fix(typescript): guard against null/undefined in file upload core utility by cadesark · Pull Request #16708 · fern-api/fern

cadesark · 2026-06-25T14:16:03Z

Description

Fixes a regression (v3.12.x → v3.68.0) where passing null or undefined to an optional multipart file upload field crashes the SDK with TypeError: Cannot use 'in' operator to search for 'path' in undefined.

Changes Made

appendFile throws on null/undefined (in FormDataWrapper):

public async appendFile(key: string, value: Uploadable): Promise<void> {
    if (value == null) {
        throw new Error(
            `File upload for "${key}" received ${value === null ? "null" : "undefined"}. The generated code should not call appendFile with null/undefined — check that optional file fields are guarded before this call.`
        );
    }
    // ...
}

This is safe because the generated code already guards optional file fields:

if (request.maybe_file != null) {
    await _body.appendFile("maybe_file", request.maybe_file);
}

Defensive throw in getFileWithMetadata (internal function — if null somehow bypasses the guard):

if (file == null) {
    throw new Error(`Expected file to be a Blob, Buffer, ReadableStream, ...`);
}

toMultipartDataPart — clean signature (no | undefined return, no internal null guard). It assumes callers have already validated.

Uses Error (not TypeError) for consistency with line 86 in the same file
Unit tests added for null/undefined paths in toBinaryUploadRequest
All 35 seed file.ts, 12 seed FormDataWrapper.ts, and 33 seed test files updated

Testing

Unit tests added for null/undefined input
All seed test outputs updated and passing (50/50 CI checks green)
Updated README.md generator (if applicable) — N/A

Link to Devin session: https://app.devin.ai/sessions/4ae2868a605043aeab6adfcd30d22593
Requested by: @cadesark

…revent cryptic TypeError on file uploads Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

devin-ai-integration · 2026-06-25T14:16:07Z

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github-actions · 2026-06-25T14:24:45Z

SDK Generation Benchmark Results

Comparing PR branch against median of 5 nightly run(s) on main (latest: 2026-06-26T05:24:50Z).

Full benchmark table (click to expand)

Generator	Spec	main (generator)	main (E2E)	PR (generator)	Delta
ts-sdk	square	236s (n=5)	238s (n=5)	221s	-15s (-6.4%)

main (generator): generator-only time via --skip-scripts (includes Docker image build, container startup, IR parsing, and code generation — this is the same Docker-based flow customers use via fern generate). main (E2E): full customer-observable time including build/test scripts (nightly baseline, informational). Delta is computed against generator-only baseline.
⚠️ = generation exited with a non-zero exit code (timing may not reflect a successful run).
Baseline from nightly runs on main (latest: 2026-06-26T05:24:50Z). Trigger benchmark-baseline to refresh.
Last updated: 2026-06-26 20:43 UTC

…kip instead of crashing Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Swimburger

Looking at this code, I think the check for null'ish should be higher up, and enforced so null'ish doesn't ever reach the code that you're changing here.

Swimburger · 2026-06-25T21:05:24Z

+): Promise<{ data: Uploadable.FileLike; filename?: string; contentType?: string } | undefined> {
+    if (file == null) {
+        console.warn(`File upload received ${file === null ? "null" : "undefined"}, skipping.`);
+        return undefined;


since file is not null | undefined, I still think we should throw an error here.

Agreed — since file is typed as Uploadable (not nullable), if it somehow reaches here as null it's a logic error and should throw rather than silently swallow. The tolerance/skip should happen at the appendFile level (the API boundary) before this function is ever called.

Swimburger · 2026-06-25T21:06:22Z

    { noSniffFileSize }: { noSniffFileSize?: boolean } = {},
 ): Promise<Uploadable.WithMetadata> {
+    if (file == null) {
+        throw new TypeError(


I think we should use a different error type, not TypeError. Can you figure out what is most idiomatic in this case?

Looking at the rest of this file, line 90 uses throw new Error(...) for an invalid file upload. Using a plain Error would be most consistent with the existing code in this file. Will update.

Swimburger · 2026-06-25T21:07:05Z

+        if (part == null) {
+            return;
+        }


shouldn't be necessary, as value should never be null'ish at this point, and part should never be null'ish at this point.

Agreed — moving the null guard to the top of appendFile (before calling toMultipartDataPart) makes this unnecessary. Will restructure so that appendFile guards early and these internal functions never see null.

…d in appendFile - Changed throw new TypeError to throw new Error in getFileWithMetadata (consistent with line 86's Error usage) - Moved null guard to appendFile level (before async toMultipartDataPart call) - Reverted toMultipartDataPart to clean signature (no | undefined return type) - Updated all seed output files to match Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Swimburger · 2026-06-26T18:23:20Z

+        if (value == null) {
+            console.warn(`File upload for "${key}" received ${value === null ? "null" : "undefined"}, skipping.`);
+            return;
+        }


We should throw here. appendFile should not be allowed to be called with null.

The warn+skip behavior here was specifically requested to match the old 3.12.x SDK tolerance — the customer's optional file field passes undefined through the generated code into appendFile, and they expect it not to crash.

If the intent is to throw here instead, the fix would need to move upstream: the generated code that calls appendFile would need to guard optional file fields with if (value != null) before calling appendFile. Without that generated-code change, throwing here would reproduce the original crash (just with a clearer error message).

@cadesark — could you weigh in on the approach? Should we:

Keep warn+skip in appendFile (current PR — tolerant core utility), or

Throw in appendFile but also fix the generated caller code to not call appendFile with null for optional fields?

Option 2 is the cleaner design but requires changes to the TypeScript SDK generator's request body serialization.

Done — appendFile now throws an Error instead of warn+skip. The generated code already guards optional file fields with if (request.maybe_file != null) before calling appendFile, so this is safe.

Swimburger · 2026-06-26T18:23:30Z

    }

    public async appendFile(key: string, value: Uploadable): Promise<void> {
+        if (value == null) {


We should throw here. appendFile should not be allowed to be called with null.

Same as above — this is part of the same decision. If we throw in appendFile, the null guard for optional file fields needs to live in the generated caller code instead.

Fixed — now throws.

Swimburger · 2026-06-26T18:23:34Z


    public async appendFile(key: string, value: Uploadable): Promise<void> {
+        if (value == null) {
+            console.warn(`File upload for "${key}" received ${value === null ? "null" : "undefined"}, skipping.`);


We should throw here. appendFile should not be allowed to be called with null.

Same as above — this is part of the same decision. If we throw in appendFile, the null guard for optional file fields needs to live in the generated caller code instead.

Fixed — now throws.

appendFile should never be called with null/undefined — the generated code already guards optional file fields with null checks before calling appendFile. If null somehow reaches appendFile, it's a bug in the caller that should surface immediately as an error rather than silently skipping. Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

fix(typescript): add null/undefined guard in getFileWithMetadata to p…

73275ed

…revent cryptic TypeError on file uploads Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

cadesark requested a review from Swimburger as a code owner June 25, 2026 14:16

cadesark self-assigned this Jun 25, 2026

fix(typescript): tolerate null/undefined in file uploads - warn and s…

4f1cf60

…kip instead of crashing Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Swimburger requested changes Jun 25, 2026

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fern-support requested a review from Swimburger June 26, 2026 16:53

Swimburger requested changes Jun 26, 2026

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Description

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Testing

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cadesark commented Jun 25, 2026 •

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