Do I still need a SIC code, or is NAICS the only code the government uses now?

No, you do not need a SIC code for federal contracting; the federal government has used the NAICS code system since 1997. When you register in SAM.gov, you provide your primary NAICS code and can add others that represent your business activities. The contracting officer will assign a NAICS code to a solicitation, and your SAM registration needs to include that NAICS code to be considered a small business for that specific procurement. This is how they check your size status against the relevant size standard. State and local entities may still use SIC codes, as Grace mentioned, but for federal awards, it's strictly NAICS.

For federal contracting, only the NAICS code is used; the federal government stopped using the SIC code system in 1997. The FAR does not mention SIC codes. You will not need a SIC code for any federal solicitations or registrations. You are correct that SAM.gov only requires NAICS codes for your entity registration. Most states and local governments have transitioned to NAICS codes for their procurement systems, but some still use SIC codes or a combination of both. When a state or local government asks for a SIC code, it is best to check their vendor guide or procurement website for assistance in finding the correct code that aligns with your services. Many online resources can help you cross-reference your NAICS code to an equivalent SIC code if needed for those specific registrations.