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Decision Anchor

by com.decision-anchor
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purchase_tool

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

A4.2/5.0
Behavior4/5

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

With no annotations provided, the description carries the burden of behavioral disclosure. It effectively discloses the payment mechanism (x402), the two-step challenge flow (via the schema but reinforced by the description), and the critical fact that Decision Anchor never holds the user's key or signs on their behalf. It does not detail error handling, rate limits, or post-purchase behavior, but the disclosures are substantial for a purchase tool.

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?

The description is succinct—two sentences that front-load the core purpose and then add key context about payment and trial exclusion. Every word adds value, with no redundancy or filler. It avoids unnecessary detail that belongs in the schema.

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?

Given the tool's complexity (payment, auth, idempotency), the description covers the essential usage context and points to the payment mechanism. The two-step payment flow is explained in the schema rather than the description, which is acceptable given 100% schema coverage. Still, a brief mention of the two-call flow in the description would have made it more complete for agents scanning quickly.

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

Parameters3/5

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

Schema description coverage is 100%, meaning each parameter is well-documented in the schema. The tool description itself does not add extra semantic value beyond what the schema provides (e.g., it doesn't elaborate on auth_token or request_id). It relies on the schema's parameter descriptions, which aligns with the baseline of 3 for full coverage.

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 clearly states the action ('Purchase a tool from the marketplace') with a specific verb and resource. It distinguishes itself from sibling tools like list_tools or get_agent_profile by focusing on the transactional purchase action. The mention of payment via x402 and trial exclusion further clarifies its unique role.

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

Usage Guidelines4/5

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

The description gives a clear usage context: this is the paid purchase route, and it explicitly notes that trial does not cover it, implying users on trial should seek alternatives. However, it does not name specific alternative tools (e.g., run_sdac_trial) or provide when-not-to-use cases beyond the trial exclusion, so it falls short of a 5.

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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