fix(schema): support list-form type arrays in JSON schema conversion

[DrewWhittleNZ]

Fixes #7280. Supersedes #7058 (closed by the first-time-contributor bot for lacking a linked issue -- opening this one instead per its instructions, now with #7280 attached).

Disclosure: I'm not a developer and didn't write this fix by hand -- I found and diagnosed the bug while evaluating crewai + MCP for my own project, then used Claude (Anthropic's AI assistant) to trace the root cause against your source, prepare the fix, and write tests. Per your CONTRIBUTING.md, this should carry the llm-generated label -- I don't have permission to add it myself as an external contributor, so flagging it here for a maintainer to apply.

What

_json_schema_to_pydantic_type (in crewai/utilities/pydantic_schema_utils.py) already handles anyOf/oneOf for nullable unions -- the form Pydantic's own schema generation produces for Optional[T] fields. But it has no handling for the other, equally valid JSON Schema way of expressing the same thing: a list-form type array, e.g. {"type": ["string", "null"]}. This is what .NET/System.Text.Json-based schema generators produce instead of anyOf, so any MCP tool schema coming from a non-Python server using this form crashes create_model_from_schema outright:

raise ValueError(f"Unsupported JSON schema type: {type_} from {json_schema}")
# ValueError: Unsupported JSON schema type: ['string', 'null'] from
# {'description': 'Optional start date filter in YYYY-MM-DD format',
#  'type': ['string', 'null'], 'format': 'date-time', 'default': None}

Since this happens during MCPServerAdapter's initialization (converting every tool's inputSchema up front), one tool with this pattern takes down the entire adapter connection, not just that one tool -- every other tool on the server becomes unusable too.

Confirmed against a real server

Found this evaluating Equibles (a self-hosted SEC-data MCP server) for my own project. Several of its tools (e.g. ListCompanyDocuments's startDate/endDate filters) use exactly this pattern. MCPServerAdapter couldn't connect to it at all as a result -- reproduced identically on both Windows and macOS, with the underlying mcp Python SDK confirmed working fine directly (connects and lists all 64 tools in well under a second), isolating the fault specifically to this schema conversion step.

Fix

Two commits:

1. List-form type array support -- mirrors the existing anyOf/oneOf handling directly above it: treat each entry in a list-form type the same way an anyOf member is handled, building a Union of the corresponding Python types. A single-element list collapses to that one type via typing.Union's own behavior, and a "null" entry resolves to None.
2. Format-override fix (addressing CodeRabbit's review comment on fix(schema): support list-form type arrays in JSON schema conversion #7058) -- the format-mapping override in _json_schema_to_pydantic_field was unconditionally replacing the entire resolved type with FORMAT_TYPE_MAP's type, even when that type was a Union built from a list-form type rather than a plain str. For {"type": ["string", "null"], "format": "date-time"}, this collapsed Union[str, None] down to plain datetime, silently dropping the null option -- invisible for most optional fields (masked by the Optional[...] rewrap for non-required fields), but a required-but-nullable formatted field would wrongly reject None. The same override also dropped any non-string members of a multi-type array (e.g. ["string", "integer", "null"]) regardless of required status. Now the override only replaces the str member specifically, leaving null/other members untouched.

Testing

• 6 new tests in TestUnionTypes (test_pydantic_schema_utils.py): nullable string with a format (the exact real-world shape, including confirming the pre-existing FORMAT_TYPE_MAP datetime coercion still applies once the crash is gone), nullable string with no format, a 3-member list (["string", "integer", "null"]), a single-element list, a required nullable formatted field (the case the format-override bug affected), and a multi-type array with a format (verifying the non-string member survives).
• Full existing test suite for this file: 89/89 passing (83 pre-existing + 6 new).
• ruff check / ruff format --check: clean.
• Reproduced the exact real-world crashing schema fragment directly against create_model_from_schema before the fix (confirmed it raised) and after (confirmed it now builds and validates correctly, including the format="date-time" coercion).
• Reproduced the real end-to-end failure through MCPServerAdapter against the live Equibles server before this fix, confirming the connection crash.
• Ran a full real crew (researcher + analyst agents, local Ollama LLM) end-to-end against the live Equibles server with this fix applied, on a separate physical machine from where the fix was written: all 64 tools loaded with no crash, the researcher agent made real tool calls against live data, and the crew completed successfully with a final summary.
• Rebased onto current main (65 commits) and re-ran the full suite clean.

[coderabbitai]

📝 Walkthrough

Walkthrough

The schema converter now supports JSON Schema type arrays. It builds Pydantic unions, preserves nullable and non-string members, and applies formats only to string members. New tests cover nullable, required, multi-member, single-member, and formatted arrays.

Changes

JSON Schema type array conversion

Layer / File(s)	Summary
Type array and format conversion lib/crewai/src/crewai/utilities/pydantic_schema_utils.py	The converter recursively maps list-form type values to Pydantic types and combines them into unions. Single-member arrays collapse to one type. Format conversion changes only string members within a union.
Type array conversion tests lib/crewai/tests/utilities/test_pydantic_schema_utils.py	Tests verify nullable, multi-member, single-member, formatted, and required type-array fields. They also verify None handling and date-time conversion.

Merge Risk: 🟡 Moderate · up to ffb31

List-form types now load successfully, but schemas combining unions with string or numeric constraints may silently accept invalid tool inputs. Constraint preservation should be fixed before merge.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 10 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the primary change: support for list-form JSON Schema type arrays during schema conversion.
Description check	✅ Passed	The description provides the linked issue, problem statement, implementation details, testing evidence, and compatibility context. It is detailed and substantially covers the repository template, alth…
Linked Issues check	✅ Passed	The implementation addresses issue #7280 by converting list-form JSON Schema type arrays into Python unions, preserving null and other union members during format conversion, and adding tests for the …
Out of Scope Changes check	✅ Passed	The changes are limited to JSON Schema conversion logic and focused tests directly related to issue #7280. No unrelated code changes are identified.

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Actionable comments posted: 1

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Inline comments:
In `@lib/crewai/src/crewai/utilities/pydantic_schema_utils.py`:
- Line 1253: Update the union construction in the schema conversion method to
preserve and apply member-specific string and numeric constraints instead of
returning unconstrained member types. Ensure nullable unions retain constraints
such as minLength on the string branch and numeric bounds on the numeric branch,
and add tests covering nullable string and numeric constrained schemas.

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