Agency MBS TBA Market: Forward Trading and Dollar Rolls

Key Takeaways

• TBA trades specify issuer, coupon, maturity, settlement month, price, and par amount; the exact pools are identified only 48 hours before settlement on notification day.
• The cheapest-to-deliver convention lets the short deliver any Good Delivery-eligible pool, keeping the market fungible and liquid across ~90% of agency MBS volume.
• Pools with prepayment protection, low loan balance, geographic concentration, high LTV, trade above TBA at a "pay-up" reflecting their superior collateral.
• The dollar roll, selling current-month TBA and buying next-month at a lower price, is a key financing tool; the drop reflects the cost of deferred settlement versus repo rates.

What It Is

Agency MBS are pass-through securities backed by residential mortgages and guaranteed by a US government agency (Ginnie Mae) or government-sponsored enterprise (Fannie Mae or Freddie Mac). Monthly principal and interest flow from borrowers through the servicer and trustee to investors.

The TBA market is the forward trading convention for these securities. Buyer and seller agree on issuer, coupon, maturity, settlement month, price, and par amount. The specific pools that will be delivered are identified only 48 hours before settlement, on the notification date. Until then, the trade is fungible.

The Intuition

Mortgages are not standardized at origination. A 6 percent 30-year pool delivered in June could be 10,000 different loans across 50 states. If every trade required identifying pools up front, liquidity would collapse into thousands of CUSIPs that rarely trade.

TBA solves this by letting traders quote a single forward contract for each combination of issuer, coupon, and maturity. The cheapest-to-deliver convention means the short can deliver any pool that meets Good Delivery standards, which keeps the market deep. Roughly 90 percent of agency MBS volume trades TBA rather than on a specified-pool basis, according to SIFMA.

How It Works

A TBA trade specifies six parameters:

Issuer (FN, FH, GN), Coupon (e.g. 5.5%), Maturity (30yr, 15yr),
Settlement month, Price, Par amount

Settlement follows a monthly schedule published by SIFMA. Each class (30-year conventional, 15-year conventional, Ginnie I, Ginnie II) settles on a different day of the month.

Good Delivery rules cap which pools can satisfy the short. The key constraints from the SIFMA guidelines include:

• Minimum pool size (typically 500,000 dollars or at least three pools per million).
• A 0.01 percent pool factor tolerance on current face versus agreed par. Delivered current face cannot differ from the trade amount by more than one basis point.
• No more than three pools per one million dollars of par.
• Issuer and coupon must match exactly.

The short chooses the worst-delivering pools that still meet these rules. That optionality is priced into the TBA versus specified-pool basis, often called pay-up. A pool with strong prepayment protection (low loan balance, NY state geography, high LTV) trades above TBA; a generic pool trades at TBA.

Dollar roll is the other central TBA mechanic. An investor sells the current month's TBA and simultaneously buys the next month's at a lower price. The price drop is the drop, which represents the coupon and carry the buyer forgoes by settling a month later. A positive drop means financing through the roll is cheap relative to repo, often driven by Fed purchases or tight specified-pool supply.

Worked Example

A money manager wants 50 million dollars of Fannie Mae 30-year 5.5 percent coupon MBS for June settlement. On April 10, the trader buys 50 million FNCL 5.5 June TBA at 99-16 (99.50).

On the June notification date (two business days before settlement), the dealer identifies the delivery: 12 pools totaling 49,998,750 in current face, a factor shortfall of 0.0025 percent, well inside the 0.01 percent tolerance. The manager pays 99.50 percent of the delivered par plus accrued interest from month-begin to settlement.

If the manager wants to defer taking delivery, they roll: sell the June TBA at 99-16 and buy July TBA at, say, 99-12. The 4/32 drop equals the cost of one more month of carry. If prevailing repo is higher than that implied financing rate, the roll is said to be trading special and rolling is cheaper than owning pools outright.

Common Mistakes

• Treating TBA price as a specified-pool price. A pool with prepayment protection can trade one to four points above TBA. Paying TBA for strong collateral means overpaying the dealer.
• Ignoring the drop. Money-market style investors sometimes roll mechanically and miss that a negative drop (or very small drop) indicates dealer funding stress, not free carry.
• Confusing FNCL and FNCI. FNCL is the 30-year conventional ticker; FNCI is 15-year. Their dollar rolls, payups, and OAS profiles are not substitutes.
• Underestimating convexity costs. Agency MBS are negatively convex because homeowners prepay when rates fall. Hedging a TBA position with Treasuries and ignoring the prepayment option leads to persistent slippage when rates move.
• Assuming Ginnie and conventional are fungible. Ginnie Mae MBS are backed by the full faith and credit of the US government and settle separately from Fannie and Freddie UMBS. A Ginnie II 5.5 is a different TBA contract from a FNCL 5.5.

Frequently Asked Questions

Why does the TBA market exist instead of trading individual pools? Residential mortgages are highly heterogeneous, each loan has a different borrower, property, geographic location, and prepayment propensity, making it impossible to standardize individual pools for liquid trading. The TBA convention creates a single forward contract for each issuer-coupon-maturity combination, allowing the seller to deliver any conforming pool 48 hours before settlement. This transforms a fragmented market of hundreds of thousands of CUSIPs into a handful of highly liquid contracts and supports the Fed's ability to conduct large-scale MBS purchase programs efficiently.

What does it mean for a pool to trade at a "pay-up" over TBA? A specified pool trades at a pay-up when buyers will pay a premium above the TBA price for its superior prepayment characteristics. Borrowers in certain pools, those with low original loan balances, state-specific geographic concentrations, or higher loan-to-value ratios, prepay more slowly because they have fewer refinancing options. That slower prepayment translates into longer expected duration and more predictable cash flows, which is valuable to investors who would otherwise face reinvestment risk when rates fall. The pay-up compensates the seller for giving up the cheapest-to-deliver optionality.

How does a dollar roll work and when is it attractive? A dollar roll involves simultaneously selling a TBA for the current settlement month and buying the same TBA for the next month at a lower price. The price difference, called the drop, represents the coupon income and carry the investor forgoes by delaying settlement by one month. The dollar roll is attractive when the implied financing rate, derived from the drop, is below the prevailing repo rate, meaning rolling is cheaper than owning pools and financing them in repo. When the Fed is actively buying MBS, demand for current-month delivery pushes the drop wider and makes dollar rolls trading special, effectively offering below-market financing to rolling holders.

What are SIFMA Good Delivery rules and why do they matter? SIFMA Good Delivery rules define which pools can satisfy a TBA trade. Key constraints include minimum pool size requirements, a 0.01 percent tolerance on current face versus the agreed par amount, and a limit of three pools per million dollars of par. The rules create enough standardization that any eligible pool can substitute for another, preserving fungibility. Without these rules, sellers would cherry-pick the worst pools and buyers would demand specific pools, collapsing the TBA market back into pool-by-pool trading with narrow liquidity.

How does negative convexity in agency MBS create hedging challenges? Agency MBS are negatively convex because homeowners tend to refinance, essentially prepaying the bond, when interest rates fall. When rates decline, MBS duration shortens as prepayment speeds up, reducing the price appreciation relative to a Treasury of equal initial duration. When rates rise, prepayments slow, duration extends, and the bond falls more in price than a non-prepayable bond would. This asymmetric duration behavior requires active hedging with Treasuries, interest rate swaps, or options; a static Treasury hedge will drift out of alignment whenever rates move significantly.

Sources

1. SIFMA. US Mortgage-Backed Securities Statistics and TBA Market Fact Sheet. https://www.sifma.org/resources/research/us-mortgage-backed-securities-statistics/
2. Fannie Mae. MBS Prospectus and Disclosures. https://capitalmarkets.fanniemae.com/mortgage-backed-securities
3. Freddie Mac. Participation Certificate and UMBS Disclosures. https://mf.freddiemac.com/investors/
4. Federal Reserve Bank of New York. Agency Mortgage-Backed Securities Operations. https://www.newyorkfed.org/markets/ambs
5. SIFMA. Uniform Practices for the Clearance and Settlement of MBS (Good Delivery Guidelines). https://www.sifma.org/resources/general/us-mbs-good-delivery-guidelines/

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.