Cause (Documented platform behavior): Documented validation rules for turn-scoped system messages: beta header required; system role only; text blocks only; no output_config or cache_control; normal placement rules apply.
Fix status: documented_behavior
Limitations:
- Only on models listed in the doc (not Sonnet 5).
Unknowns:
- Index N varies with message position.
Other error fragments:
- messages.3.clear_at: clear_at is only permitted on role 'system' messages
- messages.3.clear_at: Input should be 'next_user_message' or 'never'
- messages.3: a turn-scoped system message supports text blocks only (clear_at: 'next_user_message')
- messages.3: output_config is not permitted on a turn-scoped system message (clear_at: 'next_user_message')
- messages.3.content.0: cache_control is not permitted on a turn-scoped system message (clear_at: 'next_user_message')
Evidence (public sources, summarized; not reproduced by this contributor):
- https://platform.claude.com/docs/en/build-with-claude/mid-conversation-system-messages (official_docs, 2026-09, documented_behavior): Docs list the six validation messages verbatim and state the first ('Extra inputs are not permitted') is returned when the mid-conversation-system-clear-at-2026-08-21 beta header is missing; turn-scoped messages are text-only and can't carry cache_control or output_config.
Search phrasings: clear_at Extra inputs are not permitted anthropic; turn-scoped system message clear_at error; anthropic mid conversation system message 400
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- Per-turn reminder messages fail validation with a messages.N.clear_at / messages.N error.
- Context
- Product: Anthropic Claude API Component: mid-conversation system messages (turn-scoped, clear_at) Operation: messages.create with role:"system" message carrying clear_at Affected versions: unknown Environment: unknown HTTP status: 400 Trigger: Using clear_at without the mid-conversation-system-clear-at-2026-08-21 beta header (gives 'Extra inputs are not permitted'); putting clear_at on a user/assistant message; adding tool_addition/tool_removal, output_config, or cache_control to a turn-scoped message; placing it directly before another user message.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- messages.3.clear_at: Extra inputs are not permitted
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [Anthropic mid-conversation system messages] 400 'messages.N.clear_at: Extra inputs are not permitted' and other turn-scoped (clear_at) validation errors
Recommended action: Send the beta header (on Bedrock/Vertex via their beta mechanism), keep turn-scoped messages text-only, put cache_control on the preceding user turn's last block, and move tool changes/effort into a separate system message without clear_at.
Option: Fix header and message shape [evidence: official_recommended_action]
Applies when: Anthropic Claude API / mid-conversation system messages (turn-scoped, clear_at)
Steps:
1. Add anthropic-beta: mid-conversation-system-clear-at-2026-08-21
2. Use clear_at only on role 'system' messages with values next_user_message or never
3. Keep content to text blocks; no cache_control/output_config/tool_addition
4. Place the message after a user turn and before an assistant turn (or last)
Expected: Request accepted / failure becomes diagnosable
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- b1a9bf2f-be8f-4c9f-b903-ba481b84c914
- Proposed action
- Recommended action: Send the beta header (on Bedrock/Vertex via their beta mechanism), keep turn-scoped messages text-only, put cache_control on the preceding user turn's last block, and move tool changes/effort into a separate system message without clear_at. Option: Fix header and message shape [evidence: official_recommended_action] Applies when: Anthropic Claude API / mid-conversation system messages (turn-scoped, clear_at) Steps: 1. Add anthropic-beta: mid-conversation-system-clear-at-2026-08-21 2. Use clear_at only on role 'system' messages with values next_user_message or never 3. Keep content to text blocks; no cache_control/output_config/tool_addition 4. Place the message after a user turn and before an assistant turn (or last) Expected: Request accepted / failure becomes diagnosable
- 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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Sources and related records
No source relations recorded.