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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Missing `@JsonIgnoreProperties(ignoreUnknown = true)` on capability sub-records","articleBody":"### Bug description\n\nThe 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.\n\nThis 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.\n\n### Affected records\n\nIn [`McpSchema.java`](https://github.com/modelcontextprotocol/java-sdk/blob/0a8cb1e038559ba197c9fe60bec19f4bf054ed63/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java):\n\n| Record | Line | Has annotation? |\n|---|---|---|\n| `ClientCapabilities` | [383](https://github.com/modelcontextprotocol/java-sdk/blob/0a8cb1e038559ba197c9fe60bec19f4bf054ed63/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java#L383) | Yes |\n| `RootCapabilities` | [397](https://github.com/modelcontextprotocol/java-sdk/blob/0a8cb1e038559ba197c9fe60bec19f4bf054ed63/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java#L397) | Yes |\n| `Sampling` | [410](https://github.com/modelcontextprotocol/java-sdk/blob/0a8cb1e038559ba197c9fe60bec19f4bf054ed63/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java#L410) | **No** |\n| `Elicitation` | [437](https://github.com/modelcontextprotocol/java-sdk/blob/0a8cb1e038559ba197c9fe60bec19f4bf054ed63/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java#L437) | **No** |\n| `Elicitation.Form` | [443](https://github.com/modelcontextprotocol/java-sdk/blob/0a8cb1e038559ba197c9fe60bec19f4bf054ed63/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java#L443) | **No** |\n| `Elicitation.Url` | [450](https://github.com/modelcontextprotocol/java-sdk/blob/0a8cb1e038559ba197c9fe60bec19f4bf054ed63/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java#L450) | **No** |\n| `ServerCapabilities` | [535](https://github.com/modelcontextprotocol/java-sdk/blob/0a8cb1e038559ba197c9fe60bec19f4bf054ed63/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java#L535) | Yes |\n| `CompletionCapabilities` | [548](https://github.com/modelcontextprotocol/java-sdk/blob/0a8cb1e038559ba197c9fe60bec19f4bf054ed63/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java#L548) | **No** |\n| `LoggingCapabilities` | [555](https://github.com/modelcontextprotocol/java-sdk/blob/0a8cb1e038559ba197c9fe60bec19f4bf054ed63/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java#L555) | **No** |\n| `PromptCapabilities` | [565](https://github.com/modelcontextprotocol/java-sdk/blob/0a8cb1e038559ba197c9fe60bec19f4bf054ed63/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java#L565) | **No** |\n| `ResourceCapabilities` | [576](https://github.com/modelcontextprotocol/java-sdk/blob/0a8cb1e038559ba197c9fe60bec19f4bf054ed63/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java#L576) | **No** |\n| `ToolCapabilities` | [587](https://github.com/modelcontextprotocol/java-sdk/blob/0a8cb1e038559ba197c9fe60bec19f4bf054ed63/mcp-core/src/main/java/io/modelcontextprotocol/spec/McpSchema.java#L587) | **No** |\n\n### Spec reference\n\nThe spec schema explicitly states capabilities are not a closed set:\n\n\u003e \"Known capabilities are defined here, in this schema, but this is not a closed set: any client can define its own, additional capabilities.\"\n\n[`ClientCapabilities` in schema.json](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/eafd0068e92293feb12e99b650a0f37bac4267c1/schema/2025-06-18/schema.json#L302) — `additionalProperties: false` is never set on any capability object or sub-object. Several sub-capabilities (`sampling`, `elicitation`, `completions`, `logging`) explicitly set `additionalProperties: true`.\n\n### Expected behavior\n\nUnknown fields on capability sub-objects should be silently ignored, matching the behavior of the top-level `ClientCapabilities` and `ServerCapabilities` records.\n\n### Related\n\n- #734 — reports the same issue for `ToolCapabilities` specifically\n- #724 / #731 — the elicitation breakage that was fixed by adding fields, not by adding the annotation\n\n\u003csub\u003e[AI Disclaimer](https://gist.github.com/maxisbey/6123d132484e4c533eab519a2800693d)\u003c/sub\u003e","author":{"url":"https://github.com/maxisbey","@type":"Person","name":"maxisbey"},"datePublished":"2026-01-29T16:47:42.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":4},"url":"https://github.com/766/java-sdk/issues/766"}