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 Intense-Visions/harness-engineering --skill api-backward-compatibilitygit clone --depth 1 https://github.com/Intense-Visions/harness-engineeringWrote 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/intense-visions/harness-engineering/api-backward-compatibility)<a href="https://agentmods.dev/skills/intense-visions/harness-engineering/api-backward-compatibility"><img src="https://agentmods.dev/badge/skills/intense-visions/harness-engineering/api-backward-compatibility.svg" alt="Measured on agentmods" height="20"></a>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 | $0.00000 | $0.02689 |
| Opus 5 | $0.00000 | $0.01345 |
| Sonnet 5 | $0.00000 | $0.00538 |
| Haiku 4.5 | $0.00000 | $0.00269 |
Grade A, and why
api-backward-compatibility 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 3d 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 — 171 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API Backward Compatibility
BACKWARD COMPATIBILITY IS THE GUARANTEE THAT EXISTING CLIENTS CONTINUE TO WORK WITHOUT MODIFICATION AFTER AN API CHANGE — MASTERING THE ADDITIVE CHANGE RULES, POSTEL'S LAW, AND BREAKING CHANGE TAXONOMY LETS TEAMS EVOLVE APIS RAPIDLY WITHOUT FORCING CONSUMERS INTO LOCKSTEP MIGRATIONS.
When to Use
- Reviewing a PR that modifies an existing API endpoint to determine whether the change is breaking
- Designing an API evolution policy for a platform that ships multiple services to external consumers
- Implementing automated breaking-change detection in a CI pipeline using schema diffing tools
- Auditing an API whose consumers report unexpected failures after a non-major release
- Establishing which changes require a new URL version versus which can be deployed to the current version
- Training a team on the additive-only change model before their first API public launch
- Choosing between evolving an existing endpoint and introducing a parallel endpoint for a new behavior
Instructions
Key Concepts
-
Additive change rules — Changes are backward compatible when they only add, never remove or modify existing contracts. Safe additive changes: new optional request fields (with documented defaults), new response fields (existing clients ignore unknown fields), new endpoints, new enum values in responses (clients must handle unknown values), new optional query parameters, and relaxed validation (accepting previously rejected inputs). Each of these leaves existing clients working without modification.
-
Breaking change taxonomy — A breaking change is anything that causes an existing valid client to fail or behave differently without a code change. Categories: (a) Removed fields, endpoints, or enum values; (b) Changed field type (string → integer); (c) Changed field semantics (timestamp format, currency unit, pagination behavior); (d) Added required request fields without defaults; (e) Narrowed validation (rejecting previously accepted inputs); (f) Changed HTTP status codes for existing conditions; (g) Changed authentication requirements; (h) Reordered array responses where order was previously stable and clients depended on it.
What ships with it
1 file beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
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.
- 3d ago First seen · 171 lines · 0 tokens per session scan A fda64ec1eed9
api-backward-compatibility is a skill published in the GitHub repository Intense-Visions/harness-engineering (20 stars, last pushed yesterday), licensed MIT. It costs nothing until one of its globs matches a file; then it loads 2,689 tokens. 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 2026-09-03.
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