Missing `@JsonIgnoreProperties(ignoreUnknown = true)` on capability sub-records

Bug description

The top-level ClientCapabilities and ServerCapabilities records have @JsonIgnoreProperties(ignoreUnknown = true), but their nested sub-records do not. Since the ObjectMapper is constructed with default settings (FAIL_ON_UNKNOWN_PROPERTIES = true), any unknown field on a capability sub-object causes a deserialization failure.

This already caused a real breakage when the elicitation capability gained form and url fields (#724, fixed in #731), but the systemic issue remains — the next spec addition to any capability sub-object will break older SDK versions again.

Affected records

In McpSchema.java:

Spec reference

The spec schema explicitly states capabilities are not a closed set:

"Known capabilities are defined here, in this schema, but this is not a closed set: any client can define its own, additional capabilities."

ClientCapabilities in schema.json — additionalProperties: false is never set on any capability object or sub-object. Several sub-capabilities (sampling, elicitation, completions, logging) explicitly set additionalProperties: true.

Expected behavior

Unknown fields on capability sub-objects should be silently ignored, matching the behavior of the top-level ClientCapabilities and ServerCapabilities records.

Related

• #734 — reports the same issue for ToolCapabilities specifically
• #724 / #731 — the elicitation breakage that was fixed by adding fields, not by adding the annotation

_{AI Disclaimer}

Start in mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java and review the listed capability records, comparing them with ClientCapabilities and ServerCapabilities. Done means unknown fields on every affected capability sub-record are ignored during deserialization, including nested elicitation records.