Tool Use Schema Validation Error — Fix (2026)

The Error

Error 400: tools[0].input_schema: 'anyOf' is not supported. Use 'oneOf' or separate tool definitions instead.

This error occurs when your tool definition’s input_schema uses JSON Schema features that the Anthropic API does not support, such as anyOf, $ref, or patternProperties.

The Fix

  1. Replace unsupported schema features with supported ones:
# Before (fails):
tools = [{"name": "search", "input_schema": {
    "type": "object",
    "properties": {"query": {"anyOf": [{"type": "string"}, {"type": "array"}]}}
}}]
# After (works):
tools = [{"name": "search", "input_schema": {
    "type": "object",
    "properties": {"query": {"type": "string", "description": "Search query as a string"}},
    "required": ["query"]
}}]
  1. Validate your schema before sending:
python3 -c "
import json
schema = {
    'type': 'object',
    'properties': {
        'query': {'type': 'string', 'description': 'The search term'}
    },
    'required': ['query']
}
print(json.dumps(schema, indent=2))
print('Schema is valid')
"
  1. Test the tool definition:
claude "use the search tool to find python docs" --tools '[{"name":"search","description":"Search the web","input_schema":{"type":"object","properties":{"query":{"type":"string"}},"required":["query"]}}]'

Why This Happens

The Anthropic API supports a subset of JSON Schema for tool definitions. Features like anyOf, $ref, allOf, patternProperties, and recursive schemas are not supported. The API validates schemas strictly at request time and rejects unsupported keywords.

If That Doesn’t Work

  • Simplify nested schemas by flattening them into a single-level object.
  • Replace enum arrays with a description that lists valid values.
  • If your schema is generated from TypeScript types, manually simplify the output.

Prevention

Add this to your CLAUDE.md:

# Tool Schemas
- Only use: type, properties, required, description, enum in tool input_schema.
- Avoid: anyOf, allOf, oneOf, $ref, patternProperties, additionalProperties.
- Test all tool definitions with a minimal API call before production use.

See Also

This fix also applies if you see these related error messages:

  • SyntaxError: Unexpected token in JSON at position 0
  • JSON.parse: unexpected character at line 1 column 1
  • Error: invalid JSON response from API
  • TS2307: Cannot find module
  • TS2322: Type 'string' is not assignable to type 'number'

Frequently Asked Questions

Why does JSON parsing fail on API responses?

JSON parse failures on API responses typically indicate a network issue where an intermediate proxy returned an HTML error page instead of JSON. Check the raw response by enabling debug logging with CLAUDE_LOG_LEVEL=debug to see the actual content received.

How do I fix corrupted JSON config files?

Open the file in a text editor and look for common issues: trailing commas, missing quotes, or truncated content (from a crash during write). Use python3 -m json.tool < file.json to validate and identify the exact parse error location.

Can Claude Code handle JSON files with comments?

Standard JSON does not support comments. If your project uses JSONC (JSON with Comments), Claude Code handles it when reading via tools. For configuration files like tsconfig.json that support JSONC, Claude Code strips comments before parsing.

Why does Claude Code generate TypeScript errors?

Claude Code generates code based on the types and patterns it sees in your codebase. If your tsconfig uses strict mode but Claude Code does not see the full type definitions, it may produce code that fails type checking. Add the relevant type information to your prompt or CLAUDE.md.

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Database Operations in Claude Code

When using Claude Code for database-related tasks, configure proper safety rails:

Read-only by default. Add to your CLAUDE.md: “Never run destructive database operations (DROP, TRUNCATE, DELETE without WHERE) without explicit confirmation.” This prevents accidental data loss during development.

Migration management. Claude Code can generate Prisma migrations, but review them before applying. The generated SQL may include operations that are safe in development but dangerous in production (column drops, type changes).

# Safe workflow for Claude Code database changes:
# 1. Claude generates the migration
npx prisma migrate dev --name add-user-preferences --create-only
# 2. Review the generated SQL
cat prisma/migrations/*/migration.sql
# 3. Apply after review
npx prisma migrate dev

Query optimization. Claude Code generates queries that work but may not be optimal. For production code, ask Claude to analyze query performance: “Check if this query uses an index and estimate the execution time for 1 million rows.”

Common Database Mistakes in Claude Code

  1. N+1 queries in loops. Claude generates a loop that queries the database on each iteration. Fix: batch the IDs and use a single WHERE IN query.
  2. Missing transactions. Multi-table updates without a transaction can leave data inconsistent. Fix: wrap related operations in prisma.$transaction().
  3. Exposing sensitive fields. Claude includes all columns in SELECT queries. Fix: specify a select clause that excludes passwords, tokens, and internal IDs.