fix(filesystem): head returns CRLF lines normalized to LF, like tail

.changeset/5187-filesystem-head-crlf.md

@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/server-filesystem": patch
+---
+
+`read_text_file` with `head` no longer leaves a `\r` at the end of each line of a CRLF file. Like `tail`, it returns lines normalized to LF, including a `\r\n` pair split across a read chunk. LF output is unchanged.

src/filesystem/__tests__/server-tools.test.ts

@@ -365,6 +365,35 @@ describe("read_text_file and read_file", () => {
     expect(textOf(result)).toBe("three\nfour");
   });
 
+  describe("head line endings (#5187)", () => {
+    async function head(content: string, n: number): Promise<string> {
+      const target = path.join(dir, "head.txt");
+      await fs.writeFile(target, content);
+      return textOf(
+        await call(client, "read_text_file", { path: target, head: n }),
+      );
+    }
+
+    it("returns the first lines of an LF file unchanged", async () => {
+      expect(await head("a\nb\nc\n", 2)).toBe("a\nb");
+    });
+
+    it("returns the first lines of a CRLF file without their \\r", async () => {
+      expect(await head("a\r\nb\r\nc\r\n", 2)).toBe("a\nb");
+    });
+
+    it("returns a CRLF file with no final newline normalized to LF", async () => {
+      expect(await head("a\r\nb\r\nc", 5)).toBe("a\nb\nc");
+    });
+
+    // headFile reads 1 KB chunks from the start. Here the first chunk ends
+    // on the "\r" of a "\r\n" pair whose "\n" opens the second chunk.
+    it("joins a CRLF pair split across a chunk boundary", async () => {
+      const content = "C".repeat(1023) + "\r\nnext\r\n";
+      expect(await head(content, 2)).toBe("C".repeat(1023) + "\nnext");
+    });
+  });
+
   describe("tail at the end of the file (#5187)", () => {
     async function tail(content: string, n: number): Promise<string> {
       const target = path.join(dir, "tail.txt");

src/filesystem/lib.ts

@@ -765,7 +765,11 @@ export async function headFile(
         const completeLines = buffer.slice(0, newLineIndex).split("\n");
         buffer = buffer.slice(newLineIndex + 1);
         for (const line of completeLines) {
-          lines.push(line);
+          // Each of these lines ended in "\n"; drop the "\r" of a CRLF ending
+          // so head normalizes line endings the way tailFile does. A "\r"
+          // whose "\n" is still in the next chunk stays in the buffer until
+          // that "\n" arrives, so a pair split across chunks is handled too.
+          lines.push(line.endsWith("\r") ? line.slice(0, -1) : line);
           if (lines.length >= numLines) break;
         }
       }