Message Batch Processing - undocumented character limit for custom_id parameter

[PaulBFB]

creating a request via

import anthropic
from anthropic.types.message_create_params import MessageCreateParamsNonStreaming
from anthropic.types.messages.batch_create_params import Request

client = anthropic.Anthropic()

message_batch = client.messages.batches.create(
    requests=[
        Request(
            custom_id=f"{'a'*65}",
            params=MessageCreateParamsNonStreaming(
                model="claude-opus-4-20250514",
                max_tokens=1024,
                messages=[{
                    "role": "user",
                    "content": "Hello, world",
                }]
            )
        ),
        Request(
            custom_id=f"{'b'*65}",
            params=MessageCreateParamsNonStreaming(
                model="claude-opus-4-20250514",
                max_tokens=1024,
                messages=[{
                    "role": "user",
                    "content": "Hi again, friend",
                }]
            )
        )
    ]
)

Will cause

BadRequestError: Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': 'requests.0.custom_id: String should have at most 64 characters'}}

I checked the documentation (first with cmd-f and then using claude research ;) ) for the character limit for the custom_id parameter of 64 characters and it's not listed anywhere.

[kevinc13]

Hey Paul, thanks for raising the issue!

We document the 64 character limit in the API reference, but it's unfortunately a bit hidden in the child attributes dropdown.

Let me know if you have any more concerns / feedback

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.

[nightcityblade]

Unclaiming this — I noticed there is already a linked PR, so I won’t duplicate the work. Sorry for the noise.