Feature: Add sessionID to experimental.chat.system.transform hook for tracing integration #6142

Summary

When building observability plugins (e.g., Langfuse, OpenTelemetry exporters), it's valuable to capture the full system prompt for each LLM request and associate it with the correct trace/session. The experimental.chat.system.transform hook provides access to the system prompt, but currently lacks the sessionID needed to correlate it with a specific session.

Current Behavior

In packages/opencode/src/session/llm.ts line ~65:

await Plugin.trigger("experimental.chat.system.transform", {}, { system })

The input object is empty {}, so plugins cannot determine which session the system prompt belongs to.

Proposed Change

Pass the sessionID in the input:

await Plugin.trigger("experimental.chat.system.transform", { sessionID: input.sessionID }, { system })

And update the type definition in packages/plugin/src/index.ts:

"experimental.chat.system.transform"?: (
  input: { sessionID: string },
  output: { system: string[] },
) => Promise<void>

Use Case

This enables tracing plugins to:

1. Capture the full system prompt (including environment context, file tree, custom instructions)
2. Associate it with the correct Langfuse trace or OpenTelemetry span
3. Provide complete observability into what context the LLM received

Additional Context

I'm building a Langfuse exporter plugin for OpenCode. Currently I can capture:

• Session events
• User messages with content
• Assistant responses with token usage
• Tool calls with input/output

But the system prompt is missing from traces, which makes debugging and analysis incomplete.

Note: Similar issues were searched before filing. This report was created with AI assistance.