The "invalid outputSchema: JSON Schema declares an unsupported dialect" error is a client-side schema validation failure: a strict JSON Schema 2020-12 validator (Claude Desktop, for example) refuses a tool whose `outputSchema` declares `"$schema": "http://json-schema.org/draft-07/schema#"`. `tools/list` succeeds; `tools/call` fails.
The cause is in `@modelcontextprotocol/sdk` (TypeScript, 1.30.1 and earlier): `McpServer` never passes a `target` when converting Zod schemas, and `mapMiniTarget()` falls back to `draft-7`, so every `inputSchema` and `outputSchema` is stamped draft-07.
No released SDK version fixes it yet — PR #2085 and PR #2653 are still unmerged, and issues #2721 and #2084 are open.
Workaround 1: remove `outputSchema` from the tool and keep returning `structuredContent` (valid per the MCP spec); validate the result with Zod yourself, because the SDK stops doing it.
Workaround 2: use `patch-package` to change the `mapMiniTarget()` fallback to `draft-2020-12` (Zod v4), or to strip the `$schema` line in the Zod v3 branch, until a fixed release ships.
Verify in one line: `npx @modelcontextprotocol/inspector --cli node dist/server.js --method tools/list | grep '\$schema'` shows which dialect your server emits.
MCP Error: Invalid outputSchema, JSON Schema Declares an Unsupported Dialect
If you build or run an MCP server on the MCP TypeScript SDK (@modelcontextprotocol/sdk; new to MCP? Start with A Gentle Introduction to MCP) and one of its tools declares an outputSchema, there is a good chance a client somewhere is quietly rejecting every call to it. The tool shows up fine in tools/list. The server logs look healthy. And then tools/call fails with this:
Tool 'perplexity_search' has an invalid outputSchema: JSON Schema declares an
unsupported dialect ("$schema": "http://json-schema.org/draft-07/schema#").
The default validator supports JSON Schema 2020-12 only.
Swap the tool name and it is the same story everywhere: Perplexity’s official MCP server hits it on all four of its tools, and — a bit awkwardly — so does the reference @modelcontextprotocol/server-filesystem running inside Claude Desktop. This is not one server’s bug, so there is no point hunting for it in yours.
The “unsupported dialect” error is a client-side schema validation failure: the client refuses to trust a tool’s outputSchema — the optional JSON Schema describing the tool’s structuredContent result — because its $schema names a JSON Schema dialect (draft-07) that the client’s validator does not support.
The server hands over a draft-07 outputSchema and tools/list looks healthy; the strict client only rejects it when tools/call runs.
In other words, nothing is wrong with the listing step or with your handler. The failure sits in the gap between the two calls, where a 2020-12-only client validator meets the draft-07 $schema label the SDK stamped on the outputSchema.
The cause, how to check which dialect your server emits, the SDK status and two workarounds follow below.
References
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References
About Elena
Elena, a PhD in Computer Science, simplifies AI concepts and helps you use machine learning.
Citation
Elena Daehnhardt. (2026) 'MCP Error: Invalid outputSchema, JSON Schema Declares an Unsupported Dialect', daehnhardt.com, 27 September 2026. Available at: https://daehnhardt.com/blog/2026/09/27/mcp-error-invalid-outputschema-json-schema-declares-an-unsupported-dialect/
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