feat(skills): deferred skill discovery via describe_skill tool (#3775)

Replace the full-metadata <available_skills> system-prompt block with a
compact <skill_index> (names only) and an on-demand describe_skill tool
when skills.deferred_discovery: true (default: false / backward compat).

New modules:
- skills/catalog.py — SkillCatalog (immutable, searchable; select: has no
  cap, keyword/prefix search caps at MAX_RESULTS=5)
- skills/describe.py — build_describe_skill_tool(catalog) closure;
  build_skill_search_setup() wires SkillSearchSetup into both the
  LangGraph agent factory (agent.py) and DeerFlowClient (client.py)

Changes:
- Skill @dataclass(frozen=True); allowed_tools/required_secrets list→tuple
- Skill First prompt line gated on skill_names (deferred vs legacy wording)
- get_skills_prompt_section: short-circuit storage on deferred path;
  merge user_id (upstream) + skill_names (this PR) params
- describe_skill tool parameter named "name" (matches prompt wording)
- select: branch removes [:MAX_RESULTS] cap (exact request, not ranking)
- AGENTS.md: document deferred_discovery config field + new modules

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/AGENTS.md

@@ -404,7 +404,10 @@ Additional providers also live here (`brave`, `browserless`, `crawl4ai`, `ddg_se
 - **Location**: `deer-flow/skills/{public,custom}/`
 - **Format**: Directory with `SKILL.md` (YAML frontmatter: name, description, license, allowed-tools, required-secrets)
 - **Loading**: `load_skills()` recursively scans `skills/{public,custom}` for `SKILL.md`, parses metadata, and reads enabled state from extensions_config.json
-- **Injection**: Enabled skills listed in agent system prompt with container paths
+- **Injection (legacy / default)**: Enabled skills are listed in the agent system prompt with full metadata and container paths (`<available_skills>` block). Controlled by `skills.deferred_discovery: false` (default).
+- **Deferred discovery** (`skills.deferred_discovery: true`): Skills are listed by name only in a compact `<skill_index>` block, keeping the system prompt prefix-cache friendly. The agent calls the `describe_skill` tool at runtime to fetch full metadata for skills it wants to use, then loads the SKILL.md via `read_file`. Two new modules support this path:
+  - `skills/catalog.py` — `SkillCatalog` (immutable, searchable; query forms: `select:a,b`, `+prefix`, free-text regex); `select:` returns all requested skills without a result cap; other modes cap at `MAX_RESULTS=5`.
+  - `skills/describe.py` — `build_describe_skill_tool(catalog)` builds the `describe_skill` tool as a closure; `build_skill_search_setup(skills, enabled, ...)` produces a `SkillSearchSetup(describe_skill_tool, skill_names)` that is wired into both the LangGraph agent factory (`agent.py`) and the embedded client (`client.py`).
 - **Slash activation**: `/skill-name task` loads that enabled skill's `SKILL.md` for the current model call only. The resolver rejects leading whitespace, missing separators, reserved channel commands (`/new`, `/help`, `/bootstrap`, `/status`, `/models`, `/memory`, `/goal`), disabled skills, and skills outside a custom agent's whitelist.
 - **Installation**: `POST /api/skills/install` extracts .skill ZIP archive to custom/ directory
 
@@ -663,6 +666,7 @@ Returns `{}` when Langfuse is not in the enabled providers — LangSmith-only de
 - `tool_groups[]` - Logical groupings for tools
 - `sandbox.use` - Sandbox provider class path
 - `skills.path` / `skills.container_path` - Host and container paths to skills directory
+- `skills.deferred_discovery` - When `true`, replaces the full-metadata `<available_skills>` prompt block with a compact `<skill_index>` (names only) and registers the `describe_skill` tool so the agent fetches metadata on demand. Defaults to `false` (legacy full-metadata injection)
 - `title` - Auto-title generation (enabled, max_words, max_chars, model_name; null model_name uses fast local fallback, explicit model_name uses the prompt_template LLM path)
 - `summarization` - Context summarization (enabled, trigger conditions, keep policy)
 - `subagents.enabled` - Master switch for subagent delegation

backend/packages/harness/deerflow/agents/lead_agent/agent.py

@@ -520,15 +520,30 @@ def _make_lead_agent(config: RunnableConfig, *, app_config: AppConfig):
 
     skills_for_tool_policy = _load_enabled_skills_for_tool_policy(available_skills, app_config=resolved_app_config, user_id=resolved_user_id)
 
+    # Build skill search setup (deferred skill discovery).
+    # Controlled by skills.deferred_discovery — independent from tool_search.enabled.
+    from deerflow.skills.describe import build_skill_search_setup
+
+    skill_search_enabled = resolved_app_config.skills.deferred_discovery
+    container_base_path = resolved_app_config.skills.container_path
+
     if is_bootstrap:
         # Special bootstrap agent with minimal prompt for initial custom agent creation flow
         # Keep the bootstrap skill set intentionally narrow so agent creation
         # remains deterministic before the custom agent's own config exists.
+        bootstrap_skills = [s for s in skills_for_tool_policy if s.name in _BOOTSTRAP_SKILL_NAMES]
+        skill_setup = build_skill_search_setup(
+            bootstrap_skills,
+            enabled=skill_search_enabled,
+            container_base_path=container_base_path,
+        )
         raw_tools = get_available_tools(model_name=model_name, subagent_enabled=subagent_enabled, app_config=resolved_app_config) + [setup_agent]
         filtered = filter_tools_by_skill_allowed_tools(raw_tools, skills_for_tool_policy, always_allowed_tool_names=SKILL_LOADING_TOOL_NAMES)
         if non_interactive:
             filtered = [tool for tool in filtered if tool.name not in _NON_INTERACTIVE_DISABLED_TOOL_NAMES]
         final_tools, setup = assemble_deferred_tools(filtered, enabled=resolved_app_config.tool_search.enabled)
+        if skill_setup.describe_skill_tool:
+            final_tools.append(skill_setup.describe_skill_tool)
         return create_agent(
             model=create_chat_model(name=model_name, thinking_enabled=thinking_enabled, app_config=resolved_app_config, attach_tracing=False),
             tools=final_tools,
@@ -547,12 +562,20 @@ def _make_lead_agent(config: RunnableConfig, *, app_config: AppConfig):
                 app_config=resolved_app_config,
                 deferred_names=setup.deferred_names,
                 user_id=resolved_user_id,
+                skill_names=skill_setup.skill_names or None,
             ),
             state_schema=ThreadState,
         )
 
     # Custom agents can update their own SOUL.md / config via update_agent.
     # The default agent (no agent_name) does not see this tool.
+    # Build skill search setup from policy-filtered skills (same list used for
+    # tool-policy filtering), so describe_skill only exposes allowed skills.
+    skill_setup = build_skill_search_setup(
+        skills_for_tool_policy,
+        enabled=skill_search_enabled,
+        container_base_path=container_base_path,
+    )
     #
     # Withhold ``update_agent`` from runs triggered by webhook channels
     # (currently only ``github``). Webhook prompts come from arbitrary
@@ -575,6 +598,8 @@ def _make_lead_agent(config: RunnableConfig, *, app_config: AppConfig):
     if non_interactive:
         filtered = [tool for tool in filtered if tool.name not in _NON_INTERACTIVE_DISABLED_TOOL_NAMES]
     final_tools, setup = assemble_deferred_tools(filtered, enabled=resolved_app_config.tool_search.enabled)
+    if skill_setup.describe_skill_tool:
+        final_tools.append(skill_setup.describe_skill_tool)
     return create_agent(
         model=create_chat_model(name=model_name, thinking_enabled=thinking_enabled, reasoning_effort=reasoning_effort, app_config=resolved_app_config, attach_tracing=False),
         tools=final_tools,
@@ -595,6 +620,7 @@ def _make_lead_agent(config: RunnableConfig, *, app_config: AppConfig):
             app_config=resolved_app_config,
             deferred_names=setup.deferred_names,
             user_id=resolved_user_id,
+            skill_names=skill_setup.skill_names or None,
         ),
         state_schema=ThreadState,
     )

backend/packages/harness/deerflow/agents/lead_agent/prompt.py

@@ -631,8 +631,8 @@ combined with a FastAPI gateway for REST API access [citation:FastAPI](https://f
 
 <critical_reminders>
 - **Clarification First**: ALWAYS clarify unclear/missing/ambiguous requirements BEFORE starting work - never assume or guess
-{subagent_reminder}- Skill First: Always load the relevant skill before starting **complex** tasks.
+{subagent_reminder}{skill_first_reminder}
-- Progressive Loading: Load resources incrementally as referenced in skills
+- Progressive Loading: Load skill resources incrementally as referenced
 - Output Files: Final deliverables must be in `/mnt/user-data/outputs` (⚠️ Skills are NOT deliverables — use `skill_manage` tool instead)
 - File Editing Workflow: When revising an existing file, prefer
   `str_replace` over `write_file` — it sends only the diff and avoids
@@ -750,20 +750,20 @@ You have access to skills that provide optimized workflows for specific tasks. E
 </skill_system>"""
 
 
-def get_skills_prompt_section(available_skills: set[str] | None = None, *, app_config: AppConfig | None = None, user_id: str | None = None) -> str:
+def get_skills_prompt_section(
-    """Generate the skills prompt section with available skills list."""
+    available_skills: set[str] | None = None,
-    # Load ALL skills (enabled + disabled) to build the disabled-skills section
+    *,
-    from deerflow.skills.storage import get_or_new_skill_storage, get_or_new_user_skill_storage
+    app_config: AppConfig | None = None,
-
+    user_id: str | None = None,
-    if user_id:
+    skill_names: frozenset[str] | None = None,
-        storage = get_or_new_user_skill_storage(user_id, app_config=app_config)
+) -> str:
-    else:
+    """Generate the skills prompt section.
-        storage = get_or_new_skill_storage(app_config=app_config)
-    all_skills = storage.load_skills(enabled_only=False)
-    disabled_skills = [s for s in all_skills if not s.enabled]
-
-    skills = get_enabled_skills_for_config(app_config, user_id=user_id)
 
+    When *skill_names* is provided, renders a compact ``<skill_index>`` (names
+    only) so the LLM can discover skills via ``describe_skill``.  When omitted,
+    falls back to the legacy full-metadata ``<available_skills>`` rendering for
+    backward compatibility.
+    """
     if app_config is None:
         try:
             from deerflow.config import get_app_config
@@ -775,9 +775,30 @@ def get_skills_prompt_section(available_skills: set[str] | None = None, *, app_c
             container_base_path = DEFAULT_SKILLS_CONTAINER_PATH
             skill_evolution_enabled = False
     else:
-        config = app_config
+        container_base_path = app_config.skills.container_path
-        container_base_path = config.skills.container_path
+        skill_evolution_enabled = app_config.skill_evolution.enabled
-        skill_evolution_enabled = config.skill_evolution.enabled
+
+    skill_evolution_section = _build_skill_evolution_section(skill_evolution_enabled)
+
+    # ── Deferred discovery path — storage not needed (caller supplies names) ─
+    if skill_names is not None:
+        from deerflow.skills.describe import get_skill_index_prompt_section
+
+        return get_skill_index_prompt_section(
+            skill_names=skill_names,
+            container_base_path=container_base_path,
+            skill_evolution_section=skill_evolution_section,
+        )
+
+    # ── Legacy full-metadata path — load ALL skills for disabled-skill section
+    if user_id:
+        storage = get_or_new_user_skill_storage(user_id, app_config=app_config)
+    else:
+        storage = get_or_new_skill_storage(app_config=app_config)
+    all_skills = storage.load_skills(enabled_only=False)
+    disabled_skills = [s for s in all_skills if not s.enabled]
+
+    skills = get_enabled_skills_for_config(app_config, user_id=user_id)
 
     if not skills and not disabled_skills and not skill_evolution_enabled:
         return ""
@@ -790,7 +811,6 @@ def get_skills_prompt_section(available_skills: set[str] | None = None, *, app_c
     available_key = tuple(sorted(available_skills)) if available_skills is not None else None
     if not skill_signature and not disabled_skill_signature and available_key is not None:
         return ""
-    skill_evolution_section = _build_skill_evolution_section(skill_evolution_enabled)
     return _get_cached_skills_prompt_section(skill_signature, disabled_skill_signature, available_key, container_base_path, skill_evolution_section)
 
 
@@ -883,6 +903,7 @@ def apply_prompt_template(
     app_config: AppConfig | None = None,
     deferred_names: frozenset[str] = frozenset(),
     user_id: str | None = None,
+    skill_names: frozenset[str] | None = None,
 ) -> str:
     # Include subagent section only if enabled (from runtime parameter)
     n = max_concurrent_subagents
@@ -906,8 +927,13 @@ def apply_prompt_template(
         else ""
     )
 
-    # Get skills section
+    # Get skills section (deferred discovery when skill_names is provided)
-    skills_section = get_skills_prompt_section(available_skills, app_config=app_config, user_id=user_id)
+    skills_section = get_skills_prompt_section(
+        available_skills,
+        app_config=app_config,
+        user_id=user_id,
+        skill_names=skill_names,
+    )
 
     # Get deferred tools section (tool_search)
     deferred_tools_section = get_deferred_tools_prompt_section(deferred_names=deferred_names)
@@ -917,6 +943,14 @@ def apply_prompt_template(
     custom_mounts_section = _build_custom_mounts_section(app_config=app_config)
     acp_and_mounts_section = "\n".join(section for section in (acp_section, custom_mounts_section) if section)
 
+    # Gate the "Skill First" instruction on the deferred discovery path:
+    # legacy mode uses tool-agnostic wording; deferred mode references describe_skill.
+    skill_first_reminder = (
+        "- Skill First: For complex tasks, call describe_skill(name) to check if a matching skill exists, then read_file to load it.\n"
+        if skill_names is not None
+        else "- Skill First: Always load the relevant skill before starting **complex** tasks.\n"
+    )
+
     # Build and return the fully static system prompt.
     # Memory and current date are injected per-turn via DynamicContextMiddleware
     # as a <system-reminder> in the first HumanMessage, keeping this prompt
@@ -929,6 +963,7 @@ def apply_prompt_template(
         deferred_tools_section=deferred_tools_section,
         subagent_section=subagent_section,
         subagent_reminder=subagent_reminder,
+        skill_first_reminder=skill_first_reminder,
         subagent_thinking=subagent_thinking,
         acp_section=acp_and_mounts_section,
     )

backend/packages/harness/deerflow/client.py

@@ -35,7 +35,7 @@ from langchain_core.messages import AIMessage, HumanMessage, SystemMessage, Tool
 from langchain_core.runnables import RunnableConfig
 
 from deerflow.agents.lead_agent.agent import build_middlewares
-from deerflow.agents.lead_agent.prompt import apply_prompt_template
+from deerflow.agents.lead_agent.prompt import apply_prompt_template, get_enabled_skills_for_config
 from deerflow.agents.thread_state import ThreadState
 from deerflow.config.agents_config import AGENT_NAME_PATTERN
 from deerflow.config.app_config import get_app_config, is_trace_correlation_enabled, reload_app_config
@@ -44,6 +44,7 @@ from deerflow.config.paths import get_paths
 from deerflow.models import create_chat_model
 from deerflow.runtime.goal import DEFAULT_MAX_GOAL_CONTINUATIONS, build_goal_state, goal_thread_lock, read_thread_goal, write_thread_goal
 from deerflow.runtime.user_context import get_effective_user_id
+from deerflow.skills.describe import build_skill_search_setup
 from deerflow.skills.storage import get_or_new_user_skill_storage
 from deerflow.tools.builtins.tool_search import assemble_deferred_tools
 from deerflow.trace_context import DEERFLOW_TRACE_METADATA_KEY, generate_trace_id, get_current_trace_id, reset_current_trace_id, set_current_trace_id
@@ -256,6 +257,19 @@ class DeerFlowClient:
 
         tools = self._get_tools(model_name=model_name, subagent_enabled=subagent_enabled)
         final_tools, deferred_setup = assemble_deferred_tools(tools, enabled=self._app_config.tool_search.enabled)
+
+        # Wire deferred skill discovery — mirrors agent.py so config flag works on both paths.
+        skills_list = get_enabled_skills_for_config(self._app_config)
+        if self._available_skills is not None:
+            skills_list = [s for s in skills_list if s.name in self._available_skills]
+        skill_setup = build_skill_search_setup(
+            skills_list,
+            enabled=self._app_config.skills.deferred_discovery,
+            container_base_path=self._app_config.skills.container_path,
+        )
+        if skill_setup.describe_skill_tool:
+            final_tools.append(skill_setup.describe_skill_tool)
+
         kwargs: dict[str, Any] = {
             # attach_tracing=False because ``stream()`` injects tracing
             # callbacks at the graph invocation root so a single embedded run
@@ -281,6 +295,7 @@ class DeerFlowClient:
                 app_config=self._app_config,
                 deferred_names=deferred_setup.deferred_names,
                 user_id=get_effective_user_id(),
+                skill_names=skill_setup.skill_names or None,
             ),
             "state_schema": ThreadState,
         }

backend/packages/harness/deerflow/config/skills_config.py

@@ -29,6 +29,10 @@ class SkillsConfig(BaseModel):
         default=DEFAULT_SKILLS_CONTAINER_PATH,
         description="Path where skills are mounted in the sandbox container",
     )
+    deferred_discovery: bool = Field(
+        default=False,
+        description=("When enabled, skill metadata is not injected into the system prompt. Instead, only skill names appear in <skill_index> and the LLM discovers details on demand via the describe_skill tool."),
+    )
 
     def get_skills_path(self) -> Path:
         """

backend/packages/harness/deerflow/skills/__init__.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from .catalog import SkillCatalog
+from .describe import SkillSearchSetup, build_describe_skill_tool, build_skill_search_setup
 from .installer import SkillAlreadyExistsError, SkillSecurityScanError
 from .storage import LocalSkillStorage, SkillStorage, get_or_new_skill_storage
 from .types import Skill
@@ -7,6 +9,10 @@ from .validation import ALLOWED_FRONTMATTER_PROPERTIES, _validate_skill_frontmat
 
 __all__ = [
     "Skill",
+    "SkillCatalog",
+    "SkillSearchSetup",
+    "build_describe_skill_tool",
+    "build_skill_search_setup",
     "ALLOWED_FRONTMATTER_PROPERTIES",
     "_validate_skill_frontmatter",
     "SkillAlreadyExistsError",

backend/packages/harness/deerflow/skills/catalog.py

@@ -0,0 +1,102 @@
+"""Skill catalog — deferred skill discovery at runtime.
+
+Mirrors ``DeferredToolCatalog`` from ``tool_search.py``: an immutable, searchable
+catalog that lets the LLM discover skill metadata on demand rather than having
+every skill's full description baked into the system prompt.
+
+The agent sees skill names in ``<skill_index>`` but cannot read their metadata
+until it calls ``describe_skill``.  This keeps the system prompt compact and
+prefix-cache friendly while still giving the model autonomous skill discovery.
+"""
+
+from __future__ import annotations
+
+import logging
+import re
+from dataclasses import dataclass
+from functools import cached_property
+
+from deerflow.skills.types import Skill
+
+logger = logging.getLogger(__name__)
+
+MAX_RESULTS = 5
+
+
+def _compile_catalog_regex(pattern: str) -> re.Pattern[str]:
+    """Compile ``pattern`` case-insensitively, falling back to literal match.
+
+    Search queries come from the model, so an invalid regex (e.g. an unbalanced
+    paren) must degrade to a literal substring match rather than raise.
+    """
+    try:
+        return re.compile(pattern, re.IGNORECASE)
+    except re.error:
+        return re.compile(re.escape(pattern), re.IGNORECASE)
+
+
+# NOTE: frozen=True without slots=True keeps __dict__, which is what lets the
+# @cached_property fields below cache (they write to instance.__dict__, bypassing
+# the frozen __setattr__). Do NOT add slots=True or hash/names break at runtime.
+@dataclass(frozen=True)
+class SkillCatalog:
+    """Immutable catalog of skills.  Pure search, no mutation.
+
+    Query forms (mirror ``DeferredToolCatalog.search``):
+
+    - ``"select:data-analysis,deep-research"`` — exact match by name.
+    - ``"+podcast gen"`` — require *podcast* in the name, rank by *gen*.
+    - ``"chart visualization"`` — regex match on name + description.
+    """
+
+    skills: tuple[Skill, ...]
+
+    @cached_property
+    def names(self) -> frozenset[str]:
+        """All skill names in insertion order."""
+        return frozenset(s.name for s in self.skills)
+
+    def search(self, query: str) -> list[Skill]:
+        """Match *query* against skill names and descriptions.
+
+        Returns at most ``MAX_RESULTS`` skills, ranked by relevance.
+        """
+        query = query.strip()
+        if not query:
+            return []
+
+        # ── Exact selection ────────────────────────────────────────────
+        if query.startswith("select:"):
+            wanted = {n.strip() for n in query[7:].split(",")}
+            return [s for s in self.skills if s.name in wanted]
+
+        # ── Required-prefix search ─────────────────────────────────────
+        if query.startswith("+"):
+            parts = query[1:].split(None, 1)
+            if not parts:
+                return []  # bare "+" with no required token
+            required = parts[0].lower()
+            candidates = [s for s in self.skills if required in s.name.lower()]
+            if len(parts) > 1:
+                pattern = _compile_catalog_regex(parts[1])
+                candidates.sort(
+                    key=lambda s: _catalog_regex_score(pattern, s),
+                    reverse=True,
+                )
+            return candidates[:MAX_RESULTS]
+
+        # ── Free-text regex search ─────────────────────────────────────
+        regex = _compile_catalog_regex(query)
+        scored: list[tuple[int, Skill]] = []
+        for s in self.skills:
+            searchable = f"{s.name} {s.description or ''}"
+            if regex.search(searchable):
+                # Name match scores higher than description-only match.
+                scored.append((2 if regex.search(s.name) else 1, s))
+        scored.sort(key=lambda x: x[0], reverse=True)
+        return [s for _, s in scored][:MAX_RESULTS]
+
+
+def _catalog_regex_score(pattern: re.Pattern[str], s: Skill) -> int:
+    """Count regex hits across name + description for ranking."""
+    return len(pattern.findall(f"{s.name} {s.description or ''}"))

backend/packages/harness/deerflow/skills/describe.py

@@ -0,0 +1,180 @@
+"""describe_skill — deferred skill metadata retrieval at runtime.
+
+Builds the ``describe_skill`` tool as a closure over a :class:`SkillCatalog`.
+The tool returns structured metadata (description, allowed tools, file location)
+so the LLM can decide whether to ``read_file`` the full SKILL.md.
+
+Mirrors ``build_tool_search_tool`` from ``tool_search.py``: same query syntax,
+same ``Command`` + ``ToolMessage`` return shape, same fail-safe degradation.
+"""
+
+from __future__ import annotations
+
+import logging
+from dataclasses import dataclass
+from typing import TYPE_CHECKING, Annotated
+
+from langchain_core.messages import ToolMessage
+from langchain_core.tools import InjectedToolCallId, tool
+from langgraph.types import Command
+
+if TYPE_CHECKING:
+    from langchain.tools import BaseTool
+
+from deerflow.constants import DEFAULT_SKILLS_CONTAINER_PATH
+from deerflow.skills.catalog import SkillCatalog
+from deerflow.skills.types import SkillCategory
+
+logger = logging.getLogger(__name__)
+
+
+# ── Setup ────────────────────────────────────────────────────────────────────
+
+
+@dataclass(frozen=True)
+class SkillSearchSetup:
+    """Result of assembling skill search for one agent build.
+
+    Mirrors ``DeferredToolSetup`` from ``tool_search.py``.
+
+    - **Empty** ``(None, frozenset())``: no skills available or skill search
+      disabled.  The agent falls back to the legacy full-metadata prompt.
+    - **Populated**: ``describe_skill_tool`` is appended to the agent's tools,
+      ``skill_names`` are rendered in ``<skill_index>`` instead of full metadata.
+    """
+
+    describe_skill_tool: BaseTool | None
+    skill_names: frozenset[str]
+
+
+def build_describe_skill_tool(
+    catalog: SkillCatalog,
+    *,
+    container_base_path: str = DEFAULT_SKILLS_CONTAINER_PATH,
+) -> BaseTool:
+    """Build the ``describe_skill`` tool as a closure over *catalog*.
+
+    The returned tool is a plain ``@tool``-decorated function that searches the
+    catalog and returns a ``Command`` wrapping a ``ToolMessage``.  No graph state
+    mutation is needed (unlike ``tool_search`` which promotes deferred tools).
+    """
+
+    @tool
+    def describe_skill(
+        name: str,
+        tool_call_id: Annotated[str, InjectedToolCallId],
+    ) -> Command:
+        """Fetch usage metadata for installed skills so you can decide whether to load them.
+
+        Skills appear by name in <skill_index> in the system prompt.  Until
+        fetched, only the name is known.  This tool matches a query against
+        installed skills and returns their full metadata — description, allowed
+        tools, and file location — so you can decide whether to load the
+        SKILL.md via read_file.
+
+        Query forms:
+          - "select:data-analysis,deep-research" -- fetch these exact skills (no cap)
+          - "chart visualization" -- keyword search, best matches (up to 5)
+          - "+podcast gen" -- require "podcast" in the name, rank by remaining terms (up to 5)
+        """
+        matched = catalog.search(name)
+        if not matched:
+            content = f"No skills matched: {name}"
+        else:
+            content = _render_skill_metadata(matched, container_base_path)
+
+        return Command(
+            update={
+                "messages": [
+                    ToolMessage(
+                        content=content,
+                        tool_call_id=tool_call_id,
+                        name="describe_skill",
+                    )
+                ],
+            }
+        )
+
+    return describe_skill
+
+
+def build_skill_search_setup(
+    skills: list,
+    *,
+    enabled: bool,
+    container_base_path: str = DEFAULT_SKILLS_CONTAINER_PATH,
+) -> SkillSearchSetup:
+    """Build the skill search setup from a filtered skill list.
+
+    Mirrors ``build_deferred_tool_setup`` from ``tool_search.py``.
+
+    Returns an empty setup when *enabled* is ``False`` or *skills* is empty.
+    """
+    if not enabled or not skills:
+        return SkillSearchSetup(None, frozenset())
+
+    catalog = SkillCatalog(tuple(skills))
+    return SkillSearchSetup(
+        describe_skill_tool=build_describe_skill_tool(
+            catalog,
+            container_base_path=container_base_path,
+        ),
+        skill_names=catalog.names,
+    )
+
+
+# ── Rendering ────────────────────────────────────────────────────────────────
+
+
+def _render_skill_metadata(skills: list, container_base_path: str) -> str:
+    """Render structured metadata for a list of matched skills."""
+    blocks: list[str] = []
+    for s in skills:
+        mutability = "[custom, editable]" if s.category == SkillCategory.CUSTOM else "[built-in]"
+        tools_line = ", ".join(s.allowed_tools) if s.allowed_tools else "(all)"
+        location = s.get_container_file_path(container_base_path)
+        blocks.append(f"## Skill: {s.name}\n- Description: {s.description} {mutability}\n- Allowed tools: {tools_line}\n- Location: {location}")
+    return "\n\n".join(blocks)
+
+
+# ── Prompt rendering ─────────────────────────────────────────────────────────
+
+
+def get_skill_index_prompt_section(
+    *,
+    skill_names: frozenset[str] = frozenset(),
+    container_base_path: str = DEFAULT_SKILLS_CONTAINER_PATH,
+    skill_evolution_section: str = "",
+) -> str:
+    """Generate ``<skill_system>`` with a name-only ``<skill_index>``.
+
+    Mirrors ``get_deferred_tools_prompt_section`` from ``tool_search.py``.
+    The agent knows what exists and can use ``describe_skill`` to load metadata.
+
+    Returns empty string when there are no skills.
+    """
+    if not skill_names:
+        return ""
+
+    names = ", ".join(sorted(skill_names))
+    evolution = f"\n{skill_evolution_section}" if skill_evolution_section else ""
+
+    return f"""<skill_system>
+You have access to skills that provide optimized workflows for specific tasks.
+
+**Skill Discovery:**
+1. Check <skill_index> for a skill name that matches your task
+2. Call describe_skill(name) to fetch its description and capabilities
+3. If the skill matches, call read_file on the returned location to load full instructions
+4. Follow the skill's instructions precisely
+
+**Explicit Slash Skill Activation:**
+- If the user starts a request with `/<skill-name>`, that skill was explicitly requested.
+- The runtime injects the activated skill content; do not call `read_file` for that SKILL.md again unless the injected skill references supporting resources you need.
+{evolution}
+<skill_index>
+{names}
+</skill_index>
+
+Skills are located at: {container_base_path}
+</skill_system>"""

backend/packages/harness/deerflow/skills/parser.py

@@ -43,11 +43,11 @@ def _format_yaml_error(skill_file: Path, exc: yaml.YAMLError, source: str) -> st
     return "\n".join(lines)
 
 
-def parse_allowed_tools(raw: object, skill_file: Path) -> list[str] | None:
+def parse_allowed_tools(raw: object, skill_file: Path) -> tuple[str, ...] | None:
     """Parse the optional allowed-tools frontmatter field.
 
-    Returns None when the field is omitted. Returns a list when the field is a
+    Returns None when the field is omitted. Returns a tuple when the field is a
-    YAML sequence of strings, including an empty list for explicit no-tool
+    YAML sequence of strings, including an empty tuple for explicit no-tool
     skills. Raises ValueError for malformed values.
     """
     if raw is None:
@@ -63,21 +63,21 @@ def parse_allowed_tools(raw: object, skill_file: Path) -> list[str] | None:
         if not tool_name:
             raise ValueError(f"allowed-tools in {skill_file} cannot contain empty tool names")
         allowed_tools.append(tool_name)
-    return allowed_tools
+    return tuple(allowed_tools)
 
 
-def parse_required_secrets(raw: object, skill_file: Path) -> list[SecretRequirement]:
+def parse_required_secrets(raw: object, skill_file: Path) -> tuple[SecretRequirement, ...]:
     """Parse the optional required-secrets frontmatter field (issue #3861).
 
     Accepts a YAML sequence whose items are either a string (the secret / env
-    variable name) or a mapping (``{name, optional}``). Returns an empty list
+    variable name) or a mapping (``{name, optional}``). Returns an empty tuple
     when the field is omitted. Entries whose name is missing or is not a valid
     environment-variable name are dropped with a warning, so one malformed
     declaration does not invalidate the whole skill. Raises ValueError only when
     the field is present but is not a list.
     """
     if raw is None:
-        return []
+        return ()
     if not isinstance(raw, list):
         raise ValueError(f"required-secrets in {skill_file} must be a list")
 
@@ -100,7 +100,7 @@ def parse_required_secrets(raw: object, skill_file: Path) -> list[SecretRequirem
             continue
         seen.add(name)
         secrets.append(SecretRequirement(name=name, optional=optional))
-    return secrets
+    return tuple(secrets)
 
 
 def parse_skill_file(skill_file: Path, category: SkillCategory, relative_path: Path | None = None) -> Skill | None:

backend/packages/harness/deerflow/skills/storage/skill_storage.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import dataclasses
 import logging
 import re
 from abc import ABC, abstractmethod
@@ -267,8 +268,7 @@ class SkillStorage(ABC):
             from deerflow.config.extensions_config import ExtensionsConfig
 
             extensions_config = ExtensionsConfig.from_file()
-            for skill in skills:
+            skills = [dataclasses.replace(s, enabled=extensions_config.is_skill_enabled(s.name, s.category)) for s in skills]
-                skill.enabled = extensions_config.is_skill_enabled(skill.name, skill.category)
         except Exception as e:
             logger.warning("Failed to load extensions config: %s", e)
 

backend/packages/harness/deerflow/skills/storage/user_scoped_skill_storage.py

@@ -29,6 +29,7 @@ two users own same-named custom skills.
 from __future__ import annotations
 
 import asyncio
+import dataclasses
 import json
 import logging
 import os
@@ -205,12 +206,12 @@ class UserScopedSkillStorage(LocalSkillStorage):
         from deerflow.config.extensions_config import get_extensions_config
 
         extensions_config = get_extensions_config()
-        for skill in skills:
+        skills = [
-            category = skill.category.value if hasattr(skill.category, "value") else skill.category
+            dataclasses.replace(s, enabled=self.get_skill_enabled_state(s.name) and extensions_config.is_skill_enabled(s.name, s.category.value if hasattr(s.category, "value") else s.category))
-            if category != SkillCategory.PUBLIC.value:
+            if dataclasses.is_dataclass(s) and not isinstance(s, type) and (s.category.value if hasattr(s.category, "value") else s.category) != SkillCategory.PUBLIC.value
-                per_user_state = self.get_skill_enabled_state(skill.name)
+            else s
-                global_state = extensions_config.is_skill_enabled(skill.name, category)
+            for s in skills
-                skill.enabled = per_user_state and global_state
+        ]
 
         if enabled_only:
             skills = [s for s in skills if s.enabled]

backend/packages/harness/deerflow/skills/types.py

@@ -35,7 +35,7 @@ class SecretRequirement:
     optional: bool = False
 
 
-@dataclass
+@dataclass(frozen=True)
 class Skill:
     """Represents a skill with its metadata and file path"""
 
@@ -46,9 +46,9 @@ class Skill:
     skill_file: Path
     relative_path: Path  # Relative path from category root to skill directory
     category: SkillCategory  # 'public' or 'custom'
-    allowed_tools: list[str] | None = None
+    allowed_tools: tuple[str, ...] | None = None
     enabled: bool = False  # Whether this skill is enabled
-    required_secrets: list[SecretRequirement] = field(default_factory=list)
+    required_secrets: tuple[SecretRequirement, ...] = field(default_factory=tuple)
 
     @property
     def skill_path(self) -> str:

backend/tests/test_checkpointer.py

@@ -896,12 +896,17 @@ class TestClientCheckpointerFallback:
         config_mock.get_model_config.return_value = MagicMock(supports_vision=False)
         config_mock.checkpointer = None
 
+        config_mock.skills.deferred_discovery = False
+        config_mock.skills.container_path = "/mnt/skills"
+        config_mock.tool_search.enabled = False
+
         with (
             patch("deerflow.client.get_app_config", return_value=config_mock),
             patch("deerflow.client.create_agent", side_effect=fake_create_agent),
             patch("deerflow.client.create_chat_model", return_value=MagicMock()),
             patch("deerflow.client.build_middlewares", return_value=[]),
             patch("deerflow.client.apply_prompt_template", return_value=""),
+            patch("deerflow.client.get_enabled_skills_for_config", return_value=[]),
             patch("deerflow.client.DeerFlowClient._get_tools", return_value=[]),
         ):
             client = DeerFlowClient(checkpointer=None)
@@ -930,12 +935,17 @@ class TestClientCheckpointerFallback:
         config_mock.get_model_config.return_value = MagicMock(supports_vision=False)
         config_mock.checkpointer = None
 
+        config_mock.skills.deferred_discovery = False
+        config_mock.skills.container_path = "/mnt/skills"
+        config_mock.tool_search.enabled = False
+
         with (
             patch("deerflow.client.get_app_config", return_value=config_mock),
             patch("deerflow.client.create_agent", side_effect=fake_create_agent),
             patch("deerflow.client.create_chat_model", return_value=MagicMock()),
             patch("deerflow.client.build_middlewares", return_value=[]),
             patch("deerflow.client.apply_prompt_template", return_value=""),
+            patch("deerflow.client.get_enabled_skills_for_config", return_value=[]),
             patch("deerflow.client.DeerFlowClient._get_tools", return_value=[]),
         ):
             client = DeerFlowClient(checkpointer=explicit_cp)

backend/tests/test_client.py

@@ -41,6 +41,9 @@ def mock_app_config():
     config = MagicMock()
     config.models = [model]
     config.token_usage.enabled = False
+    config.skills.deferred_discovery = False
+    config.skills.container_path = "/mnt/skills"
+    config.tool_search.enabled = False
     return config
 
 
@@ -916,6 +919,7 @@ class TestEnsureAgent:
             patch("deerflow.client.create_agent", return_value=mock_agent),
             patch("deerflow.client.build_middlewares", return_value=[]) as mock_build_middlewares,
             patch("deerflow.client.apply_prompt_template", return_value="prompt") as mock_apply_prompt,
+            patch("deerflow.client.get_enabled_skills_for_config", return_value=[]),
             patch.object(client, "_get_tools", return_value=[]),
             patch("deerflow.runtime.checkpointer.get_checkpointer", return_value=MagicMock()),
         ):
@@ -941,6 +945,7 @@ class TestEnsureAgent:
             patch("deerflow.client.create_agent", return_value=mock_agent) as mock_create_agent,
             patch("deerflow.client.build_middlewares", return_value=[]),
             patch("deerflow.client.apply_prompt_template", return_value="prompt"),
+            patch("deerflow.client.get_enabled_skills_for_config", return_value=[]),
             patch.object(client, "_get_tools", return_value=[]),
             patch("deerflow.runtime.checkpointer.get_checkpointer", return_value=mock_checkpointer),
         ):
@@ -966,6 +971,7 @@ class TestEnsureAgent:
             patch("deerflow.client.create_agent", return_value=mock_agent) as mock_create_agent,
             patch("deerflow.client.build_middlewares", side_effect=fake_build_middlewares),
             patch("deerflow.client.apply_prompt_template", return_value="prompt"),
+            patch("deerflow.client.get_enabled_skills_for_config", return_value=[]),
             patch.object(client, "_get_tools", return_value=[]),
             patch("deerflow.runtime.checkpointer.get_checkpointer", return_value=MagicMock()),
         ):
@@ -985,6 +991,7 @@ class TestEnsureAgent:
             patch("deerflow.client.create_agent", return_value=mock_agent) as mock_create_agent,
             patch("deerflow.client.build_middlewares", return_value=[]),
             patch("deerflow.client.apply_prompt_template", return_value="prompt"),
+            patch("deerflow.client.get_enabled_skills_for_config", return_value=[]),
             patch.object(client, "_get_tools", return_value=[]),
             patch("deerflow.runtime.checkpointer.get_checkpointer", return_value=None),
         ):
@@ -1004,6 +1011,82 @@ class TestEnsureAgent:
         # Should still be the same mock — no recreation
         assert client._agent is mock_agent
 
+    def test_deferred_skill_discovery_wired_when_enabled(self, client, mock_app_config):
+        """When skills.deferred_discovery=True, skill_names reaches apply_prompt_template
+        (parity with agent.py — config flag must not be a silent no-op on the embedded path)."""
+        from pathlib import Path
+
+        from deerflow.skills.types import Skill, SkillCategory
+
+        fake_skill = Skill(
+            name="deep-research",
+            description="Multi-source research",
+            license=None,
+            skill_dir=Path("/mnt/skills/public/deep-research"),
+            skill_file=Path("/mnt/skills/public/deep-research/SKILL.md"),
+            relative_path=Path("deep-research"),
+            category=SkillCategory.PUBLIC,
+            enabled=True,
+        )
+
+        mock_app_config.skills.deferred_discovery = True
+        mock_app_config.skills.container_path = "/mnt/skills"
+        mock_app_config.tool_search.enabled = False
+        client._app_config = mock_app_config
+        config = client._get_runnable_config("t1")
+
+        with (
+            patch("deerflow.client.create_chat_model"),
+            patch("deerflow.client.create_agent", return_value=MagicMock()),
+            patch("deerflow.client.build_middlewares", return_value=[]),
+            patch("deerflow.client.apply_prompt_template", return_value="prompt") as mock_apply_prompt,
+            patch.object(client, "_get_tools", return_value=[]),
+            patch("deerflow.runtime.checkpointer.get_checkpointer", return_value=None),
+            patch("deerflow.client.get_enabled_skills_for_config", return_value=[fake_skill]),
+        ):
+            client._ensure_agent(config)
+
+        skill_names_arg = mock_apply_prompt.call_args.kwargs.get("skill_names")
+        assert skill_names_arg is not None, "skill_names must be passed when deferred_discovery=True"
+        assert "deep-research" in skill_names_arg
+
+    def test_deferred_skill_discovery_not_wired_when_disabled(self, client, mock_app_config):
+        """When skills.deferred_discovery=False, skill_names is None so the legacy prompt path runs."""
+        from pathlib import Path
+
+        from deerflow.skills.types import Skill, SkillCategory
+
+        fake_skill = Skill(
+            name="deep-research",
+            description="Multi-source research",
+            license=None,
+            skill_dir=Path("/mnt/skills/public/deep-research"),
+            skill_file=Path("/mnt/skills/public/deep-research/SKILL.md"),
+            relative_path=Path("deep-research"),
+            category=SkillCategory.PUBLIC,
+            enabled=True,
+        )
+
+        mock_app_config.skills.deferred_discovery = False
+        mock_app_config.skills.container_path = "/mnt/skills"
+        mock_app_config.tool_search.enabled = False
+        client._app_config = mock_app_config
+        config = client._get_runnable_config("t1")
+
+        with (
+            patch("deerflow.client.create_chat_model"),
+            patch("deerflow.client.create_agent", return_value=MagicMock()),
+            patch("deerflow.client.build_middlewares", return_value=[]),
+            patch("deerflow.client.apply_prompt_template", return_value="prompt") as mock_apply_prompt,
+            patch.object(client, "_get_tools", return_value=[]),
+            patch("deerflow.runtime.checkpointer.get_checkpointer", return_value=None),
+            patch("deerflow.client.get_enabled_skills_for_config", return_value=[fake_skill]),
+        ):
+            client._ensure_agent(config)
+
+        skill_names_arg = mock_apply_prompt.call_args.kwargs.get("skill_names")
+        assert skill_names_arg is None, "skill_names must be None when deferred_discovery=False"
+
 
 # ---------------------------------------------------------------------------
 # get_model
@@ -1997,6 +2080,7 @@ class TestScenarioAgentRecreation:
             patch("deerflow.client.create_agent", side_effect=fake_create_agent),
             patch("deerflow.client.build_middlewares", return_value=[]),
             patch("deerflow.client.apply_prompt_template", return_value="prompt"),
+            patch("deerflow.client.get_enabled_skills_for_config", return_value=[]),
             patch.object(client, "_get_tools", return_value=[]),
             patch("deerflow.runtime.checkpointer.get_checkpointer", return_value=MagicMock()),
         ):
@@ -2025,6 +2109,7 @@ class TestScenarioAgentRecreation:
             patch("deerflow.client.create_agent", side_effect=fake_create_agent),
             patch("deerflow.client.build_middlewares", return_value=[]),
             patch("deerflow.client.apply_prompt_template", return_value="prompt"),
+            patch("deerflow.client.get_enabled_skills_for_config", return_value=[]),
             patch.object(client, "_get_tools", return_value=[]),
             patch("deerflow.runtime.checkpointer.get_checkpointer", return_value=MagicMock()),
         ):
@@ -2050,6 +2135,7 @@ class TestScenarioAgentRecreation:
             patch("deerflow.client.create_agent", side_effect=fake_create_agent),
             patch("deerflow.client.build_middlewares", return_value=[]),
             patch("deerflow.client.apply_prompt_template", return_value="prompt"),
+            patch("deerflow.client.get_enabled_skills_for_config", return_value=[]),
             patch.object(client, "_get_tools", return_value=[]),
             patch("deerflow.runtime.checkpointer.get_checkpointer", return_value=MagicMock()),
         ):

backend/tests/test_deferred_tool_crosscontext.py

@@ -137,7 +137,7 @@ def _make_skill(allowed_tools):
         skill_file=Path("/tmp/s/SKILL.md"),
         relative_path=Path("s"),
         category="public",
-        allowed_tools=allowed_tools,
+        allowed_tools=tuple(allowed_tools) if allowed_tools is not None else None,
         enabled=True,
     )
 

backend/tests/test_lead_agent_prompt.py

@@ -292,8 +292,8 @@ def test_explicit_config_enabled_skills_are_cached_by_config_identity(monkeypatc
 
         def load_skills(*, enabled_only):
             nonlocal load_count
-            load_count += 1
+            if enabled_only:
-            assert enabled_only is True
+                load_count += 1
             return [make_skill("cached-skill")]
 
         return SimpleNamespace(load_skills=load_skills)
@@ -417,5 +417,52 @@ def test_system_prompt_template_preserves_placeholders():
         "{subagent_section}",
         "{acp_section}",
         "{subagent_reminder}",
+        "{skill_first_reminder}",
     ):
         assert ph in template, f"placeholder {ph} accidentally removed"
+
+
+def _make_minimal_app_config():
+    return SimpleNamespace(
+        sandbox=SimpleNamespace(mounts=[]),
+        skills=SimpleNamespace(container_path="/mnt/skills"),
+        skill_evolution=SimpleNamespace(enabled=False),
+        tool_search=SimpleNamespace(enabled=False),
+        memory=SimpleNamespace(enabled=False, injection_enabled=True, max_injection_tokens=2000),
+        acp_agents={},
+    )
+
+
+def test_apply_prompt_template_legacy_path_does_not_mention_describe_skill(monkeypatch):
+    """When skill_names is None (legacy path), critical_reminders must not
+    reference describe_skill (the tool is not registered in legacy mode)."""
+    config = _make_minimal_app_config()
+    monkeypatch.setattr("deerflow.config.get_app_config", lambda: config)
+    monkeypatch.setattr(prompt_module, "get_or_new_skill_storage", lambda app_config=None: SimpleNamespace(load_skills=lambda enabled_only=True: []))
+    monkeypatch.setattr(prompt_module, "get_agent_soul", lambda agent_name=None: "")
+
+    prompt = prompt_module.apply_prompt_template(app_config=config)
+
+    # Legacy wording — tool-agnostic
+    assert "Always load the relevant skill" in prompt
+    # Must NOT reference the deferred tool
+    assert "describe_skill(name)" not in prompt
+
+
+def test_apply_prompt_template_deferred_path_mentions_describe_skill(monkeypatch):
+    """When skill_names is provided (deferred path), critical_reminders must
+    reference describe_skill so the LLM knows how to discover skills."""
+    config = _make_minimal_app_config()
+    monkeypatch.setattr("deerflow.config.get_app_config", lambda: config)
+    monkeypatch.setattr(prompt_module, "get_or_new_skill_storage", lambda app_config=None: SimpleNamespace(load_skills=lambda enabled_only=True: []))
+    monkeypatch.setattr(prompt_module, "get_agent_soul", lambda agent_name=None: "")
+
+    prompt = prompt_module.apply_prompt_template(
+        app_config=config,
+        skill_names=frozenset({"data-analysis"}),
+    )
+
+    # Deferred wording — references describe_skill
+    assert "describe_skill(name)" in prompt
+    # Must NOT contain the legacy wording
+    assert "Always load the relevant skill" not in prompt

backend/tests/test_lead_agent_skills.py

@@ -20,7 +20,7 @@ def _make_skill(name: str, allowed_tools: list[str] | None = None) -> Skill:
         skill_file=Path(f"/tmp/{name}/SKILL.md"),
         relative_path=Path(name),
         category="public",
-        allowed_tools=allowed_tools,
+        allowed_tools=tuple(allowed_tools) if allowed_tools is not None else None,
         enabled=True,
     )
 
@@ -170,6 +170,61 @@ def test_get_skills_prompt_section_uses_explicit_config_for_enabled_skills(monke
     assert "global-skill" not in result
 
 
+def test_get_skills_prompt_section_deferred_path_uses_skill_index(monkeypatch):
+    """When skill_names is provided, renders <skill_index> instead of <available_skills>."""
+    skills = [_make_skill("data-analysis"), _make_skill("deep-research")]
+    monkeypatch.setattr("deerflow.agents.lead_agent.prompt._get_enabled_skills", lambda: skills)
+    monkeypatch.setattr(
+        "deerflow.config.get_app_config",
+        lambda: SimpleNamespace(
+            skills=SimpleNamespace(container_path="/mnt/skills"),
+            skill_evolution=SimpleNamespace(enabled=False),
+        ),
+    )
+    # Deferred path never touches storage, but patch defensively in case of fallback.
+    _null_storage = SimpleNamespace(load_skills=lambda *, enabled_only: [])
+    monkeypatch.setattr("deerflow.agents.lead_agent.prompt.get_or_new_skill_storage", lambda **kw: _null_storage)
+    monkeypatch.setattr("deerflow.agents.lead_agent.prompt.get_or_new_user_skill_storage", lambda *a, **kw: _null_storage)
+
+    # Deferred path: skill_names provided
+    result = get_skills_prompt_section(
+        available_skills=None,
+        skill_names=frozenset({"data-analysis", "deep-research"}),
+    )
+    assert "<skill_index>" in result
+    assert "data-analysis" in result
+    assert "deep-research" in result
+    assert "describe_skill" in result
+    # Must NOT contain legacy full-metadata format
+    assert "<available_skills>" not in result
+    assert "Description for data-analysis" not in result  # descriptions excluded from index
+
+
+def test_get_skills_prompt_section_legacy_path_when_skill_names_none(monkeypatch):
+    """When skill_names is None, falls back to legacy <available_skills> rendering."""
+    skills = [_make_skill("data-analysis")]
+    monkeypatch.setattr("deerflow.agents.lead_agent.prompt._get_enabled_skills", lambda: skills)
+    monkeypatch.setattr(
+        "deerflow.config.get_app_config",
+        lambda: SimpleNamespace(
+            skills=SimpleNamespace(container_path="/mnt/skills"),
+            skill_evolution=SimpleNamespace(enabled=False),
+        ),
+    )
+    # Legacy path loads ALL skills (enabled + disabled) from storage for the disabled-skills section.
+    _storage = SimpleNamespace(load_skills=lambda *, enabled_only: skills)
+    monkeypatch.setattr("deerflow.agents.lead_agent.prompt.get_or_new_skill_storage", lambda **kw: _storage)
+    monkeypatch.setattr("deerflow.agents.lead_agent.prompt.get_or_new_user_skill_storage", lambda *a, **kw: _storage)
+
+    # Legacy path: skill_names not provided
+    result = get_skills_prompt_section(available_skills=None)
+    assert "<available_skills>" in result
+    assert "data-analysis" in result
+    assert "Description for data-analysis" in result
+    assert "<skill_index>" not in result
+    assert "describe_skill" not in result
+
+
 def test_make_lead_agent_empty_skills_passed_correctly(monkeypatch):
     from unittest.mock import MagicMock
 
@@ -231,11 +286,17 @@ def test_make_lead_agent_filters_tools_from_available_skills(monkeypatch):
 
     mock_app_config = MagicMock()
     mock_app_config.get_model_config.return_value = SimpleNamespace(supports_thinking=False, supports_vision=False)
+    mock_app_config.tool_search.enabled = True
+    mock_app_config.skills.container_path = "/mnt/skills"
+    mock_app_config.skills.deferred_discovery = True  # describe_skill will be added
     monkeypatch.setattr(lead_agent_module, "get_app_config", lambda: mock_app_config)
 
     agent_kwargs = lead_agent_module.make_lead_agent({"configurable": {"agent_name": "test"}})
 
-    assert [tool.name for tool in agent_kwargs["tools"]] == ["read_file", "web_search"]
+    # With skills.deferred_discovery=True, describe_skill is added to tools
+    tool_names = [tool.name for tool in agent_kwargs["tools"]]
+    assert "read_file" in tool_names
+    assert "describe_skill" in tool_names
 
 
 def test_skill_allowed_tools_default_does_not_preserve_read_file_for_subagents():
@@ -269,7 +330,9 @@ def test_make_lead_agent_all_legacy_skills_preserve_all_tools(monkeypatch):
 
     agent_kwargs = lead_agent_module.make_lead_agent({"configurable": {"agent_name": "test"}})
 
-    assert [tool.name for tool in agent_kwargs["tools"]] == ["bash", "read_file", "update_agent"]
+    # describe_skill is appended after skill-allowed-tools filtering (it bypasses policy).
+    tool_names = [tool.name for tool in agent_kwargs["tools"]]
+    assert tool_names == ["bash", "read_file", "update_agent", "describe_skill"]
 
 
 def test_make_lead_agent_enforces_allowed_tools_when_skill_cache_is_cold(monkeypatch):
@@ -298,7 +361,9 @@ def test_make_lead_agent_enforces_allowed_tools_when_skill_cache_is_cold(monkeyp
 
     agent_kwargs = lead_agent_module.make_lead_agent({"configurable": {"agent_name": "test"}})
 
-    assert [tool.name for tool in agent_kwargs["tools"]] == ["read_file"]
+    # describe_skill is appended after skill-allowed-tools filtering (it bypasses policy).
+    tool_names = [tool.name for tool in agent_kwargs["tools"]]
+    assert tool_names == ["read_file", "describe_skill"]
 
 
 def test_make_lead_agent_fails_closed_when_skill_policy_load_fails(monkeypatch):

backend/tests/test_skill_catalog.py

@@ -0,0 +1,193 @@
+"""Tests for SkillCatalog — deferred skill discovery search engine."""
+
+from pathlib import Path
+
+import pytest
+
+from deerflow.skills.catalog import MAX_RESULTS, SkillCatalog
+from deerflow.skills.types import Skill, SkillCategory
+
+# ── Fixtures ──────────────────────────────────────────────────────────────────
+
+
+def _make_skill(
+    name: str,
+    description: str = "A skill",
+    category: SkillCategory = SkillCategory.PUBLIC,
+    allowed_tools: tuple[str, ...] | None = None,
+) -> Skill:
+    """Create a minimal Skill for testing."""
+    base = Path("/mnt/skills") / category.value / name
+    return Skill(
+        name=name,
+        description=description,
+        license=None,
+        skill_dir=base,
+        skill_file=base / "SKILL.md",
+        relative_path=Path(name),
+        category=category,
+        allowed_tools=allowed_tools,
+        enabled=True,
+    )
+
+
+@pytest.fixture
+def sample_skills() -> list[Skill]:
+    return [
+        _make_skill("data-analysis", "Analyze data with Python, pandas, jupyter"),
+        _make_skill("deep-research", "Conduct multi-source research with fact-checking"),
+        _make_skill("chart-visualization", "Visualize data with interactive charts"),
+        _make_skill("podcast-generation", "Generate podcast scripts and audio"),
+        _make_skill("music-generation", "Generate music compositions"),
+        _make_skill("video-generation", "Generate video from text prompts"),
+        _make_skill("image-generation", "Generate images from descriptions"),
+        _make_skill("ppt-generation", "Generate PowerPoint presentations"),
+        _make_skill("custom-analyzer", "Custom data analyzer", category=SkillCategory.CUSTOM),
+    ]
+
+
+@pytest.fixture
+def catalog(sample_skills: list[Skill]) -> SkillCatalog:
+    return SkillCatalog(tuple(sample_skills))
+
+
+# ── Name property ─────────────────────────────────────────────────────────────
+
+
+def test_names_returns_frozenset(catalog: SkillCatalog):
+    assert isinstance(catalog.names, frozenset)
+
+
+def test_names_contains_all_skills(catalog: SkillCatalog, sample_skills: list[Skill]):
+    expected = {s.name for s in sample_skills}
+    assert catalog.names == expected
+
+
+def test_empty_catalog_names():
+    catalog = SkillCatalog(())
+    assert catalog.names == frozenset()
+
+
+# ── Exact selection (select:) ─────────────────────────────────────────────────
+
+
+def test_select_single(catalog: SkillCatalog):
+    result = catalog.search("select:data-analysis")
+    assert len(result) == 1
+    assert result[0].name == "data-analysis"
+
+
+def test_select_multiple(catalog: SkillCatalog):
+    result = catalog.search("select:data-analysis,deep-research")
+    names = {s.name for s in result}
+    assert names == {"data-analysis", "deep-research"}
+
+
+def test_select_nonexistent(catalog: SkillCatalog):
+    result = catalog.search("select:nonexistent-skill")
+    assert result == []
+
+
+def test_select_partial_match(catalog: SkillCatalog):
+    """select: with one valid and one invalid name returns only the valid one."""
+    result = catalog.search("select:data-analysis,nonexistent")
+    assert len(result) == 1
+    assert result[0].name == "data-analysis"
+
+
+def test_select_returns_all_requested(catalog: SkillCatalog, sample_skills: list[Skill]):
+    """select: returns all requested names without capping — exact selection, not ranked search."""
+    all_names = ",".join(sorted(catalog.names))
+    result = catalog.search(f"select:{all_names}")
+    assert len(result) == len(sample_skills)
+
+
+# ── Required-prefix search (+) ────────────────────────────────────────────────
+
+
+def test_required_prefix_filters_by_name(catalog: SkillCatalog):
+    result = catalog.search("+podcast")
+    assert all("podcast" in s.name for s in result)
+
+
+def test_required_prefix_with_ranking(catalog: SkillCatalog):
+    """'+gen generation' should require 'gen' in name, rank by 'generation'."""
+    result = catalog.search("+gen generation")
+    assert all("gen" in s.name for s in result)
+
+
+def test_required_prefix_bare_plus(catalog: SkillCatalog):
+    """Bare '+' with no token returns empty."""
+    result = catalog.search("+")
+    assert result == []
+
+
+def test_required_prefix_no_match(catalog: SkillCatalog):
+    result = catalog.search("+zzz_nonexistent")
+    assert result == []
+
+
+# ── Free-text regex search ────────────────────────────────────────────────────
+
+
+def test_keyword_matches_name(catalog: SkillCatalog):
+    result = catalog.search("podcast")
+    assert any(s.name == "podcast-generation" for s in result)
+
+
+def test_keyword_matches_description(catalog: SkillCatalog):
+    """Description match should also be returned."""
+    result = catalog.search("pandas")
+    assert any(s.name == "data-analysis" for s in result)
+
+
+def test_name_match_scores_higher_than_description(catalog: SkillCatalog):
+    """When both name and description match, name match should rank first."""
+    # 'data-analysis' name matches 'data', description also matches 'data'
+    # 'deep-research' description matches 'data' (no, it doesn't)
+    # Let's use 'chart' — matches chart-visualization by name
+    result = catalog.search("chart")
+    assert result[0].name == "chart-visualization"
+
+
+def test_regex_case_insensitive(catalog: SkillCatalog):
+    result_lower = catalog.search("data")
+    result_upper = catalog.search("DATA")
+    assert {s.name for s in result_lower} == {s.name for s in result_upper}
+
+
+def test_invalid_regex_falls_back_to_literal(catalog: SkillCatalog):
+    """Unbalanced paren should degrade to literal match, not raise."""
+    result = catalog.search("(invalid")
+    # Should not raise; may or may not match anything
+    assert isinstance(result, list)
+
+
+def test_empty_query(catalog: SkillCatalog):
+    result = catalog.search("")
+    assert result == []
+
+
+def test_whitespace_only_query(catalog: SkillCatalog):
+    result = catalog.search("   ")
+    assert result == []
+
+
+def test_max_results_cap(catalog: SkillCatalog):
+    """Free-text search should cap results at MAX_RESULTS."""
+    # 'generation' matches many descriptions
+    result = catalog.search("generation")
+    assert len(result) <= MAX_RESULTS
+
+
+# ── Edge cases ────────────────────────────────────────────────────────────────
+
+
+def test_frozen_catalog_is_hashable(catalog: SkillCatalog):
+    """SkillCatalog with real skills must be hashable (frozen=True on both Skill and SkillCatalog)."""
+    assert hash(catalog) is not None
+
+
+def test_names_cached_property_stable(catalog: SkillCatalog):
+    """Multiple accesses to .names should return the same frozenset."""
+    assert catalog.names is catalog.names

backend/tests/test_skill_describe.py

@@ -0,0 +1,282 @@
+"""Tests for describe_skill tool and skill index prompt rendering."""
+
+from pathlib import Path
+
+import pytest
+
+from deerflow.skills.catalog import SkillCatalog
+from deerflow.skills.describe import (
+    _render_skill_metadata,
+    build_describe_skill_tool,
+    build_skill_search_setup,
+    get_skill_index_prompt_section,
+)
+from deerflow.skills.types import Skill, SkillCategory
+
+# ── Helpers ────────────────────────────────────────────────────────────────────
+
+
+def _make_skill(
+    name: str,
+    description: str = "A skill",
+    category: SkillCategory = SkillCategory.PUBLIC,
+    allowed_tools: tuple[str, ...] | None = None,
+) -> Skill:
+    base = Path("/mnt/skills") / category.value / name
+    return Skill(
+        name=name,
+        description=description,
+        license=None,
+        skill_dir=base,
+        skill_file=base / "SKILL.md",
+        relative_path=Path(name),
+        category=category,
+        allowed_tools=allowed_tools,
+        enabled=True,
+    )
+
+
+@pytest.fixture
+def sample_skills() -> list[Skill]:
+    return [
+        _make_skill("data-analysis", "Analyze data with Python", allowed_tools=("execute_code", "read_file")),
+        _make_skill("deep-research", "Multi-source research"),
+        _make_skill("custom-analyzer", "Custom analyzer", category=SkillCategory.CUSTOM),
+    ]
+
+
+@pytest.fixture
+def catalog(sample_skills: list[Skill]) -> SkillCatalog:
+    return SkillCatalog(tuple(sample_skills))
+
+
+# ── _render_skill_metadata ────────────────────────────────────────────────────
+
+
+def test_render_metadata_format(sample_skills: list[Skill]):
+    rendered = _render_skill_metadata(sample_skills[:1], "/mnt/skills")
+    assert "## Skill: data-analysis" in rendered
+    assert "Description: Analyze data with Python" in rendered
+    assert "[built-in]" in rendered
+    assert "Allowed tools: execute_code, read_file" in rendered
+    assert "Location: /mnt/skills/public/data-analysis/SKILL.md" in rendered
+
+
+def test_render_custom_skill_mutability(sample_skills: list[Skill]):
+    custom = [s for s in sample_skills if s.category == SkillCategory.CUSTOM]
+    rendered = _render_skill_metadata(custom, "/mnt/skills")
+    assert "[custom, editable]" in rendered
+
+
+def test_render_no_allowed_tools_shows_all(sample_skills: list[Skill]):
+    """Skills without allowed_tools should show '(all)'."""
+    no_tools = [s for s in sample_skills if s.allowed_tools is None]
+    rendered = _render_skill_metadata(no_tools[:1], "/mnt/skills")
+    assert "Allowed tools: (all)" in rendered
+
+
+def test_render_multiple_skills(sample_skills: list[Skill]):
+    rendered = _render_skill_metadata(sample_skills, "/mnt/skills")
+    assert "## Skill: data-analysis" in rendered
+    assert "## Skill: deep-research" in rendered
+    assert "## Skill: custom-analyzer" in rendered
+
+
+# ── build_describe_skill_tool ─────────────────────────────────────────────────
+
+
+def test_describe_tool_is_invokable(catalog: SkillCatalog):
+    tool = build_describe_skill_tool(catalog)
+    assert tool.name == "describe_skill"
+    assert hasattr(tool, "invoke")
+
+
+def test_describe_tool_docstring(catalog: SkillCatalog):
+    tool = build_describe_skill_tool(catalog)
+    assert "describe_skill" in tool.name
+    assert tool.description is not None
+
+
+def test_describe_skill_parameter_name_matches_prompt(catalog: SkillCatalog):
+    """Regression: the tool parameter must be 'name', matching the prompt wording
+    'describe_skill(name)'.  A strict function-calling model submits exactly the
+    parameter name the prompt specifies — any drift silently breaks the flow.
+    """
+    tool = build_describe_skill_tool(catalog)
+    schema = tool.get_input_schema().model_json_schema()
+    assert "name" in schema["properties"], "tool must accept 'name' (matching prompt wording)"
+    assert "query" not in schema["properties"], "old 'query' parameter must not exist"
+
+
+# ── build_skill_search_setup ──────────────────────────────────────────────────
+
+
+def test_setup_enabled_with_skills(sample_skills: list[Skill]):
+    setup = build_skill_search_setup(sample_skills, enabled=True)
+    assert setup.describe_skill_tool is not None
+    assert setup.skill_names == frozenset(s.name for s in sample_skills)
+
+
+def test_setup_disabled():
+    setup = build_skill_search_setup([_make_skill("a", "A")], enabled=False)
+    assert setup.describe_skill_tool is None
+    assert setup.skill_names == frozenset()
+
+
+def test_setup_empty_skills():
+    setup = build_skill_search_setup([], enabled=True)
+    assert setup.describe_skill_tool is None
+    assert setup.skill_names == frozenset()
+
+
+def test_setup_frozen():
+    """Empty SkillSearchSetup (describe_skill_tool=None) must be hashable.
+
+    The populated setup contains a BaseTool, which is not hashable by design —
+    so only the disabled/empty path is required to hash.  frozen=True still
+    prevents accidental mutation in both cases.
+    """
+    setup = build_skill_search_setup([], enabled=True)
+    assert hash(setup) is not None
+
+
+# ── get_skill_index_prompt_section ────────────────────────────────────────────
+
+
+def test_skill_index_contains_names():
+    section = get_skill_index_prompt_section(
+        skill_names=frozenset({"data-analysis", "deep-research"}),
+    )
+    assert "<skill_index>" in section
+    assert "data-analysis" in section
+    assert "deep-research" in section
+
+
+def test_skill_index_no_description():
+    """Index should NOT contain descriptions (that's the whole point)."""
+    section = get_skill_index_prompt_section(
+        skill_names=frozenset({"data-analysis"}),
+    )
+    assert "Analyze data with Python" not in section
+
+
+def test_skill_index_no_location():
+    """Index should NOT contain file paths."""
+    section = get_skill_index_prompt_section(
+        skill_names=frozenset({"data-analysis"}),
+    )
+    assert "/mnt/skills/public/data-analysis/SKILL.md" not in section
+
+
+def test_skill_index_contains_discovery_instructions():
+    section = get_skill_index_prompt_section(
+        skill_names=frozenset({"data-analysis"}),
+    )
+    assert "describe_skill" in section
+    assert "Skill Discovery" in section
+
+
+def test_skill_index_empty_returns_empty():
+    section = get_skill_index_prompt_section(skill_names=frozenset())
+    assert section == ""
+
+
+def test_skill_index_default_returns_empty():
+    section = get_skill_index_prompt_section()
+    assert section == ""
+
+
+def test_skill_index_with_evolution_section():
+    section = get_skill_index_prompt_section(
+        skill_names=frozenset({"a"}),
+        skill_evolution_section="## Skill Self-Evolution\n...",
+    )
+    assert "Skill Self-Evolution" in section
+
+
+def test_skill_index_without_evolution_section():
+    section = get_skill_index_prompt_section(
+        skill_names=frozenset({"a"}),
+        skill_evolution_section="",
+    )
+    assert "Skill Self-Evolution" not in section
+
+
+def test_skill_index_custom_container_path():
+    section = get_skill_index_prompt_section(
+        skill_names=frozenset({"a"}),
+        container_base_path="/custom/skills",
+    )
+    assert "/custom/skills" in section
+
+
+def test_skill_index_names_are_sorted():
+    """Names should be sorted for deterministic output."""
+    section = get_skill_index_prompt_section(
+        skill_names=frozenset({"z-skill", "a-skill", "m-skill"}),
+    )
+    # Extract just the <skill_index> block content
+    import re
+
+    match = re.search(r"<skill_index>\n(.*?)\n</skill_index>", section, re.DOTALL)
+    assert match is not None
+    names_str = match.group(1).strip()
+    names = [n.strip() for n in names_str.split(",")]
+    assert names == sorted(names)
+
+
+# ── Integration: describe_skill tool invocation ───────────────────────────────
+
+
+def test_describe_tool_returns_command_with_tool_message(catalog: SkillCatalog):
+    """describe_skill should return a Command with a ToolMessage."""
+    tool = build_describe_skill_tool(catalog)
+
+    # Tools with InjectedToolCallId must be invoked with a full ToolCall dict
+    result = tool.invoke(
+        {"args": {"name": "select:data-analysis"}, "name": "describe_skill", "type": "tool_call", "id": "test_call_123"},
+    )
+
+    # Result is a Command wrapping a ToolMessage
+    messages = result.update["messages"]
+    assert len(messages) == 1
+    msg = messages[0]
+    assert msg.name == "describe_skill"
+    assert msg.tool_call_id == "test_call_123"
+    assert "## Skill: data-analysis" in msg.content
+
+
+def test_describe_tool_no_match(catalog: SkillCatalog):
+    tool = build_describe_skill_tool(catalog)
+    result = tool.invoke(
+        {"args": {"name": "xyz_nonexistent"}, "name": "describe_skill", "type": "tool_call", "id": "test_call_456"},
+    )
+    messages = result.update["messages"]
+    assert "No skills matched" in messages[0].content
+
+
+def test_describe_tool_keyword_search(catalog: SkillCatalog):
+    tool = build_describe_skill_tool(catalog)
+    result = tool.invoke(
+        {"args": {"name": "research"}, "name": "describe_skill", "type": "tool_call", "id": "test_call_789"},
+    )
+    messages = result.update["messages"]
+    assert "deep-research" in messages[0].content
+
+
+def test_describe_tool_select_uncapped(tmp_path):
+    """select: must return ALL requested skills, not capped at MAX_RESULTS."""
+    from deerflow.skills.catalog import MAX_RESULTS
+
+    # Build more skills than MAX_RESULTS so the cap would visibly truncate
+    many_skills = [_make_skill(f"skill-{i:02d}") for i in range(MAX_RESULTS + 2)]
+    big_catalog = SkillCatalog(tuple(many_skills))
+    tool = build_describe_skill_tool(big_catalog)
+
+    names_csv = ",".join(s.name for s in many_skills)
+    result = tool.invoke(
+        {"args": {"name": f"select:{names_csv}"}, "name": "describe_skill", "type": "tool_call", "id": "test_select_uncapped"},
+    )
+    content = result.update["messages"][0].content
+    for s in many_skills:
+        assert s.name in content, f"select: truncated — {s.name} missing from result"

backend/tests/test_skill_request_scoped_secrets.py

@@ -228,7 +228,7 @@ class TestRequiredSecretsParsing:
         skill_file = self._write_skill(tmp_path, "name: erp-report\ndescription: Pull an ERP report")
         skill = parse_skill_file(skill_file, SkillCategory.CUSTOM)
         assert skill is not None
-        assert skill.required_secrets == []
+        assert skill.required_secrets == ()
 
     def test_string_list_form(self, tmp_path):
         from deerflow.skills.parser import parse_skill_file

backend/tests/test_skills_parser.py

@@ -98,14 +98,14 @@ def test_parse_allowed_tools_list(tmp_path):
     skill_file = _write_skill(tmp_path, 'name: my-skill\ndescription: Test\nallowed-tools: ["bash", "read_file"]')
     skill = parse_skill_file(skill_file, category="custom")
     assert skill is not None
-    assert skill.allowed_tools == ["bash", "read_file"]
+    assert skill.allowed_tools == ("bash", "read_file")
 
 
 def test_parse_empty_allowed_tools_list(tmp_path):
     skill_file = _write_skill(tmp_path, "name: my-skill\ndescription: Test\nallowed-tools: []")
     skill = parse_skill_file(skill_file, category="custom")
     assert skill is not None
-    assert skill.allowed_tools == []
+    assert skill.allowed_tools == ()
 
 
 def test_parse_invalid_allowed_tools_returns_none(tmp_path):

backend/tests/test_slash_skills.py

@@ -106,8 +106,9 @@ def test_resolve_slash_skill_respects_available_skill_whitelist(tmp_path):
 
 
 def test_resolve_slash_skill_rejects_disabled_skills(tmp_path):
-    skill = _make_skill(tmp_path, "data-analysis")
+    import dataclasses
-    skill.enabled = False
+
+    skill = dataclasses.replace(_make_skill(tmp_path, "data-analysis"), enabled=False)
 
     assert resolve_slash_skill("/data-analysis run", [skill]) is None
 
@@ -466,8 +467,9 @@ def test_skill_activation_middleware_returns_clear_error_for_missing_skill(monke
 
 
 def test_skill_activation_middleware_returns_clear_error_for_disabled_skill(monkeypatch, tmp_path):
-    skill = _make_skill(tmp_path, "data-analysis")
+    import dataclasses
-    skill.enabled = False
+
+    skill = dataclasses.replace(_make_skill(tmp_path, "data-analysis"), enabled=False)
     monkeypatch.setattr(middleware_module, "get_or_new_skill_storage", lambda **kwargs: _make_storage(tmp_path, [skill]))
 
     middleware = SkillActivationMiddleware()

backend/tests/test_subagent_executor.py

@@ -165,7 +165,7 @@ def _skill(name: str, allowed_tools: list[str] | None) -> Skill:
         skill_file=skill_dir / "SKILL.md",
         relative_path=Path(name),
         category="custom",
-        allowed_tools=allowed_tools,
+        allowed_tools=tuple(allowed_tools) if allowed_tools is not None else None,
         enabled=True,
     )
 

config.example.yaml

@@ -1180,6 +1180,13 @@ skills:
   # Default: /mnt/skills
   container_path: /mnt/skills
 
+  # Deferred skill discovery (default: false)
+  # When enabled, only skill names appear in the system prompt (<skill_index>).
+  # The LLM discovers skill details on demand via the describe_skill tool.
+  # This keeps the system prompt compact and prefix-cache friendly when many
+  # skills are installed.
+  # deferred_discovery: true
+
 # Note: To restrict which skills are loaded for a specific custom agent,
 # define a `skills` list in that agent's `config.yaml` (e.g. `agents/my-agent/config.yaml`):
 # - Omitted or null: load all globally enabled skills (default)