From b7047e77cf0625241bd177f17bbe35a1d4ae642c Mon Sep 17 00:00:00 2001
From: Recoordinate <296084221+latent-9@users.noreply.github.com>
Date: Thu, 10 Sep 2026 05:00:31 +1200
Subject: [PATCH] fix(server): return INVALID_PARAMS for unknown prompt in
 prompts/get (#955)

## Problem

`prompts/get` for a name that is not registered responds with JSON-RPC
`-32603` (INTERNAL_ERROR).

`handleGetPrompt` in `Server.kt` throws a bare
`IllegalArgumentException` when the prompt is missing.
`Protocol.respondWithError` maps any non-`McpException` cause to
`INTERNAL_ERROR`, so the client receives `-32603` for what is a
client-side bad parameter.

The sibling handler `handleReadResource` in the same file already throws
a typed `McpException(RESOURCE_NOT_FOUND)` for the equivalent not-found
case, which reaches the client as `-32002`. The two not-found paths are
inconsistent.

In JSON-RPC, `-32603` means the server hit an internal fault. Requesting
a prompt name that does not exist is a client error (an invalid `name`
parameter), so a client cannot tell "the prompt does not exist" apart
from "the server failed" and may retry a request that can never succeed.

## Change

Throw a typed `McpException(INVALID_PARAMS)` from `handleGetPrompt`,
consistent with `handleReadResource`. The message is unchanged.

`AbstractPromptIntegrationTest.testNonExistentPrompt` asserted `-32603`;
it now asserts `INVALID_PARAMS`.

If you would prefer a dedicated error code for this case instead of
`INVALID_PARAMS`, I am glad to adjust.
---
 .../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 eaa51ad..fb97782 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 51a2389..6f7711f 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)