From e2248b0f80517e94fead66dab6768ce5cf12755b Mon Sep 17 00:00:00 2001
From: Rob Findley <rfindley@google.com>
Date: Fri, 9 May 2025 14:47:48 +0000
Subject: [PATCH] internal/mcp/design: add a section on cancellation

Change-Id: If18a7d8efa37a6f34ae42f4d1896464800ebcdc7
Reviewed-on: https://go-review.googlesource.com/c/tools/+/671435
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
---
 design/design.md | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/design/design.md b/design/design.md
index 10e2fb3..c4b6637 100644
--- a/design/design.md
+++ b/design/design.md
@@ -420,7 +420,20 @@ func (*Server) Run(context.Context, Transport)
 
 ### Cancellation
 
-<!-- TODO: a brief section discussing how cancellation is handled. -->
+Cancellation is implemented transparently using context cancellation. The user
+can cancel an operation by cancelling the associated context:
+
+```go
+ctx, cancel := context.WithCancel(ctx)
+go session.CallTool(ctx, "slow", map[string]any{})
+cancel()
+```
+
+When this client call is cancelled, a `"notifications/cancelled"` notification
+is sent to the server. However, the client call returns immediately with
+`ctx.Err()`: it does not wait for the result from the server.
+
+The server observes a client cancellation as cancelled context.
 
 ### Progress handling