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feat: add extensions attribute to ClientCapabilities and ServerCapabilities #1064

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feat: add extensions attribute to ClientCapabilities and ServerCapabilities #1064

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660de90
feat: add extensions to ClientCapabilities
Jul 17, 2026
77d6921
feat: add extensions to ServerCapabilities
Jul 17, 2026
39cdec7
refactor: drop per-entry extension() builder helper
Jul 17, 2026
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43 changes: 39 additions & 4 deletions mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java
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Original file line number Diff line number Diff line change
Expand Up @@ -577,14 +577,23 @@ public InitializeResult build() {
* @param roots Present if the client supports listing roots
* @param sampling Present if the client supports sampling from an LLM
* @param elicitation Present if the client supports elicitation from the server
* @param extensions Present if the client supports MCP extensions, keyed by extension
* identifier
*/
@JsonInclude(JsonInclude.Include.NON_ABSENT)
@JsonIgnoreProperties(ignoreUnknown = true)
public record ClientCapabilities( // @formatter:off
@JsonProperty("experimental") Map<String, Object> experimental,
@JsonProperty("roots") RootCapabilities roots,
@JsonProperty("sampling") Sampling sampling,
@JsonProperty("elicitation") Elicitation elicitation) { // @formatter:on
@JsonProperty("elicitation") Elicitation elicitation,
@JsonProperty("extensions") Map<String, Object> extensions) { // @formatter:on

// Keep the old constructor so existing callers still compile
public ClientCapabilities(Map<String, Object> experimental, RootCapabilities roots, Sampling sampling,
Elicitation elicitation) {
this(experimental, roots, sampling, elicitation, null);
}

/**
* Present if the client supports listing roots.
Expand Down Expand Up @@ -723,6 +732,8 @@ public static class Builder {

private Elicitation elicitation;

private Map<String, Object> extensions;

public Builder experimental(Map<String, Object> experimental) {
this.experimental = experimental;
return this;
Expand Down Expand Up @@ -765,8 +776,13 @@ public Builder elicitation(Elicitation elicitation) {
return this;
}

public Builder extensions(Map<String, Object> extensions) {
this.extensions = extensions;
return this;
}

public ClientCapabilities build() {
return new ClientCapabilities(experimental, roots, sampling, elicitation);
return new ClientCapabilities(experimental, roots, sampling, elicitation, extensions);
}

}
Expand All @@ -785,6 +801,8 @@ public ClientCapabilities build() {
* @param prompts Present if the server offers any prompt templates
* @param resources Present if the server offers any resources to read
* @param tools Present if the server offers any tools to call
* @param extensions Present if the server supports MCP extensions, keyed by extension
* identifier
*/
@JsonInclude(JsonInclude.Include.NON_ABSENT)
@JsonIgnoreProperties(ignoreUnknown = true)
Expand All @@ -794,7 +812,15 @@ public record ServerCapabilities( // @formatter:off
@JsonProperty("logging") LoggingCapabilities logging,
@JsonProperty("prompts") PromptCapabilities prompts,
@JsonProperty("resources") ResourceCapabilities resources,
@JsonProperty("tools") ToolCapabilities tools) { // @formatter:on
@JsonProperty("tools") ToolCapabilities tools,
@JsonProperty("extensions") Map<String, Object> extensions) { // @formatter:on

// Keep the old constructor so existing callers still compile
public ServerCapabilities(CompletionCapabilities completions, Map<String, Object> experimental,
LoggingCapabilities logging, PromptCapabilities prompts, ResourceCapabilities resources,
ToolCapabilities tools) {
this(completions, experimental, logging, prompts, resources, tools, null);
}

/**
* Present if the server supports argument autocompletion suggestions.
Expand Down Expand Up @@ -923,6 +949,7 @@ public Builder mutate() {
builder.prompts = this.prompts;
builder.resources = this.resources;
builder.tools = this.tools;
builder.extensions = this.extensions;
return builder;
}

Expand All @@ -944,6 +971,8 @@ public static class Builder {

private ToolCapabilities tools;

private Map<String, Object> extensions;

public Builder completions() {
this.completions = new CompletionCapabilities();
return this;
Expand Down Expand Up @@ -974,8 +1003,14 @@ public Builder tools(Boolean listChanged) {
return this;
}

public Builder extensions(Map<String, Object> extensions) {
this.extensions = extensions;
return this;
}

public ServerCapabilities build() {
return new ServerCapabilities(completions, experimental, logging, prompts, resources, tools);
return new ServerCapabilities(completions, experimental, logging, prompts, resources, tools,
extensions);
}

}
Expand Down
70 changes: 70 additions & 0 deletions mcp-test/src/test/java/io/modelcontextprotocol/spec/SchemaEvolutionTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@

import java.io.IOException;
import java.util.List;
import java.util.Map;

import io.modelcontextprotocol.json.McpJsonMapper;
import org.junit.jupiter.api.Test;
Expand Down Expand Up @@ -143,6 +144,75 @@ void serverCapabilitiesUnknownFieldsIgnored() throws IOException {
assertThat(caps.tools().listChanged()).isTrue();
}

@Test
void serverCapabilitiesWithoutExtensionsDeserializesAsNull() throws IOException {
String json = """
{"tools":{"listChanged":true}}
""";
McpSchema.ServerCapabilities caps = mapper.readValue(json, McpSchema.ServerCapabilities.class);
assertThat(caps.extensions()).isNull();
}

@Test
void serverCapabilitiesWithNullExtensionsOmitsFieldOnWire() throws IOException {
String json = mapper.writeValueAsString(McpSchema.ServerCapabilities.builder().tools(true).build());
assertThat(json).doesNotContain("extensions");
}

@Test
void serverCapabilitiesExtensionsRoundTrip() throws IOException {
Map<String, Object> extensions = Map.of("com.example/ext-with-settings", Map.of("maxDepth", 3),
"com.example/ext-without-settings", Map.of());
McpSchema.ServerCapabilities caps = McpSchema.ServerCapabilities.builder().extensions(extensions).build();

String json = mapper.writeValueAsString(caps);
assertThat(json).contains("\"com.example/ext-without-settings\":{}");

McpSchema.ServerCapabilities parsed = mapper.readValue(json, McpSchema.ServerCapabilities.class);
assertThat(parsed.extensions()).isEqualTo(extensions);
}

// -----------------------------------------------------------------------
// ClientCapabilities.extensions
// -----------------------------------------------------------------------

@Test
void clientCapabilitiesWithoutExtensionsDeserializesAsNull() throws IOException {
String json = """
{"roots":{"listChanged":true},"sampling":{}}
""";
McpSchema.ClientCapabilities caps = mapper.readValue(json, McpSchema.ClientCapabilities.class);
assertThat(caps.extensions()).isNull();
}

@Test
void clientCapabilitiesWithNullExtensionsOmitsFieldOnWire() throws IOException {
String json = mapper.writeValueAsString(McpSchema.ClientCapabilities.builder().sampling().build());
assertThat(json).doesNotContain("extensions");
}

@Test
void clientCapabilitiesWithExtensionsUnknownFieldsIgnored() throws IOException {
String json = """
{"extensions":{"io.modelcontextprotocol/tasks":{}},"futureCapability":{"a":1}}
""";
McpSchema.ClientCapabilities caps = mapper.readValue(json, McpSchema.ClientCapabilities.class);
assertThat(caps.extensions()).containsOnlyKeys("io.modelcontextprotocol/tasks");
}

@Test
void clientCapabilitiesExtensionsRoundTrip() throws IOException {
Map<String, Object> extensions = Map.of("com.example/ext-with-settings", Map.of("maxDepth", 3),
"com.example/ext-without-settings", Map.of());
McpSchema.ClientCapabilities caps = McpSchema.ClientCapabilities.builder().extensions(extensions).build();

String json = mapper.writeValueAsString(caps);
assertThat(json).contains("\"com.example/ext-without-settings\":{}");

McpSchema.ClientCapabilities parsed = mapper.readValue(json, McpSchema.ClientCapabilities.class);
assertThat(parsed.extensions()).isEqualTo(extensions);
}

// -----------------------------------------------------------------------
// JSONRPCError
// -----------------------------------------------------------------------
Expand Down