Betas

A beta is an early API feature that a request switches on with a value in the anthropic-beta header. Claude Code keeps a list of these values and sends the ones that fit the model and the provider. If a proxy rejects a request with a beta error, set CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS=1 to stop the pre-release values.

ANTHROPIC_BETAS adds values (comma list), and so does the --betas flag. CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS=1 turns off the experimental betas. DISABLE_INTERLEAVED_THINKING turns off one. The betas go in the anthropic-beta header, or in the betas field of the body on Bedrock.

About the List

Claude Code joins the active values into the anthropic-beta header (or into the betas field of the body on Bedrock). A row is Documented when a docs page other than the changelog and the Agent SDK pages names the value. Few values have that. Source: Environment Variables for ANTHROPIC_BETAS and CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS, and the program for the rest.

How Claude Code Builds the List

Claude Code takes these steps for each request.

1. It adds the betas that fit the model and the provider.
2. It adds the values of ANTHROPIC_BETAS, a comma list, unless the experimental betas are off. It drops a value that has afk-mode, and it skips mid-conversation-system-2026-04-07, because that feature is generally available.
3. It removes context-1m-2025-08-07 when the model has the 1M window already.
4. For an agent query, it adds claude-code-20250219 if the list lacks it.
5. It adds the betas that depend on the turn, and then the values of the flag --betas. On a third-party provider it keeps only the allowed values of --betas.

CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS or a HIPAA policy turns the experimental betas off. Then Claude Code ignores ANTHROPIC_BETAS and --betas, and it leaves out the betas that only the Anthropic API takes.

Registry

The Registry Name column is the internal name of the value in the program.

Header Value	Status	Registry Name	When Claude Code Sends It
claude-code-20250219	Undocumented	claude_code	For every model whose name has no haiku, and for agent queries.
oauth-2025-04-20	Undocumented	oauth_auth	When you sign in with a subscription (OAuth).
interleaved-thinking-2025-05-14	Documented	interleaved_thinking	For each model except claude-3-*, unless DISABLE_INTERLEAVED_THINKING is set. The docs name it as interleaved-thinking.
context-1m-2025-08-07	Undocumented	long_context	When the model name has the [1m] suffix, also for a model with a 1M window. Never without the suffix (see Probes).
context-management-2025-06-27	Undocumented	context_management	On the Anthropic API, on Anthropic-run clouds, and on Foundry, for each model except claude-3-*.
structured-outputs-2025-12-15	Undocumented	structured_outputs	On the Anthropic API for a model that supports it, behind an experiment.
web-search-2025-03-05	Undocumented	web_search	On Vertex for models with web search there, and on Foundry.
advanced-tool-use-2025-11-20	Undocumented	tool_search	With tool search, as a second header value.
tool-search-tool-2025-10-19	Undocumented	tool_search	With tool search. On Bedrock it goes in the body instead.
effort-2025-11-24	Undocumented	effort	The program lists it, and no source states when it is sent.
task-budgets-2026-03-13	Undocumented	task_budgets	The program lists it, and no source states when it is sent.
prompt-caching-scope-2026-01-05	Undocumented	prompt_caching_scope	On the Anthropic API and Anthropic-run clouds, and when MCP tools are in the tool list.
prompt-caching-evict-2026-05-12	Undocumented	prompt_caching_evict	When a message has the evict_on_complete cache control.
cache-keepalive-2026-09-03	Undocumented	cache_keepalive	When the keep-alive switch is on. It needs a subscriber login (see Probes).
extended-cache-ttl-2025-04-11	Documented	extended_cache_ttl	When the cache time is 1h on the Anthropic API. The docs name it as extended-cache-ttl.
fast-mode-2026-02-01	Undocumented	speed	The program lists it, and no source states when it is sent.
redact-thinking-2026-02-12	Undocumented	redact_thinking	On the Anthropic API when interleaved thinking is on and the session is interactive.
thinking-resumption-2026-07-17	Undocumented	thinking_resumption	The program lists it, and no source states when it is sent.
thinking-token-count-2026-05-13	Undocumented	thinking_token_count	With interleaved thinking on the Anthropic API, and on Bedrock and Mantle for some models.
afk-mode-2026-01-31	Undocumented	afk_mode	For auto mode.
advisor-tool-2026-03-01	Undocumented	advisor_tool	The program lists it, and no source states when it is sent.
cache-diagnosis-2026-04-07	Undocumented	cache_diagnosis	The program lists it, and no source states when it is sent.
context-hint-2026-04-09	Undocumented	context_hint	The program lists it, and no source states when it is sent.
mcp-servers-2025-12-04	Undocumented	mcp_servers	The program lists it, and no source states when it is sent.
files-api-2025-04-14	Undocumented	files_api	The program lists it, and no source states when it is sent.
environments-2025-11-01	Undocumented	environments	The program lists it, and no source states when it is sent.
ccr-byoc-2025-07-29	Documented	ccr_byoc	The program lists it, and no source states when it is sent. The docs name it in Self-Hosted Environments Testing.
per-turn-control-2026-07-01	Undocumented	per_message_effort	For models that take the effort with each message.
timing-2026-09-09	Undocumented	per_turn_timing	When the time goes with each message.
mid-conversation-tool-changes-2026-07-01	Undocumented	mid_conv_tool_change	For models that allow tool changes in the middle of a conversation.
inline-tools-2026-09-15	Undocumented	inline_tools	With the previous value, when a second check passes. CLAUDE_CODE_INLINE_TOOLS=1 also adds it (see Probes).
server-side-fallback-2026-06-01	Undocumented	server_side_fallback	The program lists it, and no source states when it is sent.
server-side-fallback-2026-07-01	Undocumented	server_side_fallback_category	The program lists it, and no source states when it is sent.
fallback-credit-2026-06-01	Undocumented	fallback_credit	The program lists it, and no source states when it is sent.
auto-mode-classifier-2026-07-16	Undocumented	auto_mode_classifier	The program lists it, and no source states when it is sent.
dangerous-tool-use-2026-09-03	Undocumented	dangerous_tool_use	The program lists it, and no source states when it is sent.
thinking-display-updates-2026-08-18	Undocumented	thinking_display_updates	The program lists it, and no source states when it is sent.
thinking-binding-controls-2026-08-01	Undocumented	thinking_binding_controls	The program lists it, and no source states when it is sent.
message-threads-2026-08-12	Undocumented	message_threads	The program lists it, and no source states when it is sent.
mid-conversation-system-clear-at-2026-08-21	Undocumented	mid_conversation_system_clear_at	For the wire shape that keeps reminders.

The registry also holds two internal switches, x-cc-internal-mid-conv-cache-promotion and x-cc-internal-mid-conv-cache-promotion-ok. They are Undocumented, and Claude Code does not send them as header values.

Other Beta Values in the Program

These values belong to the embedded Anthropic SDK client or to the provider code. Claude Code does not put them in the model request. All of them are Undocumented.

Value	Where It Is Used
managed-agents-2026-04-01	The endpoints /v1/agents, /v1/sessions, and /v1/environments.
dreaming-2026-04-21	The endpoint /v1/dreams.
user-profiles-2026-08-18	The endpoint /v1/user_profiles.
agent-memory-2026-07-22	The endpoint /v1/memory_stores.
message-batches-2024-09-24	The batch endpoint.
token-counting-2024-11-01	The endpoint /v1/messages/count_tokens.
mcp-tunnels-2026-06-22	The endpoint /v1/tunnels.
oidc-federation-2026-04-01	The federation endpoints.
ccr-triggers-2026-01-30	The endpoint /v1/code/triggers.
bedrock-2023-05-31	The anthropic_version of the Bedrock body.
vertex-2023-10-16	The anthropic_version of the Vertex body.

Observed Request

A transcript of 2.1.304 records each request in an api-request line. One such line, read on a local machine with the model claude-opus-5-5, held 20 betas. The list ran from claude-code-20250219 and oauth-2025-04-20 to message-threads-2026-08-12. This is one observation and not a rule. See Transcript Format.

Example

Add a beta by hand, then turn all experimental betas off for a proxy that rejects them.

Betas from the shell

# Add one value to the header
ANTHROPIC_BETAS="files-api-2025-04-14" claude -p "hi"

# Stop the experimental values (also ignores ANTHROPIC_BETAS and --betas)
CLAUDE_CODE_DISABLE_EXPERIMENTAL_BETAS=1 claude -p "hi"

Related Pages

• Endpoints and Headers
• Request and Cache
• Probes
• Environment Variables