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apply_discount

Idempotent

Apply a discount or promo code to the cart. Checkout guide: read the MCP resource trusteed://guides/checkout. CREDENTIAL: this store needs one. Call the create_sandbox_key tool first (it is in this tool list and needs no credential), then pass the key you get back as the agent_key argument — or as an Authorization: Bearer header if your client can set headers. Do not ask a person to log in: there is no human login for this store.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
sourceNoSet to 'ucp_identity' to apply a loyalty discount derived from a UCP identity token instead of a promo code.
cart_idYesCart session ID
agent_keyNoSandbox credential from `create_sandbox_key`. Use this when your client cannot set an `Authorization: Bearer` header.
discount_codeNoDiscount or promo code to apply (required unless source is ucp_identity)
identity_tokenNoUCP identity bearer token (required when source is ucp_identity).
idempotency_keyNoREQUIRED for this write unless sent as the `Idempotency-Key` header or `params._meta["trusteed/idempotency-key"]`. 1-128 chars from [A-Za-z0-9._:/-]. Reuse the same value when retrying the same operation.

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault
cart_idYes
currencyYes
discount_codeYes
discount_centsYes
new_total_centsYes

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed3 schema fields changed
    • changedInput schema / properties / idempotency_key / description
      Previous value: -"Idempotency key for this call (UUID-v4 or bounded ASCII token, 1-128 chars). Equivalent to the `Idempotency-Key` header — send it here if your client cannot set headers."New value: +"REQUIRED for this write unless sent as the `Idempotency-Key` header or `params._meta[\"trusteed/idempotency-key\"]`. 1-128 chars from [A-Za-z0-9._:/-]. Reuse the same value when retrying the same operation."
    • addedInput schema / properties / idempotency_key / maxLength
      Added value: +128
    • addedInput schema / properties / idempotency_key / pattern
      Added value: +"^[A-Za-z0-9._:/-]{1,128}$"
  2. First observed

TDQS

A3.9/5.0
Behavior4/5

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

Annotations already declare readOnly=false, idempotent=true, and destructive=false, so the safety profile is covered. The description adds genuinely useful operational context beyond that: the credential requirement, the absence of human login, and the two ways to supply the key.

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

Conciseness4/5

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

The purpose is front-loaded in the first sentence, followed by credential mechanics. It is slightly verbose on the credential workflow, but every sentence carries actionable information rather than 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?

With annotations covering safety semantics, an output schema handling return values, and full schema description coverage, the description's remaining job is the credential/auth workflow, which it explains thoroughly. It is complete enough for an agent to invoke the tool correctly without guesswork.

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%, so the schema already documents cart_id, agent_key, discount_code, identity_token, source, and idempotency_key in detail. The description largely restates the agent_key mechanism rather than adding format or constraint detail beyond the schema, so the baseline 3 is appropriate.

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

Purpose4/5

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

The first sentence gives a specific verb and resource: 'Apply a discount or promo code to the cart.' It is unambiguous what operation is performed, though it does not explicitly distinguish itself from cart-mutation siblings like the checkout tools.

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?

It provides a clear prerequisite chain (call create_sandbox_key first, then pass the key as agent_key or an Authorization header) and points to the checkout guide resource. It does not, however, state when to prefer this tool over alternatives or any conditions under which it should not be used.

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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