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+---
+name: building-seq-plug-in-apps
+description: Use this when developing plug-in Seq inputs (for ingestion) and outputs (for alert notifications or streamed events).
+license: Apache-2.0
+metadata:
+ author: Datalust and Contributors
+---
+
+This skill covers building [Seq](https://datalust.co/seq) apps (plugins) using the `Seq.Apps` runtime API. Seq apps are .NET libraries that extend Seq with custom event processing (output apps) or event generation (input apps).
+
+## Project structure
+
+A Seq app follows this layout:
+
+```
+src/
+  Seq.App.{Name}/
+    Seq.App.{Name}.csproj          # Main library
+    {Name}App.cs                   # SeqApp subclass (entry point)
+    ...                            # Supporting types
+    Resources/                     # Embedded resources (default templates, etc.)
+  Seq.App.{Name}.SmokeTest/        # Optional console app for manual testing
+    Program.cs
+test/
+  Seq.App.{Name}.Tests/
+    {Name}AppTests.cs              # App-level integration tests
+    ...Tests.cs                    # Unit tests for components
+    Support/
+      Test{Gateway}.cs             # Test doubles
+      Some.cs                      # LogEvent factory helpers
+```
+
+Input apps use `Seq.Input.{Name}` naming.
+
+### `.csproj` conventions
+
+```xml
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFramework>net8.0</TargetFramework>
+    <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
+  <ItemGroup>
+    <PackageReference Include="Seq.Apps" Version="..." />
+    <!-- Add Seq.Syntax (project or package reference) if using the Seq template language -->
+  </ItemGroup>
+</Project>
+```
+
+Embedded resources for default templates use `LogicalName` for clean resource stream access:
+
+```xml
+<ItemGroup>
+  <EmbeddedResource Include="./Resources/DefaultHtmlTemplate.txt" LogicalName="DefaultHtmlTemplate" />
+</ItemGroup>
+```
+
+Test projects target the latest stable .NET release, and use xUnit.
+
+The runtime pins its own copies of `Seq.Apps`, `Serilog`, and `Seq.Syntax` — the app's copies of these assemblies are ignored at load time. Other dependencies are loaded from the app's package.
+
+### Namespace disambiguation
+
+Projects named `Seq.App.{Name}` have a root namespace that collides with `Seq.Apps`. Use `global::` where `Seq.Apps.App` or `Seq.Apps.Host` is referenced as a type within a `Seq.App.*` namespace:
+
+```csharp
+public static LogEventPropertyValue? MyAppHost(global::Seq.Apps.Host host) { ... }
+public static LogEventPropertyValue? MyAppInstance(global::Seq.Apps.App app) { ... }
+```
+
+Use `InternalsVisibleTo` in the main project to expose `internal` constructors and types to the test project.
+
+## App class structure
+
+### Output apps (event subscribers)
+
+Output apps derive from `SeqApp` and implement `ISubscribeToAsync<LogEvent>`:
+
+```csharp
+using Seq.Apps;
+using Serilog.Events;
+
+[SeqApp("App Name",
+    Description = "A short sentence describing what the app does.")]
+public class MyApp : SeqApp, ISubscribeToAsync<LogEvent>, IDisposable
+{
+    readonly IMyGateway _gateway;
+    MyMessageFactory? _messageFactory;
+
+    // Public parameterless constructor for production use
+    public MyApp() : this(new MyHttpGateway()) { }
+
+    // Internal constructor for testing (dependency injection)
+    internal MyApp(IMyGateway gateway) { _gateway = gateway; }
+
+    // Settings (see "App settings" section below)
+
+    protected override void OnAttached()
+    {
+        // Validate required settings, resolve defaults, create factories
+    }
+
+    public async Task OnAsync(Event<LogEvent> evt)
+    {
+        // Process the event: render message, send via gateway
+    }
+
+    public void Dispose()
+    {
+        (_gateway as IDisposable)?.Dispose();
+    }
+}
+```
+
+The `SeqApp` attribute's `Name` parameter is the display name in Seq's UI. Keep it short (1-3 words). `Description` is a single sentence shown below the name.
+
+### Output apps using raw JSON (`ISubscribeToJsonAsync`)
+
+Apps that don't need Serilog `LogEvent` deserialization can implement `ISubscribeToJsonAsync` instead. This receives each event as a raw [CLEF](https://clef-json.org) JSON string, skipping `LogEvent`/`MessageTemplate`/`LogEventProperty` construction entirely:
+
+```csharp
+[SeqApp("App Name",
+    Description = "A short sentence describing what the app does.")]
+public class MyApp : SeqApp, ISubscribeToJsonAsync, IAsyncDisposable
+{
+    public async Task OnAsync(string json)
+    {
+        // Parse and process the raw CLEF JSON
+    }
+
+    public async ValueTask DisposeAsync() { }
+}
+```
+
+Prefer `ISubscribeToJsonAsync` for apps that forward or transform events without rendering human-readable output (exporters, relays, bridges), especially at high volume. It avoids the Serilog dependency entirely. The trade-off is that the `Event<LogEvent>` wrapper (`Id`, `EventType`, `Timestamp`) and `Seq.Syntax` template rendering are not available; the app must extract CLEF fields (`@t`, `@l`, `@mt`, `@x`, etc.) from the JSON directly.
+
+Stick with `ISubscribeToAsync<LogEvent>` when the app renders human-readable output using `Seq.Syntax` templates (emails, chat messages, tickets) or inspects structured property values using the Serilog type system (`ScalarValue`, `SequenceValue`, `StructureValue`).
+
+### Input apps (event publishers)
+
+Input apps implement `IPublishJson`:
+
+```csharp
+[SeqApp("My Input",
+    Description = "Periodically does X and publishes metrics to Seq.")]
+public class MyInput : SeqApp, IPublishJson, IDisposable
+{
+    public void Start(TextWriter inputWriter)
+    {
+        // Begin producing events; write CLEF JSON lines to inputWriter
+        // Return immediately; use background tasks for ongoing work
+    }
+
+    public void Stop()
+    {
+        // Block until publishing has stopped
+    }
+}
+```
+
+The app must synchronize writes to `inputWriter` so that events are not interleaved (e.g., use a `lock` when multiple background threads produce events).
+
+### Lifecycle
+
+1. Seq instantiates the app using the public parameterless constructor
+2. The runtime sets `[SeqAppSetting]` properties via reflection, matched by **property name**
+3. `Attach()` is called, making `App`, `Host`, and `Log` available
+4. `OnAttached()` runs — validate settings and initialize here
+5. For subscribers: `OnAsync()` is called for each matching event
+6. For publishers: `Start()` is called, then `Stop()` on shutdown
+7. `Dispose()` if implemented
+
+Each app instance runs as a separate `seqcli` process. Initialization, event dispatch, and disposal are single-threaded. Events are dispatched one at a time — if `OnAsync()` is slow, backpressure is applied naturally. Apps may create their own background threads for ingestion.
+
+## App settings
+
+Settings are public properties on the app class decorated with `[SeqAppSetting]`. They appear in Seq's UI for the user to configure. The runtime injects values by matching on the **property name** (not `DisplayName`), using `Enum.Parse` for enum types and `Convert.ChangeType` for everything else.
+
+### `SeqAppSetting` attribute properties
+
+| Property | Type | Purpose |
+|---|---|---|
+| `DisplayName` | `string` | User-facing label in the UI. If omitted, the property name is used. |
+| `HelpText` | `string` | Descriptive text shown to the user. Should explain the setting's purpose and give examples. |
+| `IsOptional` | `bool` | If `true`, the user can leave the field blank. Non-optional settings that are missing cause a startup error before `Attach()` is called. |
+| `InputType` | `SettingInputType` | Controls the UI input widget: `Text`, `LongText`, `Checkbox`, `Integer`, `Decimal`, `Password`. If `Unspecified`, the runtime chooses based on the property type. |
+| `Syntax` | `string` | Syntax highlighting in the UI. `"template"` for fields accepting Seq template expressions (in `{braces}`), `"code"` for structured values (JSON, headers). Omit for plain values. |
+| `IsInvocationParameter` | `bool` | Marks settings that can be overridden per-invocation (e.g., destination address, channel). |
+
+### Quality guidelines for settings
+
+**Display names** should be concise, title-case-ish, and match how users think about the concept:
+
+| Good | Avoid |
+|---|---|
+| Bot token | BotToken |
+| Chat ID | Telegram Chat Identifier |
+| Silent notification | DisableNotification |
+| Body is plain text | BodyIsPlainText |
+| Connection security | ProtocolSecurity |
+
+**Help text** should:
+- Start with a clear description of the setting's purpose
+- Include concrete examples where helpful (e.g., format strings, URL patterns, expected values)
+- Mention defaults explicitly (e.g., "The default is `Etc/UTC`.", "The default is `POST`.")
+- Use backticks for code/values in help text
+- Note when template syntax is supported: "Template syntax is supported."
+- For password/token fields, explain how to obtain the value
+
+```csharp
+[SeqAppSetting(
+    DisplayName = "Time zone",
+    IsOptional = true,
+    HelpText = "The IANA time zone name used when formatting dates and times " +
+               "(e.g. Australia/Brisbane). The default is Etc/UTC.")]
+public string? TimeZoneName { get; set; }
+
+[SeqAppSetting(
+    DisplayName = "Bot token",
+    InputType = SettingInputType.Password,
+    HelpText = "The bot's API token, in the format 123456:ABC-DEF1234ghIkl-zyx57W2v1u123ew11. " +
+               "You can create a bot and obtain a token via @BotFather on Telegram.")]
+public string? BotToken { get; set; }
+```
+
+### Enum settings
+
+When a setting has a fixed set of valid values, use an `enum` type. The runtime presents enum settings as a dropdown and handles validation automatically. Apply `[Description]` to enum members to control the dropdown text when the member name doesn't read naturally:
+
+```csharp
+using System.ComponentModel;
+
+public enum OtlpExportProtocol
+{
+    [Description("HTTP/Protobuf")]
+    HttpProtobuf,
+    [Description("gRPC")]
+    Grpc
+}
+
+[SeqAppSetting(
+    DisplayName = "Protocol",
+    HelpText = "The OTLP export protocol. The default is `HTTP/Protobuf`.",
+    IsOptional = true)]
+public OtlpExportProtocol Protocol { get; set; } = OtlpExportProtocol.HttpProtobuf;
+```
+
+### Setting validation
+
+Validate required settings in `OnAttached()` and throw `ArgumentException` with a message that names the setting using its display name in backticks:
+
+```csharp
+protected override void OnAttached()
+{
+    var botToken = NormalizeOption(BotToken)
+        ?? throw new ArgumentException("A `Bot token` must be supplied.");
+    var chatId = NormalizeOption(ChatId)
+        ?? throw new ArgumentException("A `Chat ID` must be supplied.");
+    // ...
+}
+
+// Treat empty strings as null for optional settings
+static string? NormalizeOption(string? s) => s == "" ? null : s;
+```
+
+## Gateway pattern (testability)
+
+External I/O (HTTP APIs, SMTP, cloud services) is abstracted behind an interface so tests can run without network access.
+
+```csharp
+// Interface
+interface IMyGateway
+{
+    Task<MyResponse> SendAsync(string param1, string param2, CancellationToken cancel);
+}
+
+// Production implementation
+class MyHttpGateway : IMyGateway, IDisposable
+{
+    readonly HttpClient _httpClient = new();
+
+    public async Task<MyResponse> SendAsync(...)
+    {
+        // HTTP calls, JSON serialization, response parsing
+    }
+
+    public void Dispose() => _httpClient.Dispose();
+}
+
+// Test double
+class TestMyGateway : IMyGateway
+{
+    public List<(string Param1, string Param2)> Sent { get; } = [];
+    public MyResponse NextResponse { get; set; } = new() { Ok = true };
+
+    public Task<MyResponse> SendAsync(string param1, string param2, CancellationToken cancel)
+    {
+        Sent.Add((param1, param2));
+        return Task.FromResult(NextResponse);
+    }
+}
+```
+
+The app class has dual constructors:
+
+```csharp
+public MyApp() : this(new MyHttpGateway()) { }       // Production
+internal MyApp(IMyGateway gateway) { _gateway = gateway; }  // Testing
+```
+
+Use a single long-lived `HttpClient` per gateway. Use `System.Text.Json` for serialization — no third-party HTTP or serialization libraries.
+
+## Template support (Seq.Syntax)
+
+Apps that support user-customizable output should use the Seq template language via `Seq.Syntax`. This is the same expression language used in Seq's UI.
+
+### Template compilation
+
+Templates are compiled into `ExpressionTemplate` instances in `OnAttached()` or a factory constructor:
+
+```csharp
+using Seq.Syntax.Templates;
+using Seq.Syntax.Templates.Encoding;
+
+var template = new ExpressionTemplate(
+    templateText,
+    nameResolver: new OrderedNameResolver(new NameResolver[]
+    {
+        new StaticMemberNameResolver(typeof(MyBuiltInFunctions)),
+        myAppNameResolver
+    }),
+    encoder: isPlainText ? null : new MyHtmlEncoder());
+```
+
+### Template rendering
+
+Render a template against a `LogEvent`:
+
+```csharp
+static string Format(ExpressionTemplate template, LogEvent evt)
+{
+    var writer = new StringWriter();
+    template.Format(evt, writer);
+    return writer.ToString();
+}
+```
+
+### Custom encoding
+
+When output is HTML (or another format requiring escaping), implement `TemplateOutputEncoder`. The encoder is applied **only to interpolated values**, not to template literal text:
+
+```csharp
+class TemplateOutputTelegramHtmlEncoder : TemplateOutputEncoder
+{
+    public override string Encode(string value)
+    {
+        return value
+            .Replace("&", "&amp;")
+            .Replace("<", "&lt;")
+            .Replace(">", "&gt;");
+    }
+}
+```
+
+### Built-in functions and name resolution
+
+Apps can expose custom functions and properties within templates through a `NameResolver`:
+
+```csharp
+public static class MyAppBuiltInFunctions
+{
+    // Available as UriEncode(value) in templates
+    public static LogEventPropertyValue? UriEncode(LogEventPropertyValue? value) { ... }
+
+    // Bound to @Host in templates via the name resolver
+    public static LogEventPropertyValue? MyAppHost(Host host) { ... }
+
+    // Bound to @App in templates via the name resolver
+    public static LogEventPropertyValue? MyAppInstance(App app) { ... }
+}
+```
+
+The name resolver maps `@Host` and `@App` to the built-in functions and binds the actual `Host` and `App` instances as function parameters. See `TelegramAppNameResolver` or `MailAppNameResolver` in the reference apps for the implementation pattern.
+
+### Default templates
+
+Provide sensible default templates as embedded resources. Templates should:
+- Handle both regular events and alert notifications (`@EventType = 0xA1E77001`)
+- Include level indicators appropriate to the output format
+- Show timestamp, message, properties, and exceptions
+- Link back to the event in Seq using `@Host.BaseUri`
+
+For HTML templates, use only tags supported by the target (e.g., Telegram supports only `<b>`, `<i>`, `<code>`, `<pre>`, `<a>`).
+
+## Error handling
+
+All exceptions thrown from `OnAsync()` are caught per-event by the runtime, logged to the app's diagnostic stream, and recorded against the event. The app continues processing the next event regardless.
+
+| Scenario | Pattern |
+|---|---|
+| Missing required settings | `ArgumentException` from `OnAttached()` — Seq reports as a configuration error |
+| Invalid template syntax | `ArgumentException` from `ExpressionTemplate` constructor — fails at attachment time |
+| Transient API errors (rate limits) | Log via `Log.Warning()`, do **not** throw — avoids per-event failure noise |
+| Permanent API errors (auth, bad request) | Throw `SeqAppException` with the error details — Seq records against the event |
+| Network failures | Let `HttpRequestException` propagate — Seq records against the event |
+
+```csharp
+public async Task OnAsync(Event<LogEvent> evt)
+{
+    var response = await _gateway.SendAsync(...);
+    if (!response.Ok)
+    {
+        if (response.ErrorCode == 429) // Rate limited
+        {
+            Log.Warning("Rate limit exceeded; retry after {RetryAfter}s", response.RetryAfter);
+            return; // Don't throw
+        }
+
+        throw new SeqAppException($"API error {response.ErrorCode}: {response.Description}");
+    }
+}
+```
+
+## Testing
+
+### Test infrastructure
+
+**`TestAppHost`** (from `Seq.Apps.Testing`): Provides a minimal `IAppHost` with default `App` (id `"app-1"`, base URI `https://seq.example.com`), `Host`, and `Logger` instances. Used to attach the app in tests.
+
+### Test pattern
+
+Setting properties are set directly on the app instance before calling `Attach()`, since the runtime injects them by reflection in production but tests bypass that:
+
+```csharp
+[Fact]
+public async Task EventsAreSentWithCorrectParameters()
+{
+    var gateway = new TestMyGateway();
+    var app = new MyApp(gateway);
+    app.BotToken = "test-token";
+    app.ChatId = "12345";
+    app.Attach(new TestAppHost());
+
+    await app.OnAsync(Some.InformationEvent());
+
+    Assert.Single(gateway.Sent);
+    Assert.Equal("12345", gateway.Sent[0].ChatId);
+}
+```
+
+For `ISubscribeToJsonAsync` apps, pass a CLEF JSON string directly:
+
+```csharp
+await app.OnAsync("""{"@t":"2024-01-15T10:30:00Z","@mt":"Test event"}""");
+```
+
+A `Some` helper class is typically used to create `Event<LogEvent>` test fixtures for `ISubscribeToAsync<LogEvent>` apps.
+
+### Test categories
+
+**App-level tests** exercise the full path from `OnAsync()` through the gateway using test doubles:
+
+- Happy path: event flows end-to-end and the gateway receives the expected payload
+- Different log levels produce appropriate output (level indicators, severity mapping)
+- Alert events (`EventType = 0xA1E77001`) are handled distinctly from regular events
+- Events with exceptions (`@x`) include the exception in output
+- Events with missing optional properties degrade gracefully
+- Structured property values (sequences, structures, dictionaries) render correctly
+
+**Configuration tests** cover `OnAttached()`:
+
+- Each required setting, when missing or empty, throws `ArgumentException` naming the setting
+- Invalid combinations and out-of-range values are rejected
+- Optional settings fall back to documented defaults
+
+**Template/rendering tests** (for apps using `Seq.Syntax`):
+
+- Default templates render sensibly for both regular events and alerts
+- HTML/encoded output: interpolated values are encoded, literal template text is not
+- Special characters in property values don't break the output format
+- Built-in functions (`@Host`, `@App`, custom functions) resolve correctly
+- Output truncation at size limits preserves valid output
+
+**Error handling tests**:
+
+- Gateway errors throw `SeqAppException` with a diagnostic message
+- Transient errors (rate limits) log but do not throw
+- Error context is sufficient for diagnosis (status codes, response excerpts)
+
+**For `ISubscribeToJsonAsync` apps**, test with realistic CLEF inputs covering minimal events (`@t` + `@mt`), events with all optional fields (`@l`, `@x`, `@i`, `@tr`, `@sp`, `@st`), and properties containing special characters, nested structures, and arrays.
+
+### Smoke test project
+
+An optional console app for manual end-to-end verification against real services. Reads settings from `SEQ_APP_SETTING_{PROPERTYNAME}` environment variables (uppercase, no underscores in the property name part).
+
+Take care that the smoke test project doesn't exit or assume completion before asynchronous background processes complete, e.g. flushing buffered output to disk or remote APIs.
+
+## Runtime API reference
+
+### Key types from `Seq.Apps` (in `seq-apps-runtime`)
+
+| Type | Purpose |
+|---|---|
+| `SeqApp` | Abstract base class. Provides `App`, `Host`, `Log` after attachment. |
+| `SeqAppAttribute` | Marks the entry point class. `Name` (short UI label), `Description` (sentence), `AllowReprocessing` (default `false`; if `true`, the app will receive events that it produced itself). |
+| `SeqAppSettingAttribute` | Marks configurable properties. See "App settings" section. |
+| `ISubscribeToAsync<LogEvent>` | For output apps processing Serilog `LogEvent` objects. |
+| `ISubscribeToJsonAsync` | For output apps processing raw CLEF JSON strings. |
+| `IPublishJson` | For input apps that generate events. `Start(TextWriter)` / `Stop()`. |
+| `Event<TData>` | Wrapper with `Id`, `EventType`, `Timestamp`, `Data`. Alert events have `EventType = 0xA1E77001`. |
+| `App` | `Id`, `Title`, `Settings` (`IReadOnlyDictionary<string, string>`), `StoragePath`. |
+| `Host` | `BaseUri`, `InstanceName` (nullable). |
+| `SeqAppException` | Throw for app-specific errors; Seq records against the event. |
+| `SettingInputType` | Enum: `Unspecified`, `Text`, `LongText`, `Checkbox`, `Integer`, `Decimal`, `Password`. |
+
+### Important constants
+
+- Alert event type: `0xA1E77001`
+- Default time zone: `Etc/UTC`
+- Default date/time format: `o` (ISO-8601 round-trip)
+
+## References
+
+- [CLEF specification](https://clef-json.org) — the Compact Log Event Format (`@t`, `@mt`, `@m`, `@l`, `@x`, `@i`, `@r`)
+- [Posting raw events](https://docs.datalust.co/docs/posting-raw-events) — CLEF reference including Seq trace extensions (`@tr`, `@sp`, `@ps`, `@st`, `@sc`, `@ra`, `@sk`)
+- [Template syntax](https://docs.datalust.co/docs/template-syntax) — documentation for the Seq template language used in app settings
+- [seq-apps-runtime](https://github.com/datalust/seq-apps-runtime) — source code for the `Seq.Apps` API (`SeqApp`, `ISubscribeToAsync<LogEvent>`, `ISubscribeToJsonAsync`, etc.)
+- [seqcli](https://github.com/datalust/seqcli) — source code for the `seqcli app run` command that Seq uses to host apps at runtime
+- [seq-app-mail](https://github.com/datalust/seq-app-mail) — canonical output app example (email); also the home of `Seq.Syntax` source code
+- [seq-input-healthcheck](https://github.com/datalust/seq-input-healthcheck) — canonical input app example (HTTP health checks)