Add missing usage_update type and forward-compatible SessionUpdate deserialization

[Copilot]

SDK crashed on usage_update notifications. Type exists in ACP unstable schema v0.9.1 but wasn't implemented. Also adds forward compatibility for unknown SessionUpdate types.

Changes

Missing schema types:

• SessionUpdate.UsageUpdate - tracks token usage (used: Long, size: Long, cost: Cost?)
• Cost - cost representation (amount: Double, currency: String)

Forward compatibility:

• SessionUpdate.UnknownSessionUpdate - captures unknown types with full JSON preservation
• Custom SessionUpdateSerializer - explicitly handles 11 known types, falls back to UnknownSessionUpdate

Supporting changes:

• Updated API dump for binary compatibility tracking
• Fixed exhaustive when in sample code
• Version bump to 0.15.4

Example

// Before: crash
val json = """{"sessionUpdate":"usage_update","used":0,"size":200000,"cost":{"amount":0,"currency":"USD"}}"""
ACPJson.decodeFromString(SessionUpdate.serializer(), json)  // ❌ JsonDecodingException

// After: graceful handling
val update = ACPJson.decodeFromString(SessionUpdate.serializer(), json)
when (update) {
    is SessionUpdate.UsageUpdate -> handleUsage(update)
    is SessionUpdate.UnknownSessionUpdate -> log("Unknown: ${update.sessionUpdateType}")
    // ... other types
}

Pattern mirrors existing AuthMethod.UnknownAuthMethod approach.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Deserialization fails on usage_update</issue_title>
<issue_description>I have following error working with opencode:
2026-02-20 11:35:40,794 [7698226] INFO - #c.i.m.l.a.a.c.s.AcpModelManager - Agent acp.registry.opencode initial model: Amazon Bedrock/Claude Sonnet 4.6 (US) (high) (from saved settings)

2026-02-20 11:35:40,794 [7698226] INFO - #c.i.m.l.a.a.c.AcpSessionLifecycleManager - [OpenCode] Reusing existing session for chat cd9e1168-6855-4b54-8020-4c2b29d64192

2026-02-20 11:35:41,036 [7698468] SEVERE - com.agentclientprotocol.protocol.Protocol - Error handling notification MethodName(name=session/update)

kotlinx.serialization.json.internal.JsonDecodingException: Serializer for subclass 'usage_update' is not found in the polymorphic scope of 'SessionUpdate'.

Check if class with serial name 'usage_update' exists and serializer is registered in a corresponding SerializersModule.

To be registered automatically, class 'usage_update' has to be '@serializable', and the base class 'SessionUpdate' has to be sealed and '@serializable'.

JSON input: {"sessionUpdate":"usage_update","used":0,"size":200000,"cost":{"amount":0,"currency":"USD"}}

at kotlinx.serialization.json.internal.JsonExceptionsKt.JsonDecodingException(JsonExceptions.kt:24)

at kotlinx.serialization.json.internal.JsonExceptionsKt.JsonDecodingException(JsonExceptions.kt:32)

at kotlinx.serialization.json.internal.AbstractJsonTreeDecoder.decodeSerializableValue(TreeJsonDecoder.kt:370)

at kotlinx.serialization.internal.TaggedDecoder.decodeSerializableValue(Tagged.kt:207)

at kotlinx.serialization.internal.TaggedDecoder.decodeSerializableElement$lambda$1(Tagged.kt:279)

at kotlinx.serialization.internal.TaggedDecoder.tagBlock(Tagged.kt:294)

at kotlinx.serialization.internal.TaggedDecoder.decodeSerializableElement(Tagged.kt:279)

at com.agentclientprotocol.model.SessionNotification$$serializer.deserialize(Requests.kt:805)

at com.agentclientprotocol.model.SessionNotification$$serializer.deserialize(Requests.kt:805)

at kotlinx.serialization.json.internal.AbstractJsonTreeDecoder.decodeSerializableValue(TreeJsonDecoder.kt:351)

at kotlinx.serialization.json.internal.TreeJsonDecoderKt.readJson(TreeJsonDecoder.kt:25)

at kotlinx.serialization.json.Json.decodeFromJsonElement(Json.kt:170)

at com.agentclientprotocol.protocol.Protocol_extensionsKt$setNotificationHandler$1.invokeSuspend(Protocol.extensions.kt:101)

at com.agentclientprotocol.protocol.Protocol_extensionsKt$setNotificationHandler$1.invoke(Protocol.extensions.kt)

at com.agentclientprotocol.protocol.Protocol_extensionsKt$setNotificationHandler$1.invoke(Protocol.extensions.kt)

at com.agentclientprotocol.protocol.Protocol$setNotificationHandlerRaw$1$1$1.invokeSuspend(Protocol.kt:293)

at com.agentclientprotocol.protocol.Protocol$setNotificationHandlerRaw$1$1$1.invoke(Protocol.kt)

at com.agentclientprotocol.protocol.Protocol$setNotificationHandlerRaw$1$1$1.invoke(Protocol.kt)

at kotlinx.coroutines.intrinsics.UndispatchedKt.startUndispatchedOrReturn(Undispatched.kt:44)

at kotlinx.coroutines.BuildersKt__Builders_commonKt.withContext(Builders.common.kt:157)

at kotlinx.coroutines.BuildersKt.withContext(Unknown Source)

at com.agentclientprotocol.protocol.Protocol$setNotificationHandlerRaw$1$1.invokeSuspend(Protocol.kt:292)

at com.agentclientprotocol.protocol.Protocol$setNotificationHandlerRaw$1$1.invoke(Protocol.kt)

at com.agentclientprotocol.protocol.Protocol$setNotificationHandlerRaw$1$1.invoke(Protocol.kt)

at com.agentclientprotocol.protocol.Protocol.handleNotification(Protocol.kt:441)

at com.agentclientprotocol.protocol.Protocol.handleIncomingMessage(Protocol.kt:353)

at com.agentclientprotocol.protocol.Protocol.access$handleIncomingMessage(Protocol.kt:122)

at com.agentclientprotocol.protocol.Protocol$start$2.invokeSuspend(Protocol.kt:172)

at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:34)

at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:100)

at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:610)

at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runDefaultDispatcherTask(CoroutineScheduler.kt:1194)

at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:906)

at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:775)

at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:762)

2026-02-20 11:35:41,037 [7698469] INFO - #c.i.m.l.c.n.ChatNotificationService - Send notification for agent ChatSessionAgentId acp.registry.opencode: null -> TaskFinished

2026-02-20 11:35:42,474 [7699906] INFO - #c.i.w.i.i.j.s.JpsGlobalModelSynchronizerImpl - Saving global entities com.intellij.platform.workspace.jps.entities.SdkEntity to files

2026-02-20 11:35:42,480 [7699912] INFO - #c.i.w.i.i.j.s.JpsGlobalModelSynchronize...

• Fixes Deserialization fails on usage_update #67

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[nerzhulart]

LGTM

[nerzhulart]

@copilot отдуши братан

[nerzhulart]

@copilot promote minor version

[Copilot]

@copilot promote minor version

Version promoted from 0.15.3 to 0.16.0 in commit 4250273.