GSD Pi is a command-line coding agent and project workflow system that plans, implements, verifies, and tracks software work through milestones, tasks, Git worktrees, and stored project notes. It is for developers who want structured, longer-running agent sessions using different model providers. The catalogue entries extend GSD Pi with skills and agents.
Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add open-gsd/gsd-pi --skill design-an-interfacegit clone --depth 1 https://github.com/open-gsd/gsd-piWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/open-gsd/gsd-pi/design-an-interface)<a href="https://agentmods.dev/skills/open-gsd/gsd-pi/design-an-interface"><img src="https://agentmods.dev/badge/skills/open-gsd/gsd-pi/design-an-interface.svg" alt="Measured on agentmods" height="20"></a>- NVIDIA SkillSpector pass
What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00099 | $0.01222 |
| Opus 5 | $0.00049 | $0.00611 |
| Sonnet 5 | $0.00020 | $0.00244 |
| Haiku 4.5 | $0.00010 | $0.00122 |
Grade A, and why
design-an-interface scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 8d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 103 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Typical invocation points:
- A slice plan says "add module X" and the shape of X is not obvious.
- Two callers are about to grow coupled to an interface that has not been designed on purpose.
- A refactor surfaces a seam that needs to be re-cut deliberately.
<core_principle> RADICALLY DIFFERENT, NOT VARIANTS. Three designs with different method names are one design. Three designs that hide different things, expose different axes of control, or invert the caller/callee relationship are three designs. Enforce real divergence by assigning each sub-agent a constraint that would force a different shape.
INTERFACE, NOT IMPLEMENTATION. Do not estimate effort. Do not argue about how hard it is to build. Argue about how the interface behaves from the caller's side and what it hides internally. </core_principle>
Step 1: Gather requirements
Before spawning anything, answer in the conversation:
- What problem does the module solve?
- Who calls it? (other modules, external users, tests)
- What are the key operations?
- What constraints exist? (performance, compatibility, existing patterns)
- What should stay hidden inside vs be exposed?
If the user did not supply all of these, ask — one round, 1–3 questions max, per the ask-vs-infer rule.
Step 2: Spawn parallel design agents
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 8d ago First seen · 103 lines · 99 tokens per session scan A fbbdac702f61
design-an-interface is a skill published in the GitHub repository open-gsd/gsd-pi (1,205 stars, last pushed yesterday), licensed MIT. It adds 99 tokens to every session and 1,222 once invoked, about $0.0005 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
Other skills, from other repositories
gsd-sketch
Sketch UI/design ideas with throwaway HTML mockups, or propose what to sketch next (frontier mode).
gsd-ui-phase
Generate UI design contract (UI-SPEC.md) for frontend phases.
gsd-ui-review
Retroactive 6-pillar visual audit of implemented frontend code.
gsd-graphify
Build, query, and inspect the project knowledge graph in .planning/graphs/.
gsd-mempalace-capture
File a phase artifact into MemPalace; mirror decision facts into its temporal KG.
gsd-quick
Execute a quick task with GSD guarantees (atomic commits, state tracking) but skip optional agents.