fermi

Buy Fermi credits

buy_credits

Destructive

Cost: charges the saved card $0.50 per credit (see GET /api/v1/billing/pricing).

Purchase credits using a saved payment method (off-session).

Requires a prior Stripe Checkout session to have saved a card.
Credits are $0.50 each. Use GET /api/v1/billing/pricing to confirm
the current price.

Args:
    credits: Number of credits to purchase (minimum 1).
    api_key: Your Fermi API key from POST /api/v1/accounts. Required.

Returns:
    Dict with credits_purchased, new_balance, and payment_status, plus
    the `billing` block every Fermi tool response carries. Its
    `credit_balance` is the POST-purchase balance, and `charged_to` is
    "none": this call spends money, not credits.

Raises:
    ToolError: if authentication fails, no payment method is on file,
        or the card is declined.

Input Schema

Name	Required	Description	Default
`api_key`	Yes
`credits`	Yes

Output Schema

Name	Required	Description	Default
No arguments

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedOct 28, 2026

changedOutput schema / additionalProperties

Previous value: -{
-  "anyOf": [
-    {
-      "type": "integer"
-    },
-    {
-      "type": "string"
-    }
-  ]
-}New value: +true

First observedOct 28, 2026

TDQS

A4.6/5.0

Behavior5/5

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

Annotations already declare non-readOnly, destructive, non-idempotent, open-world, but the description adds the details that actually matter for a money-spending call: it charges the saved card real currency (not credits), priced at $0.50/credit with a pricing endpoint to verify, plus the three failure modes (auth failure, no payment method, declined card). That is substantive context beyond the annotation flags.

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?

Front-loaded with cost, which is the most decision-relevant fact, and organized into Args/Returns/Raises. Some waste: the $0.50 price is stated twice (lead line and 'Credits are $0.50 each'), and the Returns block restates what the output schema already declares.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a two-parameter mutation that spends real money, everything an agent needs to call it safely is present: price, prerequisite, minimum quantity, credential source, and error conditions. The output schema exists, so the Returns narrative is redundant but harmless rather than missing.

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 0%, so the description must carry the load and it largely does: 'credits' gains a minimum-1 constraint not present in the schema, and 'api_key' gains provenance (from POST /api/v1/accounts) and an explicit required marker. Minor gap: nothing about upper bounds or behavior of non-integer values, though the JSON type covers that.

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 and resource ('Purchase credits using a saved payment method') and immediately names the money model ($0.50 per credit) and the off-session mechanism. The sibling tools (estimate, get_estimate, whoami, feedback) are all read/identity helpers with no overlap, so the agent can route unambiguously.

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?

Explicit prerequisite: 'Requires a prior Stripe Checkout session to have saved a card', which tells the agent when the call will fail. There are no true alternative tools for this action, so the absence of an explicit 'use X instead' clause is not a meaningful gap.

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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