MCP: outputSchema anyOf/oneOf rejected — valid JSON Schema combinators cause entire server to fail #57643

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1. opencode-agent commented on Oct 21, 2026

   

   opencode-agentBot

   on Oct 21, 2026 – with OpenCode Agent

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   Thanks for the detailed report. I reproduced this on v2.1.0, and it still happens on the latest dev (15adc3d91).

   What I did:

   • I set up a minimal local MCP server with two tools. One has an ordinary outputSchema (type: "object"). The other uses $defs plus a top-level anyOf with no type, like cqs_callers.
   • opencode mcp list reports failed: Invalid result for tools/list ... path ["tools",1,"outputSchema","type"] ... expected "object", and neither tool is available.
   • Adding a top-level "type": "object" to the second schema makes the server connect.

   The check comes from the MCP SDK's validation of the tools/list result, and OpenCode has no fallback for it, so one tool's output schema takes down the whole server. The spec does declare outputSchema with type: "object". Even so, failing every tool on the server is clearly worse than skipping or loosening just that tool's schema, and OpenCode v1 used to tolerate this kind of error.

   The script below reproduces it. Related: #56116 (a tool call hard-fails when the result doesn't match its output schema).

   Reproduction script (repro.sh)

   #!/usr/bin/env bash
   # Repro for anomalyco/opencode#57643: an MCP tool whose outputSchema is $defs + anyOf (no top-level "type")
   # makes the whole server fail tools/list validation, so none of its tools load.
   # Requires: opencode v2 on PATH (bun add -g @opencode/cli@2.1.0), node.
   set -euo pipefail
   DIR=$(mktemp -d); cd "$DIR"; git init -q
   cat > server.mjs <<'EOF'
   let buf = ""
   const send = (m) => process.stdout.write(JSON.stringify(m) + "\n")
   process.stdin.on("data", (c) => {
     buf += c
     let i
     while ((i = buf.indexOf("\n")) >= 0) {
       const line = buf.slice(0, i).trim(); buf = buf.slice(i + 1)
       if (!line) continue
       const m = JSON.parse(line)
       if (m.id === undefined) continue
       if (m.method === "initialize")
         send({ jsonrpc: "2.0", id: m.id, result: { protocolVersion: m.params?.protocolVersion ?? "2025-06-18", capabilities: { tools: {} }, serverInfo: { name: "anyof-repro", version: "1.0.0" } } })
       else if (m.method === "tools/list")
         send({ jsonrpc: "2.0", id: m.id, result: { tools: [
           { name: "plain", inputSchema: { type: "object", properties: {} },
             outputSchema: { type: "object", properties: { ok: { type: "boolean" } } } },
           { name: "callers", inputSchema: { type: "object", properties: {} },
             outputSchema: {
               $defs: {
                 CallersOutput: { type: "object", properties: { callers: { type: "array", items: { type: "string" } } } },
                 KindFallbackOutput: { type: "object", properties: { kind: { type: "string" } } },
               },
               anyOf: [ { $ref: "#/$defs/CallersOutput" }, { $ref: "#/$defs/KindFallbackOutput" } ],
             } },
         ] } })
       else send({ jsonrpc: "2.0", id: m.id, error: { code: -32601, message: "Method not found" } })
     }
   })
   EOF
   echo "{ \"mcp\": { \"servers\": { \"anyof\": { \"type\": \"local\", \"command\": [\"node\", \"$DIR/server.mjs\"] } } } }" > opencode.json
   opencode --version
   # The first call can race service startup and print "No MCP servers configured"; ask twice.
   opencode mcp list || true; sleep 3; opencode mcp list
   # Expected: "anyof connected" with 2 tools (or at least the "plain" tool).
   # Actual:   "✗ anyof failed: Invalid result for tools/list ... path [tools,1,outputSchema,type] expected \"object\""

   Run it: bash repro.sh

   👍React with 👍1Reacted by bmironov
2. opencode-agent

   added

   reproduced

   on Oct 21, 2026