What does nuget:NU1301 mean?

NuGet could not use one of the configured package sources because the source URL or local path was unavailable, invalid, or unreadable during restore.

How do I prevent nuget:NU1301 in CI?

Version-control a minimal `NuGet.Config` with only approved sources. Remove stale local paths and validate feed reachability in CI before restore.

How can I manually validate nuget:NU1301?

Re-run the failing command and confirm the original code/message is gone. Confirm expected artifacts or outputs exist (packages restored, build/publish succeeds).

NU1301: Unable to load the service index for source

Fix it fast

Most likely: One configured NuGet source is unreachable, invalid, blocked by proxy/TLS/auth, or points at a local path that does not exist on this machine.

1. Confirm this is your error

error NU1301: Unable to load the service index for source https://api.nuget.org/v3/index.json.

2. Check the cause

dotnet nuget list source
dotnet restore -v minimal
dotnet nuget locals all --list

3. Apply the safe fix

# If the failing source is stale or wrong, disable it or fix the URL/path in NuGet.Config.
dotnet nuget disable source <source-name>

# If credentials or cache are stale, clear caches after fixing the source/auth config.
dotnet nuget locals all --clear
dotnet restore

4. Verify it works

dotnet restore -v minimal
dotnet build --no-restore

Don't use unsafe shortcuts

Do not keep a broken local folder source in repo or machine-level NuGet.Config.
Do not mask a feed outage by adding random public sources.
Do not clear caches repeatedly until you have checked the exact failing source URL or path.

Where the Request Failed

NuGet is telling you the request failed before it got a clean response back. Treat the connection path and the failing environment as the first suspects, not the package or image name.

Restore connectivity to the package source

List configured sources first:dotnet nuget list source

If the failing source is local, fix the path or remove that source from NuGet.Config.

If the failing source is remote, confirm the exact v3 URL is reachable from the same machine or container and fix URL, proxy, TLS, or auth as needed.

Clear caches, then retry:dotnet nuget locals all --clear, then dotnet restore

Validation

Re-run the failing command and confirm the original code/message is gone, and confirm expected artifacts or outputs exist (packages restored, build/publish succeeds).

Why the Local Machine Blocked It

Usually this comes down to a configured local source path no longer exists on the machine or agent, the remote feed URL is wrong, blocked, or not serving a valid NuGet v3 service index, or TLS, proxy, or feed authentication problems prevent NuGet from reading the source.

Prove the Failing Environment Can Reach It

dotnet restore -v minimal succeeds without NU1301.

The failing source is either reachable and valid, or no longer configured for this build.

Mechanism

NuGet checks each configured source before it can resolve package metadata and discover download endpoints. If a configured local source path is missing, or a remote source URL cannot be reached or read correctly, restore can stop with NU1301.

Prevent Repeat Connectivity Failures

To prevent this, version-control a minimal NuGet.Config with only approved sources, and remove stale local paths and validate feed reachability in CI before restore.

Docs and source code

NuGet.Protocol service index failure message

User-facing message template. - GitHub

  <data name="Log_FailedToReadServiceIndex" xml:space="preserve">
    <value>Unable to load the service index for source {0}.</value>
  </data>
  <data name="Log_RetryingServiceIndex" xml:space="preserve">
    <value>Retrying service index request for source '{0}'.</value>