0000798244 2024-08-31 2024-08-31 0000798244 cik0000798244:S000000765Member 2024-08-31 2024-08-31 0000798244 cik0000798244:S000000765Member cik0000798244:C000002210Member 2024-08-31 2024-08-31 0000798244 cik0000798244:S000000765Member cik0000798244:C000002215Member 2024-08-31 2024-08-31 0000798244 cik0000798244:S000000765Member cik0000798244:C000002216Member 2024-08-31 2024-08-31 0000798244 cik0000798244:S000000765Member cik0000798244:C000002217Member 2024-08-31 2024-08-31 0000798244 cik0000798244:S000000765Member cik0000798244:C000002219Member 2024-08-31 2024-08-31 0000798244 cik0000798244:S000000765Member cik0000798244:C000002221Member 2024-08-31 2024-08-31 0000798244 cik0000798244:S000000765Member cik0000798244:C000002222Member 2024-08-31 2024-08-31 0000798244 cik0000798244:S000000765Member cik0000798244:C000002211Member 2024-08-31 2024-08-31 0000798244 cik0000798244:S000000765Member cik0000798244:C000075013Member 2024-08-31 2024-08-31 iso4217:USD pure utr:Y SUPPLEMENT TO THE PROSPECTUSSupplement dated July 28, 2025, to the Prospectus dated December 27, 2024.MFS® New Discovery Fund<div style="border-bottom:3pt; border-bottom-style:solid; border-bottom-color:#999999; padding:0pt 0pt 1pt 0pt;"><div style="font-size:5.0pt; font-family:Serif; font-style:normal; text-align:left; color:#000000; font-weight:normal;"></div></div> Effective August 1, 2025, the sub-sections entitled "Fees and Expenses" and "Example" under the main heading entitled "Summary of Key Information" are restated in their entirety as follows: 2024-12-27 MFS® New Discovery Fund Fees and Expenses This table describes the fees and expenses that you may pay when you buy, hold, and sell shares of the fund. Investors may also pay commissions or other fees to their financial intermediaries when they buy, hold, and sell shares of the fund, which are not reflected below. You may qualify for sales charge reductions if, with respect to Class A shares, you and certain members of your family invest, or agree to invest in the future, at least $50,000 in MFS funds. More information about these and other waivers and reductions is available from your financial intermediary and in “Sales Charges and Waivers and Reductions” on page 10 and “Appendix A – Waivers and Reductions of Sales Charges” on page A-1 of the fund’s prospectus. You may qualify for sales charge reductions if, with respect to Class A shares, you and certain members of your family invest, or agree to invest in the future, at least $50,000 in MFS funds. 50000 Shareholder Fees (fees paid directly from your investment): 0.0575 0 0 0 0 0 0 0 0 0.0100 0.0400 0.0100 0 0 0 0 0 0 Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment): 0.0084 0.0084 0.0084 0.0084 0.0084 0.0084 0.0084 0.0084 0.0084 0.0025 0.0100 0.0100 0 0.0100 0.0050 0.0025 0 0 0.0020 0.0020 0.0020 0.0020 0.0020 0.0020 0.0020 0.0020 0.0007 0.0129 0.0204 0.0204 0.0104 0.0204 0.0154 0.0129 0.0104 0.0091 -0.0008 -0.0008 -0.0008 -0.0008 -0.0008 -0.0008 -0.0008 -0.0008 -0.0006 0.0121 0.0196 0.0196 0.0096 0.0196 0.0146 0.0121 0.0096 0.0085 Example This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes that: you invest $10,000 in the fund for the time periods indicated and you redeem your shares at the end of the time periods (unless otherwise indicated); your investment has a 5% return each year; and the fund’s operating expenses remain the same. Although your actual costs will likely be higher or lower, under these assumptions your costs would be: 691 950 1232 2033 599 929 1288 2167 199 629 1088 2167 299 629 1088 2167 199 629 1088 2167 98 320 563 1261 199 629 1088 2360 149 476 829 1825 123 398 697 1547 98 320 563 1261 87 281 495 1111 N-1A 2025-07-28 497 2025-07-28 2024-08-31 0000798244 2025-07-28 false ~ http://xbrl.sec.gov/rr/role/ShareholderFeesData row dei_DocumentInformationDocumentAxis compact cik0000798244_S000000765SummaryMember row dei_LegalEntityAxis compact cik0000798244_S000000765Member ~ ~ http://xbrl.sec.gov/rr/role/OperatingExpensesData row dei_DocumentInformationDocumentAxis compact cik0000798244_S000000765SummaryMember row dei_LegalEntityAxis compact cik0000798244_S000000765Member ~ ~ http://xbrl.sec.gov/rr/role/ExpenseExample row dei_DocumentInformationDocumentAxis compact cik0000798244_S000000765SummaryMember row dei_LegalEntityAxis compact cik0000798244_S000000765Member ~ ~ http://xbrl.sec.gov/rr/role/ExpenseExampleNoRedemption row dei_DocumentInformationDocumentAxis compact cik0000798244_S000000765SummaryMember row dei_LegalEntityAxis compact cik0000798244_S000000765Member ~ MFS SERIES TRUST I This contingent deferred sales charge (CDSC) applies to shares purchased without an initial sales charge and redeemed within 18 months of purchase. December 31, 2026 December 31, 2026 December 31, 2026 December 31, 2026 December 31, 2026 December 31, 2026 December 31, 2026 December 31, 2026 December 31, 2026 This contingent deferred sales charge (CDSC) applies to shares purchased without an initial sales charge and redeemed within 18 months of purchase. Massachusetts Financial Services Company (MFS) has agreed in writing to bear the fund’s expenses, excluding interest, taxes, extraordinary expenses, brokerage and transaction costs, certain tax reclaim recovery expenses (including contingency fees and closing agreement expenses), and investment-related expenses (such as interest and borrowing expenses incurred in connection with the fund’s investment activity), such that “Total Annual Fund Operating Expenses” do not exceed 1.21% of the class’ average daily net assets annually for each of Class A and Class R3 shares, 1.96% of the class’ average daily net assets annually for each of Class B, Class C, and Class R1 shares, 0.96% of the class’ average daily net assets annually for each of Class I and Class R4 shares, 1.46% of the class’ average daily net assets annually for Class R2 shares, and 0.85% of the class’ average daily net assets annually for Class R6 shares. This written agreement will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least December 31, 2026. Shares automatically convert to Class A shares approximately eight years after purchase; therefore, the expense examples reflect Class A share expenses after eight years.