[TypeScript] Error "Namespace not found for return type" for NOTION.COM api

[WolfgangHG]

What are you generating using Kiota, clients or plugins?

API Client/SDK

In what context or format are you using Kiota?

Nuget tool

Client library/SDK language

None

Describe the bug

Found as part of #7774

It fails with an error:

crit: Kiota.Builder.KiotaBuilder[0]
      error generating the client: Namespace not found for return type
      System.InvalidOperationException: Namespace not found for return type
         at Kiota.Builder.Writers.TypeScript.CodeFunctionWriter.FindFunctionInNameSpace(String functionName, CodeElement codeElement, CodeType returnType) in E:\Projekte\Github\kiota\src\Kiota.Builder\Writers\TypeScript\CodeFunctionWriter.cs:line 443
         at Kiota.Builder.Writers.TypeScript.CodeFunctionWriter.GetDeserializerFunctionName(CodeElement codeElement, CodeType returnType) in E:\Projekte\Github\kiota\src\Kiota.Builder\Writers\TypeScript\CodeFunctionWriter.cs:line 436
         at Kiota.Builder.Writers.TypeScript.CodeFunctionWriter.WriteComposedTypeDeserializer(CodeFunction codeElement, LanguageWriter writer, CodeParameter composedParam, CodeFile codeFile) in E:\Projekte\Github\kiota\src\Kiota.Builder\Writers\TypeScript\CodeFunctionWriter.cs:line 154
         at Kiota.Builder.Writers.TypeScript.CodeFunctionWriter.WriteDeserializerFunction(CodeFunction codeFunction, CodeFile codeFile, LanguageWriter writer) in E:\Projekte\Github\kiota\src\Kiota.Builder\Writers\TypeScript\CodeFunctionWriter.cs:line 667
         at Kiota.Builder.Writers.TypeScript.CodeFunctionWriter.WriteCodeElement(CodeFunction codeElement, LanguageWriter writer) in E:\Projekte\Github\kiota\src\Kiota.Builder\Writers\TypeScript\CodeFunctionWriter.cs:line 115
         at Kiota.Builder.Writers.LanguageWriter.Write[T](T code) in E:\Projekte\Github\kiota\src\Kiota.Builder\Writers\LanguageWriter.cs:line 149
         at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCode(LanguageWriter writer, CodeElement element) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 83
         at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCode(LanguageWriter writer, CodeElement element) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 88
         at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCodeNamespaceToSingleFileAsync(LanguageWriter writer, CodeElement codeElement, String outputFile, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 35
         at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCodeNamespaceToSingleFileAsync(LanguageWriter writer, CodeElement codeElement, String outputFile, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 37
         at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCodeNamespaceToFilePerClassAsync(LanguageWriter writer, CodeNamespace currentNamespace, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 55
         at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCodeNamespaceToFilePerClassAsync(LanguageWriter writer, CodeNamespace currentNamespace, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 59
         at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCodeNamespaceToFilePerClassAsync(LanguageWriter writer, CodeNamespace currentNamespace, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 59
         at Kiota.Builder.CodeRenderers.CodeRenderer.RenderCodeNamespaceToFilePerClassAsync(LanguageWriter writer, CodeNamespace currentNamespace, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\CodeRenderers\CodeRenderer.cs:line 59
         at Kiota.Builder.KiotaBuilder.CreateLanguageSourceFilesAsync(GenerationLanguage language, CodeNamespace generatedCode, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\KiotaBuilder.cs:line 668
         at Kiota.Builder.KiotaBuilder.<>c__DisplayClass21_0.<<GenerateClientAsync>b__0>d.MoveNext() in E:\Projekte\Github\kiota\src\Kiota.Builder\KiotaBuilder.cs:line 311
      --- End of stack trace from previous location ---
         at Kiota.Builder.KiotaBuilder.GenerateConsumerAsync(Func`5 innerGenerationSteps, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\KiotaBuilder.cs:line 348
         at Kiota.Builder.KiotaBuilder.GenerateConsumerAsync(Func`5 innerGenerationSteps, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\KiotaBuilder.cs:line 368
         at Kiota.Builder.KiotaBuilder.GenerateClientAsync(CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\Kiota.Builder\KiotaBuilder.cs:line 285
         at kiota.Handlers.KiotaGenerateCommandHandler.InvokeAsync(ParseResult parseResult, CancellationToken cancellationToken) in E:\Projekte\Github\kiota\src\kiota\Handlers\KiotaGenerateCommandHandler.cs:line 166

Expected behavior

No error, client should be generated

How to reproduce

After having built Kiota, execute this:

.\it\generate-code.ps1 -descriptionUrl https://developers.notion.com/openapi.json  -language "typescript" -dev

Open API description file

https://developers.notion.com/openapi.json

Kiota Version

source code build dated 2026-06-15

Latest Kiota version known to work for scenario above?(Not required)

No response

Known Workarounds

No response

Configuration

No response

Debug output

No response

Other information

No response

[WolfgangHG]

Debugging reveals these parameter values for "CodeFunctionWriter.FindFunctionInNameSpace":

functionName = deserializeIntoDateOnly
codeElement.Name = deserializeIntoDateResponse_end
returnType.Name = DateOnly

This is coming from this snippet of the API description:

  "dateResponse": {
	"type": "object",
	"properties": {
	  "start": {
		"type": "string",
		"format": "date",
		"description": "The start date of the date object."
	  },
	  "end": {
		"oneOf": [
		  {
			"type": "string",
			"format": "date"
		  },
		  {
			"type": "null"
		  }
		],
		"description": "The end date of the date object, if any."
	  },
	  "time_zone": {
		"oneOf": [
		  {
			"$ref": "#/components/schemas/timeZoneRequest"
		  },
		  {
			"type": "null"
		  }
		],
		"description": "The time zone of the date object."
	  }
	},
	"additionalProperties": false,
	"required": [
	  "start",
	  "end",
	  "time_zone"
	]
  },