---
title: "Express cache lifetimes as non-negative integers"
description: "Resource example ttlMs values are non-negative integer milliseconds. This value constraint also applies when an older protocol version permits the field without defining it. Replac"
source: "https://mcpdesc.org/docs/rules/0.9.0-rc.3/mcp/cacheable-result-ttl-ms/"
---

Resource example ttlMs values are non-negative integer milliseconds. This value constraint also applies when an older protocol version permits the field without defining it. Replace fractions or negative values with the intended duration rather than relying on automatic rounding.

## Requirement

ttlMs in a Resource example result is a non-negative integer when present.

## Details

**Rule ID:** `mcp.cacheable-result.ttl-ms`  
**Owner:** mcp  
**Surface:** CacheableResult · **Path:** `ttlMs`  
**Kind:** structural  
**Requirement level:** must  
**Protocol versions:** independent of the MCP protocol version  
**Document schema locations:** `#/$defs/completedReadResourceResultExample/properties/ttlMs`  
**Status:** arbitrated · **Enforcement:** internal

## Required by

* **MCP Description 0.9.0, Sections 3.6 and 10.4.4**  
  * 3\. Document Structure: [req-mcp-integer-numbers](https://mcpdesc.org/docs/specification/0.9.0-rc.3/document-structure#req-mcp-integer-numbers)
  * 1. Resources and Resource Templates: [req-resource-example-result-shape](https://mcpdesc.org/docs/specification/0.9.0-rc.3/resources#req-resource-example-result-shape)
* **Proposal 0029, Integer MCP Numbers**  
  * [proposal-snapshots/0029-integer-mcp-numbers.md](https://github.com/mcpdesc/mcpdesc-specification/blob/v0.9.0-rc.3/spec/draft/proposal-snapshots/0029-integer-mcp-numbers.md)

## Based on

* **Tier 1 — MCP schema.ts and normative specification text:** MCP 2026-07-28 schema.ts, interface CacheableResult, ttlMs (unannotated number, @minimum 0)  
  * [validation/mcp-schemas/2026-07-28/schema.ts#L1081-L1094](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/schema/2026-07-28/schema.ts#L1081-L1094)
* **Tier 3 — Generated MCP schema.json:** MCP 2026-07-28 generated schema.json, ReadResourceResult.ttlMs typed integer  
  * [validation/mcp-schemas/2026-07-28/schema.json#L2625-L2629](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/dae694f61a31f7a3ec2af3aff709e11f70f61e48/schema/2026-07-28/schema.json#L2625-L2629)
* **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: `2026-07-28`

**Conflict:** schema.ts declares ttlMs 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.

**Behavior:** Reject a non-integer or negative ttlMs with schema-validation.

**Rationale:** A millisecond duration is an integer in MCP’s generated schema; accepting fractions would let examples disagree with MCP’s types.

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

## Notes

MCP defines ttlMs from 2026-07-28 only. In earlier protocol versions it is a field not defined by the MCP protocol version (mcp.result.additional-properties.preserve), and Section 3.6 keeps the same value constraint so that one example can serve several protocol versions.

## Examples

### Valid example

Source: [expected-valid/named-resource-examples.json](https://github.com/mcpdesc/mcpdesc-specification/blob/v0.9.0-rc.3/spec/draft/fixtures/expected-valid/named-resource-examples.json) · Selected: `/resources/1/examples/two-files/result/ttlMs`

```json
60000
```

### Invalid example

Source: [expected-invalid/fractional-ttl-ms.json](https://github.com/mcpdesc/mcpdesc-specification/blob/v0.9.0-rc.3/spec/draft/fixtures/expected-invalid/fractional-ttl-ms.json) · Selected: `/resources/0/examples/cached/result/ttlMs`

```json
60000.5
```

### Invalid example

Source: [expected-invalid/resource-example-cache-fields.json](https://github.com/mcpdesc/mcpdesc-specification/blob/v0.9.0-rc.3/spec/draft/fixtures/expected-invalid/resource-example-cache-fields.json) · Selected: `/resources/1/examples/invalid/result/ttlMs`

```json
-1
```

## Rule Source Code

[result.yaml](https://github.com/mcpdesc/mcpdesc-specification/blob/v0.9.0-rc.3/spec/draft/validation/rules/result.yaml) at immutable tag [v0.9.0-rc.3](https://github.com/mcpdesc/mcpdesc-specification/blob/v0.9.0-rc.3/spec/draft).