ct600

Introduction

A utility for managing UK Corporation Tax submission.

Caveat

This probably won't do what you want, unless you are prepared to do a fair amount of work and data validation. It works for me, and my workflow running a small business.

I am registered as an HMRC developer and requested credentials for live submission. To use this code you need to go through the same process.

Overview

This code is intended to be used with the following workflow:

• Previously, you will have created company accounts, and corporation tax computations filing in iXBRL format.
• The first step is to use ct600 to create what I am calling a "form values" file. This is a text file with 1 line for each CT600 box that you would fill in. This process is partly automated, it takes as input the Computations iXBRL and fills in some basic CT600 fields. For my small business with no loss to record, this step produces a form values file which is mostly ready to use.
• The second step is to modify the form values file to match what you want to submit. You need a text editor to do this. You definitely need to add boxes 975, 980 and 985 (name, date, status) because these aren't filled in for you.
• The final step is to submit the accounts, computations and CT600 data in the forms values files.

Input: iXBRL files

See Company Tax Returns and Corporation Tax online: support for software developers

This is based on my understand of HMRC's requirements, this is not advice you should use in isolation. Go and read the documentation above.

The accounts file needs to match what HMRC expect. ct600 does not parse this file in any way, so it just needs to match what HMRC expect, which is basically you submit what you would be submitted to Companies House at annual filing.

The tax computations file needs to include computations data as well as detailed profit and loss (DPL).

I use ixbrl-reporter-jsonnet to create iXBRL files from GnuCash.

Creating a form values file

The command is of the form:

ct600 --computations ct.html --output-form-values > form-values.yaml

You can skip this step and create the form values file manually if you want, but doing it this way provides a way to validate that the computations file matches what is in the form values file.

The output will contain a field for every single box in the CT600 form, most of which will be blank. You can delete the blank lines if you want.

Modifying the form values file

You will probably need to have access to the published CT600 form, and guidance for completing it. We're not actually going to be filling in the form here, just making sure the form values file contains a value for all the boxes you want to fill in.

See CT600

Configuration

Configuration is a JSON file passed via --config. Three config files are provided for different scenarios:

File	Purpose	gateway-test	class	url
config-test.json	Local testing	"1"	HMRC-CT-CT600-TIL	http://localhost:8081/
config-til.json	Test-In-Live (real HMRC, test data)	"0"	HMRC-CT-CT600-TIL	HMRC production
config-prod.json	Production (real submission)	"0"	HMRC-CT-CT600	HMRC production

All three contain placeholder credentials — replace username, password and vendor-id with your own before use.

Warning: config-prod.json submits real tax returns. Do not use it unless you intend to file.

You can also create your own config.json — it is gitignored and will not be committed.

Safe defaults

If a config file omits these fields, the defaults are safe:

Field	Default
url	http://localhost:8081/
class	HMRC-CT-CT600-TIL
gateway-test	"1"

Running with an incomplete config will never accidentally hit production HMRC.

Config file fields

{
    "company-type": 6,
    "declaration-name": "Sarah McAcre",
    "declaration-status": "Director",
    "username": "govidgoeshere",
    "password": "passwordgoeshere",
    "gateway-test": "1",
    "class": "HMRC-CT-CT600-TIL",
    "vendor-id": "1234",
    "url": "http://localhost:8081/",
    "title": "Ms",
    "first-name": "Sarah",
    "second-name": "McAcre",
    "email": "sarah@example.org",
    "phone": "447900123456"
}

Submission

There are three ways to test and submit, in order of risk.

1. Local test service (recommended first step)

This is optional and slightly involved to set up, but it is the recommended way to get a feel for how the whole submission process works without touching HMRC at all.

The package includes a basic corptax emulator with schema validation. Install the package, then run:

corptax-test-service

In a separate terminal, submit to it using config-test.json:

ct600 \
    --config config-test.json \
    --accounts accts.html \
    --computations ct.html \
    --form-values form-values.yaml \
    --submit

Expected output:

IRmark is hOgMwO+75eJbBax/OhPZy/NszxE=
Correlation ID is 1E242
Endpoint is http://localhost:8082/
Poll...
Poll...
Poll...
Poll...
Submitted successfully.
Delete request...
Completed.

2. Test-In-Live (real HMRC, test data)

Test-In-Live submits to the real HMRC endpoint using the HMRC-CT-CT600-TIL message class. HMRC accepts and validates the submission but does not process it as a real tax return. If the submission is successful, you also get an email confirming this. I think this is sent to the email address associated with the HMRC account.

You need real Government Gateway credentials (username and password) for this — these are the same credentials you use to log in to HMRC online services.

Warning: Government Gateway credentials are stored in clear text in the config file. Do not commit them to version control. Use your own config.json (which is gitignored) or be careful with config-til.json.

The required config settings are:

Field	Value
gateway-test	"0"
class	HMRC-CT-CT600-TIL
url	https://transaction-engine.tax.service.gov.uk/submission

These are already set in config-til.json.

ct600 \
    --config config-til.json \
    --accounts accts.html \
    --computations ct.html \
    --form-values form-values.yaml \
    --submit

3. Production

Warning: This submits a real Corporation Tax return to HMRC. Make sure your data is correct before submitting.

You need real Government Gateway credentials, as with Test-In-Live. The same warning about storing credentials in clear text applies.

The required config settings are:

Field	Value
gateway-test	"0"
class	HMRC-CT-CT600
url	https://transaction-engine.tax.service.gov.uk/submission

These are already set in config-prod.json.

ct600 \
    --config config-prod.json \
    --accounts accts.html \
    --computations ct.html \
    --form-values form-values.yaml \
    --submit

The output is verbose. You'll know if the submission was successful.

If there are errors, the output is not parsed particularly well, but there will be clues about what the errors are.

If the submission works, you can use this repo to take the form values file and overlay the data on a CT600 PDF file as a record of what you submitted:

https://github.com/cybermaggedon/ct600-fill

Installing

pip3 install .

Credentials

For Test-In-Live and production filing, the username and password in your config file are just your Government Gateway credentials — the same ones you use to log in to HMRC online services. You do not need to register as a developer to use Test-In-Live or to file live returns.

The vendor-id is a developer credential. I am registered as an HMRC developer and have been issued a vendor ID. HMRC does not permit these credentials to be shared publicly.

HMRC also provides separate test systems intended for developers. You should only use these if you have been given access through the developer hub:

https://developer.service.hmrc.gov.uk/api-documentation/docs/using-the-hub

Usage

usage: ct600 [-h] [--config CONFIG] [--accounts ACCOUNTS]
             [--computations COMPUTATIONS] [--form-values FORM_VALUES]
             [--attachment ATTACHMENT] [--output-ct] [--output-values]
             [--output-form-values] [--submit] [--data-request]

Submission to HMRC Corporation Tax API

options:
  -h, --help            show this help message and exit
  --config CONFIG, -c CONFIG
                        Configuration file (default: config.json)
  --accounts ACCOUNTS, -a ACCOUNTS
                        Company accounts iXBRL file
  --computations COMPUTATIONS, --comps COMPUTATIONS, -t COMPUTATIONS
                        Corporation tax computations iXBRL file
  --form-values FORM_VALUES, --ct600 FORM_VALUES, -f FORM_VALUES
                        CT600 form values YAML file
  --attachment ATTACHMENT, -m ATTACHMENT
                        Corporation tax computations iXBRL file
  --output-ct, -p       Just output CT message, no submission
  --output-values       Just output some data values (debug)
  --output-form-values  Output CT600 form values from computations
  --submit              Submit the CT message
  --data-request        Perform a data request for outstanding items

Licences, Compliance, etc.

Warranty

This code comes with no warranty whatsoever. See the LICENSE file for details. Further, I am not an accountant. It is possible that this code could be useful to you in meeting regulatory reporting requirements for your business. It is also possible that the software could report misleading information which could land you in a lot of trouble if used for regulatory purposes. Really, you should check with a qualified accountant.

Licence

Copyright (C) 2020-2026, Cyberapocalypse Limited

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see https://www.gnu.org/licenses/.

Support

You're welcome to come discuss on Discord

https://discord.gg/3cAvPASS6p