# Tutorial: make one resilient edit

In a few minutes, you will make a small edit that survives harmless whitespace drift, confirm the result, and undo it.
This uses the CLI.
The same core is available from Python and MCP.

## 1. Install and create a file

```bash
uv add intact
mkdir -p /tmp/tact-tutorial && cd /tmp/tact-tutorial
printf 'DEBUG = True\nTIMEOUT = 30\n' > config.py
```

## 2. Read before you edit

```bash
uv run tact read config.py
```

The result includes a SHA-256 for the bytes it read.
You can supply that hash later as an advisory `--sha` check, but the important first habit is simpler: read the current file before proposing an edit.

## 3. Simulate harmless drift

Suppose a formatter or another author removes one space after you read the file:

```bash
sed -i 's/DEBUG = True/DEBUG=True/' config.py
```

A byte-exact editor looking for `DEBUG = True` would now refuse the change.
Tact can identify the same line under its whitespace-tolerant rungs, while still reporting what happened.

## 4. Edit and inspect the result

```bash
uv run tact edit config.py --old 'DEBUG = True' --new 'DEBUG = False'
cat config.py
```

The command should report a non-`EXACT` match and the file should contain `DEBUG=False`.
If there were two plausible matches, it would refuse and ask you to provide `--near LINE` rather than choose one for you.

## 5. Undo the write

```bash
uv run tact undo config.py
cat config.py
```

Undo restores the pre-edit state, including the simulated spacing.
It journals the state it replaces, so repeating `undo` can restore the edit.

## Next steps

- [How-to guide](howto.md) for patches, anchors, delta reads, and MCP setup.
- [Reference](reference.md) for command shapes, `--json`, exit codes, and library mappings.
- [Explanation](explanation.md) for the ladder and its deliberate refusal boundaries.