themortgagellm^™

Response

Freddie Mac Single-Class MBS book, snapshot-by-snapshot, January through July 2026:

What’s in the read:

• Pool count grew 8.5% while UPB stayed flat. Net issuance of ~36K new pools over 7 months, but the outstanding dollar balance barely moved — amortization + prepayment on the existing book roughly matched the ~$45B/month of new issuance. The typical new pool is smaller (~$13.8M average) than the older pools running off (which had aggregated more UPB pre-paydown).
• March→April was the biggest pool-count jump (+26,111 in one month) vs 2–4K/mo the rest of the window. Worth a look — that could be seasonal Q1 origination catching up on delivery, a policy trigger (e.g., LLPA update taking effect that moved a batch of loans through), or a subtype expansion (adding pools that weren’t previously in the Single-Class disclosure scope). Grep the April issue-date distribution to isolate.
• Rate creep is 3.5 bps over 7 months — slow-motion. The book is dominated by older sub-4% pools that are still holding on because refinance incentive at 5.2% is limited. As those run off gradually, the WA current rate drifts up toward the coupon of newly-issued pools.
• WA FICO 734→735 is credit-stable. The one-point tick likely reflects the mix of new issuance skewing marginally higher-FICO than the paydown, but it’s within the noise floor of book-wide WA drift. No credit-selection story from this window.

Reference: mortgage.fre_mbs_pool_details, seven consecutive first-of-month snapshots derived from the Freddie Monthly Security Factor File. The snapshot date is fixed at file-generation time by parsing the file’s own Security Factor Date column, so download timing doesn’t shift the reported month. Freddie exposes the trailing 6 months on the MBS Securities portal at any given time.