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* Update claude-api skill: Managed Agents self-hosted memory stores and web tool domain settings, cost-optimize subcommand, Admin API reference, ASCII-only text Managed Agents: self-hosted sandboxes can now attach memory stores via the SDK worker (handle_item, ANTHROPIC_WORK_SECRET, sync options, troubleshooting); web_search/web_fetch accept allowed_domains/blocked_domains/user_location/max_content_tokens on the toolset configs entry and are not governed by environment networking; typed per-tool config unions; Console session viewer notes; packages caveat under limited networking; Claude Platform on AWS self-hosted worker auth. New shared/cost-optimization.md backing a cost-optimize subcommand, and new shared/admin-api.md covering client.beta.organization in all SDKs and the CLI. Prompt caching gains TTL selection, automatic vs explicit breakpoint guidance, workspace isolation, and verification guidance. Sonnet 5 pricing is the permanent $2/$10 list price. Advisor pairing no longer excludes Claude Fable 5 for Managed Agents. Reviewer-only HTML comments are stripped from the published files. All files are now plain ASCII in prose (em dashes, arrows, emoji callouts, and box-drawing replaced with ASCII equivalents), matching the source so future syncs diff cleanly. No-Verification-Needed: documentation-only change to skill reference content * Update claude-api skill: Claude Fable 5.1 / Mythos 5.1 catalog rows, Files and Skills APIs out of beta Claude Fable 5.1 (claude-fable-5-1) and Claude Mythos 5.1 (claude-mythos-5-1) become the default Fable-tier models throughout the skill; Claude Fable 5 and Mythos 5 stay selectable by id with their own catalog rows. Feature-support lists that named Fable 5 now read Fable 5/5.1. The Files API and Skills API are out of beta: examples use client.files.* / client.skills.* with no beta header, and the API-drift table points at the beta-to-GA migration docs. No-Verification-Needed: documentation-only change to skill reference content * Update claude-api skill: Claude Fable 5.1 / Mythos 5.1 migration section and API changes Adds a "Migrating to Claude Fable 5.1 from Claude Fable 5" section to shared/model-migration.md: three breaking changes (forced tool_choice any/tool returns 400; thinking blocks are preserved only for the model that produced them or a newer one; and only in the conversation that produced them, so edited history replayed with thinking blocks is rejected), what carries over from Fable 5, the Opus 5 path, Mythos 5.1 notes, capability improvements, prompt-tunable behavioral shifts, and a migration checklist. New API features documented: per-message effort (mid-conversation-output-config beta), turn-scoped mid-conversation system messages with clear_at, progress updates between tool calls via thinking.display "updates", thinking block_binding controls, and the 0.025x cache-read rate on Fable 5.1 with a max_tokens: 0 keep-alive that usually beats the 1-hour TTL. Error catalog, prompt-caching, tool-use, platform-availability, cost-optimization, and prompt-audit are updated to match; SKILL.md routes migration and prompting questions to the new section. No-Verification-Needed: documentation-only change to skill reference content * Update claude-api skill: Claude Fable 5.1 launch-day hedges and migration-path table rows Adds claude-fable-5 -> claude-fable-5-1 and claude-mythos-5 -> claude-mythos-5-1 rows to the migration-path and model-ID mapping tables (including the Bedrock IDs), and updates the refusal-fallback example to the 5.1 model id. Hedges three claims until the launch docs confirm them: Task Budgets support on Claude Fable 5.1, whether Claude Mythos 5.1 shares the 0.025x cache-read rate, and the fallback-credit wording. The block_binding error row now says to send the controls beta header only where that beta is offered and to fall back to strip-and-retry elsewhere. Cross-references within the migration section point at the history-editing check directly. No-Verification-Needed: documentation-only change to skill reference content
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1.6 KiB
Token Counting
Use the count_tokens endpoint (POST /v1/messages/count_tokens) for accurate
token counts against Claude models. Token counts are model-specific - pass
the same model ID you'll use for inference.
Do not use tiktoken. It's OpenAI's tokenizer. It undercounts Claude
tokens by ~15-20% on typical text, and by much more on code or non-English
input. Any estimate from tiktoken, gpt-tokenizer, or similar is wrong for
Claude.
Count a file or string
from anthropic import Anthropic
client = Anthropic()
resp = client.messages.count_tokens(
model="claude-opus-5",
messages=[{"role": "user", "content": open("CLAUDE.md").read()}],
)
print(resp.input_tokens)
TypeScript: await client.messages.countTokens({model, messages}) ->
.input_tokens. See {lang}/claude-api/README.md for other SDKs.
CLI
ant messages count-tokens --model claude-opus-5 \
--message '{role: user, content: "@./CLAUDE.md"}' \
--transform input_tokens -r
Diffing a file across two versions
The endpoint is stateless - count each version separately and subtract:
from anthropic import Anthropic
import subprocess
client = Anthropic()
def count(text: str) -> int:
return client.messages.count_tokens(
model="claude-opus-5",
messages=[{"role": "user", "content": text}],
).input_tokens
before = subprocess.check_output(["git", "show", "HEAD:CLAUDE.md"], text=True)
after = open("CLAUDE.md").read()
print(count(after) - count(before))
Full docs: see the Token Counting entry in shared/live-sources.md.