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Title: Pass json-schema-ref-no-deref conformance scenario (SEP-2106) by Kludex · Pull Request #2924 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: Pass json-schema-ref-no-deref conformance scenario (SEP-2106) by Kludex · Pull Request #2924 · modelcontextprotocol/python-sdk

X Title: Pass json-schema-ref-no-deref conformance scenario (SEP-2106) by Kludex · Pull Request #2924 · modelcontextprotocol/python-sdk

Description: Closes #2903. SEP-2106 permits the full JSON Schema 2020-12 vocabulary in tool schemas, including $ref. A $ref resolving to a network URI is an SSRF / fetch-DoS vector. The SEP requires: Implementations MUST NOT automatically dereference a $ref that is not a same-document reference. A schema that fails to validate due to an unresolved external $ref SHOULD be rejected rather than treated as permissive. The SDK already satisfies the MUST NOT: there is no $ref-fetching code anywhere, and jsonschema's referencing backend raises Unresolvable rather than performing an HTTP GET. So no $ref-resolution change is needed. What changed Added a json-schema-ref-no-deref driver handler to the conformance client. Its absence (not actual dereferencing) was why the scenario failed; the SDK was already compliant. Removed the scenario from both expected-failures baselines. Added a client test confirming a network $ref in an output schema, under a property the structured content does not set, is validated without being dereferenced. Scope notes I deliberately did not add eager registration-time rejection of external $refs. The SEP only forbids dereferencing them; an external $ref that is never fetched is legitimate 2020-12 (the SEP lists $ref among the keywords it enables). Eager rejection would forbid valid schemas and go beyond the spec. The issue text says "server rejects unresolvable external $ref at registration"; this PR follows the SEP's narrower, conditional SHOULD instead. Happy to revisit if maintainers prefer the stricter reading. The structuredContent widening to any JSON value already landed in #2849; I audited the consumers and found no object/truthiness assumptions. Composition-keyword resource bounds (SEP SHOULD: schema depth / validation-time limits) are not implemented here. Verification json-schema-ref-no-deref passes on both the 2025 and 2026-07-28 client legs locally, with "Baseline check passed: all failures are expected" (no stale-entry error). Full suite green, 100% coverage, strict-no-cover/pyright/ruff clean. AI Disclaimer This PR was developed with the assistance of either Claude or Codex. I've reviewed and verified the changes.

Open Graph Description: Closes #2903. SEP-2106 permits the full JSON Schema 2020-12 vocabulary in tool schemas, including $ref. A $ref resolving to a network URI is an SSRF / fetch-DoS vector. The SEP requires: Implement...

X Description: Closes #2903. SEP-2106 permits the full JSON Schema 2020-12 vocabulary in tool schemas, including $ref. A $ref resolving to a network URI is an SSRF / fetch-DoS vector. The SEP requires: Implement...

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33774ff Reject external JSON Schema $ref in tool schemas (SEP-2106) Kludex Jun 20, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2924/commits/33774ff1553ba75a71813428e30304a835378e78
69ea4fb Drop conformance burn-down and migration docs from this PR Kludex Jun 20, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2924/commits/69ea4fb7656aa0730bf1689d64acaed2a48e4c2f
b0786d1 Reject external $ref via the tool schema generator Kludex Jun 20, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2924/commits/b0786d1628a794accc13fe9f7ce566aece70de52
9b8d0aa Inline the tool schema generator into func_metadata Kludex Jun 20, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2924/commits/9b8d0aa73dc0882fa900f1ba0209aec2d4356975
f7366a6 Make ExternalSchemaRefError a hard error, not a ValueError Kludex Jun 20, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2924/commits/f7366a6a4244a4b6486594bb7592ec5da690546d
4d8f900 Burn down json-schema-ref-no-deref conformance scenario Kludex Jun 20, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2924/commits/4d8f900f9c264611808575e7d03e81e96246e5b0
a53b892 Align SEP-2106 $ref handling to the spec Kludex Jun 20, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2924/commits/a53b89242492f59276cf1e8cf81cc3b9b6864489
7a2e7ab Reuse the initialize handler for json-schema-ref-no-deref Kludex Jun 20, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2924/commits/7a2e7abe1ee3a4e3064c43b7393226032032710a
a06d8e1 Give json-schema-ref-no-deref its own handler Kludex Jun 20, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2924/commits/a06d8e1d7cf2fc30b9a99d1f4fddba47b7921e3e
8e255ad Merge remote-tracking branch 'origin/main' into sep-2106-json-schema-ref Kludex Jun 20, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2924/commits/8e255ada033706d483d5c0a8e94a2367c64b68f2
2c47fc4 Guard the no-deref test with a socket tripwire Kludex Jun 20, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2924/commits/2c47fc4e7deb3bee5037c5ffab260bb6d8960d35
c7522f2 Drop the Python no-deref unit test Kludex Jun 20, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2924/commits/c7522f232492378a9dff0e2b74e60fac55f0189f
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