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Decision Anchor

by com.decision-anchor
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Purchase Marketplace Tool

purchase_tool
Destructive

Purchase a tool from the marketplace. The tool creator earns DAC from your purchase. Paid via x402; Trial does not cover this route.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
tool_idYesTool ID to purchase
auth_tokenNoYour DA agent auth token. Optional when this connection already carries one (Authorization: Bearer header on the remote server, or DA_AUTH_TOKEN for a local stdio server); an explicit value takes precedence.
request_idNoOptional idempotency key: must be a UUID (the server rejects non-UUID values). Auto-generated if omitted.
payment_signatureNoOptional x402 payment payload (base64), required only for paid calls. Omit it on the first call: the tool returns the payment challenge. Sign that challenge with your own wallet, then call this tool again with identical arguments plus this field. Decision Anchor never holds your key and never signs on your behalf.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed2 schema fields changed
    • changedInput schema / properties / auth_token / description
      Previous value: -"Your DA agent auth token"New value: +"Your DA agent auth token. Optional when this connection already carries one (Authorization: Bearer header on the remote server, or DA_AUTH_TOKEN for a local stdio server); an explicit value takes precedence."
    • changedInput schema / required
      Previous value: -[
-  "auth_token",
-  "tool_id"
-]New value: +[
+  "tool_id"
+]
  2. Changed1 schema field changed
    • changedInput schema / properties / request_id / description
      Previous value: -"Optional idempotency key — must be a UUID (the server rejects non-UUID values). Auto-generated if omitted."New value: +"Optional idempotency key: must be a UUID (the server rejects non-UUID values). Auto-generated if omitted."
  3. Changed1 schema field changed
    • changedInput schema / properties / request_id / description
      Previous value: -"Idempotency key"New value: +"Optional idempotency key — must be a UUID (the server rejects non-UUID values). Auto-generated if omitted."
  4. Changed1 schema field changed
    • addedInput schema / properties / payment_signature
      Added value: +{
+  "description": "Optional x402 payment payload (base64), required only for paid calls. Omit it on the first call: the tool returns the payment challenge. Sign that challenge with your own wallet, then call this tool again with identical arguments plus this field. Decision Anchor never holds your key and never signs on your behalf.",
+  "type": "string"
+}
  5. First observed

TDQS

A4.4/5.0
Behavior5/5

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

Annotations declare destructiveHint=true and readOnlyHint=false, and the description corroborates this is a spend/purchase operation where the creator earns DAC. It adds substantive context beyond annotations: the payment mechanism (x402), the two-call challenge/sign flow, and the explicit trust boundary that Decision Anchor never holds the key or signs on the user's behalf.

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 tightly scoped sentences: purpose, economic effect, and payment mechanism/limitation. No filler; every clause carries information an agent needs to decide whether and how to call it.

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?

Covers purpose, payment flow constraint, and trust boundary. No output schema exists, so return values aren't described, though the description does hint at the challenge response on the first call. Slightly more on the two-step invocation could help, but it's largely complete for a purchase tool.

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 coverage is 100%, so baseline is 3. The description adds meaning by explaining the paid-call flow (first call returns the challenge, second call submits the signature) and confirming the trial route can't substitute, which enriches understanding of payment_signature beyond the schema text.

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?

States a specific verb (Purchase) and resource (a tool from the marketplace), immediately distinguishing it from siblings like list_tools, register_tool, and run_sdac_trial. An agent can tell exactly what this tool does.

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

Usage Guidelines3/5

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

The description notes the trial route doesn't cover this path and mentions x402 payment, implying this is the paid acquisition path vs. run_sdac_trial. However, it never explicitly says 'use this instead of X' or names the alternative route by name, leaving some inference required.

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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