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feat(core): add extensions field to ClientCapabilities and ServerCapabilities #678

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21 changes: 13 additions & 8 deletions kotlin-sdk-core/api/kotlin-sdk-core.api
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Original file line number Diff line number Diff line change
Expand Up @@ -606,17 +606,19 @@ public final class io/modelcontextprotocol/kotlin/sdk/types/CancelledNotificatio
public final class io/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities {
public static final field Companion Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities$Companion;
public fun <init> ()V
public fun <init> (Lkotlinx/serialization/json/JsonObject;Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities$Roots;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;)V
public synthetic fun <init> (Lkotlinx/serialization/json/JsonObject;Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities$Roots;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
public fun <init> (Lkotlinx/serialization/json/JsonObject;Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities$Roots;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Ljava/util/Map;)V
public synthetic fun <init> (Lkotlinx/serialization/json/JsonObject;Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities$Roots;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Ljava/util/Map;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
public final fun component1 ()Lkotlinx/serialization/json/JsonObject;
public final fun component2 ()Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities$Roots;
public final fun component3 ()Lkotlinx/serialization/json/JsonObject;
public final fun component4 ()Lkotlinx/serialization/json/JsonObject;
public final fun copy (Lkotlinx/serialization/json/JsonObject;Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities$Roots;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;)Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities;
public static synthetic fun copy$default (Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities;Lkotlinx/serialization/json/JsonObject;Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities$Roots;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;ILjava/lang/Object;)Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities;
public final fun component5 ()Ljava/util/Map;
public final fun copy (Lkotlinx/serialization/json/JsonObject;Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities$Roots;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Ljava/util/Map;)Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities;
public static synthetic fun copy$default (Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities;Lkotlinx/serialization/json/JsonObject;Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities$Roots;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Ljava/util/Map;ILjava/lang/Object;)Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities;
public fun equals (Ljava/lang/Object;)Z
public final fun getElicitation ()Lkotlinx/serialization/json/JsonObject;
public final fun getExperimental ()Lkotlinx/serialization/json/JsonObject;
public final fun getExtensions ()Ljava/util/Map;
public final fun getRoots ()Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilities$Roots;
public final fun getSampling ()Lkotlinx/serialization/json/JsonObject;
public fun hashCode ()I
Expand Down Expand Up @@ -676,6 +678,7 @@ public final class io/modelcontextprotocol/kotlin/sdk/types/ClientCapabilitiesBu
public final fun elicitation (Lkotlinx/serialization/json/JsonObject;)V
public final fun experimental (Lkotlin/jvm/functions/Function1;)V
public final fun experimental (Lkotlinx/serialization/json/JsonObject;)V
public final fun extensions (Ljava/util/Map;)V
public final fun roots (Ljava/lang/Boolean;)V
public static synthetic fun roots$default (Lio/modelcontextprotocol/kotlin/sdk/types/ClientCapabilitiesBuilder;Ljava/lang/Boolean;ILjava/lang/Object;)V
public final fun sampling (Lkotlin/jvm/functions/Function1;)V
Expand Down Expand Up @@ -4245,19 +4248,21 @@ public final class io/modelcontextprotocol/kotlin/sdk/types/Sampling_dslKt {
public final class io/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities {
public static final field Companion Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Companion;
public fun <init> ()V
public fun <init> (Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Tools;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Resources;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Prompts;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;)V
public synthetic fun <init> (Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Tools;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Resources;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Prompts;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
public fun <init> (Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Tools;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Resources;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Prompts;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Ljava/util/Map;)V
public synthetic fun <init> (Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Tools;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Resources;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Prompts;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Ljava/util/Map;ILkotlin/jvm/internal/DefaultConstructorMarker;)V
public final fun component1 ()Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Tools;
public final fun component2 ()Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Resources;
public final fun component3 ()Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Prompts;
public final fun component4 ()Lkotlinx/serialization/json/JsonObject;
public final fun component5 ()Lkotlinx/serialization/json/JsonObject;
public final fun component6 ()Lkotlinx/serialization/json/JsonObject;
public final fun copy (Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Tools;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Resources;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Prompts;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;)Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities;
public static synthetic fun copy$default (Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Tools;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Resources;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Prompts;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;ILjava/lang/Object;)Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities;
public final fun component7 ()Ljava/util/Map;
public final fun copy (Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Tools;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Resources;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Prompts;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Ljava/util/Map;)Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities;
public static synthetic fun copy$default (Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Tools;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Resources;Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Prompts;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Lkotlinx/serialization/json/JsonObject;Ljava/util/Map;ILjava/lang/Object;)Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities;
public fun equals (Ljava/lang/Object;)Z
public final fun getCompletions ()Lkotlinx/serialization/json/JsonObject;
public final fun getExperimental ()Lkotlinx/serialization/json/JsonObject;
public final fun getExtensions ()Ljava/util/Map;
public final fun getLogging ()Lkotlinx/serialization/json/JsonObject;
public final fun getPrompts ()Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Prompts;
public final fun getResources ()Lio/modelcontextprotocol/kotlin/sdk/types/ServerCapabilities$Resources;
Expand Down
26 changes: 26 additions & 0 deletions ...k-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/capabilities.dsl.kt
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Original file line number Diff line number Diff line change
Expand Up @@ -39,6 +39,7 @@ public class ClientCapabilitiesBuilder @PublishedApi internal constructor() {
private var sampling: JsonObject? = null
private var roots: ClientCapabilities.Roots? = null
private var elicitation: JsonObject? = null
private var extensions: Map<String, JsonObject>? = null
private var experimental: JsonObject? = null

/**
Expand Down Expand Up @@ -132,6 +133,30 @@ public class ClientCapabilitiesBuilder @PublishedApi internal constructor() {
*/
public fun elicitation(block: JsonObjectBuilder.() -> Unit): Unit = elicitation(buildJsonObject(block))

/**
* Defines extensions that the client supports.
*
* Extension identifiers use the format `{vendor-prefix}/{extension-name}`,
* e.g., `"io.modelcontextprotocol/ui"`. Each value is an extension-specific
* settings object; an empty [JsonObject] indicates no settings.
*
* Example:
* ```kotlin
* capabilities {
* extensions(mapOf(
* "io.modelcontextprotocol/ui" to buildJsonObject {
* put("mimeTypes", JsonArray(listOf(JsonPrimitive("text/html"))))
* }
* ))
* }
* ```
*
* @param value The map of extension identifiers to their settings
*/
public fun extensions(value: Map<String, JsonObject>) {
this.extensions = value
}

/**
* Defines experimental, non-standard capabilities that the client supports.
*
Expand Down Expand Up @@ -177,6 +202,7 @@ public class ClientCapabilitiesBuilder @PublishedApi internal constructor() {
sampling = sampling,
roots = roots,
elicitation = elicitation,
extensions = extensions,
experimental = experimental,
)
}
8 changes: 8 additions & 0 deletions ...n-sdk-core/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/types/capabilities.kt
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Expand Up @@ -45,13 +45,17 @@ public data class Implementation(
* @property elicitation Present if the client supports elicitation from the server.
* @property experimental Experimental, non-standard capabilities that the client supports.
* Keys are capability names, values are capability-specific configuration objects.
* @property extensions Optional extensions that the client supports.
* Keys are extension identifiers (e.g., `"io.modelcontextprotocol/ui"`),
* values are extension-specific settings objects.
*/
@Serializable
public data class ClientCapabilities(
public val sampling: JsonObject? = null,
public val roots: Roots? = null,
public val elicitation: JsonObject? = null,
public val experimental: JsonObject? = null,
public val extensions: Map<String, JsonObject>? = null,
) {
Comment on lines 55 to 59
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Copilot AI Apr 7, 2026

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Adding the new extensions parameter to ClientCapabilities changes the primary constructor/signature and Kotlin default-args synthetic constructor, which is ABI-breaking for already-compiled Kotlin/Java consumers (potential NoSuchMethodError). This should be called out as a breaking change in the PR/release notes, or mitigated by preserving the old constructor signature (e.g., a deprecated secondary constructor delegating to the new one with extensions = null).

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@eritscher eritscher Apr 7, 2026 •
edited
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@kpavlov not sure if this is a false positive. It doesn't look anything has been done to preserve constructor signatures in the past. I can mention this in the PR description.

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@kpavlov kpavlov Apr 7, 2026

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true


/**
Expand Down Expand Up @@ -93,6 +97,9 @@ public data class ClientCapabilities(
* @property completions Present if the server supports argument autocompletion suggestions.
* Keys are capability names, values are capability-specific configuration objects.
* @property experimental Experimental, non-standard capabilities that the server supports.
* @property extensions Optional extensions that the server supports.
* Keys are extension identifiers (e.g., `"io.modelcontextprotocol/ui"`),
* values are extension-specific settings objects.
*/
@Serializable
public data class ServerCapabilities(
Expand All @@ -102,6 +109,7 @@ public data class ServerCapabilities(
val logging: JsonObject? = null,
val completions: JsonObject? = null,
val experimental: JsonObject? = null,
val extensions: Map<String, JsonObject>? = null,
) {

/**
Expand Down
97 changes: 97 additions & 0 deletions ...k-core/src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/types/CapabilitiesTest.kt
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Expand Up @@ -182,6 +182,53 @@ class CapabilitiesTest {
)
}

@Test
fun `should serialize ClientCapabilities with extensions`() {
val extensions = mapOf(
"io.modelcontextprotocol/ui" to buildJsonObject {
put("mimeTypes", buildJsonObject { put("text/html", true) })
},
)
val capabilities = ClientCapabilities(
extensions = extensions,
)
verifySerialization(
capabilities,
McpJson,
"""
{
"extensions": {
"io.modelcontextprotocol/ui": {
"mimeTypes": {
"text/html": true
}
}
}
}
""".trimIndent(),
)
}

@Test
fun `should serialize ClientCapabilities with empty extensions settings`() {
val capabilities = ClientCapabilities(
extensions = mapOf(
"io.modelcontextprotocol/ui" to EmptyJsonObject,
),
)
verifySerialization(
capabilities,
McpJson,
"""
{
"extensions": {
"io.modelcontextprotocol/ui": {}
}
}
""".trimIndent(),
)
}

@Test
fun `should serialize ClientCapabilities with experimental`() {
val experimental = buildJsonObject {
Expand Down Expand Up @@ -215,11 +262,15 @@ class CapabilitiesTest {
val experimental = buildJsonObject {
put("feature1", buildJsonObject { put("enabled", true) })
}
val extensions = mapOf(
"io.modelcontextprotocol/ui" to EmptyJsonObject,
)
val capabilities = ClientCapabilities(
sampling = ClientCapabilities.sampling,
roots = ClientCapabilities.Roots(listChanged = true),
elicitation = ClientCapabilities.elicitation,
experimental = experimental,
extensions = extensions,
)
verifySerialization(
capabilities,
Expand All @@ -235,6 +286,9 @@ class CapabilitiesTest {
"feature1": {
"enabled": true
}
},
"extensions": {
"io.modelcontextprotocol/ui": {}
}
}
""".trimIndent(),
Expand Down Expand Up @@ -270,6 +324,7 @@ class CapabilitiesTest {
assertNull(capabilities.roots)
assertNull(capabilities.elicitation)
assertNull(capabilities.experimental)
assertNull(capabilities.extensions)
}

@Test
Expand All @@ -278,6 +333,11 @@ class CapabilitiesTest {
sampling = ClientCapabilities.sampling,
roots = ClientCapabilities.Roots(listChanged = false),
elicitation = ClientCapabilities.elicitation,
extensions = mapOf(
"io.modelcontextprotocol/ui" to buildJsonObject {
put("mimeTypes", buildJsonObject { put("text/html", true) })
},
),
)

verifySerializationRoundTrip(original, McpJson)
Expand Down Expand Up @@ -466,6 +526,32 @@ class CapabilitiesTest {
)
}

@Test
fun `should serialize ServerCapabilities with extensions`() {
val capabilities = ServerCapabilities(
extensions = mapOf(
"io.modelcontextprotocol/ui" to EmptyJsonObject,
"com.example/custom" to buildJsonObject {
put("setting", "value")
},
),
)
verifySerialization(
capabilities,
McpJson,
"""
{
"extensions": {
"io.modelcontextprotocol/ui": {},
"com.example/custom": {
"setting": "value"
}
}
}
""".trimIndent(),
)
}

@Test
fun `should serialize ServerCapabilities with experimental`() {
val experimental = buildJsonObject {
Expand Down Expand Up @@ -499,6 +585,9 @@ class CapabilitiesTest {
val experimental = buildJsonObject {
put("feature1", buildJsonObject { put("enabled", true) })
}
val extensions = mapOf(
"io.modelcontextprotocol/ui" to EmptyJsonObject,
)
val capabilities = ServerCapabilities(
tools = ServerCapabilities.Tools(listChanged = true),
resources = ServerCapabilities.Resources(
Expand All @@ -509,6 +598,7 @@ class CapabilitiesTest {
logging = ServerCapabilities.Logging,
completions = ServerCapabilities.Completions,
experimental = experimental,
extensions = extensions,
)
verifySerialization(
capabilities,
Expand All @@ -531,6 +621,9 @@ class CapabilitiesTest {
"feature1": {
"enabled": true
}
},
"extensions": {
"io.modelcontextprotocol/ui": {}
}
}
""".trimIndent(),
Expand Down Expand Up @@ -578,6 +671,7 @@ class CapabilitiesTest {
assertNull(capabilities.logging)
assertNull(capabilities.completions)
assertNull(capabilities.experimental)
assertNull(capabilities.extensions)
}

@Test
Expand All @@ -590,6 +684,9 @@ class CapabilitiesTest {
),
prompts = ServerCapabilities.Prompts(listChanged = true),
logging = ServerCapabilities.Logging,
extensions = mapOf(
"io.modelcontextprotocol/ui" to EmptyJsonObject,
),
)

verifySerializationRoundTrip(original, McpJson)
Expand Down
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