[BUG]: Swift 6 Strict Sendable conformance

Issue Type

TLDR; Quicktype is generating invalid Swift under the Swift 6 language compiler

Generated Swift helper classes (JSONCodingKey, JSONAny, JSONNull) don't compile under Swift 6 strict concurrency, even when --sendable is passed.

Context (Environment, Version, Language)

Input Format: JSON Schema (--src-lang schema)
Output Language: Swift

CLI, npm, or app.quicktype.io: CLI
Version: 23.2.6

Description

When generating Swift code with --sendable and --struct-or-class struct, the generated model structs correctly gain Sendable conformance. However, the helper classes at the bottom of the output (JSONNull, JSONCodingKey, JSONAny) are not updated for Swift 6 strict concurrency:

JSONCodingKey is a non-final class — Swift 6 errors with non-final class 'JSONCodingKey' cannot conform to 'Sendable'
JSONAny stores Any which is not Sendable — needs @unchecked Sendable and nonisolated(unsafe) (related: [FEATURE]: Swift - Sendable JSONAny #2598)
JSONNull uses deprecated hashValue (minor, but warning noise)

This makes the output unusable with swift-tools-version: 6.0 packages out of the box.

Input Data

Any JSON Schema that produces additionalProperties / untyped fields triggers JSONAny usage. Minimal example:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "Example",
  "type": "object",
  "properties": {
    "data": {
      "type": "object",
      "additionalProperties": true
    }
  }
}

Expected Behaviour / Output

When --sendable is passed, all generated types — including JSONCodingKey, JSONNull, and JSONAny — should compile cleanly under Swift 6 strict concurrency (swift-tools-version: 6.0).

Specifically:

JSONCodingKey should be final class JSONCodingKey: CodingKey, Sendable
JSONAny should be marked @unchecked Sendable (since it wraps Any)
JSONNull should use hash(into:) instead of deprecated hashValue

Current Behaviour / Output

error: non-final class 'JSONCodingKey' cannot conform to 'Sendable'; use '@unchecked Sendable'
warning: 'Hashable.hashValue' is deprecated as a protocol requirement; conform type 'JSONNull' to 'Hashable' by implementing 'hash(into:)' instead

The generated struct models are correctly Sendable, but the helper classes break the build.

Steps to Reproduce

npm install -g quicktype
Save the minimal JSON Schema above as example.json

Run:

quicktype \
  --lang swift \
  --src-lang schema \
  --src example.json \
  --access-level public \
  --struct-or-class struct \
  --coding-keys \
  --sendable \
  -o Models.swift

Create a Swift package with swift-tools-version: 6.0 and try to build

 mkdir MyPackage && cd MyPackage
 swift package init --type library --name MyPackage
 sed -i '' 's/swift-tools-version: .*/swift-tools-version: 6.0/' Package.swift
 cp ../Models.swift Sources/MyPackage/Models.swift
 swift build

Possible Solution

I'm currently working around this by making the following changes with sed after generation:

Emit final class JSONCodingKey: CodingKey, Sendable instead of class JSONCodingKey: CodingKey
Emit public class JSONAny: Codable, @unchecked Sendable with nonisolated(unsafe) on the value property
Replace public var hashValue: Int with public func hash(into hasher: inout Hasher) {} on JSONNull

For point 2 this is would be safer as an enum for future extensibility

public enum JSONValue: Codable, Sendable {
      case null
      case bool(Bool)
      case int(Int64)
      case double(Double)
      case string(String)
      case array([JSONValue])
      case object([String: JSONValue])
  }

I've submitted a fix for this issue in PR joelklabo/quicktype#1

The fix adds Swift 6 strict concurrency support:

JSONCodingKey: final class with Sendable conformance when --sendable is used
JSONAny: @unchecked Sendable conformance when --sendable is used
JSONNull: uses hash(into:) instead of deprecated hashValue when --swift-5-support is used

This should allow generated Swift code to compile cleanly under swift-tools-version: 6.0.

Tips appreciated: clawlet@lnbits.klabo.world

Triage sweep note (assisted by Claude): confirmed reproducible on current master (v23.3.0, 68ecb284).

Root cause / findings

With --sendable, the generated model type does get marked Sendable (e.g. public struct Models: Codable, Sendable), but it holds an [String: JSONAny]? property, and the support-code types aren't Sendable-safe:

class JSONAny: Codable — no Sendable conformance
class JSONCodingKey: CodingKey — not final, no Sendable
class JSONNull: Codable, Hashable — no Sendable, and uses the deprecated public var hashValue instead of hash(into:)

A Sendable struct holding non-Sendable members fails to compile under Swift 6 strict concurrency — exactly as reported.

Root cause is in packages/quicktype-core/src/language/Swift/SwiftRenderer.ts: the sendable option only appends "Sendable" to the model protocol list (~line 392); the support-code emission (~lines 1168-1450, where JSONAny/JSONCodingKey/JSONNull are written out) never checks the option.

Assessment

Severity: medium — the generated code doesn't compile, but only when both --sendable is passed (non-default) and the project is on a Swift 6 toolchain with strict concurrency.
Difficulty: easy — localized string-emission changes: make JSONCodingKey final + Sendable, mark JSONAny @unchecked Sendable, replace hashValue with hash(into:) on JSONNull, all gated on the sendable option. Needs matching snapshot/fixture updates. Note there's already a candidate fix from the reporter at fix(swift): Add Swift 6 strict concurrency support for helper classes joelklabo/quicktype#1 (their fork) that looks like a reasonable starting point.

Reproduction

$ node dist/index.js --lang swift --src-lang schema --src example.json \
    --access-level public --struct-or-class struct --coding-keys --sendable -o Models.swift

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "Example",
  "type": "object",
  "properties": {
    "data": { "type": "object", "additionalProperties": true }
  }
}

Output contains public struct Example: Codable, Sendable alongside class JSONAny: Codable (no Sendable), non-final class JSONCodingKey: CodingKey, and class JSONNull: Codable, Hashable with public var hashValue.

Navigation Menu

Repository navigation

[BUG]: Swift 6 Strict Sendable conformance #2858

Issue Type

Context (Environment, Version, Language)

Description

Input Data

Expected Behaviour / Output

Current Behaviour / Output

Steps to Reproduce

Possible Solution

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

[BUG]: Swift 6 Strict Sendable conformance #2858

Description

Issue Type

Context (Environment, Version, Language)

Description

Input Data

Expected Behaviour / Output

Current Behaviour / Output

Steps to Reproduce

Possible Solution

Activity

joelklabo commented on Feb 15, 2026

schani commented on Jul 6, 2026

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions