diff --git a/src/infra/path-safety.test.ts b/src/infra/path-safety.test.ts
index b05eeced172..9c85fbac63e 100644
--- a/src/infra/path-safety.test.ts
+++ b/src/infra/path-safety.test.ts
@@ -3,14 +3,20 @@ import { describe, expect, it } from "vitest";
 import { isWithinDir, resolveSafeBaseDir } from "./path-safety.js";
 
 describe("path-safety", () => {
-  it("resolves safe base dir with trailing separator", () => {
-    const base = resolveSafeBaseDir("/tmp/demo");
-    expect(base.endsWith(path.sep)).toBe(true);
+  it.each([
+    { rootDir: "/tmp/demo", expected: `${path.resolve("/tmp/demo")}${path.sep}` },
+    { rootDir: `/tmp/demo${path.sep}`, expected: `${path.resolve("/tmp/demo")}${path.sep}` },
+  ])("resolves safe base dir for %j", ({ rootDir, expected }) => {
+    expect(resolveSafeBaseDir(rootDir)).toBe(expected);
   });
 
-  it("checks directory containment", () => {
-    expect(isWithinDir("/tmp/demo", "/tmp/demo")).toBe(true);
-    expect(isWithinDir("/tmp/demo", "/tmp/demo/sub/file.txt")).toBe(true);
-    expect(isWithinDir("/tmp/demo", "/tmp/demo/../escape.txt")).toBe(false);
+  it.each([
+    { rootDir: "/tmp/demo", targetPath: "/tmp/demo", expected: true },
+    { rootDir: "/tmp/demo", targetPath: "/tmp/demo/sub/file.txt", expected: true },
+    { rootDir: "/tmp/demo", targetPath: "/tmp/demo/../escape.txt", expected: false },
+    { rootDir: "/tmp/demo", targetPath: "/tmp/demo-sibling/file.txt", expected: false },
+    { rootDir: "/tmp/demo", targetPath: "sub/file.txt", expected: false },
+  ])("checks containment for %j", ({ rootDir, targetPath, expected }) => {
+    expect(isWithinDir(rootDir, targetPath)).toBe(expected);
   });
 });
diff --git a/src/infra/plain-object.test.ts b/src/infra/plain-object.test.ts
index b87e555b21a..892e5c89fab 100644
--- a/src/infra/plain-object.test.ts
+++ b/src/infra/plain-object.test.ts
@@ -2,17 +2,17 @@ import { describe, expect, it } from "vitest";
 import { isPlainObject } from "./plain-object.js";
 
 describe("isPlainObject", () => {
-  it("accepts plain objects", () => {
-    expect(isPlainObject({})).toBe(true);
-    expect(isPlainObject({ a: 1 })).toBe(true);
-  });
+  it.each([{}, { a: 1 }, Object.create(null), new (class X {})()])(
+    "accepts object-tag values: %j",
+    (value) => {
+      expect(isPlainObject(value)).toBe(true);
+    },
+  );
 
-  it("rejects non-plain values", () => {
-    expect(isPlainObject(null)).toBe(false);
-    expect(isPlainObject([])).toBe(false);
-    expect(isPlainObject(new Date())).toBe(false);
-    expect(isPlainObject(/re/)).toBe(false);
-    expect(isPlainObject("x")).toBe(false);
-    expect(isPlainObject(42)).toBe(false);
-  });
+  it.each([null, [], new Date(), /re/, "x", 42, () => null, new Map()])(
+    "rejects non-plain values: %j",
+    (value) => {
+      expect(isPlainObject(value)).toBe(false);
+    },
+  );
 });
diff --git a/src/infra/system-message.test.ts b/src/infra/system-message.test.ts
index b0c32f31c35..5cb1d4be87f 100644
--- a/src/infra/system-message.test.ts
+++ b/src/infra/system-message.test.ts
@@ -2,8 +2,21 @@ import { describe, expect, it } from "vitest";
 import { SYSTEM_MARK, hasSystemMark, prefixSystemMessage } from "./system-message.js";
 
 describe("system-message", () => {
-  it("prepends the system mark once", () => {
-    expect(prefixSystemMessage("thread notice")).toBe(`${SYSTEM_MARK} thread notice`);
+  it.each([
+    { input: "thread notice", expected: `${SYSTEM_MARK} thread notice` },
+    { input: `  thread notice  `, expected: `${SYSTEM_MARK} thread notice` },
+    { input: "   ", expected: "" },
+  ])("prefixes %j", ({ input, expected }) => {
+    expect(prefixSystemMessage(input)).toBe(expected);
+  });
+
+  it.each([
+    { input: `${SYSTEM_MARK} already prefixed`, expected: true },
+    { input: `  ${SYSTEM_MARK} hello`, expected: true },
+    { input: "", expected: false },
+    { input: "hello", expected: false },
+  ])("detects marks for %j", ({ input, expected }) => {
+    expect(hasSystemMark(input)).toBe(expected);
   });
 
   it("does not double-prefix messages that already have the mark", () => {
@@ -11,9 +24,4 @@ describe("system-message", () => {
       `${SYSTEM_MARK} already prefixed`,
     );
   });
-
-  it("detects marked system text after trim normalization", () => {
-    expect(hasSystemMark(`  ${SYSTEM_MARK} hello`)).toBe(true);
-    expect(hasSystemMark("hello")).toBe(false);
-  });
 });