[FEATURE]: Agent-level resume for subagents (resumeSessionId parameter)

[Steffen025]

Problem Statement

When building multi-phase agent workflows, there's currently no way to resume a specific subagent with its previous context. Each Task/subagent invocation starts fresh.

Use Case

Complex workflows like specification-driven development have multiple phases:

1. Specify - Architect agent creates specs
2. Clarify - Same architect resolves ambiguities
3. Implement - Engineer agent writes code
4. Validate - Same engineer runs tests

Currently, the engineer in step 4 has no memory of what they implemented in step 3. The context is lost between invocations.

Proposed Solution

Add an optional resumeSessionId parameter to subagent invocation:

// First call - returns session ID
const result = await invokeSubagent({
  agent: "engineer",
  task: "Implement the auth system based on these specs..."
});
// result.sessionId: "xyz789"

// Later - resume with full context
const continued = await invokeSubagent({
  agent: "engineer", 
  task: "Now add tests for the auth system you just built",
  resumeSessionId: "xyz789"  // Load previous session context
});

Technical Considerations

What would need to change:

1. Subagent API - Add optional resumeSessionId parameter
2. Session metadata - Store/expose child session IDs to parent
3. Context loading - When resuming, prepend previous child session events to new session
4. Session lifecycle - Make child sessions persistent and queryable by parent

Current architecture (from my understanding):

# Parent session
~/.opencode/sessions/abc123.jsonl

# Child session (ephemeral)
~/.opencode/sessions/xyz789.jsonl  # Currently no way to reference later

Alternatives Considered

1. Manual context injection - Parent could summarize and inject previous work, but loses nuance and adds token overhead
2. Session-level resume - /sessions works for whole conversations but not individual subagents
3. Longer initial prompts - Include everything upfront, but context windows have limits

Additional Context

This came up while migrating PAI (Personal AI Infrastructure) to run on OpenCode. PAI's Task tool has a resume parameter that enables stateful agent workflows.

Related: Issue #6558 (model selection for subagents) - similar pattern of adding optional Task parameters.

Happy to help with implementation if this is something the team is interested in!

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: give task/subagent an optional config to choose what model it runs on #6558: [FEATURE]: give task/subagent an optional config to choose what model it runs on - Similar pattern of adding optional Task/subagent parameters for enhanced control
• Sessions hang indefinitely when Task tool spawns subagents via REST API (opencode serve) #6573: Sessions hang indefinitely when Task tool spawns subagents via REST API - Related to subagent session handling and context persistence challenges
• Session does not automatically return to parent after subagent execution #6491: Session does not automatically return to parent after subagent execution - Related to subagent lifecycle and context management

Feel free to ignore if none of these address your specific case.