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

Open

#766 opened on Jan 29, 2026 4 comments 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

Assessment

Difficulty: 2/5
Estimated time: 1-3 hours
Newbie friendliness: 55/100

Issue type: Bug
Clarity: Clearly specified
Activity status: Stale
Tech stack: java
Domain: api

Research direction

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.

Written by the indexing model from the issue text.

Description

bug P1

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.

#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}

Dominant language: Java
Stars: 3.7k
Forks: 1.1k
Avg merge: 1d 15h
Merged PRs (30d): 9

Contributor guide

Open the contributing guide

First steps

Read the whole issue, then the project's contributing guide.
Comment on the issue to say you are picking it up — it saves two people doing the same work.
Fork the repository and make your change on a branch.
Open a pull request that references the issue number.

Similar issues

[BUG] Unified agent on Bedrock Converse with no tools is unusable: an empty `toolConfig` is emitted and Bedrock rejects the request Open

bug untriaged

Difficulty 2/5 1-3 hours Newbie friendliness 84/100

opensearch-project/ml-commons#5094 · Opened Sep 22, 2026
[Bug report] iceberg rest server load_table with refs fails precondition check "Cannot set metadata location with changes to table metadata: %s changes" for snapshots with statistics Open

bug

Difficulty 2/5 1-3 hours Newbie friendliness 85/100

apache/gravitino#13452 · Opened Sep 22, 2026
Support Test Project Creation for New Projects Open

emitter:client:csharp feature

Difficulty 2/5 1-3 hours Newbie friendliness 72/100

microsoft/typespec#12027 · 1 comment · 1 reaction · Opened Sep 22, 2026
Job worker stops polling silently when a poll throws an Error Open

affects/8.10 affects/8.9 component/clients kind/bug likelihood/mid severity/mid

Difficulty 2/5 1-3 hours Newbie friendliness 78/100

camunda/camunda#63825 · Opened Sep 22, 2026
Two open-case totals on one screen: the Programs tile says 15,858 and the nav badge says 15,868 Open

bug frontend maui-pilot

Difficulty 2/5 1-3 hours Newbie friendliness 72/100

Taleef7/workwell#651 · 1 comment · Opened Sep 22, 2026

More Java issues

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

Assessment

Research direction

Description

Bug description

Affected records

Spec reference

Expected behavior

Related

Contributor guide

First steps

More from modelcontextprotocol/java-sdk

Similar issues

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

Assessment

Research direction

Description

Bug description

Affected records

Spec reference

Expected behavior

Related

Contributor guide

First steps

More from modelcontextprotocol/java-sdk

Similar issues

Get new issues in your inbox