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ListMath: Marketplace Seller Fees

by io.github.davidemena-spec
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Find the price needed for a profit goal

find_target_price
Read-onlyIdempotent

Find the lowest list price on each US marketplace that reaches a goal: a profit in USD, a profit margin percent, or an ROI percent. Results are sorted from cheapest required price; marketplaces where the goal can't be reached are listed with the reason.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "target": "<target>"
    },
    "name": "find_target_price"
  }
}

Input Schema

NameRequiredDescriptionDefault
detailNosummary (default) returns totals and fee lines; full returns the complete REST response.summary
targetYesExactly one of profit, margin_percent or roi_percent.
packageNoPackaged unit dimensions (inches) and unit weight (pounds), used to determine the fulfillment size tier, dimensional weight (L x W x H / 139) and shipping weight. Required for amazon_fba and walmart_wfs.
item_costNoSeller's cost of the item, in USD. Default 0.
overridesNoPer-marketplace category, options or shipping values, e.g. {"ebay": {"options": {"store_subscription": "basic"}}, "etsy": {"options": {"offsite_ads_percent": 15}}}. Categories: amazon_fbm: default, 3d_printers, amazon_device_accessories, appliances_compact, appliances_full_size, automotive, baby, backpacks_handbags_luggage, base_equipment_power_tools, beauty, industrial, clothing_accessories, computers, consumer_electronics, electronics_accessories, eyewear, footwear, furniture, gift_cards, grocery, home_kitchen, jewelry, lawn_garden, lawn_mowers_snow_throwers, mattresses, media, merchant_fulfilled_services, musical_instruments_av, office_products, pet_supplies, pet_veterinary_diets, sports_outdoors, tires, tools_home_improvement, toys_games, video_games_accessories, video_game_consoles, watches | amazon_fba: default, 3d_printers, amazon_device_accessories, appliances_compact, appliances_full_size, automotive, baby, backpacks_handbags_luggage, base_equipment_power_tools, beauty, industrial, clothing_accessories, computers, consumer_electronics, electronics_accessories, eyewear, footwear, furniture, gift_cards, grocery, home_kitchen, jewelry, lawn_garden, lawn_mowers_snow_throwers, mattresses, media, merchant_fulfilled_services, musical_instruments_av, office_products, pet_supplies, pet_veterinary_diets, sports_outdoors, tires, tools_home_improvement, toys_games, video_games_accessories, video_game_consoles, watches | walmart: default, apparel, appliances_compact, appliances_major, automotive, baby, base_power_tools, beauty, books, camera_photo, collectibles, consumer_electronics, electronics_accessories, furniture, grocery, home_kitchen_decor_garden, industrial_scientific, jewelry_precious_metals, luggage_travel, music, musical_instruments, office_products, printer_cartridges, outdoor_power_tools, outdoors_sports, outdoor_optics, personal_computers, pet_supplies, plumbing_hvac, shoes_handbags_backpacks_sunglasses, software_video_games, tires_wheels, tools_home_improvement, toys_games, video_dvd, video_game_consoles, watches | walmart_wfs: default, apparel, appliances_compact, appliances_major, automotive, baby, base_power_tools, beauty, books, camera_photo, collectibles, consumer_electronics, electronics_accessories, furniture, grocery, home_kitchen_decor_garden, industrial_scientific, jewelry_precious_metals, luggage_travel, music, musical_instruments, office_products, printer_cartridges, outdoor_power_tools, outdoors_sports, outdoor_optics, personal_computers, pet_supplies, plumbing_hvac, shoes_handbags_backpacks_sunglasses, software_video_games, tires_wheels, tools_home_improvement, toys_games, video_dvd, video_game_consoles, watches | ebay: default, electronics, clothing, books_media, vinyl_records, guitars, jewelry, watches, handbags, athletic_shoes | tiktok_shop: default, jewelry_precious, jewelry_other, collectibles, pre_owned | etsy: default | facebook_marketplace: default | poshmark: default | mercari: default | depop: default | stockx: default
other_costsNoOther per-item costs (packaging, ads, etc.), in USD. Default 0.
marketplacesNoLimit to these marketplaces. Default: all. amazon_fba and walmart_wfs are skipped unless package is given.
shipping_costNoSeller's shipping label cost, in USD. Default 0.
fulfillment_dateNoamazon_fba only: ship date (YYYY-MM-DD) that selects the FBA rate card. Defaults to today (UTC). Range 2026-01-15 to 2027-10-14.
shipping_chargedNoShipping the buyer pays, in USD. Default 0.
estimated_sales_taxNoSales tax collected from the buyer, in USD (some marketplaces charge fees on it). Default 0.

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observed

TDQS

A4/5.0
Behavior4/5

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

Annotations already declare readOnlyHint, idempotentHint, non-destructive and closed-world, so the safety profile is covered. The description adds real value beyond that: results are ordered from cheapest required price, and marketplaces where the goal is unreachable are returned with the reason rather than dropped — behavior an agent cannot infer from annotations. It omits that cost inputs default to 0, which silently shapes the output.

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?

Two sentences, front-loaded with what is computed and followed by the result-shaping behavior. No filler, no restatement of the title, and every clause carries information.

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?

For an 11-parameter nested tool with no output schema, the description covers the core compute logic and the shape of the result ordering plus the failure-case reporting. The remaining gap is default-value behavior (costs default to 0) and the package gating for FBA/WFS, which an agent would need in order to interpret results correctly; the fully documented schema partially compensates.

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 all 11 parameters, the exactly-one-of target constraint, the package requirements and the overrides structure are already documented. The description restates the three target goal types without adding syntax, ranges or interaction rules beyond the schema, so baseline 3 applies.

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?

A specific verb ('Find the lowest list price') plus resource ('each US marketplace') and the three exact goal inputs (profit USD, margin percent, ROI percent). This reverse-solve framing is clearly distinct from calculate_fees and compare_marketplaces, so an agent can pick it without opening the schema.

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 goal-oriented framing implies when to use it ('reaches a goal'), but the description never names an alternative sibling such as calculate_fees or compare_marketplaces, nor states when this is the wrong choice. No prerequisites are mentioned (e.g. that amazon_fba and walmart_wfs require a package), even though that materially changes results.

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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