What documents govern a 401(k) plan?
Last updated October 2, 2026
A 401(k) runs on its plan document (usually a pre-approved adoption agreement plus basic plan document), the trust agreement, the summary plan description for participants, and amendments. Operating out of sync with these documents is among the most common — and correctable — compliance failures.
The document stack for a typical 401(k):
- Plan document: most employers adopt a pre-approved document — an adoption agreement of elections sitting on a provider's basic plan document — rather than an individually designed plan.
- Trust agreement: establishes the trust holding plan assets (often integrated with the plan document).
- SPD: the participant-facing summary ERISA requires.
- Amendments: legally required updates and discretionary design changes.
The recurring failure mode is operational: the document says one thing (a definition of compensation, an eligibility rule) and payroll does another. Periodically comparing operations to the document — and using IRS correction programs promptly when gaps appear — keeps small drift from becoming a qualification problem.
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