Express resource sizes as integers

Resource declarations and resource-link content express raw size as an integer, following MCP’s numeric schema convention. Replace a fractional value with the intended integer; the validator must not round it. No minimum is added beyond MCP’s definition.

Requirement

Resource size, in a declaration and in resource link content, is an integer when present.

Details

Rule ID: mcp.resource.size
Owner: mcp
Surface: Resource · Path: size
Kind: structural
Requirement level: must
Protocol versions: independent of the MCP protocol version
Document schema locations: #/$defs/resource/properties/size, #/$defs/toolResultContent/allOf/3/then/properties/size
Status: arbitrated · Enforcement: internal

Required by

• MCP Description 0.9.0, Sections 3.6 and 10.1.1
  • 10. Resources and Resource Templates: req-resource-size-integer
  • 3. Document Structure: req-mcp-integer-numbers
• Proposal 0029, Integer MCP Numbers
  • proposal-snapshots/0029-integer-mcp-numbers.md

Based on

• Tier 1 — MCP schema.ts and normative specification text: MCP 2024-11-05 to 2026-07-28 schema.ts, interface Resource, size (unannotated number); ResourceLink extends Resource from 2025-06-18
  • validation/mcp-schemas/2024-11-05/schema.ts#L449
  • validation/mcp-schemas/2025-03-26/schema.ts#L482
  • validation/mcp-schemas/2025-06-18/schema.ts#L556
  • validation/mcp-schemas/2025-11-25/schema.ts#L832
  • validation/mcp-schemas/2026-07-28/schema.ts#L1471
  • validation/mcp-schemas/2026-07-28/schema.ts#L1719-L1721
• Tier 3 — Generated MCP schema.json: MCP generated schema.json, Resource and ResourceLink size typed integer
  • validation/mcp-schemas/2024-11-05/schema.json#L1508-L1511
  • validation/mcp-schemas/2026-07-28/schema.json#L2760-L2763
  • validation/mcp-schemas/2026-07-28/schema.json#L2830-L2833
• Tier 3 — Generated MCP schema.json: MCP scripts/generate-schemas.ts: typescript-json-schema –defaultNumberType integer
  • https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/scripts/generate-schemas.ts#L95

Gaps and arbitrations

Arbitration: 2024-11-05, 2025-03-26, 2025-06-18, 2025-11-25, 2026-07-28

Conflict: schema.ts declares size as number, which TypeScript cannot restrict to integers; the generated schema.json types it as integer.

Authority: MCP’s schema generation configuration (–defaultNumberType integer), under which an unannotated number denotes an integer and fields that accept fractions carry @TJS-type number, such as Annotations.priority.

Behavior: Reject a non-integer size with schema-validation. No minimum is applied, as MCP defines none.

Rationale: The integer output is MCP’s deliberate convention, not a generator defect. Following it keeps native projection valid against the generated schema and matches what MCP tooling validates.

Review trigger: MCP changes its generator configuration or annotates size with @TJS-type number..

Examples

Valid example

Source: expected-valid/discriminated-unions.json · Selected: /tools/0/interactionExamples/approved/steps/1/response/content/size

12

Valid example

Source: expected-valid/named-resource-examples.json · Selected: /resources/2/size

3

Invalid example

Source: expected-invalid/fractional-resource-size.json · Selected: /resources/0/size

1048576.5

Invalid example

Source: expected-invalid/fractional-resource-link-size.json · Selected: /tools/0/examples/listing/result/content/0/size

2048.5

Invalid example

Source: expected-invalid/discriminated-resource-link-size.json · Selected: /tools/0/interactionExamples/nested/steps/0/response/content/size

1.5

Rule Source Code

resource.yaml at immutable tag v0.9.0-rc.3.