Practice 04: Durable Instructions, Reusable Workflows
Two files, and the mistake they stop repeating
Here's the entire mechanism, before the explanation of why it matters: a plain text file named CLAUDE.md, saved once in a project folder, with lines like Fiscal year starts in April, not January and All amounts are in EUR. From the next session onward, Claude Code reads that file automatically, every time, without being told to. Nobody retypes "remember, fiscal year starts in April" ever again.
The failure this prevents is invisible but expensive: re-explaining. Without a file like that, every session starts from zero. The fiscal year, the formats, the rules all get retyped, and quality depends on how patient anyone was that particular morning.
Two mechanisms fix it, and both are worth demystifying up front: they are plain text files. Nothing more exotic than that.
The handbook and the SOP
Both mechanisms are ordinary text files that live in the project folder: nothing to install, nothing beyond a text editor (or Claude Code itself, asked to create the file). Claude Code reads CLAUDE.md automatically every time it starts in that folder.
A complete, real CLAUDE.md, not a fragment: this is the whole file, paste-ready as-is into the root of a project folder:
# CLAUDE.md
## Standing rules for this project
- Fiscal year starts in April, not January.
- All amounts are in EUR. Convert anything in another currency
and show the conversion rate used.
- Exclude internal transfers between our own accounts from any
reconciliation or report.
- Never delete or overwrite source files. Write new output
files instead (e.g. `reconciled-june.xlsx`, not `june.xlsx`).
- Flag anything you are less than fully confident about in an
explicit "Needs review" section — do not silently guess.
- Dates are DD.MM.YYYY in all output, matching our reporting
standard.Save that as CLAUDE.md in a project's root folder. From the next session onward, every rule above applies automatically, and nobody has to retype "remember, fiscal year starts in April" ever again. That's the mechanism, not a promise about every future mistake: a rule written into the file gets read on every session from then on, so a correction made once stays in force for as long as the file does — the file doesn't forget, even though nothing stops someone from deleting a line later or the underlying process changing without the file being updated to match.
That's the actual shape on disk: CLAUDE.md sits in the project root and applies to everything in that folder; a skill lives in its own subfolder under .claude/skills/ and applies whenever its job is asked for by name. Both are checked into the same place the source files live, so a new colleague opening the folder inherits every rule and every recipe instantly, with nothing to be handed over verbally.
The conversation is volatile. Files are permanent.
A chat evaporates. A rule written to a file survives every session, every crash, every new colleague who picks up the work later.
A concrete number behind this
Anthropic's own finance team is reported to run on roughly 150 of these skills, written by accountants, not IT. That figure circulates as a company anecdote rather than a published, dated report, so treat the "roughly 150" as a repeated claim worth a second check before citing it to someone else, not an audited count. The mechanism it illustrates holds regardless: the domain expert writes the SOP; the agent executes it. In most companies, the person who'd write these is whoever already owns the process by hand.
Where this leads next
That's the shape of the discipline: fix a mistake once, in a file, and it stops being a mistake that recurs. The handbook and the skill file are the mechanism behind practice five as well, which is next, because once instructions are durable, the question becomes what's allowed to leave the building without a human looking at it first.
Practice 05, the final practice, and the demo that puts the handbook and the signature discipline together.
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