feat: implement proxy input file resolution and storage

- Added `inputFiles.ts` for handling input file resolution, including validation and normalization of file types.
- Introduced `proxyFileStore.ts` for managing file storage, retrieval, and deletion with owner-based isolation.
- Created tests for proxy file storage functionality in `proxyFileStore.test.ts`.
- Developed `proxyInputFileResolver.ts` to resolve input files in OpenAI and response formats.
- Added utility functions for handling reasoning signatures in `reasoningTransport.ts` and corresponding tests.
- Enhanced shared input file transformers in `inputFile.ts` for better file handling and type inference.

src/server/routes/api/test.proxy.test.ts

@@ -149,4 +149,46 @@ describe('testRoutes proxy tester transport', () => {
       status: 'pending',
     });
   });
+
+  it('allows multipart proxy uploads to /v1/files', async () => {
+    fetchMock.mockResolvedValue(new Response(JSON.stringify({
+      id: 'file_123',
+      object: 'file',
+      bytes: 8,
+      filename: 'notes.md',
+      purpose: 'assistants',
+    }), {
+      status: 200,
+      headers: { 'content-type': 'application/json' },
+    }));
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/api/test/proxy',
+      payload: {
+        method: 'POST',
+        path: '/v1/files',
+        requestKind: 'multipart',
+        stream: false,
+        jobMode: false,
+        rawMode: false,
+        multipartFields: {
+          purpose: 'assistants',
+        },
+        multipartFiles: [
+          {
+            field: 'file',
+            name: 'notes.md',
+            mimeType: 'text/markdown',
+            dataUrl: 'data:text/markdown;base64,IyBoZWxsbw==',
+          },
+        ],
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+    const [url, requestInit] = fetchMock.mock.calls[0] as [string, RequestInit & { body?: { constructor?: { name?: string } } }];
+    expect(url).toMatch(/\/v1\/files$/);
+    expect(requestInit.body?.constructor?.name).toBe('FormData');
+  });
 });

src/server/routes/api/test.test.ts

@@ -109,6 +109,46 @@ describe('/api/test proxy tester routes', () => {
     expect(fetchMock).not.toHaveBeenCalled();
   });
 
+  it('allows multipart /v1/files uploads through the proxy tester', async () => {
+    fetchMock.mockResolvedValue(new Response(JSON.stringify({
+      id: 'file_demo_123',
+      object: 'file',
+      filename: 'notes.txt',
+      bytes: 11,
+      purpose: 'user_data',
+    }), {
+      status: 200,
+      headers: { 'content-type': 'application/json' },
+    }));
+
+    const response = await app.inject({
+      method: 'POST',
+      url: '/api/test/proxy',
+      payload: {
+        method: 'POST',
+        path: '/v1/files',
+        requestKind: 'multipart',
+        multipartFields: {
+          purpose: 'user_data',
+        },
+        multipartFiles: [
+          {
+            field: 'file',
+            name: 'notes.txt',
+            mimeType: 'text/plain',
+            dataUrl: 'data:text/plain;base64,aGVsbG8gZmlsZXM=',
+          },
+        ],
+      },
+    });
+
+    expect(response.statusCode).toBe(200);
+    const [targetUrl, requestInit] = fetchMock.mock.calls[0] as [string, RequestInit & { body?: { constructor?: { name?: string } } }];
+    expect(targetUrl).toBe(`http://127.0.0.1:${config.port}/v1/files`);
+    expect(requestInit.method).toBe('POST');
+    expect(requestInit.body?.constructor?.name).toBe('FormData');
+  });
+
   it('keeps legacy /api/test/chat wrapper working for responses payloads', async () => {
     fetchMock.mockResolvedValue(new Response(JSON.stringify({
       object: 'response',

src/server/routes/api/test.ts

@@ -1,6 +1,6 @@
 import { FastifyInstance, FastifyReply, FastifyRequest } from 'fastify';
 import { randomUUID } from 'node:crypto';
-import { fetch, File, FormData } from 'undici';
+import { fetch, File as UndiciFile, FormData as UndiciFormData } from 'undici';
 import { config } from '../../config.js';
 
 type UndiciRequestInit = Parameters<typeof fetch>[1];
@@ -90,6 +90,7 @@ const jobs = new Map<string, TestProxyJob>();
 
 const ALLOWED_PROXY_PATH_PATTERNS: RegExp[] = [
   /^\/v1\/chat\/completions(?:\?.*)?$/i,
+  /^\/v1\/files(?:\/[^/?#]+(?:\/content)?)?(?:\?.*)?$/i,
   /^\/v1\/responses(?:\/compact)?(?:\?.*)?$/i,
   /^\/v1\/messages(?:\?.*)?$/i,
   /^\/v1\/embeddings(?:\?.*)?$/i,
@@ -481,7 +482,9 @@ async function buildUpstreamRequestInit(
   }
 
   if (envelope.requestKind === 'multipart') {
-    const formData = new FormData();
+    const FormDataCtor = globalThis.FormData ?? UndiciFormData;
+    const FileCtor = globalThis.File ?? UndiciFile;
+    const formData = new FormDataCtor();
     for (const [field, value] of Object.entries(envelope.multipartFields || {})) {
       formData.append(field, value);
     }
@@ -490,14 +493,14 @@ async function buildUpstreamRequestInit(
       const bytes = Uint8Array.from(decoded.buffer);
       formData.append(
         file.field,
-        new File([bytes], file.name, { type: file.mimeType || decoded.mimeType }),
+        new FileCtor([bytes], file.name, { type: file.mimeType || decoded.mimeType }),
       );
     }
 
     return {
       method: envelope.method,
       headers,
-      body: formData,
+      body: formData as any,
     };
   }