> ## Documentation Index > Fetch the complete documentation index at: https://rasa.com/docs/llms.txt > Use this file to discover all available pages before exploring further. # Skills > Build composable skills and add deterministic constraints where required. # Skills Voice and chat agents in Rasa are built from composable skills, coordinated at runtime by an orchestrator called [Mantle](https://rasa.com/docs/mantle/overview.md). Progressive control lets each skill start as plain instructions and add deterministic guarantees where the business requires them.This page builds one skill, `card_replace`, from a single file to a constrained procedure. Scroll to follow along. ## Start with `skill.md` A skill is a folder: instructions for the LLM, plus optional tools and references. At its simplest, the folder holds one file. Frontmatter with a name and description, then instructions in plain language. The folder name is the skill id; see [Project structure](https://rasa.com/docs/mantle/reference/project-structure.md) for the full layout.No flowcharts, no state machine. The LLM interprets the instructions in conversation. ## Add `tools/` Define tools in a `tools/` subfolder and they are automatically available while the skill is active. No registration. A tool returns structured data to the LLM and can store results in [memory](https://rasa.com/docs/mantle/reference/memory-yml.md) for later steps. ## Add `references/` FAQs, policies, and other reference material live in a `references/` subfolder. They are indexed at training time and retrieved on demand, so the agent can answer from them without carrying them in every prompt. ## Progressive control Everything so far is instructions an LLM interprets. Where a mistake is expensive, you constrain it, with tool constraints, skill prerequisites, or ordered blocks.These are framework guarantees enforced by the runtime, not suggestions to the model. A tool whose constraint isn't met is not available for the LLM to call; a jailbreak can't change that. ## Tool constraints A `requires` condition in the frontmatter gates when a tool becomes available. Until `selected_card_id` exists in memory, `lock_card` is removed from the tool schema entirely. The model can't call a tool it never sees.Enforced before the tool call executes ## Scoped instructions Different situations need different instructions. `if:` markers scope a paragraph to a memory value: when `replacement_reason` is `stolen`, only the stolen paragraph is visible to the LLM. Non-matching branches are stripped from the prompt entirely.Non-matching branches removed from prompt ## Skill prerequisites A `requires` block at the root of the frontmatter gates the whole skill: Mantle won't activate `card_replace` until `authenticated` is true. The only contract with the authentication skill is the memory key.Checked before the skill activates ## Ordered blocks For steps that must run in exact order, add an `ordered_block` to the instructions. The LLM decides when to invoke `@block.pick_card`; the runtime executes the steps inside it in order.Step order enforced by the runtime card\_replace/ skill.mdtools/tools.py*Optional*references/faqs.md*Optional* card\_replace/skill.md \--- name: Card Replace description: Replace a credit card -- lost, stolen, damaged, or not received \--- Help the customer replace a credit card. Check whether their account is eligible for replacement. If they have multiple cards, ask which one. Ask why they need a replacement -- the valid reasons are lost, stolen, damaged, or not received. For stolen or not-received cards, offer to lock the card while a new one ships. Once everything is gathered, ask their shipping preference, confirm the order, and process the replacement. card\_replace/tools/tools.py from rasa.mantle.tools.decorator import ToolContext, tool from rasa.mantle.tools.result import ToolResult @tool(description="List the active bank cards associated with the customer's account") async def list\_cards( account\_id: str, context: ToolContext = None, ) -> ToolResult: cards = await call\_cards\_api(account\_id) # Store in memory for later steps context.memory.set("user\_cards", cards) # Structured data goes back to the LLM return ToolResult( llm\_response=\{ "cards": cards, "total\_cards": len(cards) }, ) card\_replace/references/faqs.md Q: What if my dog ate my card? A: That counts as damaged. Q: Will my replacement card keep the same number and PIN? A: The number may change; your PIN stays the same. Q: How long does delivery take? A: 5-7 business days, or 2 with express shipping. ... card\_replace/skill.mdhard guarantee \--- name: Card Replace description: Replace a credit card -- lost, stolen, damaged, or not received tool\_constraints: \- lock\_card: requires: "session.card\_replace.selected\_card\_id" \--- Help the customer replace a credit card. Until selected\_card\_id is set, lock\_card is not in the tool schema at all. card\_replace/skill.mdscoped Ask why they need a replacement -- lost, stolen, damaged, or not received. if: session.card\_replace.replacement\_reason == 'damaged' Ask who needs a replacement: only the customer's card, an authorized user's, or both. if: session.card\_replace.replacement\_reason == 'stolen' Tell the customer the card will be locked. Confirm, then lock the card. if: session.card\_replace.replacement\_reason == 'lost' Present recent transactions and ask if they look familiar. Then handle locking. card\_replace/skill.mdhard guarantee \--- name: Card Replace description: Replace a credit card -- lost, stolen, damaged, or not received requires: "session.project.authenticated" tool\_constraints: \- lock\_card: requires: "session.card\_replace.selected\_card\_id" \--- Checked by the runtime before the skill activates. card\_replace/skill.mdexact order \## Identify the reason Ask why they need a replacement -- lost, stolen, damaged, or not received. Once the reason is collected, invoke @block.pick\_card :::ordered\_block id=pick\_card steps: \- id: check\_eligibility execute\_tool: card\_replace\_eligibility \- id: fetch\_cards execute\_tool: get\_customer\_info \- id: select\_card instructions: show user's cards and let them pick one complete\_when: "session.card\_replace.selected\_card\_id" \- id: END ::: If a whole skill must follow a strict procedure, replace the entire body with an ordered block. Start simple, add control incrementally, and build fully deterministic skills where you need them. > **[Local Quickstart](https://rasa.com/docs/mantle/getting-started.md)** > > Build a skill and run it. 2 minutes. > **[Build Guide](https://rasa.com/docs/mantle/build-guide/tool-constraints.md)** > > The progressive control spectrum, one lever at a time. > **[Orchestration](https://rasa.com/docs/mantle/orchestration/runtime-loop.md)** > > How Mantle runs a turn and what it guarantees. > **[Reference](https://rasa.com/docs/mantle/reference/skill-md.md)** > > File formats, tools contract, runtime internals. ## Related topics - [skill.md](https://rasa.com/docs/mantle/reference/skill-md.md) - [Skills & Routing](https://rasa.com/docs/mantle/orchestration/skills.md) - [Default skills](https://rasa.com/docs/mantle/reference/default-skills.md) - [Sub-skills & Composition](https://rasa.com/docs/mantle/build-guide/sub-skills.md) - [Assertions](https://rasa.com/docs/mantle/reference/assertions.md) This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.