fix: send multipart content_type before the file part on asset upload

src/low/errors.test.ts

@@ -26,6 +26,8 @@ describe("errorFromEnvelope", () => {
     ["queue_full", 429, QueueFull],
     ["insufficient_credits", 402, InsufficientCredits],
     ["not_found", 404, NotFound],
+    ["job_not_found", 404, NotFound],
+    ["asset_not_found", 404, NotFound],
     ["unauthorized", 401, Unauthorized],
     ["forbidden", 403, Forbidden],
   ];

src/low/errors.ts

@@ -90,6 +90,11 @@ const BY_CODE: Record<string, ApiErrorClass> = {
   queue_full: QueueFull,
   insufficient_credits: InsufficientCredits,
   not_found: NotFound,
+  // public-api currently returns entity-specific 404 codes even though the spec
+  // documents the generic `not_found`; map them so callers still get a typed
+  // NotFound. (Server/spec reconciliation is a separate follow-up.)
+  job_not_found: NotFound,
+  asset_not_found: NotFound,
   unauthorized: Unauthorized,
   forbidden: Forbidden,
 };

src/low/transport.test.ts

@@ -70,6 +70,13 @@ describe("ComfyLow transport", () => {
     }
   });
 
+  it("postAssets sends content_type before the file part", async () => {
+    // The stub rejects 422 if `file` precedes `content_type` (mirrors public-api).
+    const asset = await low.postAssets(new Blob(["hello"]), "text/plain", "hi.txt");
+    expect(asset.id).toBe("asset_uploaded_01");
+    expect(server.state.uploadCount).toBe(1); // 201, not a field-order rejection
+  });
+
   it("postAssets surfaces hash_mismatch without a blind retry", async () => {
     server.state.rejectHashMismatch = true;
     await expect(

src/low/transport.ts

@@ -178,8 +178,11 @@ export class ComfyLow {
       timeoutMs?: number | null;
     } = {},
   ): Promise<Asset> {
+    // public-api streams the multipart upload and requires the metadata fields
+    // BEFORE the file part (so it can route the file stream without buffering).
+    // The `file` part MUST be appended last, or the server rejects with 422
+    // "content_type is required and must be sent before the file field".
     const form = new FormData();
-    form.append("file", file, filePath);
     form.append("content_type", contentType);
     form.append("file_path", filePath);
     if (options.expectedHash !== undefined) {
@@ -188,6 +191,7 @@ export class ComfyLow {
     for (const tag of options.tags ?? []) {
       form.append("tags", tag);
     }
+    form.append("file", file, filePath);
     const headers: Record<string, string> = {};
     if (options.idempotencyKey) {
       headers["Idempotency-Key"] = options.idempotencyKey;

test/support/stub-server.ts

@@ -385,6 +385,20 @@ export class StubServer {
       state.uploadDataEvents += 1;
     });
     state.lastUploadContentLength = Number(req.headers["content-length"] ?? body.length);
+    // Mirror public-api: the multipart is streamed, so `content_type` MUST
+    // arrive before the `file` part or the server rejects.
+    const parts = body.toString("latin1");
+    const ctIdx = parts.indexOf('name="content_type"');
+    const fileIdx = parts.indexOf('name="file"');
+    if (fileIdx !== -1 && (ctIdx === -1 || fileIdx < ctIdx)) {
+      sendError(
+        res,
+        422,
+        "invalid_body",
+        "content_type is required and must be sent before the file field",
+      );
+      return;
+    }
     if (state.rejectHashMismatch) {
       sendError(res, 409, "hash_mismatch", "bytes do not match expected_hash");
       return;