Claude Code Opus 4.8 Fast Mode Sunset: Settings and Migration

Claude Code’s Opus 4.8 fast mode is scheduled for removal on 2026-11-30. Requests that still use Opus 4.8 with fast mode will error after removal, rather than fall back to standard-speed Opus 4.8. Check your current model and fast mode state before the deadline. Move to Opus 5.5 to keep fast mode, or turn fast mode off to stay on Opus 4.8. This announcement does not establish an end date for the model itself.

Checked: 2026-11-07 JST. Local CLI: Claude Code 2.1.317. Announcement: CHANGELOG v2.1.316. Opus 5.5 minimum: v2.1.280. Interactive switching and actual model requests remain untested.

Key points:

• Removal concerns fast mode for Opus 4.8, scheduled for 2026-11-30.
• The announcement rules out automatic fallback to standard Opus 4.8.
• Check saved settings separately from the model and fast mode active in the session.
• Choose fast mode or standard-speed 4.8, and check what sets the next session’s defaults.

#What is being removed on November 30, and what happens to requests?

The official v2.1.316 CHANGELOG and fast mode guide announce deprecation on 2026-10-30 and removal on 2026-11-30. They do not specify a time or time zone for the deadline.

#Is Opus 4.8 itself being retired?

The subject is fast mode for 4.8. This announcement alone does not set a retirement date for standard-speed 4.8.

#Will requests fall back to standard speed?

The CHANGELOG explicitly says they will error without fallback. This is a documented future behavior, not an observation from after November 30. We have not verified an actual error code or message.

#How do I check my model and fast mode settings?

#Do saved settings establish the active session state?

The model guide lists the status line and /status as ways to see the active model. Use your existing session and avoid sharing account fields from /status. Open /model without an argument to inspect the picker, then close it without selecting a model.

For fast mode, inspect /fast or the ↯ icon beside the prompt. When only inspecting, close the dialog without toggling with Space or confirming with Enter. These are documented instructions; we did not measure the interactive UI.

Settings scopes include user, project and local. Usual paths are ~/.claude/settings.json, .claude/settings.json and .claude/settings.local.json in the project. Check the user settings location when CLAUDE_CONFIG_DIR is set. Managed settings and organization restrictions also exist.

The opt-in reference says fastModePerSessionOptIn:true prevents a saved fastMode:true from enabling fast mode at startup by itself. CLAUDE_CODE_DISABLE_FAST_MODE and managed settings also affect it. Saved values alone establish neither eligibility nor the session state.

#How can I inspect settings without changing defaults?

Save the read-only example and pass only the JSON files you intend to inspect. It runs with Node.js and launches neither a CLI nor networking. It displays only model, fastMode and fastModePerSessionOptIn, redacting unknown model strings. It does not infer the effective model from precedence.

node claude-code-fast-mode-check.mjs \
  "$HOME/.claude/settings.json" \
  "$PWD/.claude/settings.json" \
  "$PWD/.claude/settings.local.json"

input:1 identifies the first argument. missing-file means no file, unset means no key, and value:false preserves an explicit false. Invalid JSON or types cause exit code 2. Symlinks and paths containing symlinks are reported as symlink without being read; supply the canonical physical path. effectiveSession:unverified means the session state was not inspected.

#How do I move to Opus 5.5 and keep fast mode?

#How do I change only this session?

Check claude --version and the fast mode requirements first. If your CLI is below v2.1.280, follow the official update instructions.

The documented startup flag selects the model for this session:

claude --model claude-opus-5-5

If you already checked fast mode access and costs and chose to enable it for that session, the example is:

claude --model claude-opus-5-5 --settings '{"fastMode":true}'

In the interactive /model picker, s changes only the session; Enter also saves a default. Interactive /model claude-opus-5-5 also saves a default. Normal interactive /fast persists a user preference; turning it off removes the fastMode key. Check this persistence behavior when preserving global defaults. These are instructions for readers, not migration commands we executed.

#Why does a new session still start on Opus 4.8?

The official troubleshooting list covers project and managed settings, environment variables, organization defaults and aliases. Check startup arguments, ANTHROPIC_MODEL, ANTHROPIC_DEFAULT_MODEL, ANTHROPIC_DEFAULT_OPUS_MODEL and settings files without bypassing organization restrictions. Our example does not combine these into a precedence calculation.

When using a separate session per worktree, check Claude Code and Codex workspaces separately from model selection. Agent Skills startup context concerns context loading, separate from fast mode’s speed setting.

#How do I keep Opus 4.8 at standard speed?

The official instruction is /fast off. In a normal interactive session this also affects the saved fast preference. The documented environment variable can disable it just for the session you launch:

CLAUDE_CODE_DISABLE_FAST_MODE=1 claude --model claude-opus-4-8

Recheck /status and the fast mode display after choosing. Check the next launch too; leave it unverified if you have not observed it. Saved values, UI state and successful requests are separate results.

#How do I check pricing and eligibility?

We checked the official API price table on 2026-11-07. Units are USD per million tokens, with uncached base input pricing. These are not CLI subscription fees or actual bills.

The 4.8 fast prices apply before removal. Check caching, regional and other modifiers separately. Subscription users access fast mode through usage credits, outside subscription rate limits. Team and Enterprise require Owner enablement, and Console has access requirements too. We did not measure balances or migration savings. Keep usage display limits separate from actual billing.

#What did we verify locally, and what remains untested?

On 2026-11-07 we checked synthetic JSON files with Node.js and local CLI version/help. The settings below are fixture results, not published personal settings.

The same detector rejected variants that added writes, dumped the whole JSON, or converted absent or invalid values into normal values. Inspecting settings does not require changing MCP or hooks. See MCP configuration for configuration sources, context handoff for pausing work, and Hooks for hook exit behavior.

Before the deadline, identify the model, check fast mode, choose an action and check what sets the next launch’s configuration.

#Frequently asked questions

#Is standard Opus 4.8 also ending on November 30?

This announcement concerns fast mode for Opus 4.8. It does not establish an end date for the model itself or guarantee how long standard Opus 4.8 will remain available.

#Will it automatically fall back to standard Opus 4.8?

The official CHANGELOG says requests using Opus 4.8 with fast mode will error after removal, with no automatic fallback to standard speed. We have not tested a request after the deadline.

#Does fastMode:true prove fast mode is active?

No. Check the session display, fastModePerSessionOptIn, the disabling environment variable, managed settings and eligibility. A saved value alone does not establish the active state.

#Can I switch without changing global defaults?

The documented --model startup flag or s in the /model picker changes only the session. In an interactive session, /model <name> also saves a default. We did not test an interactive model switch.

#Which Claude Code version supports Opus 5.5?

The documented minimum is Claude Code v2.1.280. Our version and help checks used 2.1.317; account access to the model remains unverified.

#Is fast mode included in subscription rate limits?

The official guide says subscription users access fast mode through usage credits, outside subscription rate limits. Organization enablement and access requirements also apply. We did not verify a personal balance or actual bill.

#Does this guide also cover API speed:fast?

This guide covers the Claude Code CLI. We did not verify retirement conditions or migration for API clients, SDKs or other providers. Check their respective official documentation.