diff --git a/src/infra/cli-root-options.test.ts b/src/infra/cli-root-options.test.ts
index e6907984ec0..6d7461a39e5 100644
--- a/src/infra/cli-root-options.test.ts
+++ b/src/infra/cli-root-options.test.ts
@@ -6,8 +6,11 @@ describe("isValueToken", () => {
     { value: "work", expected: true },
     { value: "-1", expected: true },
     { value: "-1.5", expected: true },
+    { value: "-0.5", expected: true },
     { value: "--", expected: false },
     { value: "--dev", expected: false },
+    { value: "-", expected: false },
+    { value: "", expected: false },
     { value: undefined, expected: false },
   ])("classifies %j", ({ value, expected }) => {
     expect(isValueToken(value)).toBe(expected);
@@ -24,6 +27,8 @@ describe("consumeRootOptionToken", () => {
     { args: ["--log-level", "-1.5"], index: 0, expected: 2 },
     { args: ["--profile", "--no-color"], index: 0, expected: 1 },
     { args: ["--profile", "--"], index: 0, expected: 1 },
+    { args: ["x", "--profile", "work"], index: 1, expected: 2 },
+    { args: ["--log-level", ""], index: 0, expected: 1 },
     { args: ["--unknown"], index: 0, expected: 0 },
     { args: [], index: 0, expected: 0 },
   ])("consumes %j at %d", ({ args, index, expected }) => {
diff --git a/src/infra/json-utf8-bytes.test.ts b/src/infra/json-utf8-bytes.test.ts
index e2f8e217be0..5009301ffd6 100644
--- a/src/infra/json-utf8-bytes.test.ts
+++ b/src/infra/json-utf8-bytes.test.ts
@@ -18,6 +18,11 @@ describe("jsonUtf8Bytes", () => {
       value: undefined,
       expected: Buffer.byteLength("undefined", "utf8"),
     },
+    {
+      name: "unicode strings",
+      value: "🙂",
+      expected: Buffer.byteLength(JSON.stringify("🙂"), "utf8"),
+    },
   ])("returns utf8 byte length for $name", ({ value, expected }) => {
     expect(jsonUtf8Bytes(value)).toBe(expected);
   });
@@ -31,4 +36,8 @@ describe("jsonUtf8Bytes", () => {
   it("uses string conversion for BigInt serialization failures", () => {
     expect(jsonUtf8Bytes(12n)).toBe(Buffer.byteLength("12", "utf8"));
   });
+
+  it("uses string conversion for symbol serialization failures", () => {
+    expect(jsonUtf8Bytes(Symbol("token"))).toBe(Buffer.byteLength("Symbol(token)", "utf8"));
+  });
 });
diff --git a/src/infra/plain-object.test.ts b/src/infra/plain-object.test.ts
index 892e5c89fab..272c7c94f9d 100644
--- a/src/infra/plain-object.test.ts
+++ b/src/infra/plain-object.test.ts
@@ -9,10 +9,21 @@ describe("isPlainObject", () => {
     },
   );
 
-  it.each([null, [], new Date(), /re/, "x", 42, () => null, new Map()])(
-    "rejects non-plain values: %j",
-    (value) => {
-      expect(isPlainObject(value)).toBe(false);
-    },
-  );
+  it.each([
+    null,
+    [],
+    new Date(),
+    /re/,
+    "x",
+    42,
+    () => null,
+    new Map(),
+    { [Symbol.toStringTag]: "Array" },
+  ])("rejects non-plain values: %j", (value) => {
+    expect(isPlainObject(value)).toBe(false);
+  });
+
+  it("accepts object-tag values with an explicit Object toStringTag", () => {
+    expect(isPlainObject({ [Symbol.toStringTag]: "Object" })).toBe(true);
+  });
 });