Add implementing-server-sent-events skill by mrsharm · Pull Request #147 · dotnet/skills

mrsharm · 2026-02-27T19:40:24Z

New Skill: implementing-server-sent-events

Adds a skill for implementing Server-Sent Events (SSE) endpoints in ASP.NET Core 8 minimal APIs.

Key Gotchas Covered

No built-in MapSSE() or MapServerSentEvents() — must use manual response writing
Must set Content-Type: text/event-stream, Cache-Control: no-cache, Connection: keep-alive
SSE format requires double newline (\n\n) to terminate events
Must call FlushAsync() after each event write (otherwise client receives nothing)
Must use HttpContext.RequestAborted for clean disconnect handling
Must set X-Accel-Buffering: no for reverse proxy compatibility
Support Last-Event-ID header for client reconnection
Must NOT return Results.Ok() after streaming events

Eval Results (3-run validation)

Overall: +13.6% improvement — PASSED
BL=4.0 SK=4.7
Pairwise: skill wins consistently

mrsharm · 2026-02-27T19:40:31Z

Eval Results: implementing-server-sent-events

3-Run Validation: +13.6% PASS

Metric	Baseline	With Skill	Change
Overall Score	4.0/5	4.7/5	+0.7
Quality (overall)	4.0/5	4.7/5	+40%
Pairwise	—	skill wins	consistent

The skill consistently improves SSE implementations by teaching the manual protocol (no built-in helper exists), proper flush behavior, disconnection handling, and reconnection support.

Model: claude-opus-4.6 (baseline + skill + judge)

Copilot

Pull request overview

Adds a new .NET skill intended to guide implementation of Server-Sent Events (SSE) endpoints in ASP.NET Core 8 minimal APIs, along with an evaluation scenario to validate the skill’s impact.

Changes:

Introduces the implementing-server-sent-events skill documentation (SSE headers, framing, flushing, disconnect handling, reconnection).
Adds a new eval.yaml scenario/rubric for the skill.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 8 comments.

File	Description
src/dotnet/tests/implementing-server-sent-events/eval.yaml	Adds an evaluation scenario and rubric for SSE endpoint implementation.
src/dotnet/skills/implementing-server-sent-events/SKILL.md	Adds a new skill doc describing how to implement SSE in ASP.NET Core minimal APIs.

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

Copilot · 2026-02-27T19:45:42Z

+    context.Response.Headers.ContentType = "text/event-stream";
+    context.Response.Headers.CacheControl = "no-cache";


Same compile issue here: context.Response.Headers.ContentType / .CacheControl are not valid APIs on IHeaderDictionary. Update this snippet to use context.Response.ContentType / Headers["Cache-Control"] (or typed headers) so the example builds.

Suggested change

context.Response.Headers.ContentType = "text/event-stream";

context.Response.Headers.CacheControl = "no-cache";

context.Response.ContentType = "text/event-stream";

context.Response.Headers["Cache-Control"] = "no-cache";

Copilot · 2026-02-27T19:45:42Z

+    context.Response.Headers.ContentType = "text/event-stream";
+    context.Response.Headers.CacheControl = "no-cache";
+    context.Response.Headers["Connection"] = "keep-alive";


Same header API issue in this snippet: context.Response.Headers.ContentType / .CacheControl won’t compile. Please update consistently across all examples (Step 5/6) to avoid copy/paste failures.

Copilot · 2026-02-27T19:45:42Z

+// Option B: Use StreamWriter with AutoFlush = true
+await using var writer = new StreamWriter(context.Response.Body, leaveOpen: true);
+writer.AutoFlush = true;  // Flushes after every Write
+await writer.WriteLineAsync($"data: {msg}\n");  // Note: WriteLine adds one \n, we add one more


StreamWriter.WriteLineAsync uses the platform newline by default (often \r\n), and this example also appends an extra \n, which can produce confusing line endings for SSE. Prefer writing explicit \n (or set writer.NewLine = "\n" and use WriteAsync) to keep the SSE framing unambiguous cross-platform.

Suggested change

await writer.WriteLineAsync($"data: {msg}\n"); // Note: WriteLine adds one \n, we add one more

await writer.WriteAsync($"data: {msg}\n\n"); // Explicit \n\n for SSE event framing

Copilot · 2026-02-27T19:45:43Z

+    await context.Response.WriteAsync($"retry: 5000\n\n");
+    await context.Response.Body.FlushAsync();
+
+    var startFrom = lastEventId != null ? int.Parse(lastEventId) + 1 : 0;


int.Parse(lastEventId) will throw if the client sends a non-integer Last-Event-ID header (or an out-of-range value), turning a reconnect into a 500. Use int.TryParse and fall back to a safe default (or return 400) to make the reconnection example robust.

Suggested change

var startFrom = lastEventId != null ? int.Parse(lastEventId) + 1 : 0;

var startFrom = 0;

if (!string.IsNullOrEmpty(lastEventId) && int.TryParse(lastEventId, out var parsedId) && parsedId >= 0)

{

startFrom = parsedId + 1;

}

Copilot · 2026-02-27T19:45:43Z

+## When Not to Use
+- Bidirectional communication needed → use WebSockets
+- Binary data streaming → use WebSockets or gRPC streaming
+- Need more than 6 concurrent connections per domain in HTTP/1.1 → use HTTP/2
+


This skill doc doesn’t include an explicit “Validation” section/checklist. CONTRIBUTING.md asks that changes include validation steps a reviewer can follow; consider adding a short section (e.g., how to run the sample, curl an SSE stream, verify Last-Event-ID behavior, and test via nginx) so the skill is verifiable.

Copilot · 2026-02-27T19:45:43Z

+scenarios:
+  - name: "Implement SSE notification endpoint in ASP.NET Core 8 minimal API"


This eval file is under src/dotnet/tests/..., but repo convention (and CI) expects evals at tests/<plugin>/<skill-name>/eval.yaml (e.g. tests/dotnet/implementing-server-sent-events/eval.yaml). With the current location, the evaluation workflow that runs --tests-dir ./tests/dotnet will not pick up these scenarios.

Copilot · 2026-02-27T19:45:43Z

+---
+name: implementing-server-sent-events
+description: Implement Server-Sent Events (SSE) endpoints in ASP.NET Core. Use when building real-time streaming from server to client without WebSockets.
+---


This SKILL.md is placed under src/dotnet/skills/..., but the repository layout expects skills under plugins/<plugin>/skills/<skill-name>/SKILL.md so they are packaged and discovered (e.g. plugins/dotnet/skills/implementing-server-sent-events/SKILL.md). As-is, the evaluation workflow that validates ./plugins/dotnet/skills will not discover or test this skill.

Copilot · 2026-02-27T19:45:43Z

+    context.Response.Headers.ContentType = "text/event-stream";
+    context.Response.Headers.CacheControl = "no-cache";


These snippets use context.Response.Headers.ContentType / .CacheControl, but HttpResponse.Headers is an IHeaderDictionary and doesn't expose those properties. This will not compile in ASP.NET Core; set context.Response.ContentType (or Headers["Content-Type"]) and Headers["Cache-Control"] (or use context.Response.GetTypedHeaders()).

Suggested change

context.Response.Headers.ContentType = "text/event-stream";

context.Response.Headers.CacheControl = "no-cache";

context.Response.ContentType = "text/event-stream";

context.Response.Headers["Cache-Control"] = "no-cache";

danmoseley · 2026-03-03T21:28:55Z

+
+# Implementing Server-Sent Events (SSE) in ASP.NET Core
+
+## When to Use


move when to use and when not to use entirely or largely into the description, which is the thing the ai reads when deciding whether or not to load. probably shouldn't have this section here.

danmoseley · 2026-03-03T21:30:35Z

+// Each event MUST end with TWO newlines (\n\n)
+
+// Simple data event:
+await context.Response.WriteAsync($"data: {message}\n\n", ct);


should {message} be sanitized for newlines here and below. AI says - risk of injection attack

danmoseley · 2026-03-03T21:31:18Z

needs CODEOWNERS entry

danmoseley · 2026-03-03T21:31:51Z

+
+# Implementing Server-Sent Events (SSE) in ASP.NET Core
+
+## When to Use


SSE endpoints hold long-lived HTTP connections. Without connection limits, an attacker (or misbehaving client) can exhaust server resources by opening many concurrent connections. Consider adding a note about:

Enforcing per-client or total connection limits (e.g., via middleware or a SemaphoreSlim)

Requiring authentication before establishing the SSE connection

Setting an idle timeout to drop stale connections

This is especially relevant for a skill doc since agents will copy the pattern as-is without thinking about production hardening.

danmoseley · 2026-03-03T21:31:51Z

+4. **Not handling RequestAborted**: The loop runs forever if you don't check `context.RequestAborted`. `OperationCanceledException` on disconnect is normal.
+5. **Returning a result after streaming**: Don't `return Results.Ok()` after writing SSE events — the response body is already being written.
+6. **Missing Content-Type header**: Must be exactly `text/event-stream`, not `application/json` or `text/plain`.
+7. **Missing X-Accel-Buffering: no**: Reverse proxies (nginx) buffer responses by default, breaking SSE.


Worth adding a note about CORS configuration. SSE endpoints are frequently consumed cross-origin via the browser EventSource API, which does not support custom headers. Without app.UseCors(...) or a CORS policy on the endpoint, cross-origin clients will silently fail. This is a common deployment surprise.

BrennanConroy · 2026-03-03T23:48:07Z

+
+### Step 1: CRITICAL — There Is No Built-In MapSSE() or MapServerSentEvents()
+
+ASP.NET Core has NO built-in SSE endpoint helper. You must manually write the SSE protocol.


Wrong. We have TypedResults.ServerSentEvents
https://learn.microsoft.com/en-us/aspnet/core/release-notes/aspnetcore-10.0?view=aspnetcore-10.0#support-for-server-sent-events-sse

Most of this Skill should probably be rewritten to use TypedResults.ServerSentEvents

mrsharm · 2026-03-06T17:23:18Z

Closing: replaced by new PR from mrsharm/skills with plugins/ directory structure.

Add implementing-server-sent-events skill with eval scenario

178dfb4

Copilot AI review requested due to automatic review settings February 27, 2026 19:40

Copilot started reviewing on behalf of mrsharm February 27, 2026 19:40 View session

Copilot AI reviewed Feb 27, 2026

View reviewed changes

jeffschwMSFT requested review from BrennanConroy, SamMonoRT, artl93 and danmoseley March 2, 2026 16:12

jeffschwMSFT added the area-aspnet label Mar 2, 2026

danmoseley reviewed Mar 3, 2026

View reviewed changes

BrennanConroy reviewed Mar 3, 2026

View reviewed changes

mrsharm closed this Mar 6, 2026

mrsharm mentioned this pull request Mar 6, 2026

Add implementing-server-sent-events skill #265

Closed

		context.Response.Headers.ContentType = "text/event-stream";
		context.Response.Headers.CacheControl = "no-cache";

	await writer.WriteLineAsync($"data: {msg}\n"); // Note: WriteLine adds one \n, we add one more
	await writer.WriteAsync($"data: {msg}\n\n"); // Explicit \n\n for SSE event framing

-    var startFrom = lastEventId != null ? int.Parse(lastEventId) + 1 : 0;
+    var startFrom = 0;
+    if (!string.IsNullOrEmpty(lastEventId) && int.TryParse(lastEventId, out var parsedId) && parsedId >= 0)
+    {
+        startFrom = parsedId + 1;
+    }

		scenarios:
		- name: "Implement SSE notification endpoint in ASP.NET Core 8 minimal API"


		# Implementing Server-Sent Events (SSE) in ASP.NET Core

		## When to Use


		### Step 1: CRITICAL — There Is No Built-In MapSSE() or MapServerSentEvents()

		ASP.NET Core has NO built-in SSE endpoint helper. You must manually write the SSE protocol.

Uh oh!

Conversation

mrsharm commented Feb 27, 2026

New Skill: implementing-server-sent-events

Key Gotchas Covered

Eval Results (3-run validation)

Uh oh!

mrsharm commented Feb 27, 2026

Eval Results: implementing-server-sent-events

3-Run Validation: +13.6% PASS

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Reviewed changes

Uh oh!

Copilot AI Feb 27, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Feb 27, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Feb 27, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Feb 27, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Feb 27, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Feb 27, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Feb 27, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Feb 27, 2026

Choose a reason for hiding this comment

Uh oh!

danmoseley Mar 3, 2026

Choose a reason for hiding this comment

Uh oh!

danmoseley Mar 3, 2026

Choose a reason for hiding this comment

Uh oh!

danmoseley commented Mar 3, 2026

Uh oh!

danmoseley Mar 3, 2026

Choose a reason for hiding this comment

Uh oh!

danmoseley Mar 3, 2026

Choose a reason for hiding this comment

Uh oh!

danmoseley Mar 3, 2026

Choose a reason for hiding this comment

Uh oh!

danmoseley Mar 3, 2026

Choose a reason for hiding this comment

Uh oh!

BrennanConroy Mar 3, 2026

Choose a reason for hiding this comment

Uh oh!

mrsharm commented Mar 6, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants