0001562565 2026-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member 2026-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:C000255892Member 2026-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:MunicipalMarketVolatilityMember 2026-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:InterestRateChangesMember 2026-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:PrepaymentMember 2026-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:IssuerSpecificChangesMember 2026-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:FluctuationOfNetAssetValueAndSharePriceMember 2026-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:TradingIssuesMember 2026-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:LeverageRiskMember 2026-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:CashTransactionsRiskMember 2026-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member oef:RiskNotInsuredDepositoryInstitutionMember 2026-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member oef:RiskLoseMoneyMember 2026-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:C000255892Member 2024-01-01 2024-12-31 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:C000255892Member 2025-01-01 2025-12-31 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:C000255892Member 2025-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:C000255892Member 2023-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:C000255892Member oef:AfterTaxesOnDistributionsMember 2025-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:C000255892Member oef:AfterTaxesOnDistributionsMember 2023-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:C000255892Member oef:AfterTaxesOnDistributionsAndSalesMember 2025-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:C000255892Member oef:AfterTaxesOnDistributionsAndSalesMember 2023-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:IndexIXU23Member 2025-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:IndexIXU23Member 2023-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:IndexLB015Member 2025-05-30 2026-05-30 0001562565 fmr:FidelityMunicipalBondOpportunitiesETF-PROMember fmr:S000089370Member fmr:IndexLB015Member 2023-05-30 2026-05-30 iso4217:USD pure 485BPOS 2026-05-30 false 0001562565 N-1A Fidelity Merrimack Street Trust Fund Summary Fund: Fidelity® Municipal Bond Opportunities ETF Investment Objective Fidelity® Municipal Bond Opportunities ETF seeks to provide a high current yield exempt from federal income tax. Growth of capital may also be considered. Fee Table The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below. Shareholder fees 0 Annual Operating Expenses 0.0030 0 0.0000 0.0030 This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated: 31 97 169 381 Portfolio Turnover The fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when fund shares are held in a taxable account. These costs, which are not reflected in annual operating expenses or in the example, affect the fund's performance. During the most recent fiscal year, the fund's portfolio turnover rate was 39 % of the average value of its portfolio. 0.39 Principal Investment Strategies Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax. Municipal debt securities are issued to raise money for a variety of public and private purposes, including general financing for state and local governments, financing for eligible non-profit organizations, or financing for a specific project or public facility. Municipal debt securities in which the fund invests include securities issued by U.S. territories and possessions, general obligation bonds of municipalities, local or state governments, project or revenue-specific bonds, or pre-refunded or escrowed bonds, municipal money market securities, and synthetic securities. Municipal securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. Derivative instruments that provide investment exposure to the investments above or exposure to one or more market risk factors associated with such investments are included in the fund's 80% policy, consistent with the fund's investment policies and limitations with respect to investments in derivatives. 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). Investing up to 20% of assets in securities that are not municipal securities exempt from federal income tax, including, but not limited to, taxable municipal bonds, U.S. Treasury and Government agency issues, and investment-grade corporate bonds. Managing the fund to have similar overall interest rate risk to the supplemental index (see "Additional Index Information" section of the prospectus). Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments. Allocating assets across different market sectors and maturities. Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects. Engaging in transactions that have a leveraging effect on the fund, including investments in derivatives - such as swaps (interest rate, total return, and credit default), options, and futures contracts - and forward-settling securities, to adjust the fund's risk exposure. Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities. Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease. Prepayment. The ability of an issuer of a debt security to repay principal prior to a security's maturity can cause greater price volatility if interest rates change. 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) and certain types of other securities 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 and certain types of other securities can be more volatile due to increased sensitivity to adverse issuer, political, regulatory, market, or economic developments and can be difficult to resell. Fluctuation of Net Asset Value and Share Price. The net asset value per share (NAV) of the fund will generally fluctuate with changes in the market value of the fund's holdings. The fund's shares can be bought and sold in the secondary market at market prices. Disruptions to creations and redemptions, the existence of extreme market volatility or potential lack of an active trading market for the fund's shares may result in the fund's shares trading significantly above (at a premium) or below (at a discount) to NAV. Given the nature of the relevant markets for certain of the fund's securities, shares may trade at a larger premium or discount to the NAV than shares of other ETFs. In addition, in stressed market conditions or periods of market disruption or volatility, the market for shares may become less liquid in response to deteriorating liquidity in the markets for the fund's underlying portfolio holdings. Trading Issues. There can be no assurance that an active trading market will be maintained. Market makers and Authorized Participants are not obligated to make a market in the fund's shares or to submit purchase and redemption orders for creation units. Decisions by market makers or Authorized Participants to reduce their role with respect to market making or creation and redemption activities during times of market stress, or a decline in the number of Authorized Participants due to decisions to exit the business, bankruptcy, or other factors, could inhibit the effectiveness of the arbitrage process in maintaining the relationship between the underlying value of the fund's portfolio securities and the market price of fund shares. In addition, trading may be halted, for example, due to market conditions. Leverage Risk. Leverage can increase market exposure, magnify investment risks, and cause losses to be realized more quickly. Cash Transactions Risk. Unlike certain ETFs, the fund may effect some or all creations and redemptions using cash, rather than in-kind securities. As a result, an investment in the fund may be less tax-efficient than an investment in an ETF that distributes portfolio securities entirely in-kind. An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency .   You could lose money by investing in the fund. Performance Effective April 4, 2025, Fidelity® Municipal Core Plus Bond Fund ("Predecessor Fund") was reorganized into the fund ("Reorganization"). The Predecessor Fund's investment objective is identical to the fund's and the Predecessor Fund is managed in a manner that, in all material respects, complied with the investment guidelines and restrictions of the fund. The Predecessor Fund is designated as the accounting survivor in the Reorganization. As a result, the fund has assumed the historical performance of Fidelity® Municipal Core Plus Bond Fund, a class of shares of the Predecessor Fund, and the performance information shown below for periods prior to April 4, 2025 reflects the performance of the Predecessor Fund, which had a different fee structure than the fund. Past performance may have been different if the fund's current fee structure had been in place during the period. The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index and a hypothetical composite of market indexes over various periods of time. The hypothetical composite of market indexes has characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance . Visit www.fidelity.com for more recent performance information. The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index and a hypothetical composite of market indexes over various periods of time. Past performance (before and after taxes) is not an indication of future performance . www.fidelity.com Year-by-Year Returns 0.0184 0.0550    Highest Quarter Return 0.0305 2025-09-30    Lowest Quarter Return -0.0120 2024-12-31    Year-to-Date Return -0.0002 2026-03-31 Average Annual Returns After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares. After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares. Return Before Taxes 0.0550 0.0480 Return After Taxes on Distributions 0.0544 0.0476 Return After Taxes on Distributions and Sale of Fund Shares 0.0467 0.0449 Fidelity Municipal Bond Opportunities Composite Index℠ 0.0436 0.0373 Bloomberg Municipal Bond Index 0.0425 0.0361 2023-02-16 A From February 16, 2023 .