[BUG]: skill loading resets agent to default one

[Doster-d]

Problem

OpenCode automatically switches to the default agent, which leads to undesirable behavior.

Cause

When OpenCode uses skill_use, it does not set the original agent used when adding a skill to the prompt.

Evidence

Solution

Based on the information from this issue, the solution should be something like this:

import type { PluginInput } from '@opencode-ai/plugin';

export function createInstructionInjector(ctx: PluginInput) {
  // Message 1: Skill loading header (silent insertion - no AI response)
  const sendPrompt = async (text: string, props: { sessionId: string }) => {
    await ctx.client.session.prompt({
      path: { id: props.sessionId },
      body: {
        agent: props.agent
        noReply: true,
        parts: [{ type: 'text', text }],
      },
    });
  };
  return sendPrompt;
}

UPD1: context->props

[airtonix]

Analysis update (validated with CodeMapper + type definitions)

Thanks for the report. I rechecked the issue and the root cause is confirmed.

Root cause

skill_use and skill_resource inject skill content via createInstructionInjector in src/lib/OpenCodeChat.ts.
The injected silent prompt currently sends:

• noReply: true
• parts: [{ type: "text", text }]
• no agent field

When agent is omitted, OpenCode can fall back to the default agent, which matches the behavior reported here.

Evidence

• Code path: src/lib/OpenCodeChat.ts → createInstructionInjector / sendPrompt
• Callers: src/index.ts in both skill_use and skill_resource
• ToolContext provides the active agent: agent: string
  • @opencode-ai/plugin/dist/tool.d.ts
• Session prompt body supports agent?: string
  • @opencode-ai/sdk/dist/gen/types.gen.d.ts

Proposed fix

1. Update sendPrompt props in src/lib/OpenCodeChat.ts to include agent:
   • from { sessionId: string }
   • to { sessionId: string; agent: string }
2. Forward agent in the prompt body:
   • body: { agent: props.agent, noReply: true, parts: [...] }
3. Update both call sites in src/index.ts (skill_use, skill_resource) to pass toolCtx.agent.
4. Add a regression test for createInstructionInjector that asserts session.prompt receives body.agent and keeps noReply: true.

This preserves the currently active agent during skill injection and prevents fallback to the default agent.

[airtonix]

Opened PR #30 to fix this: #30