diff --git a/src/platforms/curseforge/curseforge-error.ts b/src/platforms/curseforge/curseforge-error.ts
new file mode 100644
index 0000000..ce7b08f
--- /dev/null
+++ b/src/platforms/curseforge/curseforge-error.ts
@@ -0,0 +1,78 @@
+/**
+ * Error object returned by CurseForge Upload API.
+ */
+export interface CurseForgeError {
+    /**
+     * Error code returned by CurseForge Upload API.
+     */
+    errorCode: number;
+
+    /**
+     * Error message returned by CurseForge Upload API.
+     */
+    errorMessage: string;
+}
+
+/**
+ * Checks if an object is a {@link CurseForgeError}.
+ *
+ * @param error - The object to check.
+ *
+ * @returns `true` if the object matches the structure of a CurseForgeError; otherwise, `false`.
+ */
+export function isCurseForgeError(error: unknown): error is CurseForgeError {
+    const e = error as CurseForgeError;
+    return (
+        e &&
+        typeof e.errorCode === "number" &&
+        typeof e.errorMessage === "string"
+    );
+}
+
+/**
+ * Error code for an invalid project slug.
+ */
+const INVALID_PROJECT_SLUG_ERROR_CODE = 1018;
+
+/**
+ * Regular expression to match invalid project slug errors.
+ */
+const INVALID_PROJECT_SLUG_REGEX = /Invalid slug in project relations: '([^']*)'/;
+
+/**
+ * Checks if an error is an invalid project slug error.
+ *
+ * @param error - The error to check.
+ *
+ * @returns `true` if the error is an invalid project slug error; otherwise, `false`.
+ */
+export function isInvalidProjectSlugCurseForgeError(error: unknown): error is CurseForgeError {
+    return isCurseForgeError(error) && error.errorCode === INVALID_PROJECT_SLUG_ERROR_CODE;
+}
+
+/**
+ * Extracts the invalid project slug from an error.
+ *
+ * @param error - The error to extract the invalid project slug from.
+ *
+ * @returns The invalid project slug, or `undefined` if the error is not an invalid project slug error.
+ */
+export function getInvalidProjectSlug(error: unknown): string | undefined {
+    return isInvalidProjectSlugCurseForgeError(error) ? error.errorMessage.match(INVALID_PROJECT_SLUG_REGEX)?.[1] : undefined;
+}
+
+/**
+ * Error code for an invalid game version ID.
+ */
+const INVALID_GAME_VERSION_ID_ERROR_CODE = 1009;
+
+/**
+ * Checks if an error is an invalid game version ID error.
+ *
+ * @param error - The error to check.
+ *
+ * @returns `true` if the error is an invalid game version ID error; otherwise, `false`.
+ */
+export function isInvalidGameVersionIdCurseForgeError(error: unknown): error is CurseForgeError {
+    return isCurseForgeError(error) && error.errorCode === INVALID_GAME_VERSION_ID_ERROR_CODE;
+}