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By the end of this page a paid MCP tool is cataloged in the Bazaar, keyed on its resource URL and tool name, and callable by an agent that discovers it through search. This uses describeTool from @rail402.dev/sdk (also exported from @rail402.dev/seller-helpers).

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MCP tools are first-class resources

An MCP tool is a resource type in its own right, keyed on the pair of resource.url and toolName, because one MCP endpoint serves many tools. That key is what lets an agent find one specific tool, and what lets you list several tools from the same endpoint without collision. You declare a tool the same way you describe an HTTP endpoint, with a toolName, a description for the tool, and a description for every parameter.
The description and every parameter description are what search ranks and what an agent reads to build a valid call, exactly as for an HTTP endpoint. Write them for a reader who has never seen your tool.

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The resource URL must be addressable

The resource URL is the addressable HTTP endpoint that serves the tool. It cannot be an mcp:// URL. An mcp:// URL (the default resource URL some MCP clients construct) has no host to key on, so the facilitator rejects the listing with bazaar_mcp_resource_url_not_addressable. Use the public HTTPS endpoint that serves the tool.
The declaration must also carry toolName. Without it there is no key to distinguish this tool from the others on the same endpoint, and the facilitator rejects the entry with a coded, non-null reason. See Errors.

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Cataloging works the same way

An MCP tool is cataloged on the same settlement-gated, hybrid path as any other resource: a provisional listing at verify, a confirmed and owned listing at settlement. Nothing about MCP changes when a listing appears or who owns it. See Get discovered for the full flow.

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Try it end to end

The examples/mcp-tool-seller example in the Rail402 repository runs a paid MCP tool that catalogs itself, gets found by natural-language search, and gets called and paid, end to end on testnet.

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Next steps

Get discovered

Write parameter descriptions an agent can act on, and see what the facilitator catalogs and when.

Meter usage with upto

Bill a tool for actual usage against a buyer-authorized ceiling.

Buyer quickstart

See the other side: an agent discovers your tool and pays to call it.

Bazaar trust model

How MCP tools are keyed, owned, and protected from squatting.

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When it fails

An mcp:// resource URL is rejected with bazaar_mcp_resource_url_not_addressable, and a listing without a toolName is rejected with a coded reason. Every code and its reason is in Errors.