diff --git a/src/shared/ignore-rules.test.ts b/src/shared/ignore-rules.test.ts
new file mode 100644
index 000000000000..fe70e6938210
--- /dev/null
+++ b/src/shared/ignore-rules.test.ts
@@ -0,0 +1,97 @@
+import { mkdirSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import ignore from "ignore";
+import { describe, expect, it } from "vitest";
+import { addIgnoreRules } from "./ignore-rules.js";
+
+function writeIgnoreTree(root: string, rules: string, dir = root) {
+  mkdirSync(dir, { recursive: true });
+  writeFileSync(join(dir, ".gitignore"), rules);
+}
+
+function buildMatcher(root: string, dir: string) {
+  const ig = ignore();
+  addIgnoreRules(ig, join(root, dir), root);
+  return ig;
+}
+
+describe("addIgnoreRules", () => {
+  it("ignores nested slash-free patterns at any depth", () => {
+    const root = join(tmpdir(), "ignore-rules-test-1");
+    writeIgnoreTree(root, "*.log\n", join(root, "sub"));
+    writeFileSync(join(root, "sub", "y.log"), "");
+    mkdirSync(join(root, "sub", "deep"), { recursive: true });
+    writeFileSync(join(root, "sub", "deep", "x.log"), "");
+
+    const ig = buildMatcher(root, "sub");
+
+    expect(ig.ignores("sub/y.log")).toBe(true);
+    expect(ig.ignores("sub/deep/x.log")).toBe(true);
+  });
+
+  it("keeps anchored patterns relative to the ignore file directory", () => {
+    const root = join(tmpdir(), "ignore-rules-test-2");
+    writeIgnoreTree(root, "dir/*.log\n", join(root, "sub"));
+    mkdirSync(join(root, "sub", "dir"), { recursive: true });
+    writeFileSync(join(root, "sub", "dir", "x.log"), "");
+    mkdirSync(join(root, "sub", "other"), { recursive: true });
+    writeFileSync(join(root, "sub", "other", "x.log"), "");
+
+    const ig = buildMatcher(root, "sub");
+
+    expect(ig.ignores("sub/dir/x.log")).toBe(true);
+    expect(ig.ignores("sub/other/x.log")).toBe(false);
+  });
+
+  it("keeps leading-slash patterns relative to the ignore file directory", () => {
+    const root = join(tmpdir(), "ignore-rules-test-3");
+    writeIgnoreTree(root, "/dir/*.log\n", join(root, "sub"));
+    mkdirSync(join(root, "sub", "dir"), { recursive: true });
+    writeFileSync(join(root, "sub", "dir", "x.log"), "");
+
+    const ig = buildMatcher(root, "sub");
+
+    expect(ig.ignores("sub/dir/x.log")).toBe(true);
+  });
+
+  it("treats trailing-slash directory patterns as unanchored", () => {
+    const root = join(tmpdir(), "ignore-rules-test-4");
+    writeIgnoreTree(root, "node_modules/\n", join(root, "sub"));
+    mkdirSync(join(root, "sub", "node_modules"), { recursive: true });
+    mkdirSync(join(root, "sub", "deep", "node_modules"), { recursive: true });
+
+    const ig = buildMatcher(root, "sub");
+
+    expect(ig.ignores("sub/node_modules/file.js")).toBe(true);
+    expect(ig.ignores("sub/deep/node_modules/file.js")).toBe(true);
+  });
+
+  it("does not corrupt escaped ! patterns at the root", () => {
+    const root = join(tmpdir(), "ignore-rules-test-5");
+    writeIgnoreTree(root, "*.txt\n\\!keep.txt\n");
+    writeFileSync(join(root, "keep.txt"), "");
+    writeFileSync(join(root, "!keep.txt"), "");
+    writeFileSync(join(root, "drop.txt"), "");
+
+    const ig = buildMatcher(root, "");
+
+    expect(ig.ignores("keep.txt")).toBe(true);
+    expect(ig.ignores("!keep.txt")).toBe(true);
+    expect(ig.ignores("drop.txt")).toBe(true);
+  });
+
+  it("does not corrupt escaped ! patterns in nested directories", () => {
+    const root = join(tmpdir(), "ignore-rules-test-6");
+    writeIgnoreTree(root, "*.txt\n\\!keep.txt\n", join(root, "sub"));
+    writeFileSync(join(root, "sub", "keep.txt"), "");
+    writeFileSync(join(root, "sub", "!keep.txt"), "");
+    writeFileSync(join(root, "sub", "drop.txt"), "");
+
+    const ig = buildMatcher(root, "sub");
+
+    expect(ig.ignores("sub/keep.txt")).toBe(true);
+    expect(ig.ignores("sub/!keep.txt")).toBe(true);
+    expect(ig.ignores("sub/drop.txt")).toBe(true);
+  });
+});
diff --git a/src/shared/ignore-rules.ts b/src/shared/ignore-rules.ts
index 5f4e3fba0da5..e262c4ab030d 100644
--- a/src/shared/ignore-rules.ts
+++ b/src/shared/ignore-rules.ts
@@ -48,14 +48,27 @@ function prefixIgnorePattern(line: string, prefix: string): string | null {
   if (pattern.startsWith("!")) {
     negated = true;
     pattern = pattern.slice(1);
-  } else if (pattern.startsWith("\\!")) {
-    pattern = pattern.slice(1);
   }
+  // Keep escaped "!" intact; the ignore library handles "\!" as a literal filename.
 
+  let anchored = false;
   if (pattern.startsWith("/")) {
+    anchored = true;
     pattern = pattern.slice(1);
   }
 
-  const prefixed = prefix ? `${prefix}${pattern}` : pattern;
+  const slashIndex = pattern.indexOf("/");
+  if (slashIndex !== -1 && slashIndex !== pattern.length - 1) {
+    anchored = true;
+  }
+
+  // A pattern without a middle/beginning slash matches at any depth below the
+  // ignore file's directory; prefix with **/ so the ignore library agrees.
+  const needsDepthGlob = prefix && !anchored;
+  const prefixed = prefix
+    ? needsDepthGlob
+      ? `${prefix}**/${pattern}`
+      : `${prefix}${pattern}`
+    : pattern;
   return negated ? `!${prefixed}` : prefixed;
 }