# Fallback

Tiny paid recovery and decision utilities for autonomous agents.

Pay-per-call tools that help AI agents find sources, diagnose failures, repair requests, and decide when to stop searching.

Catalog: https://fallback.factrail.online/catalog.json
MCP endpoint: https://fallback.factrail.online/mcp (active Streamable HTTP)
Agent skill: https://fallback.factrail.online/skill.md
HTTP execution: active
x402 payments: production mode (eip155:8453, Base mainnet, USDC)
Source (MIT): https://github.com/baronsigma/fallback-agent-tools

## When to use
- Use source_route when you know what information you need but not where to retrieve it in machine-readable form; do not use it for general research or fact verification.
- Use error_route after an API, HTTP, MCP, or tool request fails when the safe next action is unclear; do not use it when recovery is obvious or requires deeper domain reasoning.
- Read error_route evidence and limitations. It classifies supplied information only; it does not execute requests or claim undocumented fixes.

## Tools

### source_route
Find likely authoritative or machine-readable sources when you know what information you need but not where to retrieve it. Provide a goal and optional publisher domain or start URL; it returns ranked candidate routes and bounded evidence. Use it before blind searching. Do not use it when you already have the correct source or only need reasoning over supplied context.
Status: available
Price: $0.02 USD per call (x402 active)
HTTP: https://fallback.factrail.online/v1/tools/source_route
MCP: source_route

### error_route
Diagnose a failed API, HTTP, MCP, or tool request and return a bounded safe next action. Use it after a request fails when the safe next step is unclear. Provide a status, response, or error text; it does not execute requests or verify undocumented fixes.
Status: available
Price: $0.002 USD per call (x402 active)
HTTP: https://fallback.factrail.online/v1/tools/error_route
MCP: error_route

### request_repair
Repair a failed API or HTTP request using only supplied schema and error evidence. Use after error_route identifies a request-shape or input problem. Returns the smallest justified request changes and abstains when evidence is insufficient.
Status: available
Price: $0.005 USD per call (x402 active)
HTTP: https://fallback.factrail.online/v1/tools/request_repair
MCP: request_repair

### stop_search
Decide whether another search or paid retrieval is worth the cost based on the scope already checked. Use after several search or source checks when you need to decide whether to continue or return a scoped not-found result. It does not prove universal absence and performs no searches itself.
Status: available
Price: $0.003 USD per call (x402 active)
HTTP: https://fallback.factrail.online/v1/tools/stop_search
MCP: stop_search

## Related
- FACTRAIL MCP: https://factrail.online/llms.txt. Separate service for source-backed facts with support levels, unresolved fields, and receipts.