Document NU1019 error for disallowed characters in TargetFramework names

Agent-Logs-Url: https://github.com/NuGet/docs.microsoft.com-nuget/sessions/939ab205-9284-4871-b331-25cae2734b0a

Co-authored-by: nkolev92 <[email protected]>

Author: Copilot

docs/reference/Errors-and-Warnings.md

@@ -27,7 +27,7 @@ NuGet supports the following configuration properties.
 
 | Group | Error Numbers |
 | --- | --- |
-| Invalid input errors | [NU1001](./errors-and-warnings/NU1001.md), [NU1002](./errors-and-warnings/NU1002.md), [NU1003](./errors-and-warnings/NU1003.md), [NU1004](./errors-and-warnings/NU1004.md), [NU1005](./errors-and-warnings/NU1005.md), [NU1006](./errors-and-warnings/NU1006.md), [NU1007](./errors-and-warnings/NU1007.md), [NU1008](./errors-and-warnings/NU1008.md), [NU1009](./errors-and-warnings/NU1009.md), [NU1010](./errors-and-warnings/NU1010.md), [NU1011](./errors-and-warnings/NU1011.md), [NU1012](./errors-and-warnings/NU1012.md), [NU1013](./errors-and-warnings/NU1013.md), [NU1014](./errors-and-warnings/NU1014.md), [NU1015](./errors-and-warnings/NU1015.md), [NU1016](./errors-and-warnings/NU1016.md), [NU1017](./errors-and-warnings/NU1017.md) |
+| Invalid input errors | [NU1001](./errors-and-warnings/NU1001.md), [NU1002](./errors-and-warnings/NU1002.md), [NU1003](./errors-and-warnings/NU1003.md), [NU1004](./errors-and-warnings/NU1004.md), [NU1005](./errors-and-warnings/NU1005.md), [NU1006](./errors-and-warnings/NU1006.md), [NU1007](./errors-and-warnings/NU1007.md), [NU1008](./errors-and-warnings/NU1008.md), [NU1009](./errors-and-warnings/NU1009.md), [NU1010](./errors-and-warnings/NU1010.md), [NU1011](./errors-and-warnings/NU1011.md), [NU1012](./errors-and-warnings/NU1012.md), [NU1013](./errors-and-warnings/NU1013.md), [NU1014](./errors-and-warnings/NU1014.md), [NU1015](./errors-and-warnings/NU1015.md), [NU1016](./errors-and-warnings/NU1016.md), [NU1017](./errors-and-warnings/NU1017.md), [NU1019](./errors-and-warnings/NU1019.md) |
 | Missing package and project errors | [NU1100](./errors-and-warnings/NU1100.md), [NU1101](./errors-and-warnings/NU1101.md), [NU1102](./errors-and-warnings/NU1102.md), [NU1103](./errors-and-warnings/NU1103.md), [NU1104](./errors-and-warnings/NU1104.md), [NU1105](./errors-and-warnings/NU1105.md), [NU1106](./errors-and-warnings/NU1106.md), [NU1107](./errors-and-warnings/NU1107.md), [NU1108](./errors-and-warnings/NU1108.md), [NU1109](./errors-and-warnings/NU1109.md) |
 | Package source mapping errors | [NU1110](./errors-and-warnings/NU1110.md) |
 | Compatibility errors | [NU1201](./errors-and-warnings/NU1201.md), [NU1202](./errors-and-warnings/NU1202.md), [NU1203](./errors-and-warnings/NU1203.md), [NU1204](./errors-and-warnings/NU1204.md), [NU1401](./errors-and-warnings/NU1401.md), [NU1402](./errors-and-warnings/NU1402.md), [NU1403](./errors-and-warnings/NU1403.md) |

docs/reference/errors-and-warnings/NU1019.md

@@ -0,0 +1,50 @@
+---
+title: NuGet Error NU1019
+description: NU1019 error code
+author: nkolev92
+ms.author: nikolev
+ms.date: 04/02/2026
+ms.topic: reference
+f1_keywords: 
+  - "NU1019"
+---
+
+# NuGet Error NU1019
+
+## Scenario
+
+> The project 'MyProject' contains a TargetFramework 'net10.0/win' with disallowed characters. TargetFramework names must contain only ASCII characters and must not contain path separators.
+
+### Issue
+
+Your project file has a `TargetFramework` or `TargetFrameworks` value that contains disallowed characters.
+NuGet doesn't allow the following characters in target framework names:
+
+- **Path separators** (`/` or `\`): These characters break the assets file format and are always disallowed.
+- **Non-ASCII characters**: Target framework names must contain only ASCII characters (characters with a value of 127 or less).
+
+This validation is gated behind the `SdkAnalysisLevel` property:
+
+- **SDK analysis level 10.0.300 or later**: Path separators in target framework names raise an error.
+  Non-ASCII characters raise a warning.
+- **SDK analysis level 11.0.100 or later**: Both path separators and non-ASCII characters raise an error.
+
+### Solution
+
+Rename the target framework values in your project file to use only ASCII characters and remove any path separators.
+
+For example, change:
+
+```xml
+<PropertyGroup>
+  <TargetFramework>net10.0/win</TargetFramework>
+</PropertyGroup>
+```
+
+To a valid target framework name:
+
+```xml
+<PropertyGroup>
+  <TargetFramework>net10.0-windows</TargetFramework>
+</PropertyGroup>
+```