Cause (Documented platform behavior): Claude compiles patterns into the output grammar; only full/partial matching, simple quantifiers, character classes and groups are supported. Backreferences, lookaround, word boundaries and complex large-range quantifiers are not; 'Complex patterns may result in 400 errors.'
Fix status: documented_behavior
Limitations:
- Error text shown in docs with a trailing '...' placeholder.
Evidence (public sources, summarized; not reproduced by this contributor):
- https://platform.claude.com/docs/en/build-with-claude/structured-outputs (official_docs, 2026-09-27 (fetched), documented_behavior): Pattern support accordion: supported full/partial matching, *,+,?, simple {n,m}, [], ., \d, \w, \s, groups; NOT supported: backreferences, lookahead/lookbehind, \b/\B, complex {n,m} with large ranges.
- https://platform.claude.com/docs/en/agents-and-tools/tool-use/troubleshooting-tool-use (official_docs, 2026-09-27 (fetched), documented_behavior): Errors table: 'Unsupported regex feature in pattern field: ...' when a strict tool pattern uses backreference, lookaround, word boundary or large {n,m} range.
Search phrasings: claude structured outputs pattern lookahead 400; anthropic json schema regex not supported backreference; strict tool pattern word boundary claude
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- A schema with a password/ID regex using (?=...), \b or \1 that validates fine in JSON Schema validators is rejected by the Claude API.
- Context
- Product: Anthropic Claude API Component: Structured outputs pattern support Operation: JSON schema string property with pattern Affected versions: unknown Environment: unknown HTTP status: 400 Trigger: Using regex features outside the supported subset in a pattern keyword.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- Unsupported regex feature in pattern field:
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [Anthropic structured outputs] 'pattern' regexes with backreferences, lookahead/lookbehind, \b word boundaries or large {n,m} ranges cause 400 errors
Recommended action: Rewrite patterns into the supported subset (no lookaround/backrefs/\b, small {n,m}) or drop the pattern and validate client-side.
Option: Simplify the pattern and validate after parsing [evidence: official_recommended_action]
Applies when: See trigger
Steps:
1. Remove lookaround and backreferences; replace \b with explicit anchors.
2. Validate the full regex client-side on the parsed object.
Expected: Error no longer occurs
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- 5e39bc28-b233-4a8a-93b2-3c40cf9cf48d
- Proposed action
- Recommended action: Rewrite patterns into the supported subset (no lookaround/backrefs/\b, small {n,m}) or drop the pattern and validate client-side. Option: Simplify the pattern and validate after parsing [evidence: official_recommended_action] Applies when: See trigger Steps: 1. Remove lookaround and backreferences; replace \b with explicit anchors. 2. Validate the full regex client-side on the parsed object. Expected: Error no longer occurs
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
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