fix: fix broken tool spec with composition keywords

[mkmeral]

fix: Handle all JSON Schema composition keywords in tool property normalization

Description

This PR extends #1226 to handle all JSON Schema composition keywords, not just anyOf. The original fix addressed issue #1190 where MCP tools with List[str] | None parameters caused models to return string-encoded JSON ('["foo", "bar"]') instead of proper arrays.

Root Cause: _normalize_property() was adding type: "string" to properties that already had type constraints defined via composition keywords, creating invalid JSON Schema.

Fix: Skip setting default type: "string" when any composition keyword is present:

• anyOf - Union types (e.g., Optional[T], str | int)
• oneOf - Exclusive unions (exactly one must match)
• allOf - Schema intersection (all must match)
• not - Negation (value must not match)

Added COMPOSITION_KEYWORDS constant for maintainability and applied fix to both tools.py and registry.py.

Related Issues

• Fixes [BUG] normalized tool property default type incorrect for objects and arrays #1190
• Extends fix: fix normalized tool property default type incorrect for objects #1226

Changes

File	Change
src/strands/tools/tools.py	Added COMPOSITION_KEYWORDS constant, updated _normalize_property()
src/strands/tools/registry.py	Imported constant, updated validate_tool_spec()
tests/strands/tools/test_registry.py	Added unit tests for all composition keywords

Testing

Test Results

Test Suite	main	fix1190
Unit Tests (comprehensive)	26 failed, 43 passed	69 passed
Unit Tests (registry)	3 failed, 23 passed	26 passed
Integration Tests (with LLM)	7 failed, 8 passed	15 passed

Tool Types Tested

• @tool decorator with Optional[List[str]], Optional[Dict], Union types
• Class-based tools with optional parameters
• Module tools (PythonAgentTool) with anyOf in TOOL_SPEC
• MCP tool schemas with all composition keywords

Real LLM Integration Tests

Verified agent correctly passes Python types (not string-encoded JSON):

@tool
def tag_processor(item: str, tags: Optional[List[str]] = None) -> str: ...

agent("Process 'document' with tags 'important' and 'urgent'")
assert isinstance(received_tags, list)  # ✓ list, not '["important", "urgent"]'

Backwards Compatibility Verified

• Empty properties still get type: "string" ✓
• Existing types preserved (integer, boolean, array) ✓
• $ref properties not modified ✓
• Property constraints preserved (enum, default, min/max) ✓

Checklist

• I have read the CONTRIBUTING document
• I have added tests that prove my fix is effective
• My changes generate no new warnings
• I ran hatch run prepare

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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