineLimit: Math.abs(count), fromEnd: cmd === "tail" };
   }
   if (rest.length === 3 && rest[0] === "-n") {
+    // `tail -n +N` prints from line N onward, not the last N lines. Leave it to
+    // the VFS shell, whose tail implements the +N form.
+    if (cmd === "tail" && rest[1].startsWith("+")) return null;
     const count = Number(rest[1]);
     if (!Number.isFinite(count)) return null;
     return { lineLimit: Math.abs(count), fromEnd: cmd === "tail" };
diff --git a/tests/claude-code/bash-command-compiler.test.ts b/tests/claude-code/bash-command-compiler.test.ts
index 3bb90a733..0337249df 100644
--- a/tests/claude-code/bash-command-compiler.test.ts
+++ b/tests/claude-code/bash-command-compiler.test.ts
@@ -454,4 +454,22 @@ describe("bash-command-compiler execution", () => {
     );
     expect(output).toBe("/summaries/a.md");
   });
+
+  it("does not compile `tail -n +N`, which starts at line N rather than keeping the last N lines", async () => {
+    expect(parseCompiledSegment("tail -n +3 /a.md")).toBeNull();
+
+    const readVirtualPathContentsFn = vi.fn(async () => new Map([["/a.md", "l1\nl2\nl3\nl4\nl5\nl6\nl7\nl8"]]));
+    const output = await executeCompiledBashCommand(
+      { query: vi.fn() } as any,
+      "memory",
+      "sessions",
+      "tail -n +3 /a.md",
+      { readVirtualPathContentsFn: readVirtualPathContentsFn as any },
+    );
+    // GNU tail prints l3..l8 here; the compiled fast path used to answer with the
+    // last three lines (l6..l8). Returning null hands the command to the VFS shell,
+    // whose tail implements the +N form.
+    expect(output).toBeNull();
+    expect(readVirtualPathContentsFn).not.toHaveBeenCalled();
+  });
 });