Using Claude cowork. Need to upload file from sandbox to r2/s3 for attaching outlook. Is there an MCP tool call the agent can perform to received a pre signed url that it can post bytes to? I see this is possible with the API

Hi there, Looking deeper, this should be possible in a Tool Router/workbench-style session. The key is that Outlook needs a Composio storage key, not the sandbox path itself. If the file is already under

/mnt/files

, have the agent call

get_mount_file_s3_key("your-file.ext")

. If the file is elsewhere in the sandbox, have it call

upload_local_file("/path/to/your-file.ext")

and use the returned

s3key

. Then pass that into Outlook as the attachment:

{
  "attachment": {
    "name": "your-file.ext",
    "mimetype": "application/octet-stream",
    "s3key": "<returned s3key>"
  }
}

The part that will not work is passing

/mnt/files/...

,

/workspace/...

, a URL, or a local file path directly as `s3key`; the Outlook tool resolves

s3key

from Composio storage, not from the sandbox filesystem. If those helper tools are not visible in your Claude Cowork session, use the API/SDK upload flow instead, then pass the returned

{name, mimetype, s3key}

object to Outlook. Sent by Composio Support Agent

hey @PM, this is possble. Lmk if you were unable to do it. A prompt and sample response from why it wasn't able to would help

to be clear, I am not running in the Composio sandbox. what tool call would i make to achieve this

Curious why you want to upload file to r2/s3. If usecase is something like sending email in outlook with attachment then you can use Outlook send email/message tool - uploading the file to s3 and passing the url in the tool input will be handled by Composio

i am working in claude cowork, which can only run arbitrary code within a linux sandox container

checking

thanks!

quick clarification and sorry for the earlier confusion. There are two different upload flows, depending on where the file exists: If the file is already inside Composio Remote Workbench, If your Composio MCP session exposes

COMPOSIO_REMOTE_WORKBENCH

(its exposed in the Composio consumer MCP you use), call that tool and run the preloaded

upload_local_file()

Python helper with the file's local Workbench path:

result, error = upload_local_file("/tmp/report.pdf")
if error:
    raise Exception(error)

s3key = result["s3key"]

The helper uploads the file to Composio-hosted storage and returns the s3key required by the Outlook send-email tool. If the file exists only inside Claude Cowork, a Cowork-local path cannot be passed to

upload_local_file()

, because Cowork and Composio Remote Workbench are separate environments. In that case: Call POST

/api/v3/files/upload/request

([doc](https://docs.composio.dev/reference/api-reference/files)) to receive new_presigned_url and key. PUT the raw file bytes to new_presigned_url. Use the returned key as the attachment's s3key. In either flow, pass the attachment to Outlook as:

{
  "attachment": {
    "name": "report.pdf",
    "mimetype": "application/pdf",
    "s3key": "<returned s3key or key>"
  }
}

For your current Cowork setup, the custom MCP wrapper you created around /api/v3/files/upload/request is the correct approach. You do not need to provide or manage your own storage bucket.

thanks @Abhishek, the MCP wrapper we rolled out has been working well. Definitely something to consider for future MCP releases. I'm sure there are many other users with similar Cowork/Code workflows that need this.