MailerCheck MCP by usefulapi

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

Annotations already declare readOnlyHint=true, idempotentHint=true and destructiveHint=false, so the safety profile is covered. The claim that it doesn't spend verification credits reinforces that, and the list of returned fields adds some context, but there is no mention of authentication requirements or failure modes for an openWorldHint tool.

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

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

Two short sentences: the positive capability comes first, the exclusion second. No filler, nothing to trim.

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

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

With no output schema, the description carries the burden of telling the agent what comes back, and it does so by naming the three returned fields. What is missing is only peripheral: no mention of auth setup or what happens if no account is connected.

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

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

The tool takes zero parameters, so the baseline is 4 and there is nothing the description needs to disambiguate. It correctly avoids inventing inputs.

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

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

States a specific verb and resource ('Read the connected MailerCheck account') and enumerates the returned fields (name, ID, credit balance), so it is clearly not a verification tool. It does not, however, distinguish itself from the sibling mailercheck_credits, which plausibly also reports credits.

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

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

The second sentence gives a clear exclusion ('Does not verify emails or spend verification credits'), which steers an agent away from mailercheck_verify_email for read-only account inspection. It stops short of explicitly naming alternatives or stating prerequisites such as a required API key.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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

Annotations already declare readOnly, idempotent, and non-destructive, so the safety profile is covered. The description adds a genuine behavioral fact beyond them: this call does not kick off a verification, so the agent knows the balance read has no side effects. It stops short of noting rate limits or whether calls count against quota.

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

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

One front-loaded sentence with no filler: the read action and its scope come first, and the non-verification qualifier follows immediately.

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

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

For a zero-parameter read with annotations covering safety, the description is nearly complete. The only gap is that with no output schema, it doesn't say what the returned balance looks like (integer count, unit, or whether it is per-account), which an agent might want before surfacing the number.

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

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

The tool takes zero parameters, so the baseline of 4 applies; there is nothing for the schema to document or for the description to compensate for.

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

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

Names a specific verb (Read) and a specific resource (available MailerCheck verification credit balance), which is unambiguous. The clause 'without starting verification' also implicitly separates it from the verify_email sibling, though it never names that sibling.

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

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

The phrase 'without starting verification' implies the use case (check balance without consuming a verification action), but it never explicitly states when to prefer this over mailercheck_account or mailercheck_verify_email. Usage is inferred rather than directed.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false, and openWorldHint, so the safety profile is covered. The description adds one useful behavioral clarification (it does not trigger or restart verification), but says nothing about rate limits, what happens for an unknown/invalid list_id, or the shape of the returned counts.

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

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

Two short sentences, the core action is front-loaded, and the exclusion follows immediately. No filler or redundancy.

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

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

With no output schema, the description does at least indicate what comes back (status plus aggregate counts), and annotations cover safety. The main residual gap is not explaining the lone parameter or where to obtain a list_id, which matters given 0% schema description coverage.

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

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

There is one parameter and schema description coverage is 0%, so the description must compensate and does not: it never mentions list_id, its numeric-string pattern, or that it must come from an existing list (e.g., mailercheck_lists). The parameter name is largely self-explanatory, keeping this from dropping lower.

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

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

States a specific verb and resource ('Read one existing list's verification status and aggregate result counts'), scoping to a single list which implicitly separates it from the plural mailercheck_lists sibling. It does not name a sibling explicitly, so it falls short of a 5.

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

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

The exclusion 'Does not start or restart verification' usefully rules out an adjacent behavior, but no alternative tool is named and no positive when-to-use condition (e.g., 'call after mailercheck_lists to check progress') is given. Usage is implied rather than routed.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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

Annotations already declare readOnly, idempotent and non-destructive, so the safety profile is covered. The description adds genuinely new behavioral context: results are a single bounded page, pagination is handled by the MailerCheck API, and no response links are followed. That last point is valuable because it tells the agent not to expect a next-page URL.

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

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

Three short sentences, no filler, with the core read scope front-loaded before the exclusions and pagination caveat. Every sentence carries distinct information.

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

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

For a read-only list tool with no output schema, the description tells the agent what data comes back (summaries, status, statistics) and that paging is API-side, which is adequate. The remaining gap is pagination semantics, which the description hints at but does not define.

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

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

Schema description coverage is 0%, so the description must carry parameter meaning, but it only alludes to a 'bounded page' without explaining page numbering, limit, defaults, or the 1-100 cap. The agent gets no help on how to paginate from the prose.

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

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

The description uses a specific verb (read) with a specific resource (one bounded page of list summaries, processing status and statistics), which is clearly distinct from mailercheck_get_list's single-list fetch. It does not name that sibling explicitly, but the scope is unambiguous.

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

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

The negative scoping sentence ('Does not create, verify or delete lists') usefully rules out mutation, and 'one bounded page' implies enumeration. However, it never states when to choose this over mailercheck_get_list or mailercheck_verification_result, leaving the routing inference to the agent.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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

Annotations already declare readOnly, idempotent, non-destructive. The description adds behavior beyond them: that no automatic polling happens and that the caller must space out repeat calls, which is the key operational trait for an async-result endpoint. It doesn't say what a pending vs completed result looks like, leaving a gap.

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

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

Three short sentences, zero filler, with the scope limitation and the polling constraint front-loaded. Every sentence earns its place.

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

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

There is no output schema, so the description should carry return-value semantics; saying it returns 'status and result' is too thin for a polling endpoint. An agent doesn't learn which status values indicate completion or how to distinguish pending from failed, which is the main thing this tool is for.

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

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

Schema description coverage is 0% and the single parameter verification_id is undocumented in both schema and description. The description never says where the ID comes from (presumably the return of mailercheck_verify_email) or what form it takes, so it fails to compensate for the coverage gap.

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

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

States a specific verb and resource ('Read the status and result of a previously submitted asynchronous email check') and explicitly negates the adjacent write path ('Does not start new verification'), which cleanly separates it from the mailercheck_verify_email sibling.

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

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

Gives clear invocation guidance: use this only for an already-submitted check, call once and wait before polling again. It stops short of naming mailercheck_verify_email as the tool that initiates checks, so the routing is implied rather than explicit.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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

Goes well beyond the annotations: annotations only say non-readOnly, non-idempotent, openWorld. The description adds that this consumes paid credits, that confirm:true gates the spend, that the platform does not retry, and that the outcome is asynchronous and must be polled. Those are exactly the traits an agent needs to avoid duplicate charged submissions.

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

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

Four short sentences, each carrying distinct operational information (scope, cost/gate, retry policy, next step and recovery). The critical cost/confirm constraint is front-loaded in sentence two with zero filler.

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

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

No output schema exists, yet the description explains what the call returns conceptually (a verification ID) and points to the sibling that materializes it, plus how to poll and how to recover from a lost response. For a two-parameter async mutation, an agent has everything needed to invoke it correctly and safely.

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

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

Schema coverage is 0%, so the description must compensate; it does for 'confirm' by explaining that it gates credit spend, which the bare boolean default=false would not convey. 'email' is largely self-documenting from the format/pattern constraint, and the description reinforces single-email scope ('Submit one email'), but it doesn't explain what happens if confirm is omitted or the exact rejection behavior. Strong but not exhaustive.

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

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

States a specific verb and resource ('Submit one email for asynchronous verification') and immediately distinguishes itself from the sibling that reads the outcome ('Read the returned verification ID with mailercheck_verification_result'). An agent can tell it starts the job rather than fetches the result without opening either schema.

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

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

Gives explicit operational conditions: confirm:true is required, credits are spent, the call is never retried, polls should be spaced out, and lost responses should be checked in MailerCheck before resubmitting. It names the follow-up alternative (mailercheck_verification_result) and the recovery path, leaving little to inference.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.