toon-format--toon/docs/reference/spec.md

---
description: Guided tour of the TOON specification – sections, conformance checklists, media type, and versioning.
---

# Specification

The [TOON specification](https://github.com/toon-format/spec) is the authoritative reference for implementing encoders, decoders, and validators. It defines the concrete syntax, normative encoding/decoding behavior, and strict-mode validation rules.

You don't need this page to *use* TOON. It's mainly for implementers and contributors. If you're looking to learn how to use TOON, start with the [Getting Started](/guide/getting-started) guide instead.

> [!NOTE]
> The TOON specification is stable, but also an idea in progress. Nothing's set in stone – help shape where it goes by contributing to it or sharing feedback.

## Current Version

**Spec v{{ $spec.version }}** (2026-05-20) is the current published Working Draft. It is stable for implementation but not yet finalized; see "Status of This Document" in the spec for details.

## Media Type & File Extension

The spec defines a provisional media type and file extension in [§17](https://github.com/toon-format/spec/blob/main/SPEC.md#17-iana-considerations):

- **Media type:** `text/toon` (provisional, not yet IANA‑registered; UTF‑8 only)
- **File extension:** `.toon`

TOON documents are always UTF‑8 with LF (`\n`) line endings; the optional `charset` parameter, when present, is `utf-8`.

## Guided Tour of the Spec

### Core Concepts

[§1 Terminology and Conventions](https://github.com/toon-format/spec/blob/main/SPEC.md#1-terminology-and-conventions):
Defines key terms like "indentation level", "active delimiter", "strict mode", and RFC2119 keywords (MUST, SHOULD, MAY).

[§2 Data Model](https://github.com/toon-format/spec/blob/main/SPEC.md#2-data-model):
Specifies the JSON data model (objects, arrays, primitives), array/object ordering requirements, and canonical number formatting (canonical decimal for values in `[1e-6, 1e21)` or zero; exponent form permitted outside).

[§3 Encoding Normalization](https://github.com/toon-format/spec/blob/main/SPEC.md#3-encoding-normalization-reference-encoder):
Defines how non-JSON types (Date, BigInt, NaN, Infinity, undefined, etc.) are normalized before encoding. Required reading for encoder implementers.

[§4 Decoding Interpretation](https://github.com/toon-format/spec/blob/main/SPEC.md#4-decoding-interpretation-reference-decoder):
Specifies how decoders map text tokens to host values (quoted strings, unquoted primitives, numeric parsing with leading-zero handling). Decoders default to strict mode (`strict = true`) in the reference implementation; strict-mode errors are enumerated in §14.

### Syntax Rules

[§5 Concrete Syntax and Root Form](https://github.com/toon-format/spec/blob/main/SPEC.md#5-concrete-syntax-and-root-form):
Defines TOON's line-oriented, indentation-based notation and how to determine whether the root is an object, array, or primitive.

[§6 Header Syntax](https://github.com/toon-format/spec/blob/main/SPEC.md#6-header-syntax-normative):
Normative ABNF grammar for array headers: `key[N<delim?>]{fields}:`. Specifies bracket segments, delimiter symbols, and field lists.

[§7 Strings and Keys](https://github.com/toon-format/spec/blob/main/SPEC.md#7-strings-and-keys):
Complete quoting rules (when strings MUST be quoted), escape sequences (only `\\`, `\"`, `\n`, `\r`, `\t`, and `\uXXXX` for other U+0000–U+001F controls are valid), and key encoding requirements.

[§8 Objects](https://github.com/toon-format/spec/blob/main/SPEC.md#8-objects):
Object field encoding (key: value), nesting rules, key order preservation, and empty object handling.

[§9 Arrays](https://github.com/toon-format/spec/blob/main/SPEC.md#9-arrays):
Covers all array forms: primitive (inline), arrays of objects (tabular), mixed/non-uniform (list), and arrays of arrays. Includes tabular detection requirements.

[§10 Objects as List Items](https://github.com/toon-format/spec/blob/main/SPEC.md#10-objects-as-list-items):
Indentation rules for objects appearing in list items (first field on the hyphen line), including the canonical pattern when the first field is a tabular array (header on the hyphen line, rows at depth +2, sibling fields at depth +1).

[§11 Delimiters](https://github.com/toon-format/spec/blob/main/SPEC.md#11-delimiters):
Delimiter scoping (document vs active), delimiter-aware quoting, and parsing rules for comma/tab/pipe delimiters.

[§12 Indentation and Whitespace](https://github.com/toon-format/spec/blob/main/SPEC.md#12-indentation-and-whitespace):
Encoding requirements (consistent spaces, no tabs in indentation, no trailing spaces/newlines) and decoding rules (strict vs non-strict indentation handling).

### Conformance and Validation

[§13 Conformance and Options](https://github.com/toon-format/spec/blob/main/SPEC.md#13-conformance-and-options):
Defines conformance classes (encoder, decoder, validator), standardized options, and conformance checklists.

[§13.4 Key Folding and Path Expansion](https://github.com/toon-format/spec/blob/main/SPEC.md#134-key-folding-and-path-expansion):
Optional encoder feature (key folding) and decoder feature (path expansion) for collapsing/expanding dotted paths, with deep-merge semantics and strict/non-strict conflict resolution.

[§14 Strict Mode Errors and Diagnostics](https://github.com/toon-format/spec/blob/main/SPEC.md#14-strict-mode-errors-and-diagnostics-authoritative-checklist):
**Authoritative checklist** of all strict-mode errors: array count and width mismatches (§14.1), syntax and structural errors (§14.2), path expansion conflicts (§14.3), and duplicate sibling keys (§14.4).

### Implementation Guidance

[§15 Security Considerations](https://github.com/toon-format/spec/blob/main/SPEC.md#15-security-considerations):
Injection risks, quoting rules, and strict-mode checks relevant to security.

[§16 Internationalization](https://github.com/toon-format/spec/blob/main/SPEC.md#16-internationalization):
Unicode handling and locale-independent number formatting.

[§17 IANA Considerations](https://github.com/toon-format/spec/blob/main/SPEC.md#17-iana-considerations):
Media type registration plans and provisional status.

[§18 Versioning and Extensibility](https://github.com/toon-format/spec/blob/main/SPEC.md#18-versioning-and-extensibility):
How the spec evolves: major vs minor bumps and the extensibility policy.

[§19 Intellectual Property Considerations](https://github.com/toon-format/spec/blob/main/SPEC.md#19-intellectual-property-considerations):
Licensing and IP terms for the specification.

[Appendix F: Host Type Normalization Examples](https://github.com/toon-format/spec/blob/main/SPEC.md#appendix-f-host-type-normalization-examples-informative):
Non-normative guidance for Go, JavaScript, Python, Rust, and Java implementations on normalizing language-specific types.

[Appendix C: Test Suite and Compliance](https://github.com/toon-format/spec/blob/main/SPEC.md#appendix-c-test-suite-and-compliance-informative):
Reference test suite at [github.com/toon-format/spec/tree/main/tests](https://github.com/toon-format/spec/tree/main/tests) for validating implementations.

## Spec Sections at a Glance

| Section | Topic | When to Read |
|---------|-------|--------------|
| §1–4 | Data model, normalization, decoding | Implementing encoders/decoders |
| §5–6 | Syntax, headers, root form | Implementing parsers |
| §7 | Strings, keys, quoting, escaping | Implementing string handling |
| §8–10 | Objects, arrays, list items | Implementing structure encoding |
| §11–12 | Delimiters, indentation, whitespace | Implementing formatting and validation |
| §13 | Conformance, options, key folding/path expansion | Implementing options and features |
| §14 | Strict-mode errors | Implementing validators |
| §15–16 | Security, internationalization | Operational considerations |
| §17–19 | IANA, versioning, IP | Ecosystem and licensing |

## Conformance Checklists

The spec includes three conformance checklists:

### Encoder Checklist (§13.1) <sup>[↗ SPEC.md](https://github.com/toon-format/spec/blob/main/SPEC.md#131-encoder-conformance-checklist)</sup>

Key requirements:
- Produce UTF-8 with LF line endings
- Use consistent indentation (default 2 spaces, no tabs)
- Escape `\\`, `\"`, `\n`, `\r`, `\t` in quoted strings, and use `\uXXXX` for any other U+0000–U+001F control character; lone surrogates are rejected
- Quote strings with active delimiter, colon, or structural characters
- Emit array lengths `[N]` matching actual count
- Preserve object key order
- Emit numbers per §2 (canonical decimal in `[1e-6, 1e21)` or zero; exponent form permitted outside)
- Convert `-0` to `0`, `NaN`/±Infinity to `null`
- Emit booleans and null as lowercase literals (`true`, `false`, `null`)
- No trailing spaces or trailing newline
- When `keyFolding="safe"` is enabled, folding MUST follow §13.4:
  - Only fold IdentifierSegment keys (letters/digits/underscores, no dots),
  - Do not introduce collisions with existing sibling keys,
  - Do not fold segments that would require quoting.
- When `flattenDepth` is set, folding MUST stop at the configured number of segments (§13.4).

### Decoder Checklist (§13.2) <sup>[↗ SPEC.md](https://github.com/toon-format/spec/blob/main/SPEC.md#132-decoder-conformance-checklist)</sup>

Key requirements:
- Parse array headers per §6 (length, delimiter, fields)
- Split inline arrays and tabular rows using active delimiter only
- Unescape quoted strings with only valid escapes
- Type unquoted primitives: true/false/null → booleans/null, numeric → number, else → string
- Enforce strict-mode rules when `strict=true`
- Preserve array order and object key order
- When `expandPaths="safe"` is enabled, expand dotted keys into nested objects per §13.4:
  - Split on `.`, only expand when all segments are IdentifierSegments,
  - Deep-merge overlapping paths (object + object),
  - Do not perform element-wise array merges.
- With `expandPaths="safe"` and `strict=true` (default), MUST error on any expansion conflict (§14.3).
- With `expandPaths="safe"` and `strict=false`, MUST apply deterministic last-write-wins (LWW) conflict resolution (§13.4).

### Validator Checklist (§13.3) <sup>[↗ SPEC.md](https://github.com/toon-format/spec/blob/main/SPEC.md#133-validator-conformance-checklist)</sup>

Validators should verify:
- Structural conformance (headers, indentation, list markers)
- Whitespace invariants (no trailing spaces/newlines)
- Delimiter consistency between headers and rows
- Array length counts match declared `[N]`
- All strict-mode requirements (including path-expansion conflicts when enabled)

## Versioning

The spec uses semantic versioning (major.minor):
- **Major version** (e.g., v2 → v3): Breaking changes, incompatible with previous versions
- **Minor version** (e.g., v3.1 → v3.2): Clarifications, additional requirements, or backward-compatible additions

See [Appendix D: Document Changelog](https://github.com/toon-format/spec/blob/main/SPEC.md#appendix-d-document-changelog-informative) for detailed version history.

## Contributing to the Spec

The spec is community-maintained at [github.com/toon-format/spec](https://github.com/toon-format/spec). We welcome contributions of all kinds: reporting ambiguities or errors, proposing clarifications and examples, adding test cases to the reference suite, or discussing edge cases and normative behavior. Your feedback helps shape the format.