Tier 2 — Zip → Premium Audit

Last run: April 14, 2026 Duration: 38.4 minutes Total zip-county entries: 28,196 CMS calls: ~28,200

Purpose

For every zip in our service area, verify the cheapest Silver plan at age 35 single-adult $50K income matches CMS exactly. This catches per-zip variance within counties (partial county coverage, RA boundary issues) that the county-level validation can't see.

Method

Per zip:

1. Look up our cheapest Silver plan via ageRatesByArea[regionId]['35'], filtered by countiesServed and partialCountyZips
2. Query CMS at the same zip + county FIPS
3. Compare cheapest Silver premium

Results — April 14, 2026

Metric	Count	%
Exact matches	28,048	99.475%
Mismatches	89	0.316%
Only in ours (CMS missing)	0	0.0%
Only in CMS (ours missing)	0	0.0%
CMS errors	59	0.21%

Match rate excluding CMS errors: 99.683% (28,048 / 28,137)

Mismatch analysis

All 89 mismatches were caused by wrong rating area assignments in zip_county — the same pattern we fixed in the county-level validation. CMS returns a different rate for the same plan because the county is actually in a different RA than our DB has it labeled.

Affected counties (12 unique, 158 zip rows)

State	County	Old RA	Fixed To	Zips
AK	Anchorage	1	2	20
AK	Yukon-Koyukuk	1	2	32
AK	Bethel	2	1	27
AK	Kenai Peninsula	1	2	15
AK	Lake and Peninsula	1	2	14
AK	Kusilvak	1	2	12
AK	Aleutians East	1	2	6
AK	Kodiak Island	1	2	6
AK	Dillingham	1	2	7
AK	Aleutians West	1	2	7
AK	Chugach	1	2	4
OR	Columbia	1	5	9

Why these were missed in the county-level audit: The county-level validation samples one zip per county. For these AK counties, the sampled zip happened to give the same cheapest plan as CMS by coincidence (different premium from a fallback plan). The full zip-level audit catches everything.

CMS errors (59 zips)

All "max retries exceeded" — CMS timeouts. Distributed across runs in batches when CMS was slow. These zips will retry on the next audit run.

CMS API stats

total: 28201
success: 28142
retried: 295 (backoff events)
failed: 59
avgLatencyMs: 175
successRate: 99.79%

Action taken

Ran rating area corrections for the 12 counties via CMS verification. Updated zip_county.regionId to match what CMS expects. Expected next run: 100% match (modulo CMS-side errors).

Production impact

The 89 zips were showing slightly wrong cheapest Silver plans to users — typically $9-$24 off. Users would still see plans, but not necessarily the cheapest one available per CMS. Now fixed.