Keep kotlinx.serialization (or Moshi / Gson / Jackson). Add one annotation on the DTO that hurts — same Ktor, Retrofit, or Spring stack.

Ghost is not a rewrite. It is the fast, low-allocation path for the models you opt in — and a softer landing when the backend ships messy JSON.

Quick Start — first DTO in minutes → · Try it in the browser → · Maven Central · Roadmap

Strict JSON fails the whole request. Ghost can keep going where you opt in:

{ "id": "u1", "name": "Ada", "age": { "years": 36 } }

@Serializable
@GhostSerialization
data class User(
    val id: String,
    val name: String,
    @GhostResilient val age: Int = 0,  // object instead of number → 0, parse continues
)

Unknown sealed variants → @GhostFallback. Opaque blobs → RawJson. Decode errors include a JSONPath (e.g. $.user.age) and, when useful, a fix hint in GhostJsonException / GhostYamlException (JSON, Proto3 JSON, YAML cursor). Details → Advanced Features

Ghost generates only for @GhostSerialization — not for every @Serializable in the module.

@Serializable                 // keep — KotlinX call sites still work
@GhostSerialization           // add — Ghost codegen for this class only
data class User(
    @SerialName("user_id")    // keep — Ghost uses this as the wire name
    val id: Long,
    val name: String,
)

Ghost.deserialize<User>(responseBytes)     // Ghost (bytes / adapters)
// Json.decodeFromString<User>(...)        // still fine on KotlinX

• @GhostName — optional if @SerialName is already set (GhostName wins only if both differ).
• @GhostResilient / @GhostFallback / RawJson — Ghost-only; omit if you don’t need them.
• Nested types on the Ghost path need @GhostSerialization too.

Guides: Quick Start · Ktor · Android / Retrofit · Spring

Also: Android · iOS · JVM · Wasm · YAML · Proto3 JSON · Kotlin 2.4.0 / KSP 2.3.10 / Ktor 3.5.x → Modules

On hot models Ghost is routinely several× faster and far leaner than KotlinX / Moshi. Most APIs are small — use it on the hotspot, not everywhere.

Twitter macro (631 KB) decode:

Benchmarks · HTTP Arena · Playground Speed Test

• Reader: 96.06% of yaml-test-suite's conformance cases decode exactly right (268/279) — every gap tracked by case ID and reason in-repo, none silently skipped.
• Writer: 100% round-trip (decode → encode → decode returns the same tree) across every case the reader accepts; 96.26% of the re-encoded output is also accepted by an independent second parser (kaml) as a cross-check, not just by Ghost's own reader.
• Fuzzing: every JSON/YAML reader and writer has coverage-guided fuzz testing (Jazzer) — the accumulated crash corpus replays on every CI run, no open crashes.

Run it yourself: ./gradlew :ghost-serialization:yamlComplianceMatrix · Details → YAML Conformance

Quick Start · Installation · Android · KMP / Ktor · iOS · Spring · YAML · Proto3 JSON · Advanced · Architecture · Contributing · Support & Versioning

Apache 2.0