How it works

 import "npm:react@^19"
         │
         ▼                                   RESOLUTION: deterministic
   route match      "*" → fallback(cache(), race(esmSh(), jsDelivr()), unpkg())
         │
   registry lookup  react@^19 → 19.2.0 (npm registry / JSR metadata, or the lockfile)
         │
         ▼                                   ROUTING: adaptive
   strategy tree    fallback ─┬─ cache
                              ├─ race ─┬─ esm.sh   ✓ 41 ms  ← selected
                              │        └─ jsDelivr ✗ aborted
                              └─ unpkg
         │
         ▼                                   COMPILATION
   import map       "react": "https://esm.sh/[email protected]?target=es2022"
         │
         ▼
   native browser ESM

The rule mport follows is that resolution is deterministic and transport is adaptive:

• react@^19 becomes 19.2.0 once. The lockfile then pins that version together with its build, and a CDN outage can’t change it.
• Providers that transform packages, such as esm.sh and jspm, serve a different artifact from providers that serve raw npm files, such as jsDelivr and unpkg. Only providers with the same build count as mirrors of each other. Once something is locked to the npm build, failover moves between jsDelivr, unpkg and your local() copy, and never silently switches to an esm.sh build.

Native import maps vs mport

	Import map	mport
Map a bare specifier to a URL	✓	✓ (this is what it outputs)
Prefix mappings, scopes	✓	✓
Integrity	✓ (integrity)	✓ computes it with verified()
Several candidate URLs for one specifier	✗	✓ fallback, race, adaptive
Choose by health, latency or capability	✗	✓
Version ranges → exact versions	✗	✓ plus a lockfile
Retry another CDN after the browser has picked a URL	✗	only through router.import() / createImporter()

Once the browser has resolved import "react" through an import map, there is no standard hook to try another URL if that fetch fails. mport therefore resolves ahead of time (CLI or startup()), or runs every load through router.import().