writing-design-docs

Writing design docs

REQUIRED BACKGROUND: the technical-writing skill (read-first, hard rules, truth rules, style).

Overview

A design document is a proposal made discussable: conclusion first, every non-trivial choice in a Why & What box, costs named next to benefits, and fact separated from proposal.

When to invoke, and not

Invoke for anything that argues for a change or records a design: proposals, RFCs, design docs, specs, migration plans, "should we" documents. Do NOT invoke for recording an already-taken decision (recording-decisions), for procedures (writing-runbooks), or for status reports.

Under pressure: a proposal persuades with its numbers and its named costs, and the register rules hold whatever the deadline (the technical-writing rule; "punchy" is not an override). When supplied facts arrive without sources, mark them **[source wanted: ...]** and keep writing (see references/truth.md in the technical-writing skill); never invent a citation and never silently drop the fact.

Skeleton

# Title: what the document does
 
**Subtitle pinning the scope in one sentence**
 
|                |                                            |
| -------------- | ------------------------------------------ |

Structure rules

• Number chapters and cite them as ch. 7.1.
• Goals and non-goals both. The non-goals (or "explicitly not changed") section states what stays unchanged.
• Definitions before behavior when a term is ambiguous: pin "responded", "eligible", "stale" before using them.
• A grounding section pins the facts the design rests on: a fact/source table, checked against a named commit. Separate verified facts from what will be built.
• Appendices take letters (Appendix A, B) and hold what would bury the main text: config examples, glossaries, inventories.
• A fact lives in one place. Link to it; never repeat it, not even across documents in the same repo.
• Mark unfinished parts with **[DRAFT - input wanted]** instead of omitting them.
• Open questions get owners: a name, a role, or an explicit "to be filled by".
• Residual risks and what NOT to do close the document when the design ships work to others.
• No line budget, but length from repetition or emphasis goes; past roughly 800 lines, split and let the main document link to the parts.

The Why & What box

Every non-trivial choice gets one; readers react to the box, not to the conclusion.

• An alternative dismissed without its strongest argument is a strawman. Name that argument.
• Admitting what the choice does not solve makes the document more credible.
• No box for choices nobody would contest; that is noise.
• An alternative that appears nowhere else in the document does not belong in the box: the reader would never consider it. One such rejection can be justified; several short ones in a row mean the box is padded.

Tone

• The document stays a proposal: "we propose" and "whether that convinces is up to you", not "this becomes the way of working". It sets the direction and leaves the detailed choices open.
• Name what it costs. The benefits chapter ends with the price: what gets harder, what people must unlearn, which freedom disappears.
• No superlatives, no promise language. Concrete figures and verifiable statements.
• The expected outcome may be negative; say so up front: "the expected outcome is that the current queue beats the proposed rewrite; that is a useful result."

Completeness check

Before handing a spec or plan to a reviewer or executor, check the five vagueness defects:

1. Unresolved placeholders: any literal TBD, TODO, "fill in later", or clearly incomplete sentence (a marked **[DRAFT - input wanted]** block is deliberate; an unmarked gap is a defect).
2. Missing acceptance criteria: a requirement with no concrete, independently testable success condition.
3. Undefined references: a type, endpoint, component, or table mentioned but defined nowhere.
4. No verifiable output: a task producing nothing a reviewer could inspect (no file path, no command, no observable behavior).
5. What without how: an outcome with no implementable direction ("handle errors appropriately" with no definition of appropriate).

For execution plans, add per task: goal, exact files, the change shown, tests with concrete scenarios, and the verify command. Explain any confusing leftover (an odd directory name, a legacy alias) rather than leaving it puzzling.