fix: throw error when plain JSON Schema is passed as inputSchema to server.tool()#1598
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February 28, 2026 15:50
…erver.tool()
Previously, passing a plain JSON Schema object (e.g. { type: 'object', properties: {...} })
as the inputSchema parameter to server.tool() would silently misinterpret it as
ToolAnnotations, causing the schema to be dropped entirely. Tools would register
with empty parameter schemas ({ type: 'object', properties: {} }), leading to
silent data loss at runtime.
This adds a looksLikeJsonSchema() check that detects objects with JSON Schema
characteristic fields (type, properties, $schema, etc.) and throws a descriptive
error instructing users to use Zod schemas instead.
Fixes modelcontextprotocol#1585
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Summary
server.tool()silently dropsinputSchemawhen a plain JSON Schema object is passed instead of a Zod schema. The object is misinterpreted asToolAnnotations, and the tool registers with an empty parameter schema — a silent data-loss bug.Root Cause
In
mcp.ts, the tool registration overload resolution checksisZodRawShapeCompat(firstArg). When this returnsfalse(as it does for plain JSON Schema objects, which lack.parse()/.safeParse()methods), the fallbackelse ifbranch unconditionally treats the object asToolAnnotations:Fix
Added a
looksLikeJsonSchema()helper that detects objects with JSON Schema characteristic fields (type,properties,$schema,items,allOf,anyOf,oneOf). When detected, a descriptive error is thrown:Tests
Added 3 new tests:
type/properties/required$schemapropertyToolAnnotations(readOnlyHint,destructiveHint)Verification
2 consecutive clean test runs: 1553/1553 tests passed, 0 failures.
Fixes #1585