Can an employee who was enrolled in an HSA-compatible high-deductible plan for part of the tax year, contribute the maximum full-year amount?

In most cases — no. Only “eligible individuals” as described by the IRS are permitted to make contributions to an HSA and eligibility is determined monthly. An individual is eligible if, among other requirements, they are enrolled in an HSA-compatible plan on the first day of the month. An eligible individual is permitted to contribute one-twelfth of the full-year maximum each month. They are not allowed to “front-load” their contribution (except as described below).

Example
The maximum contribution in 2017 for an employee with self-only coverage, who is not yet 55 years old, is $3,400. If that employee is enrolled in an HSA-compatible plan on January 1 and transitions to a non-compatible plan on July 1, they (and their employer if the employer contributes) may only contribute $1,700 to the HSA ($3,400 divided by 12 times 6). And the contributions should have been made in increments of $283.33 per month ($3,400 divided by 12) over the six-month period.

Exception
The IRS does permit “front-loading” of the full-year contribution amount according to the “last month” rule. To qualify, the individual must be eligible on the first day of the last month of their tax-paying year (December 1 for most) and must maintain HSA-compatible coverage during a testing period that runs from the last month of their tax period (usually December) through the last day of the twelfth month following that month (usually December 31 of the following year).

Resources

IRS Publication 969, which describes Health Savings Accounts, including the last month rule, is a good reference guide.