transloadit/uppy · error · Error

Missing S3 object key for completing multipart upload

Error message

Missing S3 object key for completing multipart upload

What it means

After a successful OAuth token exchange, Companion needs req.companion.buildURL (a URL builder derived from server options) to redirect the user back to the Uppy client with the uppyAuthToken. If buildURL is missing, Companion cannot construct the redirect and returns 500.

Source

Thrown at packages/@uppy/aws-s3/src/S3Uploader.ts:344

      // after part finished uploading, update chunk state
      this.#chunkState[i].uploaded = chunk.size
      this.#chunkState[i].etag = etag
      this.#onProgress()

      if (this.#options.onPartComplete) {
        this.#options.onPartComplete({
          PartNumber: partNumber,
          ETag: etag,
        })
      }
    }

    const parts = this.#chunkState.flatMap((state, i) =>
      state.etag ? [{ partNumber: i + 1, etag: state.etag }] : [],
    )

    if (this.#key == null) {
      throw new Error('Missing S3 object key for completing multipart upload')
    }

    const { location, key } =
      await this.#options.s3Client.completeMultipartUpload({
        key: this.#key,
        uploadId: this.#uploadId!,
        parts,
        signal,
      })

    this.#onSuccess({
      location,
      key,
      uploadId: this.#uploadId,
    })
  }

  #onProgress(): void {

View on GitHub (pinned to 5d4dedd02a)

Solutions

1. Set the server host/companionUrl option so Companion can build absolute URLs
2. Review the Companion standalone/server options for missing required URL fields
3. If behind a proxy, configure server.oauthDomain and path prefixes correctly
4. Check Companion startup logs for warnings about URL configuration

Example fix

// before
companion({ server: {} })
// after
companion({ server: { host: 'https://companion.myapp.com', path: '/companion' } })

if (!process.env.COMPANION_DOMAIN) throw new Error('COMPANION_DOMAIN required for OAuth redirects')

When it happens

Trigger: The companion middleware did not initialize buildURL — typically because server.host/companionUrl or server.path options are missing or malformed, so the URL builder was never attached to req.companion.

Common situations: Self-hosting Companion with an incomplete server configuration (no companionUrl/host), misconfigured filePath vs domain, or running behind a proxy without the right server options after an upgrade that changed required options.

Related errors

• [s3mini] fileType must be a string
• File data is missing for file ${options.file.id}
• Missing S3 object key for aborting upload
• Missing S3 object key for resuming upload
• Missing S3 object key for uploading part

AI-assisted analysis of transloadit/uppy@5d4dedd02a (2026-08-28). Data as JSON: /api/errors/d5c7e46280b1af6d. Report an issue: GitHub.