Share an idea in layers. The outline, meaning the problem and the rough direction, can go out in the open: it carries little risk and it is how you get feedback and help. The details that would be hard to rebuild, such as a technical method, customer lists or unreleased numbers, you share only with people you have chosen, and with a confidentiality agreement when it matters. The exception is a technical invention you might want to patent: talk to a patent attorney or the patent office before you publish anything about it.
What is actually at risk
The common fear is that someone will steal the idea. An idea is usually worth little until somebody does the work, and the people who could do it are rarely short of ideas of their own. The real risks are narrower and worth knowing:
Information that is valuable because it is secret stops being secret.
Something you wrote in confidence is passed on by someone who never agreed to keep it to themselves.
Three layers
What to share with whom
Layer
What goes in it
Who sees it
Open
The problem, who has it, the direction you are thinking of, which competences you lack
Anyone: a post, a pitch, a public page
Chosen people
How you plan to do it, early plans, who you have talked to
People you have spoken with and have a reason to trust
Under agreement
Technical details, source code, customer data, prices, anything that is valuable because it is secret
People who have signed a confidentiality agreement
Where to share
With friends and colleagues, in person: the safest and the quickest way to get honest feedback.
In communities and groups for your field: fine for the open layer. Anything you post there can be read by strangers, copied and found by search engines.
On Bay of Ideas: you write a public side and a private side. The public side is readable by anyone, including search engines and AI assistants. The private side is only for the members you accept. You can also post anonymously, and delete the idea whenever you like. Put the open layer on the public side and the chosen-people layer on the private side.
With investors and advisers: ask what they will do with what you tell them. Some will not sign a confidentiality agreement at a first meeting, so decide beforehand what you are willing to say without one.
How to share without regret
Decide the layer before you write. It is easier to leave something out than to take it back.
Keep a dated record of what you shared, with whom and how: an e-mail or a saved message is enough. It helps if there is a dispute later about who knew what and when.
Share the part you need help with. A designer needs the user and the problem, not your financial model.
Ask for confidentiality in plain words when it matters, and put it in writing. A mutual confidentiality agreement is a normal request between businesses.
Avoid unencrypted channels for sensitive material. The Swedish Patent and Registration Office (PRV) gives the same advice to inventors.
When a confidentiality agreement helps
A confidentiality agreement, often called a non-disclosure agreement or NDA, is a contract: the person who signs promises to keep what they learn to themselves and to use it only for the agreed purpose. It binds only the people who sign. It does not protect information that was already public, and it is only as good as your ability to find out about a breach. It matters most where the secret itself is the value, for example a technical method or a customer list. The World Intellectual Property Organization lists marking documents as confidential, restricting access and confidentiality agreements as typical "reasonable steps" to keep a trade secret.
This is general information, not legal advice. If a lot depends on the idea, or you may want a patent, speak to a lawyer or a patent attorney before you share the details.