Money in Fidelity money market funds is protected by the SIPC, up to $500k.
Specifically, for their cash management accounts:
“Cash balances in the Fidelity® Cash Management Account are swept into an FDIC-Insured interest bearing account at one or more program banks and, under certain circumstances, a money market mutual fund (the "Money Market Overflow"). Deposits swept into the program bank(s) are eligible for FDIC Insurance, subject to FDIC insurance coverage limits. Balances that are swept to the Money Market Overflow are not eligible for FDIC insurance but are eligible for SIPC coverage under SIPC rules (referenced below). Fidelity automatically performs all transfers between the program banks and your account. You cannot access your funds directly from a program bank.” [1]
More detail:
“Securities Investor Protection Corporation (SIPC) - All Fidelity brokerage accounts are automatically protected by the SIPC. SIPC protects brokerage accounts of each customer when a brokerage firm is closed due to bankruptcy or other financial difficulties and customer assets are missing from accounts, including a limit of up to $500,000 in securities with a maximum of $250,000 on claims for cash awaiting investment. Money market funds held in a brokerage account are considered securities. For more information, visit sipc.org.” [2]
Specifically, for their cash management accounts: “Cash balances in the Fidelity® Cash Management Account are swept into an FDIC-Insured interest bearing account at one or more program banks and, under certain circumstances, a money market mutual fund (the "Money Market Overflow"). Deposits swept into the program bank(s) are eligible for FDIC Insurance, subject to FDIC insurance coverage limits. Balances that are swept to the Money Market Overflow are not eligible for FDIC insurance but are eligible for SIPC coverage under SIPC rules (referenced below). Fidelity automatically performs all transfers between the program banks and your account. You cannot access your funds directly from a program bank.” [1]
More detail: “Securities Investor Protection Corporation (SIPC) - All Fidelity brokerage accounts are automatically protected by the SIPC. SIPC protects brokerage accounts of each customer when a brokerage firm is closed due to bankruptcy or other financial difficulties and customer assets are missing from accounts, including a limit of up to $500,000 in securities with a maximum of $250,000 on claims for cash awaiting investment. Money market funds held in a brokerage account are considered securities. For more information, visit sipc.org.” [2]
[1] https://www.fidelity.com/cash-management/fidelity-cash-manag...
[2] http://personal.fidelity.com/misc/ekits/pdf/safeguarding_you...