Skill Fallback - PraisonAI

A skill can declare itself a fallback for a tool or server, and it’s offered only when that capability is absent.

from praisonaiagents import Agent

agent = Agent(
    name="Researcher",
    instructions="Answer research questions.",
    skills=["./skills/web-via-terminal", "./skills/summarise"],
)
agent.start("Find recent papers on diffusion models.")

The user asks for research; a fallback skill is offered only when the primary tool is absent. The fallback declaration lives in the skill itself — the agent code doesn’t change at all.

Quick Start

Add fallback_for_tools to your SKILL.md

Create a SKILL.md file with the fallback declaration:

---
name: web-via-terminal
description: Fetch the web via terminal when no web tool is available.
requires_tools: [terminal]
fallback_for_tools: [web_search, web]
---

# How to fetch the web with curl/wget when no web tool is available

When asked to retrieve a URL, use: curl -sL "<url>"

The skill becomes visible only when web_search and web are both absent, and the terminal tool is present.

Use the skill from your Agent

from praisonaiagents import Agent

agent = Agent(
    name="Researcher",
    instructions="Answer research questions.",
    skills=["./skills/web-via-terminal", "./skills/summarise"],
)

agent.start("Find recent papers on diffusion models.")

The agent automatically receives the fallback skill only when it needs it. No code changes required when you add or remove real tools.

How It Works

The filter runs in every enforcement mode — an unusable fallback is never injected into the prompt regardless of enforcement level.

Decision Table

Real tool present?	Fallback’s own `requires_*` met?	Skill offered?
✅ Yes	✅ Yes	❌ Hidden — capable agent doesn’t need the fallback
✅ Yes	❌ No	❌ Hidden
❌ No	✅ Yes	✅ Offered — this is the graceful fallback case
❌ No	❌ No	❌ Hidden — don’t inject an unusable skill

Frontmatter Reference

Frontmatter key	Type	Default	Behaviour
`fallback_for_tools`	list of tool names (or single string)	`[]`	Skill is hidden when any listed tool is present in the registry.
`fallback_for_servers`	list of MCP server names (or single string)	`[]`	Skill is hidden when any listed server is present.
`fallback-for-tools`	hyphenated alias	—	Identical to `fallback_for_tools`.
`fallback-for-servers`	hyphenated alias	—	Identical to `fallback_for_servers`.
`requires_tools`	list	`[]`	Composes with fallback — the fallback skill must still satisfy its own gates.
`requires_servers`	list	`[]`	Same.

Both underscored (fallback_for_tools) and hyphenated (fallback-for-tools) forms are accepted and treated identically.

Common Patterns

Web-via-terminal fallback

When no native web tool is available, offer a terminal-based alternative:

---
name: web-via-terminal
description: Fetch URLs via curl/wget when no web tool is available.
requires_tools: [terminal]
fallback_for_tools: [web_search, web]
---

# Fetching the web without a web tool

Use: curl -sL "<url>" | head -200

Filesystem-via-shell fallback

When no MCP filesystem server is connected, fall back to shell commands:

---
name: filesystem-via-shell
description: Read and write files via shell when no MCP filesystem server is present.
requires_tools: [terminal]
fallback_for_servers: [filesystem, mcp-filesystem]
---

# File operations via shell

Read: cat <path>
Write: echo "<content>" > <path>
List: ls -la <dir>

LLM-only summarise when no scraper

When no scraping tool is available, summarise from context only:

---
name: summarise-from-context
description: Summarise content already in context when no scraper tool is available.
fallback_for_tools: [web_scraper, scrape_url]
---

# Summarising without a scraper

Use only the text already provided in the conversation. Do not attempt to fetch URLs.

Best Practices

Keep fallback instructions tight

Fallback skills are injected when a real tool is missing. Shorter, focused instructions reduce token usage and avoid confusing the agent.

---
name: web-via-terminal
description: Fetch the web via curl when no web tool is present.
requires_tools: [terminal]
fallback_for_tools: [web_search, web]
---

Use: curl -sL "<url>" | head -200

Always pair fallback_for_tools with requires_tools

A fallback that can’t run is worse than no fallback. Use requires_tools to ensure the fallback’s own dependencies are met before it’s offered.

---
name: web-via-terminal
requires_tools: [terminal]           # must have terminal
fallback_for_tools: [web_search]     # only when web_search is absent
---

Without requires_tools: [terminal], the skill would be offered even when the terminal tool is also absent — and the agent would try to use instructions it can’t execute.

One fallback per real tool, not chains

Avoid creating a chain of fallbacks-of-fallbacks. Pick the simplest fallback and stop there.

# Good — one direct fallback
fallback_for_tools: [web_search]

# Avoid — chaining fallbacks adds complexity with minimal gain
# web-via-terminal fallback_for web_search
# web-via-python fallback_for terminal  <-- don't do this

Test by toggling tools in your agent

The fastest way to verify your fallback works is to construct an agent with and without the primary tool and check which skills are offered:

from praisonaiagents import Agent

# Agent WITH the real tool — web-via-terminal should be hidden
agent_with_web = Agent(
    name="Researcher",
    instructions="Research assistant.",
    tools=["web_search"],
    skills=["./skills/web-via-terminal"],
)

# Agent WITHOUT the real tool — web-via-terminal should appear
agent_without_web = Agent(
    name="Researcher",
    instructions="Research assistant.",
    skills=["./skills/web-via-terminal"],
)

Skill Capability Gates

The inverse concept — declare what a skill requires to be active

Agent Skills

Learn about the Agent Skills system and how to create SKILL.md files

Skill Management

Manage skills programmatically with the SkillManager API

Doctor CLI

Diagnose skill issues with the doctor command

Skill Capability Gates

Skills vs Tools

​Quick Start

​How It Works

​Decision Table

​Frontmatter Reference

​Common Patterns

​Web-via-terminal fallback

​Filesystem-via-shell fallback

​LLM-only summarise when no scraper

​Best Practices

​Related

Skill Capability Gates

Agent Skills

Skill Management

Doctor CLI

Quick Start

How It Works

Decision Table

Frontmatter Reference

Common Patterns

Web-via-terminal fallback

Filesystem-via-shell fallback

LLM-only summarise when no scraper

Best Practices

Related