Gate the API documentation with DocFX

The Kotlin backend checked its documentation with Dokka; the ASP.NET Core
API had only the compiler warning for a public declaration without a
comment. CI now builds the API reference with DocFX, every warning as an
error, and Documents/api/CheckDocumentation.cs fails when a namespace,
type or member of the reference has no description, internal and
private members included. The setup is the one Progmaweb uses: the
docfx tool pinned in .config/dotnet-tools.json, the configuration and
the namespace description under Documents/api, and the generated
reference ignored.

12 files changed, 268 insertions, 2 deletions

diff --git a/.config/dotnet-tools.json b/.config/dotnet-tools.json
new file mode 100644
index 0000000..e4d2756
--- /dev/null
+++ b/.config/dotnet-tools.json
@@ -0,0 +1,11 @@
+{
+  "version": 1,
+  "isRoot": true,
+  "tools": {
+    "docfx": {
+      "version": "2.81.0",
+      "commands": ["docfx"],
+      "rollForward": false
+    }
+  }
+}
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 4929d81..6aee49a 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -65,6 +65,12 @@ jobs:
       - run: dotnet build Progmasoft.Tala.Website.slnx --configuration Release --no-restore
       - run: dotnet test --project backend/Progmasoft.Tala.Website.Api.Tests/Progmasoft.Tala.Website.Api.Tests.csproj --configuration Release --no-build --coverlet --coverlet-output-format cobertura --results-directory TestResults
       - run: dotnet publish backend/Progmasoft.Tala.Website.Api/Progmasoft.Tala.Website.Api.csproj --configuration Release --no-restore --output publish
+      # Documentation gate. DocFX builds the API reference with every warning
+      # as an error; the coverage check then fails when a namespace, type or
+      # member of the reference has no description.
+      - run: dotnet tool restore
+      - run: dotnet docfx Documents/api/docfx.json --warningsAsErrors
+      - run: dotnet run Documents/api/CheckDocumentation.cs -- Documents/api
       - uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         with:
           name: website-backend-coverage
diff --git a/.gitignore b/.gitignore
index 8f9dffa..2aea83e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -17,3 +17,5 @@ build/
 *.user
 TestResults/
 publish/
+Documents/api/_site/
+Documents/api/reference/
diff --git a/Documents/DEVELOPMENT.md b/Documents/DEVELOPMENT.md
index 835ae05..0bf7f55 100644
--- a/Documents/DEVELOPMENT.md
+++ b/Documents/DEVELOPMENT.md
@@ -41,6 +41,10 @@ dotnet run --project backend/Progmasoft.Tala.Website.Api      # serves on 127.0.
 ```
 
 - The build treats warnings as errors and requires a documentation comment on every public declaration.
+- DocFX builds the API reference from those comments. `dotnet tool restore`, then
+  `dotnet docfx Documents/api/docfx.json --warningsAsErrors` and
+  `dotnet run Documents/api/CheckDocumentation.cs -- Documents/api`; the second fails on any namespace, type or
+  member of the reference without a description. See [the API reference](api/index.md).
 - The tests run the endpoints in an in-memory host with xUnit v3 on Microsoft.Testing.Platform.
 - `PORT` changes the port; the service binds to loopback only.
 
@@ -87,7 +91,7 @@ narrow window, and keyboard focus.
 | Job | Enforces |
 | --- | --- |
 | `frontend` | `npm ci`, `npm audit` at moderate severity, type check, tests with coverage, format check, build |
-| `backend` | locked restore, build with warnings as errors, tests with coverage, and the published application |
+| `backend` | locked restore, build with warnings as errors, tests with coverage, the published application, and the DocFX documentation gate |
 | `frontend-coverage`, `backend-coverage` | upload coverage to Codecov |
 | `dependency-review` | reviews dependency changes of a pull request |
 | CodeQL | static analysis of the workflows, TypeScript and C# |
diff --git a/Documents/api/CheckDocumentation.cs b/Documents/api/CheckDocumentation.cs
new file mode 100644
index 0000000..f6b2da8
--- /dev/null
+++ b/Documents/api/CheckDocumentation.cs
@@ -0,0 +1,118 @@
+// SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+// SPDX-License-Identifier: AGPL-3.0-or-later WITH AdditionRef-Progmasoft-Patent-Grant-1.1
+
+// Documentation coverage gate of the API reference.
+//
+// DocFX builds a reference from whatever comments exist and does not report a declaration that has none. This
+// program reads the metadata DocFX generated and fails when a namespace, type or member is undocumented, so the
+// reference cannot contain an entry without a description.
+//
+// Usage, after `dotnet docfx metadata Documents/api/docfx.json`:
+//
+//     dotnet run Documents/api/CheckDocumentation.cs -- Documents/api
+//
+// The exit code is 0 when everything is documented, 1 when something is not, and 2 when the metadata is missing.
+
+using System.Text.RegularExpressions;
+
+string root = args.Length > 0 ? args[0] : "Documents/api";
+string referenceDirectory = Path.Combine(root, "reference");
+string namespaceDirectory = Path.Combine(root, "namespaces");
+
+if (!Directory.Exists(referenceDirectory))
+{
+    Console.Error.WriteLine($"No metadata in '{referenceDirectory}'. Run 'dotnet docfx metadata' first.");
+    return 2;
+}
+
+// A namespace has no declaration that could carry a comment. Its description is an overwrite file named after it.
+HashSet<string> describedNamespaces = Directory.Exists(namespaceDirectory)
+    ? Directory.EnumerateFiles(namespaceDirectory, "*.md")
+        .Where(HasDescription)
+        .Select(path => Path.GetFileNameWithoutExtension(path)!)
+        .ToHashSet(StringComparer.Ordinal)
+    : [];
+
+List<string> undocumented = [];
+int checkedItems = 0;
+
+foreach (string file in Directory.EnumerateFiles(referenceDirectory, "*.yml").Order(StringComparer.Ordinal))
+{
+    if (Path.GetFileName(file) == "toc.yml")
+    {
+        continue;
+    }
+
+    // The items a file defines come first; the `references` section after them repeats names of other files.
+    string text = File.ReadAllText(file).ReplaceLineEndings("\n");
+    int references = text.IndexOf("\nreferences:", StringComparison.Ordinal);
+    string definitions = references < 0 ? text : text[..references];
+
+    foreach (string item in ItemStart().Split(definitions).Skip(1))
+    {
+        string uid = item[..item.IndexOf('\n')].Trim();
+        string type = ItemType().Match(item) is { Success: true } match ? match.Groups[1].Value : "Item";
+        checkedItems++;
+
+        bool documented = type == "Namespace"
+            ? describedNamespaces.Contains(uid)
+            : ItemSummary().IsMatch(item);
+        if (!documented)
+        {
+            undocumented.Add($"{type} {uid}");
+        }
+    }
+}
+
+if (checkedItems == 0)
+{
+    Console.Error.WriteLine($"The metadata in '{referenceDirectory}' defines nothing; the gate would pass vacuously.");
+    return 2;
+}
+
+if (undocumented.Count != 0)
+{
+    Console.Error.WriteLine($"{undocumented.Count} of {checkedItems} API items have no documentation:");
+    foreach (string entry in undocumented)
+    {
+        Console.Error.WriteLine($"  {entry}");
+    }
+
+    Console.Error.WriteLine("Add a <summary> to each declaration, or a file in Documents/api/namespaces for a namespace.");
+    return 1;
+}
+
+Console.WriteLine($"All {checkedItems} API items are documented.");
+return 0;
+
+// An overwrite file describes its namespace when it has text after its front matter and comment header.
+static bool HasDescription(string path)
+{
+    string text = File.ReadAllText(path).ReplaceLineEndings("\n");
+    int frontMatterEnd = text.IndexOf("\n---", 3, StringComparison.Ordinal);
+    if (!text.StartsWith("---\n", StringComparison.Ordinal) || frontMatterEnd < 0)
+    {
+        return false;
+    }
+
+    string body = HtmlComment().Replace(text[(frontMatterEnd + 4)..], string.Empty);
+    return text[..frontMatterEnd].Contains($"uid: {Path.GetFileNameWithoutExtension(path)}", StringComparison.Ordinal)
+        && !string.IsNullOrWhiteSpace(body);
+}
+
+internal static partial class Program
+{
+    // Every defined item starts with its identifier at the left margin.
+    [GeneratedRegex(@"^- uid: ", RegexOptions.Multiline)]
+    private static partial Regex ItemStart();
+
+    [GeneratedRegex(@"^  type: (\w+)", RegexOptions.Multiline)]
+    private static partial Regex ItemType();
+
+    // A summary with content: text on the same line, or a block scalar followed by an indented line.
+    [GeneratedRegex(@"^  summary: (?:[^\s>|].*|[>|][-+]?\n    \S)", RegexOptions.Multiline)]
+    private static partial Regex ItemSummary();
+
+    [GeneratedRegex(@"<!--.*?-->", RegexOptions.Singleline)]
+    private static partial Regex HtmlComment();
+}
diff --git a/Documents/api/docfx.json b/Documents/api/docfx.json
new file mode 100644
index 0000000..58bc6c9
--- /dev/null
+++ b/Documents/api/docfx.json
@@ -0,0 +1,39 @@
+{
+  "metadata": [
+    {
+      "src": [
+        {
+          "src": "../../backend/Progmasoft.Tala.Website.Api",
+          "files": [
+            "Progmasoft.Tala.Website.Api.csproj"
+          ]
+        }
+      ],
+      "dest": "reference",
+      "includePrivateMembers": true
+    }
+  ],
+  "build": {
+    "content": [
+      {
+        "files": [
+          "index.md",
+          "toc.yml",
+          "reference/**/*.yml"
+        ]
+      }
+    ],
+    "dest": "_site",
+    "globalMetadata": {
+      "_appTitle": "Tala website API",
+      "_enableSearch": false
+    },
+    "overwrite": [
+      {
+        "files": [
+          "namespaces/*.md"
+        ]
+      }
+    ]
+  }
+}
diff --git a/Documents/api/index.md b/Documents/api/index.md
new file mode 100644
index 0000000..8cbed8a
--- /dev/null
+++ b/Documents/api/index.md
@@ -0,0 +1,31 @@
+<!--
+SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+SPDX-License-Identifier: AGPL-3.0-or-later WITH AdditionRef-Progmasoft-Patent-Grant-1.1
+-->
+
+# Tala website API
+
+Reference documentation of the website's ASP.NET Core API, generated by DocFX from the XML documentation comments
+of `backend/Progmasoft.Tala.Website.Api`. The reference includes internal and private members: it documents the code
+for the people who maintain it.
+
+Build and check it from the repository root:
+
+```sh
+dotnet tool restore
+dotnet docfx Documents/api/docfx.json --warningsAsErrors
+dotnet run Documents/api/CheckDocumentation.cs -- Documents/api
+```
+
+## What the gate requires
+
+Three things fail the check, locally and in CI:
+
+- **A broken link or an unresolved reference.** DocFX runs with every warning treated as an error.
+- **An undocumented public declaration or a wrong `cref`.** The API project compiles its XML documentation, and the
+  build treats the compiler's documentation warnings as errors.
+- **Any undocumented item of the reference.** `CheckDocumentation.cs` reads the metadata DocFX generated and fails
+  when a namespace, type or member has no summary, internal and private members included.
+
+A namespace has no declaration that could hold a comment. Its description is a file in `namespaces/`, named after
+the namespace.
diff --git a/Documents/api/namespaces/Progmasoft.Tala.Website.Api.md b/Documents/api/namespaces/Progmasoft.Tala.Website.Api.md
new file mode 100644
index 0000000..de90b81
--- /dev/null
+++ b/Documents/api/namespaces/Progmasoft.Tala.Website.Api.md
@@ -0,0 +1,10 @@
+---
+uid: Progmasoft.Tala.Website.Api
+summary: *content
+---
+
+<!-- SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com> -->
+<!-- SPDX-License-Identifier: AGPL-3.0-or-later WITH AdditionRef-Progmasoft-Patent-Grant-1.1 -->
+
+The API of the language website: the site's own public state at `/api/v1/site` and a liveness probe at `/health`.
+It has no account, package, publication or registry endpoint.
diff --git a/Documents/api/packages.lock.json b/Documents/api/packages.lock.json
new file mode 100644
index 0000000..0f57a57
--- /dev/null
+++ b/Documents/api/packages.lock.json
@@ -0,0 +1,35 @@
+{
+  "version": 1,
+  "dependencies": {
+    "net10.0": {
+      "Microsoft.DotNet.ILCompiler": {
+        "type": "Direct",
+        "requested": "[10.0.10, )",
+        "resolved": "10.0.10",
+        "contentHash": "tnG8ntt/Bk6odvHREnGLMo3PEiihy5iSlIFVp0JbIo00GKtNRt2k73eKZbPqR5yaJNIa3z8R86YLwbxfqpb17g=="
+      },
+      "Microsoft.NET.ILLink.Tasks": {
+        "type": "Direct",
+        "requested": "[10.0.10, )",
+        "resolved": "10.0.10",
+        "contentHash": "f5VCIE7AJpd5YvzNTeMGVzQIgyE9tX+AreTYwQF+REbu+DZo/2Ae+jNSwhPEYrVz6RRkd7y8ubXjk6Nn6Ka+Cg=="
+      }
+    },
+    "net10.0/win-x64": {
+      "Microsoft.DotNet.ILCompiler": {
+        "type": "Direct",
+        "requested": "[10.0.10, )",
+        "resolved": "10.0.10",
+        "contentHash": "tnG8ntt/Bk6odvHREnGLMo3PEiihy5iSlIFVp0JbIo00GKtNRt2k73eKZbPqR5yaJNIa3z8R86YLwbxfqpb17g==",
+        "dependencies": {
+          "runtime.win-x64.Microsoft.DotNet.ILCompiler": "10.0.10"
+        }
+      },
+      "runtime.win-x64.Microsoft.DotNet.ILCompiler": {
+        "type": "Transitive",
+        "resolved": "10.0.10",
+        "contentHash": "e5P00ObvdmAl49KkvGvqDLE5+EXu/IdXCDsLghgKFXKrNybyxMmH12HuGrsO7LTIGjXI58tyN3Hwxwr/MR0d8A=="
+      }
+    }
+  }
+}
\ No newline at end of file
diff --git a/Documents/api/toc.yml b/Documents/api/toc.yml
new file mode 100644
index 0000000..cf49dc1
--- /dev/null
+++ b/Documents/api/toc.yml
@@ -0,0 +1,7 @@
+# SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com>
+# SPDX-License-Identifier: AGPL-3.0-or-later WITH AdditionRef-Progmasoft-Patent-Grant-1.1
+
+- name: Overview
+  href: index.md
+- name: Reference
+  href: reference/
diff --git a/README.md b/README.md
index 667d927..58fdf50 100644
--- a/README.md
+++ b/README.md
@@ -77,7 +77,8 @@ dotnet test --project backend/Progmasoft.Tala.Website.Api.Tests/Progmasoft.Tala.
 dotnet publish backend/Progmasoft.Tala.Website.Api/Progmasoft.Tala.Website.Api.csproj --configuration Release --output publish
 ```
 
-The build treats warnings as errors and requires a documentation comment on every public declaration. The server unit
+The build treats warnings as errors and requires a documentation comment on every public declaration; DocFX builds
+the API reference from those comments and fails on any undocumented item (see `Documents/api/`). The server unit
 in `ops/systemd/tala-site-api.service` runs the published DLL from `/srv/xsharp/website/current/site-api/` on the
 shared ASP.NET Core 10 runtime and binds it only to loopback port 5086. The old .NET process on loopback port 5080 in
 `current/backend/` belongs to Progmaweb's ViGet status route; the language site does not proxy to it. Committing these
diff --git a/REUSE.toml b/REUSE.toml
index a2500d3..847b5a6 100644
--- a/REUSE.toml
+++ b/REUSE.toml
@@ -5,6 +5,8 @@ SPDX-PackageDownloadLocation = "https://github.com/Progmasoft/website"
 
 [[annotations]]
 path = [
+  ".config/dotnet-tools.json",
+  "Documents/api/docfx.json",
   "Progmasoft.Tala.Website.slnx",
   "global.json",
   "backend/Progmasoft.Tala.Website.Api/appsettings.json",