Singapore Visa for a freelance Software Engineer

As you are an Indian citizen, you fall under Assessment Level I Countries and need a visa to visit Singapore. According to the Singapore high commission in India, you have to do this through an authorised visa agent as per the list on their website.

I checked the website of one of those agents, and they have detailed description on what you need to apply for a business visa. Among others you need an invitation letter from a Singapore company (your client presumably) and a cover letter stating the purpose of your visit. This second letter has to be on the letter head of your Indian company. As on how this works for a freelancer like you, I am not sure. I think your best bet would be to contact one of the visa agents and ask them.

If you are going to Singapore just to meet the client, to discuss requirements for a project for instance, you should be able to do so with a visa.

However, as you state you are going to stay there for 30 days, I suppose you are planning to work for the customer (do actual development work) during your stay.

For this, you need a Work Pass, not a visa.

You can find more information about work passes on the Singapore Ministry of Manpower website.

The various types of passes have different eligibility requirements (like qualifications, earnings, types of jobs and industries, citizenship, quotas...). In most cases you would also be salaried, not freelancing.

We're missing a lot of information about your specific situation (including qualifications, earnings...), but it would seem to me (from 10 000 feet) that an S Pass may be the most appropriate. There's a self-assessment tool available to check if you are eligible.