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Missing @JsonIgnoreProperties(ignoreUnknown = true) on capability sub-records #766
Description
Activity
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.
@maxisbey I am thinking of making the ObjectMapper not fail on unknown properties. So this would apply globally. Is this okay @chemicL @tzolov ? @rameshreddy-adutla 's fix applies annotation on each field that is affected, but is prone to failure to future fields with missed annotations.
That's what we do in the Python SDK (and is also done in the Typescript SDK), we just mark all extra fields as ignored so it won't error
- added a commit that references this issue
on May 14, 2026
Bug description
The top-level
ClientCapabilitiesandServerCapabilitiesrecords have@JsonIgnoreProperties(ignoreUnknown = true), but their nested sub-records do not. Since theObjectMapperis 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
formandurlfields (#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:ClientCapabilitiesRootCapabilitiesSamplingElicitationElicitation.FormElicitation.UrlServerCapabilitiesCompletionCapabilitiesLoggingCapabilitiesPromptCapabilitiesResourceCapabilitiesToolCapabilitiesSpec reference
The spec schema explicitly states capabilities are not a closed set:
ClientCapabilitiesin schema.json —additionalProperties: falseis never set on any capability object or sub-object. Several sub-capabilities (sampling,elicitation,completions,logging) explicitly setadditionalProperties: true.Expected behavior
Unknown fields on capability sub-objects should be silently ignored, matching the behavior of the top-level
ClientCapabilitiesandServerCapabilitiesrecords.Related
ToolCapabilitiesspecificallyInitializeRequestdoesn'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 annotationAI Disclaimer