Migrating to Claude Fable 5: The Practical Guide

Anthropic shipped Claude Fable 5 on June 9. It's a new tier above Opus: $10 per million input tokens, $50 per million output, 1M context window, 128K max output. The model ID is claude-fable-5.

If you're on Opus 4.8, migration is close to a one-line change. If you're on anything older, there's a stack of breaking changes between you and Fable 5, and one of them has a deadline this week: claude-sonnet-4-20250514 and claude-opus-4-20250514 retire on June 15, 2026.

Here's the full migration, in order.

The one-line version

response = client.messages.create(
    model="claude-fable-5",  # was: claude-opus-4-8
    max_tokens=32000,
    messages=[{"role": "user", "content": "..."}],
)

This works for most Opus 4.8 codebases. Now here's everything that can break.

1. thinking: disabled is now a 400 error

This is the one new breaking change versus Opus 4.8. Fable 5 has exactly one thinking mode: adaptive, always on. There is no way to turn it off.

# Opus 4.8: valid, runs without thinking
thinking={"type": "disabled"}

# Fable 5: 400 error. Delete the parameter entirely.

If you omit thinking, you get adaptive thinking. That's the only option. Search your codebase for "disabled" near any thinking config and remove it.

Two related carryovers from the Opus 4.7/4.8 surface, in case you skipped those releases: budget_tokens returns a 400, and so do temperature, top_p, and top_k. The effort parameter is the only depth control now.

2. Raise max_tokens on workloads that ran without thinking

Because thinking is always on, and thinking tokens count against max_tokens, any workload you previously ran with thinking disabled now needs headroom it didn't need before.

A classification task that comfortably ran at max_tokens: 500 on Opus 4.8 with thinking off can now hit the cap mid-thought. Budget for thinking plus response text, not response text alone.

3. Start effort at high, not xhigh

Effort levels are low, medium, high, xhigh, and max, set via output_config:

output_config={"effort": "high"}

Anthropic's own migration guidance is direct about this: even if you ran xhigh on Opus 4.8, start at high on Fable 5. Lower effort on Fable 5 often beats xhigh on prior models. Given that output costs $50 per million tokens, the difference between high and xhigh shows up on your bill fast. Reserve xhigh and max for work where capability genuinely matters more than cost.

4. Handle the new refusal stop reason

Fable 5 runs safety classifiers on requests and during generation, targeting three categories: offensive cyber, biology and chemistry, and attempts to extract the model's raw reasoning. When a classifier fires, you get HTTP 200 with a new shape:

{
  "stop_reason": "refusal",
  "stop_details": {
    "type": "refusal",
    "category": "cyber",
    "explanation": "..."
  }
}

The categories are "cyber", "bio", "reasoning_extraction", or null. Anthropic says fewer than 5% of sessions trigger a fallback, but the classifiers are tuned conservative and benign work trips them. Day-one reports include a base64 implementation flagged as cybersecurity and genome-alignment work force-routed away from Fable.

If your code doesn't check stop_reason, a refusal looks like a short, useless completion. At minimum, log it. In production, you want the fallback pattern: retry on Opus 4.8 automatically. That's a big enough topic that we wrote a separate guide to the new Fallback API.

The billing rule worth knowing: a request refused before any output is generated is not billed and doesn't count against rate limits.

5. Audit your prompts for "show your reasoning"

This one will bite teams with mature prompt libraries. The reasoning_extraction classifier targets attempts to pull out the model's chain of thought, and old prompts are full of phrases like "show your reasoning step by step" and "explain your thought process before answering."

On Fable 5, those instructions can trigger refusals or elevated fallback rates. The model's raw chain of thought is never returned anyway: thinking.display defaults to "omitted", and the most you can get is "summarized". So those instructions buy you nothing and cost you reliability.

Grep your prompts, skills, and CLAUDE.md files for reasoning-extraction language and cut it. If you need visibility into the model's process, read the summarized thinking blocks instead.

6. Re-baseline your costs, not just your code

Two facts to hold at once:

• Fable 5 is exactly 2x Opus 4.8 pricing across the board, including cache writes and batch.
• Token counts are roughly unchanged from Opus 4.8 (same tokenizer), and early production reports suggest Fable 5 uses substantially fewer tokens to finish the same agentic work. One pre-launch tester measured roughly half the tokens of Opus 4.8 in agentic harnesses.

So your real cost delta is workload-dependent and probably less than 2x on long agentic tasks. Don't guess. Run a week of representative traffic and compare actual spend, not unit prices.

Two pricing notes that help: the full 1M context window bills at standard rates with no long-context premium, and the prompt-cache minimum drops to 512 tokens on the Claude API (it stays 1,024 on Bedrock). Short system prompts that never cached before now do.

7. Trim your prompts. Seriously.

Anthropic's Fable 5 prompting guide makes a point that's easy to skim past: old, prescriptive prompts can make Fable 5 worse. Instruction following is strong enough that brief instructions beat enumerated rule lists, and skill files written to keep weaker models on rails now read as constraints that degrade output.

The shift in one line: give it objectives, not task lists. If your CLAUDE.md spells out a 14-step procedure for something Fable 5 can figure out, the procedure is now the bottleneck.

While you're in there: single requests can run many minutes at high effort, so raise client timeouts, stream responses, and treat long runs as async jobs rather than blocking calls.

Coming from Opus 4.7 or earlier

Apply the generations in order. Each one has its own breaking changes:

From 4.7 to 4.8: nothing breaks. Swap the ID and re-tune prompts.

From 4.6 to 4.7: remove temperature, top_p, top_k (all 400 now). Replace budget_tokens thinking with adaptive thinking plus effort. The 4.7 tokenizer produces up to 30-35% more tokens for the same text, so re-run count_tokens on your prompts and raise max_tokens and any compaction triggers accordingly.

From 4.5 or earlier to 4.6: assistant prefills return a 400 (replace with structured outputs via output_config.format). Remove beta headers that went GA (effort-2025-11-24 and friends). Stream anything above roughly 16K max_tokens. Handle the refusal and model_context_window_exceeded stop reasons.

If you're on the retiring claude-sonnet-4-20250514 or claude-opus-4-20250514, you have until June 15. That's not a Fable 5 decision, it's a "your API calls stop working" decision. Move to claude-sonnet-4-6 or claude-opus-4-8 first, then evaluate Fable 5 from stable ground.

The checklist

1. Swap model ID to claude-fable-5
2. Delete any thinking: {type: "disabled"}
3. Raise max_tokens on previously non-thinking workloads
4. Set effort to high, benchmark before going higher
5. Handle stop_reason: "refusal" and stop_details.category
6. Grep prompts for "show your reasoning" language and remove it
7. Run a cost comparison on real traffic before committing
8. Cut prescriptive procedures from prompts and skills
9. Raise client timeouts and stream long requests

One more date for your calendar: Fable 5 is included free on Pro, Max, Team, and seat-based Enterprise plans only through June 22. From June 23 it requires usage credits. If you're evaluating on a subscription, this is the week to do it.

Sources: Anthropic's migration guide, models overview, Prompting Claude Fable 5, and model deprecations.