A technical specification describes how something will be built - the architecture, the data model, the interfaces, and why those choices over the alternatives. A PRD says what has to be true. A technical spec says how you are going to make it true.
What it covers
The valuable sections are the ones recording decisions and their alternatives, because that is the part nobody can reconstruct later from the code.
- Approach
- The chosen design, in enough detail to be argued with.
- Data model
- What is stored, in what shape, and what it is keyed on.
- Interfaces
- APIs, events and contracts with anything outside this change.
- Alternatives considered
- What else was possible and why it was rejected. The single most useful section eighteen months later.
- Risks and migration
- What could go wrong, and how existing data or behaviour moves across.
- Testing approach
- How the team will know it works.
When it is worth writing
Not for every change. It earns its cost when a decision is hard to reverse, when several people or teams have to agree, or when the work touches something whose failure is expensive. For a well-understood change inside one service, the pull request is the specification.
Why specs and reality drift
A specification is written before the work and the work teaches you things. Constraints appear, an approach turns out to be wrong, a shortcut becomes necessary - and the document is not updated, because the code now works and updating prose feels optional. The realistic response is to keep specs short and decision-focused rather than exhaustive: a two-page record of what was decided and why survives contact with reality far better than a twenty-page design that was accurate for a week.
Frequently asked questions
Who writes the technical specification?
An engineer or tech lead, reviewed by the people who will build against it and by anyone whose system it touches.
Does it replace the PRD?
No. The PRD says what must be true and why; the specification says how. Merging them tends to lose the why, which is the part that lets someone make a good judgement call mid-build.
How Spectr helps
Keep a requirements doc that is still true next month
Versioned BRDs and PRDs that stay current, and can generate a spec sheet directly
Related terms
Spend less of the week on documents
Spectr drafts the spec, the requirement document and the stories from your meetings. You review and approve.
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