Why We Document Every Prompt Breakage When We Migrate Between Cursor, Claude Code, and Copilot

TL;DR: We stopped migrating AI dev tools silently after a single refactor broke three repo-specific prompts; now we log every config, prompt, and workflow dependency before switching anything.

The Context

In 2025, we treated AI coding tools as interchangeable API endpoints. If pricing changed, we switched. In 2026, that assumption failed twice: Cursor’s Composer/Auto vs Third-Party API split, and Copilot’s move to AI Credits, meant that “same tool, new tier” was not a lateral move — it was a partial rewrite. The hidden tax is not dollars. It is context that does not travel.

What We Tested

Move	Tool	Verdict	Why
Cursor → Claude Code	Terminal/CI workflows	✅	Clean migration if prompts are CLI-native; Claude Code is strongest for terminal/CI/deploy tasks
Cursor → Copilot + Fable 5	In-IDE agentic work	⚠️	Fable 5 is GA in Copilot since June 9, but Copilot’s usage-based billing requires an income-statement model, not a flat subscription model
Copilot → Cursor Third-Party API	Cost reduction attempt	⚠️	Cursor Teams Premium now spans $40–$120/user/mo; third-party API mode is a separate usage bucket with its own throttling rules
Any → Claude API direct	Headless automation	✅	Token tiers are explicit and portable: Fable 5 at $10/$50 per MTok, Opus 4.8 at $5/$25, Sonnet 4.6 at $3/$15, Haiku 4.5 at $1/$5

The Pivot Point

We moved a team from Cursor Pro to Copilot Pro mid-Q2 2026. The pricing looked equivalent. Within a week, three repo-specific composer prompts broke because Copilot’s chat context window and Fable 5 tool-calling behavior differed from Cursor’s inline composer. The team spent 14 engineer-hours fixing what should have been a 1-hour billing switch. We audited the damage: migration tax was 14× the projected savings.

What We Use Now

Every migration gets a pre-flight checklist:

1. Extract all custom prompts, rules, and slash commands from the old tool.
2. Map them to the new tool’s context window, tool-calling policy, and file-access rules.
3. Run the same 3-task sample suite on the new tier before committing.
4. Keep the old tool active at minimum tier for 30 days as a fallback.

We also require a 90-day spend audit before any move that involves usage-based billing, because “same price, different meter” is the most common migration lie.

When You’d Choose Differently

If your prompt library is generic and your workflows are standard (autocomplete, simple refactors), migration risk is low. If you have built tool-specific automations — CI pipelines tied to Cursor CLI, Copilot chat conventions, or custom slash commands — migrate as if you were relocating a small codebase, not swapping a SIM card.

Tool Crucible Rating

Overall / Ease / Value / Support — 1-5 each

• Overall: 3/5
• Ease: 2/5
• Value: 3/5
• Support: 3/5

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This is part of our AI coding tool evaluation series. See full comparison: [link]