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MCP server fails to initialize with VS Code Insiders: Unrecognized field "form" in elicitation capabilities #8015
Description
Describe the bug
When attempting to connect to the Metals MCP server from VS Code Insiders, the initialization fails with a deserialization error related to the elicitation.form capability field.
Error message
2025-12-08 10:48:35.521 [error] Error: MPC -32603: Unrecognized field "form" (class io.modelcontextprotocol.spec.McpSchema$ClientCapabilities$Elicitation), not marked as ignorable (0 known properties: ])
at [Source: UNKNOWN; byte offset: #UNKNOWN] (through reference chain: io.modelcontextprotocol.spec.McpSchema$InitializeRequest["capabilities"]->io.modelcontextprotocol.spec.McpSchema$ClientCapabilities["elicitation"]->io.modelcontextprotocol.spec.McpSchema$ClientCapabilities$Elicitation["form"])
MCP Configuration
{
"mcpServers": {
"activity-log-metals": {
"type": "sse",
"url": "http://localhost:43305/sse"
}
}
}Environment
- VS Code Version: VS Code Insiders (December 2025)
- OS: Linux
- Metals Version: 1.6.4
Analysis
The error suggests that the MCP Java SDK used by Metals doesn't recognise the form field within the elicitation capabilities that VS Code Insiders is sending during the InitializeRequest. This appears to be a version incompatibility between VS Code's MCP client implementation and the MCP SDK version used by Metals.
Expected behaviour
The Metals MCP server should successfully initialise when connecting from VS Code Insiders, either by supporting the newer elicitation.form capability or by gracefully ignoring unknown fields during deserialization.
Potential fix
Update the Model Context Protocol Java SDK dependency to a version that includes support for the elicitation capability schema, or configure Jackson to ignore unknown properties during deserialization.
Activity
tgodzik commentedon Dec 8, 2025
Thanks for reporting! I am doing an update in #8016, that should help
susuro commentedon Dec 27, 2025
After changing Metals version to "1.6.4+56-8bf8f1e0-SNAPSHOT"
I'm still getting this error:
tgodzik commentedon Dec 29, 2025
Let me double check what is going on
tgodzik commentedon Dec 29, 2025
Looks like it's still an issue in modelcontextprotocol/java-sdk#724
skylayer commentedon Jan 11, 2026
Hi @tgodzik,
Just a heads up that the upstream
modelcontextprotocol/java-sdkv0.17.1 was released recently.Release: https://github.com/modelcontextprotocol/java-sdk/releases/tag/v0.17.1
Could you please take a look? Hopefully, this unblocks the fix for Metals. Thanks!
tgodzik commentedon Jan 12, 2026
We are already on that version, so we should be good. I am planning afull release soon