StdioServerTransportProvider stops reading after malformed JSON #1172

Summary

With io.modelcontextprotocol.sdk:mcp-core:2.0.1 and the Jackson 2 mapper, a malformed JSON line terminates the server transport's inbound reader. Subsequent valid requests receive no response.

This was reproduced with a minimal SDK server, independently of any application input adapter.

Environment

• MCP Java SDK: 2.0.1
• Jackson mapper: JacksonMcpJsonMapper
• Java: 21.0.12.1
• Windows 11
• Initialized protocol version: 2025-11-25

Minimal server

Use io.modelcontextprotocol.sdk:mcp:2.0.1, io.modelcontextprotocol.sdk:mcp-json-jackson2:2.0.1, and their transitive dependencies:

import com.fasterxml.jackson.databind.ObjectMapper;
import io.modelcontextprotocol.json.jackson2.JacksonMcpJsonMapper;
import io.modelcontextprotocol.server.McpServer;
import io.modelcontextprotocol.server.transport.StdioServerTransportProvider;

public class Repro {
    public static void main(String[] args) throws Exception {
        var mapper = new JacksonMcpJsonMapper(new ObjectMapper());
        var transport = new StdioServerTransportProvider(mapper, System.in, System.out);
        var server = McpServer.sync(transport)
                .serverInfo("stdio-repro", "1.0")
                .build();
        Thread.currentThread().join();
    }
}

Keep logging on stderr. Send these newline-delimited messages to stdin in order, waiting for the initialize response before the initialized notification:

{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-11-25","capabilities":{},"clientInfo":{"name":"repro","version":"1.0"}}}
{"jsonrpc":"2.0","method":"notifications/initialized"}
{"jsonrpc":"2.0","id":2,"method":"ping"}
{broken
{"jsonrpc":"2.0","id":3,"method":"ping"}

Actual result

• Initialize succeeds.
• Ping with id 2 returns an empty result.
• The malformed line logs Error processing inbound message to stderr.
• No JSON-RPC parse-error response is returned.
• Ping with id 3 receives no response during a four-second observation window.
• The minimal process remains alive because the main thread is waiting, but its transport no longer reads requests.

The catch block in StdioServerTransportProvider at v2.0.1, lines 249–251 breaks out of the inbound loop. The same catch-and-break behavior is present on the current main branch.

Expected result

For a malformed JSON line, return a JSON-RPC parse error with id: null and code -32700, then continue reading subsequent newline-delimited requests. The later ping should still receive its response.

Any error response should use the transport's serialized output path to avoid interleaving with concurrent responses. This report concerns malformed JSON recovery; protocol/session validation should remain in the SDK.

Related report and workaround

Issue #1157 describes a related failure on the STDIO client when a response is rejected. This reproduction concerns the server receiving malformed JSON input.

Our local workaround is a bounded input adapter that rejects malformed lines before forwarding valid messages to the official SDK. Tests verify continued reading after malformed input, invalid UTF-8, and oversized lines. Recovery in the transport itself would remove the need for that application-level adapter.