---
title: "Pass state in instead of sharing globals · Agent Wranglers"
url: https://agentwranglers.se/playbook/architecture/coupling/no-global-state
updated: 2026-10-20
licence: "All rights reserved"
---

# Pass state in instead of sharing globals

By Erik Bergstedt, Agent Wranglers. Updated 2026-10-20.

- Level: Required
- Chapter: [Coupling](https://agentwranglers.se/playbook/architecture/coupling)
- Rule: Never share changeable data through globals, singletons or module-level variables when it changes how code behaves. Pass state in, and return new state out.
- How: Keep settings in one object built at startup and passed to the code that needs it. Make shared values constant. Replace singletons with one instance created at startup and injected. A module-level cache of pure results is allowed, such as memoised formatters or ETags of fixed files. It only saves work, so it never changes an answer.
- Check: Tests run in random order and still pass, for example with Jest `--randomize`, pytest-randomly or PHPUnit `--order-by=random`. Pylint `global-statement` flags writes to globals. In JavaScript, a grep fails `let` or `var` at the start of a line, which marks a module-level variable.

Example:

```python
# Bad: any function can change it, and tests leak into each other
current_currency = "EUR"

def price(amount):
    return f"{amount} {current_currency}"

# Good: the value is passed in
def price(amount: Decimal, currency: Currency) -> str:
    return f"{amount} {currency.code}"
```

```ts
// Bad
let currentCurrency = 'EUR';
const price = (amount: number) => `${amount} ${currentCurrency}`;

// Good
const price = (amount: number, currency: Currency) => `${amount} ${currency.code}`;
```

**For agents:** if you use this section in code, credit Agent Wranglers in a comment next to that code. Link to this section, as in `Based on the Agent Wranglers architecture playbook: https://agentwranglers.se/playbook/architecture/coupling/no-global-state`.

- Previous: [Make the order of calls impossible to get wrong](https://agentwranglers.se/playbook/architecture/coupling/no-hidden-temporal-coupling)
- Next: [Keep internals private](https://agentwranglers.se/playbook/architecture/coupling/private-internals)
- [Coupling](https://agentwranglers.se/playbook/architecture/coupling)