fix(openapi): preserve anyOf/oneOf/allOf in projected MCP input schema

[AlexisMarasigan]

Summary

convert_openapi_to_mcp_tools was injecting a top-level "type" field on every property without one — including properties whose schema used a JSON Schema composition keyword (anyOf, oneOf, allOf). The downstream jsonschema validator then AND'd the injected type with the existing composition, silently rejecting valid inputs at the MCP layer with Input validation error: <payload> is not of type '<picked>'.

For T | None, the bug surfaced as the nullable-rejection in #246. For T | U with both real types (e.g. dict[str, list[str]] | list[str]), the bug rejects whichever side lost the non-deterministic next(iter(set)) pick in get_single_param_type_from_schema — #307.

Fix

Skip the "type" injection when any composition keyword is already present:

_COMPOSITION_KEYWORDS = ("anyOf", "oneOf", "allOf")

def _has_composition_keyword(schema):
    return any(k in schema for k in _COMPOSITION_KEYWORDS)

# in convert_openapi_to_mcp_tools, for path/query/body params:
if not _has_composition_keyword(properties[param_name]) and "type" not in properties[param_name]:
    properties[param_name]["type"] = ...

Tests

• Two existing tests asserted the buggy behavior on Optional[ProductCategory] / Optional[float] / Optional[List[str]] / Optional[str] params (they expected the spurious "type" injection). Updated to assert the correct post-fix shape (anyOf preserved, no sibling "type").
• Added test_anyof_union_preserved_in_input_schema — pins the [BUG] Union types of two real types (non-null) rejected at MCP-server jsonschema validator with misleading "is not of type 'X'" #307 case (T | U, both real types).
• Added test_oneof_and_allof_preserved_in_input_schema — pins the same behavior for oneOf and allOf.

pytest tests/  →  71 passed

(The 14 errors in test_http_real_transport.py are pre-existing on main — RuntimeError: Event loop is closed on Python 3.13. Unrelated to this fix.)

Closes / refs

• Closes [BUG] Union types of two real types (non-null) rejected at MCP-server jsonschema validator with misleading "is not of type 'X'" #307 (non-null union rejection)
• Likely closes convert_openapi_to_mcp_tools adds "type" field, breaking nullable schemas #246 (nullable rejection — same convert.py "type"-injection bug; the existing buggy tests for it are updated here)
• May help [BUG] Union types with arrays generate invalid MCP schemas (missing 'items' property) #165 (union-with-array items missing — same area, different symptom)

🤖 Generated with Claude Code

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