### Agent System

**Lead Agent** (`packages/harness/deerflow/agents/lead_agent/agent.py`):
- Entry point: `make_lead_agent(config: RunnableConfig)` registered in `langgraph.json`
- Dynamic model selection via `create_chat_model()` with thinking/vision support
- Tools loaded via `get_available_tools()` - combines sandbox, built-in, MCP, community, and subagent tools
- System prompt generated by `apply_prompt_template()` with skills, memory, and subagent instructions

**ThreadState** (`packages/harness/deerflow/agents/thread_state.py`):
- Extends `AgentState` with: `sandbox`, `thread_data`, `title`, `artifacts`, `todos`, `uploaded_files`, `viewed_images`, `goal`, `promoted`, `delegations`, `skill_context`, `summary_text`
- Uses custom reducers: `merge_artifacts` (deduplicate), `merge_viewed_images` (merge/clear), `merge_goal` (preserve the active goal across ordinary state updates unless the goal writer replaces it), `merge_promoted` (catalog-hash-scoped deferred tool promotions), `merge_delegations` (append task delegation entries, same id latest wins, terminal status never downgraded, capped to the most recent entries), and `merge_skill_context` (dedupe active-skill references by path, keep the most recently read entries; entries store a name/path/description reference, not the SKILL.md body). `summary_text` is a LastValue channel updated by summarization and projected into model requests as durable context data instead of being stored as a `messages` item.
- Delta-mode `merge_message_writes` normalizes the current message state once,
  then folds normalized writes in order with message-ID position indexes and
  deferred tombstone compaction. It preserves public `add_messages` behavior,
  including duplicate IDs, replacement position, removal errors,
  `REMOVE_ALL_MESSAGES`, null-write errors, and missing-ID allocation order,
  without rescanning the accumulated state for every write. Keep this
  full-parity contract covered by differential tests: LangGraph's private
  `_messages_delta_reducer` is also linear, but intentionally omits some of
  those public `add_messages` semantics and cannot be substituted directly.

**Runtime Configuration** (via `config.configurable`):
- `thinking_enabled` - Enable model's extended thinking
- `model_name` - Select specific LLM model
- `is_plan_mode` - Enable TodoList middleware
- `subagent_enabled` - Enable task delegation tool
- `max_concurrent_subagents` - Per-response `task` call concurrency limit (clamped by `SubagentLimitMiddleware`)
- `max_total_subagents` - Optional per-run total delegation cap override (falls back to `subagents.max_total_per_run`, clamped to 1-50)
  Gateway and `DeerFlowClient.stream()` always provide the runtime `run_id`; custom
  graph integrations must do the same. If it is absent, enforcement deliberately
  counts the thread's full delegation ledger (fail-restrictive) and emits a warning.