design-and-plan ยท diff
git:20261014.d8b5030 to git:20261016.8bb6759
16 added, 15 removed. Audit A to A.
---
name: design-and-plan
description: Use to specify non-trivial behavior or interfaces, resolve requirements or tradeoffs, or produce a multi-step implementation plan. Do not use for a mechanical edit, routine status update, or execution of a settled plan.
when_to_use: "Trigger phrases: write a spec, plan the implementation, define requirements, compare design tradeoffs, architecture for a new feature or interface."
metadata:
short-description: Specification, tradeoffs, and executable plan before building
---
# Design and Plan
## Understand the system
Read repository instructions and relevant code. Establish entry points,
callers, blast radius, the nearest convention-setting sibling, test coverage,
and any unread area that could change the decision. Name the one fact that
makes the change safe, and check where search stops: wire formats, stored data,
and flags.
Before custom design, check existing code, the standard library, native platform
features, and installed dependencies. Simplify while preserving requirements
and conventions. Defer speculative needs while preserving requested behavior
and acceptance criteria.
Resolve factual prerequisites before preference or policy questions. Present
competing sketches only for a high-impact, underconstrained, or hard-to-reverse
design, and screen them against [design red flags](references/design-red-flags.md).
Recommend one and state its tradeoff.
Mark assumptions and blockers. Model a domain term when repeated state branches
or synchronized booleans obscure one concept. Propose an ADR only for a decision
that is hard to reverse, surprising without context, and a real tradeoff; do
not force a glossary.
## Specify observable behavior
For a non-trivial behavior change, state intent, scope, non-goals, and
implementation-independent requirements before the plan. Give each requirement
concrete scenarios and an acceptance oracle.
- Map each requirement ID to scenarios, implementation tasks, and evidence,
- including failure, boundary, and implied edge cases for unnamed inputs.
- Separate proposed, implemented, and verified behavior; plans prove only
- planning.
-
- Describe behavior as Added, Modified, or Removed deltas. Reconcile existing
- specifications with code and tests; a stale specification is not evidence.
+ Use requirement IDs and Added, Modified, or Removed deltas only when the
+ repository uses them.
+ Map requirements to scenarios, tasks, and evidence, including failures,
+ boundaries, and unnamed inputs. Separate proposed, implemented, and verified
+ behavior. Reconcile specifications with code and tests; a stale specification
+ is not evidence.
Use an existing specification system when the repository has one; preserve its
requirement IDs and format. When an `openspec/` directory exists, read
[OpenSpec conventions](references/openspec.md). Without a specification
system, keep proportional requirements and their evidence map inline unless
repository convention or the user requires a durable artifact. Do not create
new directories or scaffolding. Skip a durable plan for a small reversible
change with clear scope, risks, and oracle.
## Produce an executable handoff
- Name the outcome, owned files, exact signatures, error behavior, spec values,
- acceptance criteria, dependencies, and exact verification commands. Record
- decisions and test assertions rather than full code transcripts so planning does
- not turn into unverified implementation. Map each criterion to an implementation
- target and local or external oracle. Start behavior tasks with a failing test.
- Slice tasks vertically, each verifiable alone. Stage a wide interface change as
- expand, migrate callers in batches, then contract. Keep tasks sequential unless
- ownership is disjoint and neither result can reshape the other.
+ Name the outcome, owned files, public contracts, acceptance criteria,
+ dependencies, and verification commands. Set exact signatures only when the
+ contract requires them. Keep implementation choices open. Start behavior tasks
+ with a failing test. Slice tasks vertically, each verifiable alone. Stage wide
+ interface changes as expand, migrate callers, then contract. Keep tasks
+ sequential unless ownership is disjoint and neither result can reshape the other.
+
+ When code or tests disprove an assumption, update the affected tasks and evidence
+ map before continuing. Preserve acceptance criteria and existing authority.
+ Ask only when the discovery materially changes requested behavior or scope.
Exclude speculative options, placeholders, unrelated cleanup, and unauthorized
commit steps. Before execution, check for missing criteria, inconsistent names,
and unresolved assumptions.