fix: skip JSON pre-parsing for str union annotations in pre_parse_json#2462
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Christian-Sidak wants to merge 3 commits intomodelcontextprotocol:mainfrom
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fix: skip JSON pre-parsing for str union annotations in pre_parse_json#2462Christian-Sidak wants to merge 3 commits intomodelcontextprotocol:mainfrom
Christian-Sidak wants to merge 3 commits intomodelcontextprotocol:mainfrom
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The `pre_parse_json` method used an identity check (`field_info.annotation is not str`) to decide whether to attempt `json.loads` on string values. For union types like `str | None`, this check passed because `str | None` is not identical to `str`, causing string values to be unnecessarily JSON-parsed. This could corrupt string identifiers (UUIDs, etc.) when the parsed result was a non-scalar type like a dict or list. Replace the identity check with a `_should_pre_parse_json` helper that inspects union annotations. Unions containing only simple scalar types (str, int, float, bool, NoneType) skip pre-parsing entirely, while complex unions like `list[str] | None` still benefit from it. Closes modelcontextprotocol#1873
Remove extra blank line before _SIMPLE_TYPES and between test functions to satisfy pre-commit linting. Remove # pragma: no cover from test inner functions that are exercised by the tests.
The inner functions func_optional_str and func_optional_list are only used for metadata extraction and pre_parse_json testing -- their bodies are never actually called, so coverage cannot track them.
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Summary
pre_parse_json()usesfield_info.annotation is not strto decide whether to attemptjson.loads()on incoming string values. For union types likestr | None, this identity check passes becausestr | Noneis notstr, so string values get JSON-parsed when they should not be._should_pre_parse_json()helper that inspects union annotations: unions of only simple scalar types (str, int, float, bool, NoneType) skip pre-parsing, while complex unions likelist[str] | Nonestill get pre-parsed.Closes #1873
Test plan
test_str_union_pre_parse_preserves_strings-- verifies JSON object strings, JSON array strings, plain strings, and UUID strings are all preserved as-is when annotation isstr | Nonetest_complex_union_still_pre_parses-- verifieslist[str] | Nonestill deserializes a JSON-encoded list from a stringtest_str_union_uuid_end_to_end-- end-to-end validation that a UUID passed to astr | Noneparameter arrives intact throughcall_fn_with_arg_validation