0001562565 2026-09-18 2026-09-18 0001562565 fmr:FidelityFixed-IncomeActiveETFs-ComboPROMember fmr:S000042569Member 2026-09-18 2026-09-18 0001562565 fmr:FidelityFixed-IncomeActiveETFs-ComboPROMember fmr:S000042569Member fmr:Issuer-SpecificChangesHeaderMember 2026-09-18 2026-09-18 0001562565 fmr:FidelityFixed-IncomeActiveETFs-ComboPROMember fmr:S000042569Member fmr:Issuer-SpecificChangesTextnon-MMMember 2026-09-18 2026-09-18 0001562565 fmr:FidelityFixed-IncomeActiveETFs-ComboPROMember fmr:S000042569Member fmr:Issuer-SpecificChangesText-declinincreditqualityMember 2026-09-18 2026-09-18 0001562565 fmr:FidelityFixed-IncomeActiveETFs-ComboPROMember fmr:S000042569Member fmr:Issuer-SpecificChangesText-lowerqualitymorevolatileMember 2026-09-18 2026-09-18 0001562565 fmr:FidelityFixed-IncomeActiveETFs-ComboPROMember fmr:S000042569Member fmr:Issuer-SpecificChangesText-CLOMember 2026-09-18 2026-09-18 497 2026-09-18 false 0001562565 N-1A Fidelity Merrimack Street Trust Supplement to theFidelity® Corporate Bond ETF, Fidelity® Investment Grade Bond ETF, Fidelity® Investment Grade Securitized ETF, Fidelity® Limited Term Bond ETF, and Fidelity® Total Bond ETFDecember 30, 2025Prospectus Investing up to 30% of assets in lower-quality debt securities (those of less than investment-grade quality, also referred to as high yield debt securities or junk bonds). The following information supplements information for Fidelity® Limited Term Bond ETF found in the "Fund Summary" section under the "Principal Investment Strategies" heading. Investing in collateralized loan obligations (CLO). Issuer-Specific Changes. The value of an individual security or particular type of security can be more volatile than, and can perform differently from, the market as a whole. A decline in the credit quality of an issuer or a provider of credit support (such as guarantees) or a maturity-shortening structure (such as demand and put features) for a security can cause the price of a security to decrease. Lower-quality debt securities (those of less than investment-grade quality, also referred to as high yield debt securities or junk bonds) involve greater risk of default or price changes due to changes in the credit quality of the issuer. The value of lower-quality debt securities can be more volatile due to increased sensitivity to adverse issuer, political, regulatory, market, or economic developments. CLO tranches can experience substantial losses due to actual and anticipated defaults, as well as aversion to CLO securities as a class.