Change HTTP response default encoding from ISO-8859-1 to UTF-8

Matches System.Net.Http.HttpClient behaviour. RFC 2616 specifies
ISO-8859-1 as the default, but modern servers, browsers, and HttpClient
all default to UTF-8 in practice. This fixes garbled characters when
fetching UTF-8 content from servers that don't specify a charset.

The responseEncodingOverride parameter can still be used to override
for any response, including forcing ISO-8859-1 when needed.

Closes #1251

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

RELEASE_NOTES.md

@@ -2,6 +2,7 @@
 
 ## 8.1.0-beta
 
+- Change HTTP response default encoding from ISO-8859-1 to UTF-8 to match `System.Net.Http.HttpClient` behaviour (closes #1251)
 - Add `PreferOptionals` parameter to `JsonProvider` and `XmlProvider` (defaults to `true` to match existing behavior; set to `false` to use empty string or `NaN` for missing values, like the CsvProvider default) (closes #649)
 
 ## 8.0.0 - Feb 25 2026

src/FSharp.Data.DesignTime/Csv/CsvProvider.fs

@@ -264,7 +264,7 @@ type public CsvProvider(cfg: TypeProviderConfig) as this =
         + """</c>.</param>
            <param name='CacheRows'>Whether the rows should be caches so they can be iterated multiple times. Defaults to true. Disable for large datasets.</param>
            <param name='Culture'>The culture used for parsing numbers and dates. Defaults to the invariant culture.</param>
-           <param name='Encoding'>The encoding used to read the sample. You can specify either the character set name or the codepage number. Defaults to UTF8 for files, and to ISO-8859-1 the for HTTP requests, unless <c>charset</c> is specified in the <c>Content-Type</c> response header.</param>
+           <param name='Encoding'>The encoding used to read the sample. You can specify either the character set name or the codepage number. Defaults to UTF8 for files, and for HTTP requests when no <c>charset</c> is specified in the <c>Content-Type</c> response header.</param>
            <param name='ResolutionFolder'>A directory that is used when resolving relative file references (at design time and in hosted execution).</param>
            <param name='EmbeddedResource'>When specified, the type provider first attempts to load the sample from the specified resource 
               (e.g. 'MyCompany.MyAssembly, resource_name.csv'). This is useful when exposing types generated by the type provider.</param>

src/FSharp.Data.DesignTime/Html/HtmlProvider.fs

@@ -111,7 +111,7 @@ type public HtmlProvider(cfg: TypeProviderConfig) as this =
         + String.Join(",", TextConversions.DefaultMissingValues)
         + """</c>.</param>
            <param name='Culture'>The culture used for parsing numbers and dates. Defaults to the invariant culture.</param>
-           <param name='Encoding'>The encoding used to read the sample. You can specify either the character set name or the codepage number. Defaults to UTF8 for files, and to ISO-8859-1 the for HTTP requests, unless <c>charset</c> is specified in the <c>Content-Type</c> response header.</param>
+           <param name='Encoding'>The encoding used to read the sample. You can specify either the character set name or the codepage number. Defaults to UTF8 for files, and for HTTP requests when no <c>charset</c> is specified in the <c>Content-Type</c> response header.</param>
            <param name='ResolutionFolder'>A directory that is used when resolving relative file references (at design time and in hosted execution).</param>
            <param name='EmbeddedResource'>When specified, the type provider first attempts to load the sample from the specified resource 
               (e.g. 'MyCompany.MyAssembly, resource_name.html'). This is useful when exposing types generated by the type provider.</param>

src/FSharp.Data.DesignTime/Json/JsonProvider.fs

@@ -186,7 +186,7 @@ type public JsonProvider(cfg: TypeProviderConfig) as this =
            <param name='SampleIsList'>If true, sample should be a list of individual samples for the inference.</param>
            <param name='RootName'>The name to be used to the root type. Defaults to `Root`.</param>
            <param name='Culture'>The culture used for parsing numbers and dates. Defaults to the invariant culture.</param>
-           <param name='Encoding'>The encoding used to read the sample. You can specify either the character set name or the codepage number. Defaults to UTF8 for files, and to ISO-8859-1 the for HTTP requests, unless `charset` is specified in the `Content-Type` response header.</param>
+           <param name='Encoding'>The encoding used to read the sample. You can specify either the character set name or the codepage number. Defaults to UTF8 for files, and for HTTP requests when no `charset` is specified in the `Content-Type` response header.</param>
            <param name='ResolutionFolder'>A directory that is used when resolving relative file references (at design time and in hosted execution).</param>
            <param name='EmbeddedResource'>When specified, the type provider first attempts to load the sample from the specified resource
               (e.g. 'MyCompany.MyAssembly, resource_name.json'). This is useful when exposing types generated by the type provider.</param>

src/FSharp.Data.DesignTime/Xml/XmlProvider.fs

@@ -213,7 +213,7 @@ type public XmlProvider(cfg: TypeProviderConfig) as this =
            <param name='SampleIsList'>If true, the children of the root in the sample document represent individual samples for the inference.</param>
            <param name='Global'>If true, the inference unifies all XML elements with the same name.</param>                     
            <param name='Culture'>The culture used for parsing numbers and dates. Defaults to the invariant culture.</param>
-           <param name='Encoding'>The encoding used to read the sample. You can specify either the character set name or the codepage number. Defaults to UTF8 for files, and to ISO-8859-1 the for HTTP requests, unless <c>charset</c> is specified in the <c>Content-Type</c> response header.</param>
+           <param name='Encoding'>The encoding used to read the sample. You can specify either the character set name or the codepage number. Defaults to UTF8 for files, and for HTTP requests when no <c>charset</c> is specified in the <c>Content-Type</c> response header.</param>
            <param name='ResolutionFolder'>A directory that is used when resolving relative file references (at design time and in hosted execution).</param>
            <param name='EmbeddedResource'>When specified, the type provider first attempts to load the sample from the specified resource 
               (e.g. 'MyCompany.MyAssembly, resource_name.xml'). This is useful when exposing types generated by the type provider.</param>

src/FSharp.Data.Http/Http.fs

@@ -1376,8 +1376,8 @@ module HttpEncodings =
     /// ISO-8859-1
     let PostDefaultEncoding = Encoding.GetEncoding("ISO-8859-1") // http://stackoverflow.com/questions/708915/detecting-the-character-encoding-of-an-http-post-request/708942#708942
 
-    /// ISO-8859-1
-    let ResponseDefaultEncoding = Encoding.GetEncoding("ISO-8859-1") // http://www.ietf.org/rfc/rfc2616.txt
+    /// UTF-8 (matches System.Net.Http.HttpClient default behaviour; RFC 2616 specifies ISO-8859-1 but modern servers and browsers default to UTF-8)
+    let ResponseDefaultEncoding = Encoding.UTF8
 
     let internal getEncoding (encodingStr: string) =
         match Int32.TryParse(encodingStr, NumberStyles.Integer, CultureInfo.InvariantCulture) with

tests/FSharp.Data.Core.Tests/HttpEncodings.fs

@@ -10,8 +10,8 @@ let ``HttpEncodings.PostDefaultEncoding returns ISO-8859-1`` () =
     HttpEncodings.PostDefaultEncoding.WebName |> should equal "iso-8859-1"
 
 [<Test>]
-let ``HttpEncodings.ResponseDefaultEncoding returns ISO-8859-1`` () =
-    HttpEncodings.ResponseDefaultEncoding.WebName |> should equal "iso-8859-1"
+let ``HttpEncodings.ResponseDefaultEncoding returns UTF-8`` () =
+    HttpEncodings.ResponseDefaultEncoding.WebName |> should equal "utf-8"
 
 [<Test>]
 let ``HttpEncodings.getEncoding with valid encoding name works`` () =