Cause (Documented platform behavior): Documented validation of mid-conversation tool changes.
Fix status: documented_behavior
Limitations:
- Beta: requires inline-tools-2026-09-15 (or mid-conversation-tool-changes-2026-07-01 for by-reference only).
Unknowns:
- Exact human-readable message text not quoted in docs.
Other error fragments:
- tool_name_conflict
- available_tools_limit_exceeded
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: unresolved reference -> 400 with error.details.error_code tool_reference_unresolved; reusing a name for a different tool type -> tool_name_conflict; available_tools_limit_exceeded when >10,000 deferred/later tools or >4,194,304 bytes; tool changes not accepted right after a paused assistant turn; computer use can't be defined in a message during beta.
Search phrasings: tool_reference_unresolved anthropic; tool_name_conflict tool_addition; available_tools_limit_exceeded anthropic 400
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- Adding or swapping tools mid-conversation fails with a 400 whose error.details.error_code names the rule.
- Context
- Product: Anthropic Claude API Component: mid-conversation tool changes (tool_addition / tool_removal, inline-tools beta) Operation: messages.create with system message tool_addition/tool_removal blocks Affected versions: unknown Environment: unknown HTTP status: 400 Trigger: tool_addition references a name not declared in tools; a by-value definition reuses the name of a different tool type; more than 10,000 deferred or later-defined tools, or >4 MB of later tool definitions / rendered tool text; also some tool types (e.g. computer use) cannot be defined in a message during beta; tool_addition/tool_removal right after a paused (pause_turn) assistant turn is rejected.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- tool_reference_unresolved
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [Anthropic mid-conversation tool changes] 400 with error.details.error_code tool_reference_unresolved / tool_name_conflict / available_tools_limit_exceeded
Recommended action: Declare known tools up front in tools (defer_loading: true) and reference them; use a new name when changing tool type; resume a paused turn before sending a tool change; keep the inline tool volume under the limits.
Option: Read error.details.error_code and apply the matching rule [evidence: official_recommended_action]
Applies when: Anthropic Claude API / mid-conversation tool changes (tool_addition / tool_removal, inline-tools beta)
Steps:
1. Check error.details.error_code in the 400 body
2. tool_reference_unresolved: add the tool to tools (defer_loading: true)
3. tool_name_conflict: rename or use a newer version of the same tool type
4. available_tools_limit_exceeded: prune tools or use tool search
5. After pause_turn, resume first, then send the tool change
Expected: Request accepted / failure becomes diagnosable
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- 2f30553d-006e-4972-bc49-2cc4f0e8e799
- Proposed action
- Recommended action: Declare known tools up front in tools (defer_loading: true) and reference them; use a new name when changing tool type; resume a paused turn before sending a tool change; keep the inline tool volume under the limits. Option: Read error.details.error_code and apply the matching rule [evidence: official_recommended_action] Applies when: Anthropic Claude API / mid-conversation tool changes (tool_addition / tool_removal, inline-tools beta) Steps: 1. Check error.details.error_code in the 400 body 2. tool_reference_unresolved: add the tool to tools (defer_loading: true) 3. tool_name_conflict: rename or use a newer version of the same tool type 4. available_tools_limit_exceeded: prune tools or use tool search 5. After pause_turn, resume first, then send the tool change 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.