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fix: serialization empty response without id #592

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Merged
devcrocod merged 2 commits into from
Mar 9, 2026
Merged

fix: serialization empty response without id #592

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1a1be69
update JSONRPCMessage serializer to handle JSONRPCEmptyMessage scenarios
devcrocod Mar 6, 2026
a00a71e
reduce known SDK limitations to 12 scenarios and update conformance b…
devcrocod Mar 7, 2026
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3 changes: 1 addition & 2 deletions conformance-test/README.md
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Expand Up @@ -110,7 +110,7 @@ Tests the conformance server against all server scenarios:

## Known SDK Limitations

10 scenarios are expected to fail due to current SDK limitations (tracked in [
9 scenarios are expected to fail due to current SDK limitations (tracked in [
`conformance-baseline.yml`](conformance-baseline.yml).

| Scenario | Suite | Root Cause |
Expand All @@ -122,7 +122,6 @@ Tests the conformance server against all server scenarios:
| `elicitation-sep1034-defaults` | server | *(same as above)* |
| `elicitation-sep1330-enums` | server | *(same as above)* |
| `resources-templates-read` | server | SDK does not implement `addResourceTemplate()` with URI pattern matching; resources are looked up by exact URI |
| `initialize` | client | Conformance server sends a JSON-RPC response without `id`; `JSONRPCResponse.id` is non-nullable so deserialization fails |
| `elicitation-sep1034-client-defaults` | client | SDK does not fill in `default` values from the elicitation request schema before sending the response |
| `sse-retry` | client | Transport does not respect the SSE `retry` field timing or send `Last-Event-ID` on reconnection |

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1 change: 0 additions & 1 deletion conformance-test/conformance-baseline.yml
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Expand Up @@ -10,6 +10,5 @@ server:
- resources-templates-read

client:
- initialize
- elicitation-sep1034-client-defaults
- sse-retry
18 changes: 10 additions & 8 deletions ...in-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/serializers.kt
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Expand Up @@ -371,21 +371,23 @@ internal object ServerResultPolymorphicSerializer :
* Polymorphic serializer for [JSONRPCMessage] types.
* Determines the message type based on the presence of specific fields:
* - "error" -> JSONRPCError
* - "result" -> JSONRPCResponse
* - "result" + "id" -> JSONRPCResponse
* - "result" -> JSONRPCEmptyMessage
* - "method" + "id" -> JSONRPCRequest
* - "method" -> JSONRPCNotification
*/
internal object JSONRPCMessagePolymorphicSerializer :
JsonContentPolymorphicSerializer<JSONRPCMessage>(JSONRPCMessage::class) {
override fun selectDeserializer(element: JsonElement): DeserializationStrategy<JSONRPCMessage> {
val jsonObject = element.jsonObject
val jsonObj = element.jsonObject

@e5l e5l Mar 9, 2026

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We should check if we can fix it with schema changes. Could you log an issue to review for this case?

@devcrocod devcrocod Mar 9, 2026

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Yes, sure
we already have an issue for the json rpc response #545
I added a comment about this case

return when {
"error" in jsonObject -> JSONRPCError.serializer()
"result" in jsonObject -> JSONRPCResponse.serializer()
"method" in jsonObject && "id" in jsonObject -> JSONRPCRequest.serializer()
"method" in jsonObject -> JSONRPCNotification.serializer()
jsonObject.isEmpty() || jsonObject.keys == setOf("jsonrpc") -> JSONRPCEmptyMessage.serializer()
else -> throw SerializationException("Invalid JSONRPCMessage type: ${jsonObject.keys}")
"error" in jsonObj -> JSONRPCError.serializer()
"result" in jsonObj && "id" in jsonObj -> JSONRPCResponse.serializer()
"result" in jsonObj && jsonObj["result"]?.jsonObject?.isEmpty() == true -> JSONRPCEmptyMessage.serializer()
"method" in jsonObj && "id" in jsonObj -> JSONRPCRequest.serializer()
"method" in jsonObj -> JSONRPCNotification.serializer()
jsonObj.isEmpty() || jsonObj.keys == setOf("jsonrpc") -> JSONRPCEmptyMessage.serializer()
else -> throw SerializationException("Invalid JSONRPCMessage type: ${jsonObj.keys}")
}
}
}
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