diff --git a/packages/utils/src/GamedataLoader.ts b/packages/utils/src/GamedataLoader.ts
index cf4dadc..cf2d794 100644
--- a/packages/utils/src/GamedataLoader.ts
+++ b/packages/utils/src/GamedataLoader.ts
@@ -1,4 +1,5 @@
-import { fetchConfigJson } from './JsonParser';
+import { ConfigJsonError, fetchConfigJson, isMissingResource } from './JsonParser';
+import { NitroLogger } from './NitroLogger';
 
 export const DEFAULT_TIERS = [ 'core', 'custom', 'seasonal' ] as const;
 export type GamedataTier = typeof DEFAULT_TIERS[number] | string;
@@ -28,51 +29,69 @@ const joinUrl = (base: string, path: string): string =>
     return `${ cleanBase }${ cleanPath }`;
 };
 
-const tryFetchOrNull = async <T = any>(url: string): Promise<T | null> =>
+// Returns the parsed payload when the manifest exists, null on a clean 404.
+// Re-throws on any other error (network failure, 5xx, parse error) so callers
+// don't silently skip a tier because of a typo in manifest.json5.
+const tryFetchManifest = async <T = any>(url: string): Promise<T | null> =>
 {
     try
     {
         return await fetchConfigJson<T>(url);
     }
-    catch
+    catch(err)
     {
-        return null;
+        if(isMissingResource(err)) return null;
+        throw err;
     }
 };
 
+// Try .json5 first, then .json — both treated as optional. Anything other
+// than 404 on either bubbles up.
+const tryFetchManifestPair = async <T = any>(baseUrl: string, name: string): Promise<T | null> =>
+{
+    const json5 = await tryFetchManifest<T>(joinUrl(baseUrl, `${ name }.json5`));
+    if(json5 !== null) return json5;
+
+    return await tryFetchManifest<T>(joinUrl(baseUrl, `${ name }.json`));
+};
+
 const isPlainObject = (value: any): value is Record<string, any> => !!value && typeof value === 'object' && !Array.isArray(value);
 
-const arrayItemsLookKeyed = (arr: any[], idKeys: readonly string[]): string | null =>
+const arrayItemsLookKeyed = (arr: any[], idKeys: readonly string[], sourceLabel?: string): string | null =>
 {
     if(!arr.length) return null;
 
     for(const key of idKeys)
     {
-        let allHave = true;
+        let have = 0;
 
         for(const item of arr)
         {
-            if(!isPlainObject(item) || item[key] === undefined || item[key] === null)
-            {
-                allHave = false;
-                break;
-            }
+            if(isPlainObject(item) && item[key] !== undefined && item[key] !== null) have++;
         }
 
-        if(allHave) return key;
+        if(have === arr.length) return key;
+
+        // Heuristic: if most items are keyed but a few are not, the data is
+        // probably keyed and the outliers are bugs in the source data.
+        // Surface this so operators don't get silent duplicates after merge.
+        if(have > 0 && have / arr.length >= 0.8)
+        {
+            NitroLogger.warn(`mergeGamedata: ${ sourceLabel ? `${ sourceLabel }: ` : '' }array looks keyed by "${ key }" (${ have }/${ arr.length } items) but some entries are missing it — falling back to concat which may produce duplicates`);
+        }
     }
 
     return null;
 };
 
-export const mergeGamedata = (a: any, b: any, idKeys: readonly string[] = DEFAULT_ID_KEYS): any =>
+export const mergeGamedata = (a: any, b: any, idKeys: readonly string[] = DEFAULT_ID_KEYS, sourceLabel?: string): any =>
 {
     if(b === undefined) return a;
     if(a === undefined) return b;
 
     if(Array.isArray(a) && Array.isArray(b))
     {
-        const idKey = arrayItemsLookKeyed(a, idKeys) || arrayItemsLookKeyed(b, idKeys);
+        const idKey = arrayItemsLookKeyed(a, idKeys, sourceLabel) || arrayItemsLookKeyed(b, idKeys, sourceLabel);
 
         if(!idKey) return a.concat(b);
 
@@ -92,7 +111,7 @@ export const mergeGamedata = (a: any, b: any, idKeys: readonly string[] = DEFAUL
 
             if(at !== undefined)
             {
-                out[at] = mergeGamedata(out[at], item, idKeys);
+                out[at] = mergeGamedata(out[at], item, idKeys, sourceLabel);
             }
             else
             {
@@ -110,7 +129,7 @@ export const mergeGamedata = (a: any, b: any, idKeys: readonly string[] = DEFAUL
 
         for(const k of Object.keys(b))
         {
-            out[k] = mergeGamedata(a[k], b[k], idKeys);
+            out[k] = mergeGamedata(a[k], b[k], idKeys, sourceLabel);
         }
 
         return out;
@@ -130,6 +149,11 @@ interface RootManifest
     files?: string[];
 }
 
+// Load every file in `files` concurrently, return them in the original
+// declared order so the merge step preserves override semantics.
+const fetchFilesInOrder = async (baseUrl: string, files: readonly string[]): Promise<any[]> =>
+    Promise.all(files.map(file => fetchConfigJson(joinUrl(baseUrl, file))));
+
 export const loadGamedata = async <T = any>(url: string, options: GamedataLoadOptions = {}): Promise<T> =>
 {
     if(!url) throw new Error('loadGamedata: empty URL');
@@ -140,42 +164,47 @@ export const loadGamedata = async <T = any>(url: string, options: GamedataLoadOp
     }
 
     const idKeys = options.mergeArrayIdKeys ?? DEFAULT_ID_KEYS;
-    const rootManifest = await tryFetchOrNull<RootManifest>(joinUrl(url, 'manifest.json5'))
-        ?? await tryFetchOrNull<RootManifest>(joinUrl(url, 'manifest.json'));
+    const rootManifest = await tryFetchManifestPair<RootManifest>(url, 'manifest');
 
     const tiers = (rootManifest?.tiers && rootManifest.tiers.length)
         ? rootManifest.tiers
         : (options.tiers ?? DEFAULT_TIERS);
 
+    // Fetch root-level files in parallel with discovering each tier's
+    // manifest. Per-tier file batches stay sequenced relative to each other
+    // so override order (core → custom → seasonal) is preserved during
+    // merge, but fetches inside a tier batch run concurrently.
+    const [ rootParts, tierManifests ] = await Promise.all([
+        rootManifest?.files?.length ? fetchFilesInOrder(url, rootManifest.files) : Promise.resolve([] as any[]),
+        Promise.all(tiers.map(async tier =>
+        {
+            const tierUrl = joinUrl(url, `${ tier }/`);
+            const manifest = await tryFetchManifestPair<TierManifest>(tierUrl, 'manifest');
+
+            return { tier, tierUrl, manifest };
+        }))
+    ]);
+
     let merged: any = undefined;
 
-    if(rootManifest?.files?.length)
+    for(const part of rootParts)
     {
-        for(const file of rootManifest.files)
+        merged = (merged === undefined) ? part : mergeGamedata(merged, part, idKeys, url);
+    }
+
+    for(const { tier, tierUrl, manifest } of tierManifests)
+    {
+        if(!manifest?.files?.length) continue;
+
+        const parts = await fetchFilesInOrder(tierUrl, manifest.files);
+
+        for(const part of parts)
         {
-            const fileUrl = joinUrl(url, file);
-            const part = await fetchConfigJson(fileUrl);
-            merged = (merged === undefined) ? part : mergeGamedata(merged, part, idKeys);
+            merged = (merged === undefined) ? part : mergeGamedata(merged, part, idKeys, `${ url } (${ tier })`);
         }
     }
 
-    for(const tier of tiers)
-    {
-        const tierUrl = joinUrl(url, `${ tier }/`);
-        const tierManifest = await tryFetchOrNull<TierManifest>(joinUrl(tierUrl, 'manifest.json5'))
-            ?? await tryFetchOrNull<TierManifest>(joinUrl(tierUrl, 'manifest.json'));
-
-        if(!tierManifest?.files?.length) continue;
-
-        for(const file of tierManifest.files)
-        {
-            const fileUrl = joinUrl(tierUrl, file);
-            const part = await fetchConfigJson(fileUrl);
-            merged = (merged === undefined) ? part : mergeGamedata(merged, part, idKeys);
-        }
-    }
-
-    if(merged === undefined) throw new Error(`loadGamedata: directory mode at "${ url }" produced no data — make sure at least one tier (core/custom/seasonal) has a manifest.json5 with a 'files' array`);
+    if(merged === undefined) throw new ConfigJsonError(`loadGamedata: directory mode at "${ url }" produced no data — make sure at least one tier (core/custom/seasonal) has a manifest.json5 with a 'files' array`, 'fetch', url);
 
     return merged as T;
 };
diff --git a/packages/utils/src/JsonParser.ts b/packages/utils/src/JsonParser.ts
index c812d63..46f3631 100644
--- a/packages/utils/src/JsonParser.ts
+++ b/packages/utils/src/JsonParser.ts
@@ -5,6 +5,28 @@ declare const __NITRO_JSON_MODE__: 'legacy' | 'json5' | 'auto' | undefined;
 const JSON5_EXTENSION = /\.json5(?:[?#]|$)/i;
 const JSON5_MIME = /(?:application|text)\/(?:json5|x-json5)/i;
 
+export type ConfigJsonErrorPhase = 'fetch' | 'parse';
+
+export class ConfigJsonError extends Error
+{
+    public readonly phase: ConfigJsonErrorPhase;
+    public readonly sourceUrl: string;
+    public readonly httpStatus?: number;
+
+    constructor(message: string, phase: ConfigJsonErrorPhase, sourceUrl: string, httpStatus?: number, cause?: unknown)
+    {
+        super(message);
+        this.name = 'ConfigJsonError';
+        this.phase = phase;
+        this.sourceUrl = sourceUrl;
+        this.httpStatus = httpStatus;
+        if(cause !== undefined) (this as any).cause = cause;
+    }
+}
+
+export const isMissingResource = (err: unknown): boolean =>
+    err instanceof ConfigJsonError && err.phase === 'fetch' && err.httpStatus === 404;
+
 const resolveJsonMode = (): 'legacy' | 'json5' | 'auto' =>
 {
     try
@@ -44,9 +66,7 @@ const formatStrictError = (sourceUrl: string, err: unknown): string =>
 
 export const parseConfigJson = <T = any>(text: string, sourceUrl: string = ''): T =>
 {
-    if(text === null || text === undefined) throw new Error(`Empty response${ sourceUrl ? ` for "${ sourceUrl }"` : '' }`);
-
-    const trimmed = text.length > 0 ? text : '';
+    const trimmed = text ?? '';
     const mode = resolveJsonMode();
 
     if(mode === 'legacy')
@@ -57,7 +77,7 @@ export const parseConfigJson = <T = any>(text: string, sourceUrl: string = ''):
         }
         catch(err)
         {
-            throw new Error(formatStrictError(sourceUrl, err));
+            throw new ConfigJsonError(formatStrictError(sourceUrl, err), 'parse', sourceUrl, undefined, err);
         }
     }
 
@@ -69,7 +89,7 @@ export const parseConfigJson = <T = any>(text: string, sourceUrl: string = ''):
         }
         catch(err)
         {
-            throw new Error(formatParseError(sourceUrl, err, err));
+            throw new ConfigJsonError(formatParseError(sourceUrl, err, err), 'parse', sourceUrl, undefined, err);
         }
     }
 
@@ -90,7 +110,7 @@ export const parseConfigJson = <T = any>(text: string, sourceUrl: string = ''):
     }
     catch(json5Error)
     {
-        throw new Error(formatParseError(sourceUrl, strictError, json5Error));
+        throw new ConfigJsonError(formatParseError(sourceUrl, strictError, json5Error), 'parse', sourceUrl, undefined, json5Error);
     }
 };
 
@@ -109,7 +129,7 @@ export const parseConfigJsonFromResponse = async <T = any>(response: Response, s
         }
         catch(err)
         {
-            throw new Error(formatParseError(url, err, err));
+            throw new ConfigJsonError(formatParseError(url, err, err), 'parse', url, undefined, err);
         }
     }
 
@@ -118,9 +138,23 @@ export const parseConfigJsonFromResponse = async <T = any>(response: Response, s
 
 export const fetchConfigJson = async <T = any>(url: string, init?: RequestInit): Promise<T> =>
 {
-    const response = await fetch(url, init);
+    let response: Response | undefined;
 
-    if(!response || response.status !== 200) throw new Error(`Failed to fetch "${ url }" — server returned HTTP ${ response?.status ?? 'no response' }`);
+    try
+    {
+        response = await fetch(url, init);
+    }
+    catch(networkErr)
+    {
+        const message = (networkErr as Error)?.message || String(networkErr);
+        throw new ConfigJsonError(`Network error fetching "${ url }" — ${ message }`, 'fetch', url, undefined, networkErr);
+    }
+
+    if(!response || response.status !== 200)
+    {
+        const status = response?.status;
+        throw new ConfigJsonError(`Failed to fetch "${ url }" — server returned HTTP ${ status ?? 'no response' }`, 'fetch', url, status);
+    }
 
     return parseConfigJsonFromResponse<T>(response, url);
 };
diff --git a/yarn.lock b/yarn.lock
index 288c306..aaeae8d 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -369,7 +369,7 @@
   resolved "https://registry.yarnpkg.com/@standard-schema/spec/-/spec-1.1.0.tgz#a79b55dbaf8604812f52d140b2c9ab41bc150bb8"
   integrity sha512-l2aFy5jALhniG5HgqrD6jXLi/rUWrKvqN/qJx6yoJsgKhblVd+iqqU4RCXavm/jPityDo5TCvKMnpjKnOriy0w==
 
-"@thumbmarkjs/thumbmarkjs@^1.8.1":
+"@thumbmarkjs/thumbmarkjs@^1.9.0":
   version "1.9.0"
   resolved "https://registry.yarnpkg.com/@thumbmarkjs/thumbmarkjs/-/thumbmarkjs-1.9.0.tgz#a6444ac1f924f061cfc1507a21dcaf83ee705cab"
   integrity sha512-6LooyYk8i5L2zEZgDMLE6m2sGDcIHHBiZfxdFp0A16Q4ZXafEmhHmt+zCqQEBMiQHi+08e/v5q77IY2KhvAJwg==
@@ -527,6 +527,54 @@
     "@typescript-eslint/types" "8.59.3"
     eslint-visitor-keys "^5.0.0"
 
+"@typescript/native-preview-darwin-arm64@7.0.0-dev.20260519.1":
+  version "7.0.0-dev.20260519.1"
+  resolved "https://registry.yarnpkg.com/@typescript/native-preview-darwin-arm64/-/native-preview-darwin-arm64-7.0.0-dev.20260519.1.tgz#deaa14299366a79917a6ec19fc7f240304c36afe"
+  integrity sha512-c9zdG6sGJf25Jpz04JgE23zhYeprqFypDGuqiX94yMTvR8IWXjq3R2oMnim66YLBDon/V1nCEy6cFixeSd/4fg==
+
+"@typescript/native-preview-darwin-x64@7.0.0-dev.20260519.1":
+  version "7.0.0-dev.20260519.1"
+  resolved "https://registry.yarnpkg.com/@typescript/native-preview-darwin-x64/-/native-preview-darwin-x64-7.0.0-dev.20260519.1.tgz#a6348c134204afdcdfa8f9a0925091788fc994d7"
+  integrity sha512-N16V3wiM0tsNmSSA7nZrxqXXt5OCJxBwiCVn35rnA7fr4WzJw6rJmwf9heNNhZ6Gh4ne3+Pexajf5akzuHR75Q==
+
+"@typescript/native-preview-linux-arm64@7.0.0-dev.20260519.1":
+  version "7.0.0-dev.20260519.1"
+  resolved "https://registry.yarnpkg.com/@typescript/native-preview-linux-arm64/-/native-preview-linux-arm64-7.0.0-dev.20260519.1.tgz#04f2f0b314e646a6921cc818db6bac92364bab24"
+  integrity sha512-ltf91vAwKdbu0SlRQbFgi1h5ZrLLrBn6a4qIeN2VILGbtYrCXnARHRznLBv81yUETQ7aVr/LSQcmsWo1ejCK0w==
+
+"@typescript/native-preview-linux-arm@7.0.0-dev.20260519.1":
+  version "7.0.0-dev.20260519.1"
+  resolved "https://registry.yarnpkg.com/@typescript/native-preview-linux-arm/-/native-preview-linux-arm-7.0.0-dev.20260519.1.tgz#ad10403fb8ae6e2ed120cb9f81a029328a00e995"
+  integrity sha512-8v4BExeeuCTrhaSGfeIJqm3qQkTzlZix/Qd/FkPlWoz9f7d7COvXb3Z4qhbaVolL0MMnUvQ7m005Z4kYsZ645A==
+
+"@typescript/native-preview-linux-x64@7.0.0-dev.20260519.1":
+  version "7.0.0-dev.20260519.1"
+  resolved "https://registry.yarnpkg.com/@typescript/native-preview-linux-x64/-/native-preview-linux-x64-7.0.0-dev.20260519.1.tgz#dde79208611da3855f995e595b9b790c622f726f"
+  integrity sha512-AVD0tczTtFCHNa4RQRVPvu8Hnw4P3hQ+OlUAjnz/lHowvc6o1pYB46elMqfDuaoWqIpv+EAkAPP4ipFCofJ5IA==
+
+"@typescript/native-preview-win32-arm64@7.0.0-dev.20260519.1":
+  version "7.0.0-dev.20260519.1"
+  resolved "https://registry.yarnpkg.com/@typescript/native-preview-win32-arm64/-/native-preview-win32-arm64-7.0.0-dev.20260519.1.tgz#74d0cf98c75e2df20772ae2fa4c8139066e2cf7f"
+  integrity sha512-TM+qatljyejqjHevCta3WIH53i0oGC7K8SoJ6t+mf4cGMTpZTyd7NhC1ts7e6/aydZnG53Bsta2iQi1SMIlQEw==
+
+"@typescript/native-preview-win32-x64@7.0.0-dev.20260519.1":
+  version "7.0.0-dev.20260519.1"
+  resolved "https://registry.yarnpkg.com/@typescript/native-preview-win32-x64/-/native-preview-win32-x64-7.0.0-dev.20260519.1.tgz#a8cfe9f730399cdc21a20c40a59aee15eef75a18"
+  integrity sha512-r9LEsoY7JC/82gXo8hlOmpQaUXcqmngCVOv+mUx1UeMt9f+1S6oNO0W48o75mlBqqC7jfcMHqw8YS4LfVxPRGw==
+
+"@typescript/native-preview@^7.0.0-dev.20260510.1":
+  version "7.0.0-dev.20260519.1"
+  resolved "https://registry.yarnpkg.com/@typescript/native-preview/-/native-preview-7.0.0-dev.20260519.1.tgz#0d865f5cc6d376d896fc031dfbb38ed0004153c6"
+  integrity sha512-VVER7vFUDdfm5k3jbH5765tVEJa7+0rTUkFeXyGYrXPxpw9BIjA0QDxdtdlRyaU8MCZV9IKZUo6doxeAQRAjPg==
+  optionalDependencies:
+    "@typescript/native-preview-darwin-arm64" "7.0.0-dev.20260519.1"
+    "@typescript/native-preview-darwin-x64" "7.0.0-dev.20260519.1"
+    "@typescript/native-preview-linux-arm" "7.0.0-dev.20260519.1"
+    "@typescript/native-preview-linux-arm64" "7.0.0-dev.20260519.1"
+    "@typescript/native-preview-linux-x64" "7.0.0-dev.20260519.1"
+    "@typescript/native-preview-win32-arm64" "7.0.0-dev.20260519.1"
+    "@typescript/native-preview-win32-x64" "7.0.0-dev.20260519.1"
+
 "@vitest/coverage-v8@^4.0.18":
   version "4.1.6"
   resolved "https://registry.yarnpkg.com/@vitest/coverage-v8/-/coverage-v8-4.1.6.tgz#1eacee5def68dfcb08c3ed5355edbad2a4c869b3"
@@ -1681,15 +1729,10 @@ typescript-eslint@^8.26.1:
     "@typescript-eslint/typescript-estree" "8.59.3"
     "@typescript-eslint/utils" "8.59.3"
 
-typescript@~5.5.4:
-  version "5.5.4"
-  resolved "https://registry.yarnpkg.com/typescript/-/typescript-5.5.4.tgz#d9852d6c82bad2d2eda4fd74a5762a8f5909e9ba"
-  integrity sha512-Mtq29sKDAEYP7aljRgtPOpTvOfbwRWlS6dPRzwjdE+C0R4brX/GUyhHSecbHMFLNBLcJIPt9nl9yG5TZ1weH+Q==
-
-typescript@~5.8.2:
-  version "5.8.3"
-  resolved "https://registry.yarnpkg.com/typescript/-/typescript-5.8.3.tgz#92f8a3e5e3cf497356f4178c34cd65a7f5e8440e"
-  integrity sha512-p1diW6TqL9L07nNxvRMM7hMMw4c5XOo/1ibL4aAIGmSAt9slTE1Xgw5KWuof2uTOvCg9BY7ZRi+GaF+7sfgPeQ==
+typescript@^6.0.3:
+  version "6.0.3"
+  resolved "https://registry.yarnpkg.com/typescript/-/typescript-6.0.3.tgz#90251dc007916e972786cb94d74d15b185577d21"
+  integrity sha512-y2TvuxSZPDyQakkFRPZHKFm+KKVqIisdg9/CZwm9ftvKXLP8NRWj38/ODjNbr43SsoXqNuAisEf1GdCxqWcdBw==
 
 undici-types@~6.21.0:
   version "6.21.0"