If you've been vibe coding for a while, you've accumulated prompts. Careful, numbered, step-by-step instructions that kept earlier models on the rails. On Claude Fable 5.1, most of that scaffolding lowers the quality of what you get back.

Why

Prescriptive prompts encode your plan for how to do the task. Fable 5.1 is generally better at planning the task than your prompt is, and following your steps stops it from doing so. It also tends to follow instructions closely, so it will execute a suboptimal plan faithfully rather than improve on it.

Anthropic's own migration guidance is blunt: prompts and skills written for prior models are often too prescriptive and reduce output quality. A/B your workload with the scaffolding removed.

What to keep

The goal. What you're building and why.

The constraints. Stack, files that mustn't change, style, what's off limits.

The definition of done. Specific and checkable.

How to verify. The test command, the URL to hit, what a correct result looks like.

Communication preferences. How you want the summary written. Fable 5.1 responds very well to explicit style instructions.

What to cut

Numbered steps for how to do the task. "First read X, then check Y, then write Z." State the outcome; let it sequence.

Reasoning instructions. "Think carefully about..." — it always thinks. "Consider the edge cases..." — it does.

Defensive rules for behaviours the model no longer has. Many prompts carry lines like "don't hold your findings for the end" or "don't use bullet points" written for older models' habits. Fable 5.1's habits are different, and those rules can now push it the wrong way.

Before and after

before.txt
You are an expert senior developer. Think step by step. First, read the
existing code carefully. Then identify the function that handles
uploads. Then check whether it validates file size. If it doesn't, add
validation. Then add a test. Then run the tests. Then report back with
a detailed summary using bullet points.
after.txt
Uploads in this project (upload.js) don't validate file size, and
someone filled the disk last week.

Add server-side size validation, capped at 10MB, with a clear error to
the user. Match the existing error handling style. Don't touch anything
else in the file. Run `npm test` after.

Done means: an oversized upload is rejected with a readable message,
and the existing tests still pass.

The second is shorter, gives the reason, sets a boundary, and leaves the how to the model.

How to migrate an existing prompt

de-prescribe.txt
Here's a prompt I've been using with older models: <paste>

Rewrite it for Claude Fable 5.1. Keep: the goal, constraints, definition
of done, verification, and communication preferences. Remove: step-by-
step instructions for how to do the task, reasoning instructions, and
rules that guard against behaviours an older model had.

Show me what you cut and why. Then give me the shorter version.

Then try both on a real task and compare. Most people find the shorter one produces better work.

The instinct to add more instructions when output disappoints is the wrong instinct here. Add context — the why, the constraints — not steps.