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Coal — Payments for AI agents

by io.github.emmanuel39hanks

Download Purchased Product

download_product
Read-onlyIdempotent

Retrieve / download / get the file for a digital product after the user paid for it. Use after pay_merchant succeeds for digital goods (PDFs, ebooks, cheatsheets, datasets). Pass the on-chain txHash from pay_merchant OR a Coal checkout sessionId. Returns a verified download URL the user can click. Supported product slugs: 0g-cheatsheet (The 0G Builder's Cheatsheet, $0.10).

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
txHashNoOn-chain tx hash from pay_merchant (preferred for agent flows)
productYesProduct slug (e.g. `0g-cheatsheet`)
sessionIdNoCoal checkout session id (preferred for human-checkout flows)

TDQS

A4.6/5.0
Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With readOnlyHint, openWorldHint, and idempotentHint annotations covering safety, the description adds useful context: prerequisite of payment, return of a verified download URL, and supported product slugs. It does not mention edge cases like URL expiration, but only a minor gap.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Three sentences, each adding distinct value: action and prerequisite, authentication methods, and return type/product support. No redundant wording or filler.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

The description covers the core prerequisites (paid, product slug, auth identifiers), return value (verified download URL), and supported products. With no output schema, the only missing piece is explicit error/edge-case behavior, but the description is largely complete for its straightforward purpose.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema descriptions already cover all three parameters (100% coverage). The description adds meaning by explaining the source and preferred flow for each identifier ('On-chain txHash from pay_merchant (preferred for agent flows)', 'Coal checkout sessionId'), and lists the exact product slug with price.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific action ('Retrieve / download / get the file') for a clear resource ('digital product after the user paid for it'). It distinguishes itself from siblings like pay_merchant by requiring prior payment and from check_paywall by returning a download URL.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explicitly instructs to use 'after pay_merchant succeeds' for digital goods, and provides two authentication identifiers (txHash from pay_merchant, sessionId from Coal checkout) with preferences. This clearly defines when to use the tool versus alternatives.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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TDQS

A4/5.0
Disambiguation3/5

Several tools overlap in the product discovery and payment status domains. discover_merchants, search_products, and get_merchant_profile all relate to finding products/merchants, though descriptions clarify intended use. Similarly, check_paywall, get_checkout_status, and verify_receipt all deal with payment status/verification, creating potential agent confusion.

Naming Consistency4/5

Most tools follow a consistent lowercase verb_noun pattern (check_paywall, create_checkout, download_product, get_0g_health, etc.). The one outlier is agent_wallet_status, which is a noun phrase instead of verb-first, but it still uses underscores and fits thematically.

Tool Count4/5

13 tools is within the ideal range and the server covers payments, product discovery, and verification. A few tools like get_0g_health and setup_instructions feel tangential to the core payment flow, making the set slightly heavier than necessary but still reasonable.

Completeness4/5

The payment lifecycle is well covered: discover/search products, pay via checkout or direct merchant payment, download digital goods, and verify receipts. Minor gaps exist such as no explicit refund or transaction history tool, but agents can work around these by using existing verification and wallet status tools.

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