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

[maxisbey]

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:

Record	Line	Has annotation?
ClientCapabilities	383	Yes
RootCapabilities	397	Yes
Sampling	410	No
Elicitation	437	No
Elicitation.Form	443	No
Elicitation.Url	450	No
ServerCapabilities	535	Yes
CompletionCapabilities	548	No
LoggingCapabilities	555	No
PromptCapabilities	565	No
ResourceCapabilities	576	No
ToolCapabilities	587	No

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

• [问题] ToolCapabilities 要加上 @JsonIgnoreProperties(ignoreUnknown = true) 不然容易出错 #734 — reports the same issue for ToolCapabilities specifically
• InitializeRequest doesn't accept non-empty Elicitation, violating 2025-11-25 spec #724 / fix: Support form and url fields in Elicitation capability per 2025-11-25 spec #731 — the elicitation breakage that was fixed by adding fields, not by adding the annotation

_{AI Disclaimer}

[DevasiaThomas]

Thank you @maxisbey for raising this. My team and I are the ones's that experienced the real breakage 🙂. Appreciate the group's attention to this. Thanks!

We got folks using all kinds of clients that are on the latest protocol spec, but some Java based MCP servers ( a few owned by us) are on older versions.