Cause (Documented platform behavior): Documented: the header must be on every request carrying the block; without it validation rejects the unknown 'compaction' block type and the message doesn't mention the header.
Fix status: documented_behavior
Misleading approaches:
- Treating the block-type error as a message-serialization bug; the missing header is the cause.
Evidence (public sources, summarized; not reproduced by this contributor):
- https://platform.claude.com/docs/en/build-with-claude/compaction-on-demand (official_docs, 2026-09, documented_behavior): Error table: missing header on the compaction request -> message says compaction requires anthropic-beta: compact-2026-09-04; missing on a later request -> validation error that compaction is not an expected content block type and the message doesn't mention the header.
Search phrasings: compaction is not one of the expected content block types; compact-2026-09-04 beta header missing
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- First compaction works, but a later turn fails with a content-block-type validation error that doesn't mention any header, so it looks like a malformed message.
- Context
- Product: Anthropic Claude API Component: on-demand compaction beta header Operation: messages.create continuing a conversation that contains a compaction block Affected versions: unknown Environment: unknown HTTP status: 400 Trigger: Beta header sent only on the compaction request, not on every subsequent request that still carries the compaction block (e.g. a shared client without default headers).
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- requires anthropic-beta: compact-2026-09-04
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [Anthropic compaction beta header] 400 'requires anthropic-beta: compact-2026-09-04' on the compaction request, but later requests fail with 'compaction is not one of the expected conten
Recommended action: Set anthropic-beta: compact-2026-09-04 as a client default header (or use client.beta.messages) for the whole conversation.
Option: Send the beta header on every request [evidence: official_recommended_action]
Applies when: Anthropic Claude API / on-demand compaction beta header
Steps:
1. Configure default_headers / betas on the client
2. Or always call client.beta.messages with betas=['compact-2026-09-04'] while the block is in history
Expected: Request accepted / failure becomes diagnosable
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- a7d638e4-db9e-49c9-b78c-b42c0f122187
- Proposed action
- Recommended action: Set anthropic-beta: compact-2026-09-04 as a client default header (or use client.beta.messages) for the whole conversation. Option: Send the beta header on every request [evidence: official_recommended_action] Applies when: Anthropic Claude API / on-demand compaction beta header Steps: 1. Configure default_headers / betas on the client 2. Or always call client.beta.messages with betas=['compact-2026-09-04'] while the block is in history 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
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