## TL;DR
The configured feeds do not carry that package version. Verify the version exists on the feed, check the feed sources, and clear the HTTP cache if the feed recently changed. Then restore again.

## Error

```text
nuget 'NU1101: Unable to find package X' after a version bump
```

## Steps

1. Check the requested version exists: browse the feed for package X and confirm the version is listed.
   Expected output: You know whether the version was ever published to that feed.
2. Check the configured sources: dotnet nuget list sources.
   Expected output: You see which feeds restore actually queries.
3. Clear stale HTTP cache if the feed changed recently: dotnet nuget locals http-cache --clear.
   Expected output: The next restore queries the feed fresh.
4. Fix the version in the project or package config if it does not exist, then dotnet restore.
   Expected output: Restore finds the package and completes.
5. Build to confirm: dotnet build.
   Expected output: The build succeeds with the restored packages.

## When to use

- Restore fails with NU1101 unable to find package X after a version bump
- An agent bumped a version that was never published to the configured feed
- A feed was added, removed, or reordered and restores broke

## When not to use

- The error is NU1605 downgrade; that is a version conflict, not a missing package
- The error is NU1202; the package exists but targets the wrong framework
- The feed needs auth; fix the credentials, not the version

## Variant phrasings

### NU1101 on a private feed after a version bump
The private-feed shape; the version may exist publicly but not on the internal feed.

### Unable to find package on CI but restore works locally
CI queries a different feed set or a stale cache; align sources and clear the cache.

### NU1101 for a version that exists on nuget.org
A source mapping or disabled source is hiding the public feed; check the source list.

## Why it happens

Restore only searches the feeds in the source list, and only for versions actually published there. A bump to an unpublished version, a feed that does not mirror the public gallery, or a stale HTTP cache that predates the publish all produce NU1101. The error is about feed contents, not about your code.

## Edge cases

- Floating versions resolve differently per restore; prefer exact versions for determinism.
- Source mappings in NuGet.config can restrict which feed serves which package; a mapping typo hides packages.
- Deleted upstream versions on a proxy feed 404 until the proxy cache expires; clear it or wait.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_hu1EkILmemaN7_iTNZCcvg