Merge upstream PR #6478: Parallel subtasks, model inheritance, and command.execute.before hook

[shuv1337]

Summary

Merge sst/opencode#6478 into our fork. This PR combines three related improvements to slash commands and subtask handling:

1. Parallel subtask execution - Slash commands can spawn multiple subagents concurrently
2. Model inheritance fix - Proper model/agent context restoration after subtask completion
3. New plugin hook - command.execute.before for plugin extensibility

Problem/Context

Currently, subtasks execute sequentially and lose parent session context (agent/model) after completion. This limits the power of slash commands and plugins. The upstream PR addresses these issues by:

• Allowing parallel subtask execution via Promise.all()
• Tracking parent agent/model in SubtaskPart schema
• Adding command.execute.before hook for pre-execution plugin logic
• Fixing model resolution chain: Command Frontmatter → Agent → Session → Default

This also enables the subtask2 plugin which provides advanced slash command capabilities like multi-step workflows and parallel model comparison (A/B testing with different models).

Acceptance Criteria

• All changes from upstream PR add command.execute.before hook + allow parallel subtasks + fix model inheritance anomalyco/opencode#6478 are merged
• SubtaskPart schema includes model, parentAgent, and parentModel fields
• Subtasks execute in parallel when multiple are queued
• Parent agent/model context is restored after subtask completion
• command.execute.before hook is available in plugin system
• Model inheritance follows correct priority: command → agent → session → default
• Existing tests pass
• Manual testing confirms subtask behavior works correctly

Implementation Details

Files Modified

File	Changes
packages/opencode/src/session/message-v2.ts	Add model, parentAgent, parentModel to SubtaskPart schema
packages/opencode/src/session/prompt.ts	Refactor subtask handling for parallel execution, fix model inheritance
packages/opencode/src/tool/task.ts	Update model resolution to use ctx.extra.model
packages/plugin/src/index.ts	Add command.execute.before hook type
packages/sdk/js/src/v2/gen/types.gen.ts	Update generated types for SubtaskPart

Key Code Changes

Parallel Subtask Execution (prompt.ts):

// Filter subtasks from other task types
const subtasks = tasks.filter((t) => t.type === "subtask")
const otherTasks = tasks.filter((t) => t.type !== "subtask")

// Execute all subtasks in parallel
await Promise.all(subtasks.map(executeSubtask))

Model Inheritance Fix (prompt.ts):

// Restore parent context after subtask
agent: subtasks[0]?.parentAgent ?? lastUser.agent,
model: subtasks[0]?.parentModel ?? lastUser.model,

New Plugin Hook (index.ts):

"command.execute.before"?: (
  input: { command: string; sessionID: string; arguments: string },
  output: { parts: Part[] },
) => Promise<void>

Related PRs (Merged into anomalyco#6478)

• sst/opencode#6411 - command.execute.before and parallel subtasks
• sst/opencode#5726 - Restore session agent/model on subtask return

Tasks

• Fetch upstream changes from sst/opencode
• Cherry-pick or merge PR add command.execute.before hook + allow parallel subtasks + fix model inheritance anomalyco/opencode#6478 commits
• Resolve any conflicts with fork-specific changes
• Verify SubtaskPart schema changes in message-v2.ts
• Verify prompt.ts parallel execution logic
• Verify task.ts model resolution changes
• Verify plugin hook additions
• Run bun test in packages/opencode
• Manual test: Create subtask command and verify parallel execution
• Manual test: Verify model inheritance works correctly
• Update fork sync documentation if needed

External References

• Upstream PR: add command.execute.before hook + allow parallel subtasks + fix model inheritance anomalyco/opencode#6478
• Author: @spoons-and-mirrors
• Related Plugin: subtask2 - Advanced slash command handler that leverages these changes
• Merged PRs:
  • feat(plugin): command.execute.before and parallel subtasks anomalyco/opencode#6411
  • fix: restore session agent and model on subtask return anomalyco/opencode#5726

Notes

• PR stats: +193 additions, -130 deletions across 5 files
• The PR is currently OPEN in upstream
• Consider waiting for upstream merge or merge early if we need the functionality