From d765ba263ae999ca947f9e427a080e7b33194e84 Mon Sep 17 00:00:00 2001
From: BrianHotopp <brihoto@gmail.com>
Date: Fri, 10 Apr 2026 01:33:20 -0400
Subject: [PATCH] [2.x] fix: Return JSON-RPC error for unknown server methods
 (#9002)

The fallback ServerHandler silently dropped requests with unknown
methods, violating the JSON-RPC spec. Clients like Metals that send
unrecognized requests would never receive a response, potentially
causing timeouts or hangs.

Return a -32601 (Method not found) error response instead.

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 .../main/scala/sbt/internal/server/ServerHandler.scala |  5 ++++-
 server-test/src/test/scala/testpkg/ResponseTest.scala  | 10 ++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/main-command/src/main/scala/sbt/internal/server/ServerHandler.scala b/main-command/src/main/scala/sbt/internal/server/ServerHandler.scala
index 5356b710d..56046fa13 100644
--- a/main-command/src/main/scala/sbt/internal/server/ServerHandler.scala
+++ b/main-command/src/main/scala/sbt/internal/server/ServerHandler.scala
@@ -30,7 +30,10 @@ object ServerHandler {
 
   lazy val fallback: ServerHandler = ServerHandler({ handler =>
     ServerIntent(
-      onRequest = { case x => handler.log.debug(s"Unhandled request received: ${x.method}: $x") },
+      onRequest = { case x =>
+        handler.log.debug(s"Unhandled request received: ${x.method}: $x")
+        handler.jsonRpcRespondError(Some(x.id), -32601, s"Unknown method: ${x.method}")
+      },
       onResponse = { case x => handler.log.debug(s"Unhandled response received") },
       onNotification = { case x =>
         handler.log.debug(s"Unhandled notification received: ${x.method}: $x")
diff --git a/server-test/src/test/scala/testpkg/ResponseTest.scala b/server-test/src/test/scala/testpkg/ResponseTest.scala
index 4f248fc01..48f914932 100644
--- a/server-test/src/test/scala/testpkg/ResponseTest.scala
+++ b/server-test/src/test/scala/testpkg/ResponseTest.scala
@@ -79,6 +79,16 @@ class ResponseTest extends AbstractServerTest {
     }
   }
 
+  test("unknown method returns error") {
+    val id = svr.session.nextId()
+    svr.session.sendJsonRpc(id, "build/foo", "{}").get
+    val response = svr.session.waitForResponseMsg(10.seconds, id).get
+    assert(
+      response.error.exists(_.code == -32601),
+      s"Expected method-not-found error, got: $response"
+    )
+  }
+
   private def neverReceiveResponse(
       duration: FiniteDuration
   )(predicate: sbt.internal.protocol.JsonRpcResponseMessage => Boolean): Unit =