feat: add client-side JSON Schema 2020-12 preservation scenario

[pugsatoshi]

This PR adds a new JSON-Schema-2020-12-preservation client conformance scenario that closes Issue #101. The existing json-schema-2020-12 scenario tests server-side keyword emission; this new scenario tests the client-side preservation path that SEP-1613 and TypeScript SDK PR #1135 actually fixed (.catchall() so the parser retains the 2020-12 vocabulary).

The scenario sets up a mock MCP server (via the version-aware MockServer abstraction from #321) that advertises:

• json_schema_2020_12_tool — focal tool whose inputSchema contains the full SEP-1613 + SEP-2106 vocabulary
• json_schema_echo — permissive echo tool

The client under test calls tools/list, then round-trips the observed inputSchema back via tools/call json_schema_echo with { schema: <observed> }.
The scenario diffs the echoed schema against the canonical fixture and emits eight per-keyword checks. SEP-2106 vocabulary checks are soft-gated via sep2106KeywordCheckStatus so they only fail on DRAFT-2026-v1 runs.

Motivation and Context

Closes #101.

Issue #101 noted that the existing json-schema-2020-12 scenario covers only the server's wire output, while the actual SEP-1613 / TS SDK PR #1135 fix was on the client side.
This PR fills that gap with a dedicated client-side scenario that mirrors the server-side scenario, one check at a time.

Scope is intentionally pinned to introducedIn: '2025-11-25'.
The 2026-07-28-RC changelog (draft changelog) does not change the preservation requirements for SEP-1613 / SEP-2106, so the wire shape under test remains unchanged.
The broader 2026-07-28-RC changes (SEP-2575 stateless, SEP-2549 CacheableResult, SEP-2243 Mcp-Method MUST) are deliberately out of scope for this PR; once 2026-07-28 is finalized and lands in DATED_SPEC_VERSIONS, the same scenario can be re-targeted in a follow-up.

The focal-schema fixture (a 2020-12 inputSchema with $schema, $defs.address.$anchor, allOf/anyOf, if/then/else, additionalProperties: false) was previously embedded only in the server-side scenario's description markdown block.
This PR extracts it into an exported JSON_SCHEMA_2020_12_FIXTURE, so the new scenario can reuse it verbatim; the server-side description now derives its example from the same constant via JSON.stringify, eliminating the risk of documentation/runtime drift.

How Has This Been Tested?

• npm run check — typecheck + lint pass.
• npm test — 274/274 tests pass (6 new vitest cases for this scenario, covering no-client, compliant-on-dated, compliant-on-draft, $schema/$defs strip, SEP-2106 strip on draft, and a round-trip-fidelity sanity check).
• End-to-end against the bundled reference client (CONTRIBUTING.md "Running your scenario" form):

$ npm run build
$ node dist/index.js client \
  --command "npx tsx examples/clients/typescript/everything-client.ts" \
  --scenario json-schema-2020-12-preservation
Starting scenario: json-schema-2020-12-preservation
Checks:
2026-06-05T03:25:14.074Z [json-schema-2020-12-client-tool-found                    ] SUCCESS Client called tools/list and
the mock server advertised 'json_schema_2020_12_tool'
2026-06-05T03:25:14.074Z [json-schema-2020-12-client-echo-completed                ] SUCCESS Client called tools/call
'json_schema_echo' with a 'schema' object argument
2026-06-05T03:25:14.074Z [json-schema-2020-12-client-$schema-preserved             ] SUCCESS Client preserved $schema =
'https://json-schema.org/draft/2020-12/schema'
2026-06-05T03:25:14.074Z [json-schema-2020-12-client-$defs-preserved               ] SUCCESS Client preserved $defs with the
expected nested definitions
2026-06-05T03:25:14.074Z [json-schema-2020-12-client-additionalProperties-preserved] SUCCESS Client preserved
additionalProperties = false on the focal inputSchema
2026-06-05T03:25:14.074Z [sep-2106-client-composition-keywords-preserved           ] SUCCESS Client preserved composition
keywords (allOf/anyOf) on the focal inputSchema
2026-06-05T03:25:14.074Z [sep-2106-client-conditional-keywords-preserved           ] SUCCESS Client preserved conditional
keywords (if/then/else) on the focal inputSchema
2026-06-05T03:25:14.074Z [sep-2106-client-anchor-keyword-preserved                 ] SUCCESS Client preserved $anchor inside
$defs.address

Test Results:
Passed: 8/8, 0 failed, 0 warnings

The negative case (a client that strips $schema/$defs before echoing) is covered by src/scenarios/client/json-schema-2020-12-preservation.test.ts rather than by a separate broken example client, matching the pattern used by json-schema-ref-deref.test.ts.

src/seps/traceability.json will drift after this change picks up the six new requirement rows in sep-2106.yaml; per AGENTS.md the traceability workflow will refresh it via a follow-up PR rather than as part of this one.

Breaking Changes

None.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[pkg-pr-new]

Open in StackBlitz

npx https://pkg.pr.new/@modelcontextprotocol/conformance@335

commit: 1a82ca9

[pcarleton]

Thanks @pugsatoshi — the scenario is solid and runs green end-to-end at both 2025-11-25 and the draft spec version, including the wire-schema validation from #399. Main moved under the PR, so I pushed the fixes onto your branch rather than a review round trip:

1. Merge with current main; the draft-target unit tests now drive the stateless lifecycle (post-Validate all wire messages against the per-version spec JSON schema #399 the stateful mock requires the SEP-2549 result fields) and the test helper matches createServerStateful's new signature.
2. sep-2106.yaml: reworded the preservation rows as derived-from-Consider adding client test for JSON Schema 2020-12 preservation (SEP-1613) #101 expectations (SEP-2106 has no normative preservation MUST), and dropped three rows whose check IDs the traceability collector can't pick up.
3. Replaced stale DRAFT-2026-v1 references with 2026-07-28.

Verified: 489/489 unit tests, scenario 9/9 end-to-end at both spec versions. We also prototyped the missing ~10-35 line fixture handler in all five Tier 1 SDK conformance clients — every SDK passes 9/9 (no SDK strips any keyword), so SDK repos will get those handler patches as follow-up PRs; until then a new-scenario failure can be baselined via expected-failures.

[pcarleton]

Reviewed, fixes applied on-branch, all five Tier 1 SDKs verified 9/9 with the fixture handler prototypes.