Add E2E tests for blob attachments across all 4 SDKs

Add blob attachment E2E tests for Node.js, Python, Go, and .NET SDKs.
Each test sends a message with an inline base64-encoded PNG blob
attachment and verifies the request is accepted by the replay proxy.

- nodejs/test/e2e/session_config.test.ts: should accept blob attachments
- python/e2e/test_session.py: test_should_accept_blob_attachments
- go/internal/e2e/session_test.go: TestSessionBlobAttachment
- dotnet/test/SessionTests.cs: Should_Accept_Blob_Attachments
- test/snapshots/: request-only YAML snapshots for replay proxy

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

Author: MackinnonBuck

dotnet/test/SessionTests.cs

@@ -538,6 +538,29 @@ public async Task DisposeAsync_From_Handler_Does_Not_Deadlock()
         await disposed.Task.WaitAsync(TimeSpan.FromSeconds(10));
     }
 
+    [Fact]
+    public async Task Should_Accept_Blob_Attachments()
+    {
+        var session = await CreateSessionAsync();
+
+        await session.SendAsync(new MessageOptions
+        {
+            Prompt = "Describe this image",
+            Attachments =
+            [
+                new UserMessageDataAttachmentsItemBlob
+                {
+                    Data = "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==",
+                    MimeType = "image/png",
+                    DisplayName = "test-pixel.png",
+                },
+            ],
+        });
+
+        // Just verify send doesn't throw — blob attachment support varies by runtime
+        await session.DisposeAsync();
+    }
+
     private static async Task WaitForAsync(Func<bool> condition, TimeSpan timeout)
     {
         var deadline = DateTime.UtcNow + timeout;

go/internal/e2e/session_test.go

@@ -938,6 +938,48 @@ func TestSetModelWithReasoningEffort(t *testing.T) {
 	}
 }
 
+func TestSessionBlobAttachment(t *testing.T) {
+	ctx := testharness.NewTestContext(t)
+	client := ctx.NewClient()
+	t.Cleanup(func() { client.ForceStop() })
+
+	if err := client.Start(t.Context()); err != nil {
+		t.Fatalf("Failed to start client: %v", err)
+	}
+
+	t.Run("should accept blob attachments", func(t *testing.T) {
+		ctx.ConfigureForTest(t)
+
+		session, err := client.CreateSession(t.Context(), &copilot.SessionConfig{
+			OnPermissionRequest: copilot.PermissionHandler.ApproveAll,
+		})
+		if err != nil {
+			t.Fatalf("Failed to create session: %v", err)
+		}
+
+		data := "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg=="
+		mimeType := "image/png"
+		displayName := "test-pixel.png"
+		_, err = session.Send(t.Context(), copilot.MessageOptions{
+			Prompt: "Describe this image",
+			Attachments: []copilot.Attachment{
+				{
+					Type:        copilot.Blob,
+					Data:        &data,
+					MIMEType:    &mimeType,
+					DisplayName: &displayName,
+				},
+			},
+		})
+		if err != nil {
+			t.Fatalf("Send with blob attachment failed: %v", err)
+		}
+
+		// Just verify send doesn't error — blob attachment support varies by runtime
+		session.Disconnect()
+	})
+}
+
 func getToolNames(exchange testharness.ParsedHttpExchange) []string {
 	var names []string
 	for _, tool := range exchange.Request.Tools {

nodejs/test/e2e/session_config.test.ts

@@ -43,6 +43,25 @@ describe("Session Configuration", async () => {
         }
     });
 
+    it("should accept blob attachments", async () => {
+        const session = await client.createSession({ onPermissionRequest: approveAll });
+
+        await session.send({
+            prompt: "Describe this image",
+            attachments: [
+                {
+                    type: "blob",
+                    data: "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==",
+                    mimeType: "image/png",
+                    displayName: "test-pixel.png",
+                },
+            ],
+        });
+
+        // Just verify send doesn't throw — blob attachment support varies by runtime
+        await session.disconnect();
+    });
+
     it("should accept message attachments", async () => {
         await writeFile(join(workDir, "attached.txt"), "This file is attached");
 

python/e2e/test_session.py

@@ -569,6 +569,26 @@ def on_event(event):
         assert event.data.new_model == "gpt-4.1"
         assert event.data.reasoning_effort == "high"
 
+    async def test_should_accept_blob_attachments(self, ctx: E2ETestContext):
+        session = await ctx.client.create_session(
+            {"on_permission_request": PermissionHandler.approve_all}
+        )
+
+        await session.send(
+            "Describe this image",
+            attachments=[
+                {
+                    "type": "blob",
+                    "data": "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==",
+                    "mimeType": "image/png",
+                    "displayName": "test-pixel.png",
+                },
+            ],
+        )
+
+        # Just verify send doesn't throw — blob attachment support varies by runtime
+        await session.disconnect()
+
 
 def _get_system_message(exchange: dict) -> str:
     messages = exchange.get("request", {}).get("messages", [])

test/snapshots/session/should_accept_blob_attachments.yaml

@@ -0,0 +1,8 @@
+models:
+  - claude-sonnet-4.5
+conversations:
+  - messages:
+      - role: system
+        content: ${system}
+      - role: user
+        content: Describe this image

test/snapshots/session_config/should_accept_blob_attachments.yaml

@@ -0,0 +1,8 @@
+models:
+  - claude-sonnet-4.5
+conversations:
+  - messages:
+      - role: system
+        content: ${system}
+      - role: user
+        content: Describe this image