diff --git a/src/selfhost/config-lint.ts b/src/selfhost/config-lint.ts
index 2ff0af08ef..c063745f8a 100644
--- a/src/selfhost/config-lint.ts
+++ b/src/selfhost/config-lint.ts
@@ -73,8 +73,13 @@ function unknownTopLevelWarnings(text: string | null | undefined): string[] {
   const parsed = parseTopLevelObject(trimmed);
   if (parsed === null) return [];
   const keys = Object.keys(parsed).filter((key) => !TOP_LEVEL_FIELD_SET.has(key));
-  const retiredWarnings = keys.filter((key) => key in RETIRED_FIELD_MIGRATION_WARNINGS).map((key) => RETIRED_FIELD_MIGRATION_WARNINGS[key]!);
-  const unknown = keys.filter((key) => !(key in RETIRED_FIELD_MIGRATION_WARNINGS)).map(formatFieldName);
+  // `hasOwnProperty.call`, NOT `key in`: a manifest field named like an Object.prototype member
+  // (`constructor`, `toString`, `hasOwnProperty`, ...) would otherwise test true for the inherited
+  // property and resolve to the prototype's function instead of a real retired-field warning string,
+  // corrupting the string[] result and suppressing the genuine unknown-field warning.
+  const isRetired = (key: string): boolean => Object.prototype.hasOwnProperty.call(RETIRED_FIELD_MIGRATION_WARNINGS, key);
+  const retiredWarnings = keys.filter(isRetired).map((key) => RETIRED_FIELD_MIGRATION_WARNINGS[key]!);
+  const unknown = keys.filter((key) => !isRetired(key)).map(formatFieldName);
   return [
     ...retiredWarnings,
     ...(unknown.length > 0 ? [`Manifest contains unknown top-level field${unknown.length === 1 ? "" : "s"}: ${unknown.join(", ")}.`] : []),
diff --git a/test/unit/selfhost-config-lint.test.ts b/test/unit/selfhost-config-lint.test.ts
index 87c1d4d4cb..9ddb8297b8 100644
--- a/test/unit/selfhost-config-lint.test.ts
+++ b/test/unit/selfhost-config-lint.test.ts
@@ -159,6 +159,16 @@ unknownSecretKey: super-secret-value
     expect(JSON.stringify(result)).not.toContain("/tmp/private");
   });
 
+  it("treats a field named like an Object.prototype member as unknown, not retired", () => {
+    const result = lintManifestText("wantedPaths: [src/]\nconstructor: whatever\n");
+
+    expect(result.ok).toBe(false);
+    expect(result.recognizedFields).toEqual(["wantedPaths"]);
+    expect(result.warnings).toEqual(["Manifest contains unknown top-level field: constructor."]);
+    // Every warning must be a real string — a prototype-name collision must never leak a function.
+    expect(result.warnings.every((warning) => typeof warning === "string")).toBe(true);
+  });
+
   it("uses singular wording for one unknown top-level field", () => {
     const result = lintManifestText("wantedPaths: [src/]\nunknownSecretKey: super-secret-value\n");