[v1.x] fix(server): emit JSON Schema 2020-12 in tools/list (SEP-1613)#2085
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[v1.x] fix(server): emit JSON Schema 2020-12 in tools/list (SEP-1613)#2085rsalus wants to merge 2 commits into
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McpServer's tools/list handler called toJsonSchemaCompat() without the target option, so mapMiniTarget(undefined) fell back to 'draft-7' and every inputSchema/outputSchema advertised draft-07, contradicting SEP-1613 / MCP 2025-11-25 spec. Pass target: 'draft-2020-12' at both call sites. For the Zod v4 branch, this surfaces native 2020-12 output (including prefixItems for tuples). For the Zod v3 branch — which uses zod-to-json-schema, a library with no 2020-12 target — overlay $schema in toJsonSchemaCompat when 2020-12 is requested; the emitted keyword subset (type, properties, required, additionalProperties, oneOf/anyOf/allOf, enum, const) is unchanged between draft-07 and 2020-12. Tests cover inputSchema and outputSchema $schema across both Zod versions, plus Zod v4 tuples emitting prefixItems. Fixes modelcontextprotocol#2084
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Wrap long line in .changeset/fix-sep-1613-emit-2020-12.md to satisfy the repo's prettier config. No semantic change.
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Fixes #2084.
Summary
McpServer'stools/listhandler callstoJsonSchemaCompat()without atarget, somapMiniTarget(undefined)returns'draft-7'and everyinputSchema/outputSchemaadvertises draft-07 instead of the 2020-12 dialect SEP-1613 requires.The
target: 'draft-2020-12'plumbing was added in #1135, but themcp.tscall sites weren't updated to use it. This PR does that, plus an extra fix for the Zod v3 path that the 2-liner doesn't reach.What changed
src/server/mcp.ts— passtarget: 'draft-2020-12'at both call sites. That's the 2-line patch from the issue. For Zod v4 it's the whole fix:z4mini.toJSONSchemathen emits the right$schemaand usesprefixItemsfor tuples.src/server/zod-json-schema-compat.ts— the v3 branch needed more.zod-to-json-schemahas nodraft-2020-12target (onlyjsonSchema7andjsonSchema2019-09), so passing the option through is a silent no-op. The v3 branch now overrides$schematohttps://json-schema.org/draft/2020-12/schemawhen 2020-12 is requested.A caveat on the v3 override: the keywords
zod-to-json-schemaactually emits (type,properties,required,additionalProperties,oneOf/anyOf/allOf,enum,const) have identical semantics in draft-07 and 2020-12. The one keyword that diverges is tuples.items: [...]is a positional tuple in draft-07 but means "every item matches one of these schemas" in 2020-12. A Zod v3 tuple stamped as 2020-12 is therefore subtly wrong; an inline comment in the file flags this and points at Zod v4 for nativeprefixItems. IMO, leaving v3 stuck on draft-07 is worse than the caveat.test/server/mcp.test.ts: three new tests in the existingzodTestMatrixblock so they run on v3 and v4: inputSchema$schema, outputSchema$schema, and a v4-only tuple →prefixItemscheck..changeset/fix-sep-1613-emit-2020-12.md: patch-level changeset.Tests
npm test: 1584 / 1584 passnpm run typecheck: cleannpm run lint: cleanOn v2.0.0-alpha
maindoesn't have this bug.standardSchemaToJsonSchemahardcodestarget: 'draft-2020-12'at the conversion site, and v2 dropped Zod v3.Test plan
npm test(1584 / 1584)npm run typecheckpassesnpm run lintpasses