From da0c62dc081897b10adff640d78159e9c01b27aa Mon Sep 17 00:00:00 2001
From: latent-9 <296084221+latent-9@users.noreply.github.com>
Date: Sat, 15 Aug 2026 09:24:07 +1200
Subject: [PATCH] fix(server): return INVALID_PARAMS for unknown prompt in
 prompts/get

handleGetPrompt threw a bare IllegalArgumentException when the requested
prompt was not registered. Protocol.respondWithError maps any non-McpException
cause to INTERNAL_ERROR (-32603), so a client requesting an unknown prompt
received a server-fault code for what is a client-side bad parameter.

The sibling handler handleReadResource already throws a typed McpException
(RESOURCE_NOT_FOUND) for the equivalent not-found case. Align prompts/get by
throwing McpException(INVALID_PARAMS), and update the integration test that
asserted -32603.
---
 .../sdk/integration/kotlin/AbstractPromptIntegrationTest.kt  | 4 ++--
 .../io/modelcontextprotocol/kotlin/sdk/server/Server.kt      | 5 ++++-
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/integration-test/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/integration/kotlin/AbstractPromptIntegrationTest.kt b/integration-test/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/integration/kotlin/AbstractPromptIntegrationTest.kt
index eaa51ada1..fb97782db 100644
--- a/integration-test/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/integration/kotlin/AbstractPromptIntegrationTest.kt
+++ b/integration-test/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/integration/kotlin/AbstractPromptIntegrationTest.kt
@@ -698,8 +698,8 @@ abstract class AbstractPromptIntegrationTest : KotlinTestBase() {
 
         val expectedMessage = "Prompt not found: non-existent-prompt"
 
-        withClue("Exception code should be INTERNAL_ERROR: -32603") {
-            exception.code shouldBe -32603
+        withClue("Exception code should be INVALID_PARAMS: ${RPCError.ErrorCode.INVALID_PARAMS}") {
+            exception.code shouldBe RPCError.ErrorCode.INVALID_PARAMS
         }
         withClue("Unexpected error message for non-existent prompt") {
             exception.message shouldBe expectedMessage
diff --git a/kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/Server.kt b/kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/Server.kt
index 51a238918..6f7711fa2 100644
--- a/kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/Server.kt
+++ b/kotlin-sdk-server/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/Server.kt
@@ -682,7 +682,10 @@ public open class Server(
         val prompt = promptRegistry.get(requestParams.name)
             ?: run {
                 logger.error { "Prompt not found: ${requestParams.name}" }
-                throw IllegalArgumentException("Prompt not found: ${requestParams.name}")
+                throw McpException(
+                    code = RPCError.ErrorCode.INVALID_PARAMS,
+                    message = "Prompt not found: ${requestParams.name}",
+                )
             }
         return prompt.run {
             session.clientConnection.messageProvider(request)