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fix(server): preserve id on duplicate initialize errors #868

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fix(server): preserve id on duplicate initialize errors #868

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a2bb1bd
fix(server): preserve id on duplicate initialize errors
jstar0 Jun 30, 2026
170e4d9
Merge branch 'main' into fix/streamable-http-reject-id
devcrocod Jul 15, 2026
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18 changes: 12 additions & 6 deletions ...monMain/kotlin/io/modelcontextprotocol/kotlin/sdk/server/StreamableHttpServerTransport.kt
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Original file line number Diff line number Diff line change
Expand Up @@ -459,16 +459,18 @@ public class StreamableHttpServerTransport(private val configuration: Configurat
}

val messages = parseBody(call) ?: return
val isInitializationRequest = messages.any {
it is JSONRPCRequest && it.method == Method.Defined.Initialize.value
val initializationRequest = messages.filterIsInstance<JSONRPCRequest>().firstOrNull {
it.method == Method.Defined.Initialize.value
}
val isInitializationRequest = initializationRequest != null

if (isInitializationRequest) {
if (initialized.load() && sessionId != null) {
call.reject(
HttpStatusCode.BadRequest,
RPCError.ErrorCode.INVALID_REQUEST,
"Invalid Request: Server already initialized",
initializationRequest.id,
)
return
}
Expand Down Expand Up @@ -506,8 +508,7 @@ public class StreamableHttpServerTransport(private val configuration: Configurat
// For initialize requests, get from request params.
// For other requests, get from header (already validated).
val clientProtocolVersion = if (isInitializationRequest) {
val initRequest = messages.first() as JSONRPCRequest
(initRequest.params as? JsonObject)?.get("protocolVersion")
(initializationRequest.params as? JsonObject)?.get("protocolVersion")
?.let { McpJson.decodeFromJsonElement<String>(it) }
?: DEFAULT_NEGOTIATED_PROTOCOL_VERSION
} else {
Expand Down Expand Up @@ -913,7 +914,12 @@ public class StreamableHttpServerTransport(private val configuration: Configurat
}
}

internal suspend fun ApplicationCall.reject(status: HttpStatusCode, code: Int, message: String) {
internal suspend fun ApplicationCall.reject(
status: HttpStatusCode,
code: Int,
message: String,
id: RequestId? = null,
) {
this.response.status(status)
this.respond(JSONRPCError(id = null, error = RPCError(code = code, message = message)))
this.respond(JSONRPCError(id = id, error = RPCError(code = code, message = message)))
}
9 changes: 7 additions & 2 deletions ...est/kotlin/io/modelcontextprotocol/kotlin/sdk/server/StreamableHttpServerTransportTest.kt
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Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,7 @@ import io.modelcontextprotocol.kotlin.sdk.types.Implementation
import io.modelcontextprotocol.kotlin.sdk.types.InitializeRequest
import io.modelcontextprotocol.kotlin.sdk.types.InitializeRequestParams
import io.modelcontextprotocol.kotlin.sdk.types.InitializedNotification
import io.modelcontextprotocol.kotlin.sdk.types.JSONRPCError
import io.modelcontextprotocol.kotlin.sdk.types.JSONRPCMessage
import io.modelcontextprotocol.kotlin.sdk.types.JSONRPCRequest
import io.modelcontextprotocol.kotlin.sdk.types.JSONRPCResponse
Expand Down Expand Up @@ -163,7 +164,7 @@ class StreamableHttpServerTransportTest {
}

@Test
fun `second initialization request returns an HTTP error`() = testApplication {
fun `second initialization request returns JSON-RPC error with request id`() = testApplication {
configTestServer()

val client = createTestClient()
Expand All @@ -186,13 +187,17 @@ class StreamableHttpServerTransportTest {

firstResponse.status shouldBe HttpStatusCode.OK

val secondRequest = buildInitializeRequestPayload().copy(id = RequestId("second-init"))
val secondResponse = client.post(path) {
addStreamableHeaders()
header("mcp-session-id", firstResponse.headers[MCP_SESSION_ID_HEADER])
setBody(payload)
setBody(secondRequest)
}

secondResponse.status shouldBe HttpStatusCode.BadRequest
val error = secondResponse.body<JSONRPCError>()
error.id shouldBe secondRequest.id
error.error.message shouldBe "Invalid Request: Server already initialized"
}

@Test
Expand Down
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