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MCP tool results are collected as empty {} #22
Description
Problem
MCP tool calling is not collected to the langfuse by plugin. Opencode emits tool call result as {title:"...",output:"..."} which the plugin collects correctly, but for MCP call, opencode emits {content:[...]}, which is not compatible with current plugin behavior. So, the langfuse collect {} as output of the MCP call instead.
Cause
src/index.ts:549-561 reads output.title / output.output, then
src/langfuse.ts:913-915 does:
JSON.stringify({ title: input.title, output: input.output })Both are undefined for MCP tools, and JSON.stringify drops undefined
properties, so it's exactly "{}". Same input.output also feeds rememberToolResult({ content: input.output })
(src/langfuse.ts:932), so the content is lost there too.
Reproduce
// .opencode/plugin/probe.js
import { appendFileSync } from "node:fs";
export const ProbePlugin = async () => ({
"tool.execute.after": async (input, output) => {
appendFileSync("/tmp/probe.log", JSON.stringify({
tool: input.tool,
outputKeys: Object.keys(output ?? {}),
payload: JSON.stringify({ title: output?.title, output: output?.output }),
}) + "\n");
},
});
export default ProbePlugin;Run one native tool and one MCP tool:
{"tool":"bash","outputKeys":["title","metadata","output","attachments"],
"payload":"{\"title\":\"echo hello\",\"output\":\"hello\\n\"}"}
{"tool":"context7_resolve-library-id","outputKeys":["content"],
"payload":"{}"}Happens with every MCP server I tried (context7, playwright, sonarqube).
Note: @opencode-ai/plugin declares the hook as
output: { title: string; output: string; metadata: any }
(dist/index.d.ts:249-258), so OpenCode isn't honouring its own type here —
might need an upstream fix too.
Environment
- OpenCode: 1.18.16
- Plugin: 0.2.0
- Langfuse: Cloud (US)
Activity
bezbac commentedon Aug 18, 2026
Hi @Azther0z, thanks for creating an issue here. I just checked and noticed this in my own traces as well.
Would you be interested in contributing a fix? We just opened up the repo to pull requests a few hours ago.
If not, this is something I’m planning to take a look at in the near term.
Azther0z commentedon Aug 18, 2026
Yes, I am happy to contribute. I should have a PR up in a day or two. Thank you for the opportunity.
fix: normalize MCP tool results
Azther0z commentedon Aug 19, 2026
Submitted #31
bezbac commentedon Aug 20, 2026
This was released as part of v0.3.0.
You might need to update your OpenCode config to explicitly reference this version if you’ve already used a prior version.