Free no-auth tools for Spain: validate IBAN, EU VAT check (VIES), Modelo 303 sums, Veri*factu dates.
Scored across 6 tools
Each tool targets a distinct fiscal operation: VAT arithmetic, Modelo 303 summation, VIES lookup, IBAN check, Spanish tax ID check, and deadline lookup. The only mild overlap is calculate_vat vs calculate_modelo_303, but the descriptions explicitly cross-reference each other and clarify the boundary. The 'check' vs 'validate' verb pair (check_vat_number, validate_iban, validate_spanish_tax_id) remains distinguishable by their distinct subject matter.
All names are snake_case verb_noun following a clear pattern (calculate_, check_, get_, validate_). The verb set varies slightly (check vs validate for validation-style tools), which is a minor inconsistency but still readable and predictable.
Six tools form a tightly scoped fiscal/accounting utility set, each covering a distinct, non-redundant operation. Nothing feels padded or missing within the apparent scope.
The set covers validation and calculation helpers well, but calculate_modelo_303's description points to a get_modelo tool that is not present in the surface, leaving a dead end. There is also no tool to fetch invoices/gastos, so the referenced account-based workflow cannot actually be completed.
Add up the boxes (casillas) of the Spanish quarterly VAT return Modelo 303 with the formulas printed on the official AEAT form: 27, 45, 46, 64, 66, 87, 69 and 71. Input are the tax amounts (cuotas) per box, e.g. {"09": 17430, "29": 9870}. It only sums what it is given: it does not decide which rate or deduction applies and it is not a tax return. To compute the 303 from an existing contix account's own invoices and gastos instead of typed-in boxes, use get_modelo with model 303.
| Name | Required | Description | Default |
|---|---|---|---|
| casillas | Yes | Box number -> amount. Accepted: 152, 167, 03, 155, 06, 09, 11, 13, 15, 158, 170, 18, 21, 24, 26, 29, 31, 33, 35, 37, 39, 41, 42, 43, 44, 58, 76, 65, 77, 110, 78, 68, 108, 70, 109 |
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is known; the description reinforces this by framing the tool as a pure summation ('It only sums what it is given') that performs no interpretation. It does not describe the shape of the returned object beyond naming the computed boxes, which is a minor gap for a calculation tool with no output schema.
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 front-loaded sentences: what it sums, what the input is with an example, what it does not do, then the sibling alternative. No filler, and the scope limitation is placed before the routing hint where it matters most.
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 single-parameter, read-only calculation tool with no output schema, the description covers purpose, input format, limitations, and the alternative path. The only missing piece is what the response envelope looks like, though the enumerated formula boxes substantially telegraph the output.
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 100%, so baseline is 3, but the description adds real meaning: the values are tax amounts ('cuotas') per box rather than rates, and it supplies a concrete example object ({"09": 17430, "29": 9870}) clarifying the key format. That goes beyond the schema's accepted-box enumeration.
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 states a specific verb and resource — 'Add up the boxes (casillas) of the Spanish quarterly VAT return Modelo 303' — and enumerates the exact formula boxes (27, 45, 46, 64, 66, 87, 69, 71). It also explicitly distinguishes itself from the sibling get_modelo, so an agent can route between the two without opening a 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?
It gives both sides of the routing decision: use this tool when you have tax amounts per box already, and use get_modelo with model 303 when computing from an account's own invoices and gastos. It also sets explicit exclusions — 'it does not decide which rate or deduction applies and it is not a tax return.'
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.
Compute the taxable base, VAT (IVA) amount, optional recargo de equivalencia and optional IRPF withholding from an amount. By default importe is the taxable base (sin IVA). With incluye_iva=true, importe is the TOTAL with VAT already included and the base is derived backwards (quitar IVA / reverse VAT). It only applies the rates it is given: it does NOT decide which VAT or IRPF rate applies to a case. Returns base_imponible, cuota_iva, recargo_importe, retencion_irpf, total_con_iva and total_factura (total after subtracting the IRPF withholding).
| Name | Required | Description | Default |
|---|---|---|---|
| irpf | No | IRPF withholding rate in %, e.g. 15, 19. Default 0 | |
| importe | Yes | Amount: taxable base, or total with VAT if incluye_iva=true | |
| recargo | No | Recargo de equivalencia rate in %, e.g. 5.2, 1.4, 0.5, 1.75. Default 0 | |
| tipo_iva | Yes | VAT rate in %, e.g. 21, 10, 4, 0 | |
| incluye_iva | No | True if `importe` already includes VAT (reverse calculation) |
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint=true and openWorldHint=false, and the description adds useful behavioral context: reverse VAT calculation, rate-application boundary, and the returned total_factura after IRPF withholding. It does not discuss rounding or error behavior, so not a 5.
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?
Front-loaded with the purpose, then proceeds through default behavior, reverse calculation, rate boundary, and return values. Every sentence earns its place with no repetition or 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?
Although there is no output schema, the description lists all returned fields and explains total_factura versus total_con_iva. Combined with the read-only annotations and complete parameter schema, it gives the agent everything needed to call the tool correctly.
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 100%, so parameter meanings are already fully documented. The description restates the importe/incluye_iva relationship and adds return-field context, but adds little parameter semantics beyond the schema itself, making the baseline 3 appropriate.
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 (compute) and the exact resources it calculates: taxable base, VAT, recargo de equivalencia, and IRPF withholding. The boundary that it only applies supplied rates and does not decide which rate applies distinguishes it from rate-decision or tax-form tools.
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?
Clearly explains the default mode (importe as taxable base) and the reverse-VAT mode when incluye_iva=true. It also states a key exclusion (it does not decide applicable rates), though it does not explicitly name a sibling alternative such as calculate_modelo_303.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.
Look up an EU VAT number in the European Commission's VIES registry. Returns what the registry says: whether the number exists and, where the member state publishes it, the name and address. It draws NO tax conclusion. VIES is often down; that is reported as disponible=false, which is NOT the same as an invalid number.
| Name | Required | Description | Default |
|---|---|---|---|
| pais | No | ISO country code (e.g. DE); omit when the number carries its prefix | |
| numero | Yes | VAT number, with or without country prefix |
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint and openWorldHint, and the description adds genuinely non-obvious context: VIES is unreliable, downtime surfaces as disponible=false, and a third-party registry's answer is not a tax determination. These are failure-mode and semantic caveats an agent cannot derive from annotations or schema.
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 tight sentences, front-loaded with what the tool does, then what it returns, then the two critical caveats. Zero filler; every sentence carries decision-relevant 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?
With no output schema, the description compensates by describing return content (existence, and member-state-dependent name/address) and the special disponible=false signal. Nothing an agent needs to call and interpret this tool is missing.
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 100%, so both parameters (pais, numero) are already documented in the schema, including the prefix rule. The description adds no parameter-level detail beyond that, so the baseline 3 applies.
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 (look up an EU VAT number in the European Commission's VIES registry) and is clearly distinct from sibling accounting/validation tools like validate_iban. An agent knows exactly what domain question this tool answers.
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 strong contextual guidance on interpretation – 'It draws NO tax conclusion' and how to read an outage (disponible=false is not an invalid number). It does not explicitly name alternatives or state when-not-to-use versus siblings, 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.
Deadlines from the published Spanish rules: Veri*factu (RD 1007/2023, dates of RD-ley 15/2025) and mandatory B2B e-invoicing (RD 238/2026, counted from the ministerial order, provisional while the order is a draft). Information about the rules, not advice. Returns a date (or null) and explanatory text for each obligation (verifactu, factura_electronica and, where it applies, estados_facturas).
| Name | Required | Description | Default |
|---|---|---|---|
| forma | Yes | sociedad = company paying Impuesto sobre Sociedades; autonomo = self-employed | |
| mas_8_millones | No | Turnover above 8 million EUR last year | |
| factura_a_empresas | Yes | Invoices other businesses or professionals in Spain |
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint and openWorldHint=false, so the description focuses on what the annotations cannot say: which legal sources are used, that the e-invoicing dates are provisional while the ministerial order is a draft, and that a null date is a possible outcome. That provisionality/uncertainty disclosure is genuinely useful context a caller could not derive from structured fields.
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?
A compact paragraph that front-loads the subject (deadlines from published Spanish rules), then sources, then the return shape. It is dense with legal citations but every clause (regulation IDs, provisional status, null return, obligation keys) carries information an agent needs.
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 correctly compensates by describing the return: a date or null plus explanatory text, keyed by obligation name (verifactu, factura_electronica, estados_facturas). The one gap is that it never says which inputs trigger estados_facturas ('where it applies' is left ambiguous), though the schema hints at it via mas_8_millones.
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 100%, including an explanation of the forma enum and the mas_8_millones turnover threshold, so the schema carries parameter meaning fully. The description adds nothing about how forma / mas_8_millones / factura_a_empresas change which obligations are returned — baseline 3 is the right level.
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 (returns deadlines) and a specific, narrowly-scoped resource (Spanish Veri*factu and B2B e-invoicing obligations), citing the governing regulations. No sibling tool overlaps with this lookup, and the description makes that uniqueness obvious.
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?
Usage is only implied: an agent can infer this answers 'when do I have to comply with Veri*factu / e-invoicing'. There is no explicit statement of when to call it versus e.g. get_modelo or calculate_modelo_303, and no prerequisites mentioned beyond the schema. The scope caveat 'Information about the rules, not advice' is the only boundary drawn.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.
Check whether an IBAN is well formed: country length (SWIFT IBAN registry) and ISO 13616 check digits. For Spanish IBANs also the two account check digits (DC) and the bank (entidad) with its BIC from the Banco de España registry. A 20-digit Spanish account number (CCC) is converted to its IBAN. Pure arithmetic: it does NOT confirm that the account exists or who owns it.
| Name | Required | Description | Default |
|---|---|---|---|
| iban | Yes | IBAN or 20-digit Spanish account, spaces allowed |
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations only declare readOnlyHint and openWorldHint; the description adds substantive behavior: it is pure arithmetic against registry data, it discloses which registries are consulted (SWIFT, Banco de España), and it explicitly caps its own guarantees by saying it does not verify account existence or ownership. That last point is exactly the kind of limitation an agent needs and is not available anywhere in the structured fields.
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 dense sentences, front-loaded with the core check and then narrowing to the Spanish-specific behavior and the limitation. Every clause carries information; nothing is restated from the name or title.
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 single-parameter, read-only validation tool, the description covers inputs, checks performed, and limits. The only gap is that, with no output schema, it never indicates the shape of the result (e.g., valid/invalid plus reasons), which an agent may need to interpret the response.
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 100%, so baseline is 3. The description goes beyond the schema by clarifying that a 20-digit Spanish account (CCC) is accepted and internally converted to its IBAN, and that spaces are tolerated, which disambiguates the single parameter's accepted 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 (validate/check) and resource (IBAN), then enumerates exactly which checks are performed: SWIFT country length, ISO 13616 check digits, and Spanish DC/entidad/BIC. This is clearly distinct from siblings like check_vat_number, so an agent can route correctly without opening any 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?
The description gives a clear context of use (well-formedness validation) and draws an explicit boundary: it does NOT confirm the account exists or who owns it. It also notes the CCC-to-IBAN conversion path. It stops short of naming alternatives or stating when-not-to-use conditions, so it is strong but not fully routing-aware.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.
Two modes in one input. If valor is only 1-8 digits, computes the DNI check letter and returns the full 8-digit-plus-letter DNI (digits are left-padded with zeros). If valor is a full document, validates its check character: DNI (8 digits + letter), NIE (X/Y/Z + 7 digits + letter) or CIF (entity letter + 7 digits + check digit or letter, with the entity type per Orden EHA/451/2008). Tolerates spaces, dots, dashes and an ES prefix. Pure arithmetic: it does NOT confirm the document exists, is active or belongs to the person who gave it. In validation mode, returns valido (true/false) and, when it does not match, the correct check character (letra_correcta / control_correcto).
| Name | Required | Description | Default |
|---|---|---|---|
| valor | Yes | 1-8 digits, or a full DNI/NIE/CIF |
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Beyond the readOnlyHint/openWorldHint annotations, the description discloses critical behavior: it is pure arithmetic and does NOT confirm document existence, active status, or ownership. It also describes the return values in validation mode, including valido and corrected check characters.
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?
The description is dense but front-loaded with 'Two modes in one input,' then systematically covers calculation, validation, format tolerance, limitations, and return values. Every sentence carries useful information and there is no redundant 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?
For a single-parameter validation tool with no output schema, the description fully equips an agent to call it correctly: it explains the two modes, accepted input formats, scope limitations, and expected validation responses. Nothing essential is missing.
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 100%, so the baseline is 3, but the description adds meaningful semantics for the single `valor` parameter: accepted formats, left-padding behavior, and the distinction between numeric-only and full-document input. It could be even stronger by detailing CIF entity-letter rules, but it already adds substantial value beyond the schema.
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 states a specific verb (calculate/validate) and resource (Spanish DNI, NIE, CIF check character), and immediately explains the two input modes. It is clearly distinguishable from siblings like validate_iban or check_vat_number.
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 description gives clear usage context by explaining that 1-8 digits triggers DNI calculation while a full document triggers validation, and notes formatting tolerance. It does not explicitly compare against sibling validation tools such as check_vat_number, 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.
Recent tool additions, removals, and schema changes observed during successful MCP inspections.
calculate_vatvalidate_spanish_tax_idcalculate_modelo_303check_vat_numberget_obligation_datesvalidate_ibanSpanish e-invoicing with VeriFactu (AEAT): issue invoices, manage customers, validate NIFs.
8 invoice/payment tools; 5 free, no login. IBAN, bank changes, Peppol. Paid VAT and invoice review.
Free VAT, card fee, loan, mortgage, margin, ROI and break-even calculators plus IBAN/SWIFT checks
Spanish invoicing (Verifactu/TicketBAI), bookkeeping, bank and tax returns for autónomos and SLs.
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