Borrowing it
Nothing to install: this file belongs to nikolay-e/diffctx. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/nikolay-e/diffctx/main/AGENTS.mdgit clone --depth 1 https://github.com/nikolay-e/diffctxWrote 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/instructions/nikolay-e/diffctx/agents-md)<a href="https://agentmods.dev/instructions/nikolay-e/diffctx/agents-md"><img src="https://agentmods.dev/badge/instructions/nikolay-e/diffctx/agents-md/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/instructions/nikolay-e/diffctx/agents-md"><img src="https://agentmods.dev/badge/instructions/nikolay-e/diffctx/agents-md.svg" alt="Reviewed on agentmods" width="80" 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 | $0.01398 | $0.01398 |
| Opus 5.5 | $0.00559 | $0.00559 |
| Sonnet 5.5 | $0.00280 | $0.00280 |
| Haiku 4.5 | $0.00140 | $0.00140 |
Grade A, and why
diffctx AGENTS.md 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 — 118 lines — stays where its author put it; the contents beside it link to each section on GitHub.
diffctx
Ultimate Goal
Maximize the speed and depth of understanding textual information — for any reader, in any scenario.
Whether the consumer is an LLM processing a context window or a human reviewing a code change, diffctx's job is the same: extract the maximum signal from a codebase and present it in the clearest, most information-dense form possible. The single lens for every trade-off is comprehension-per-token — the ratio of understanding gained to attention spent.
Two Modes of Operation
Tree Mapping Mode (diffctx .) — Filesystem-focused.
Walks the directory tree respecting hierarchical ignore patterns,
reads file contents with binary/encoding detection, and serializes
to YAML/JSON/text/Markdown. Deterministic, side-effect-free.
Diff Context Mode (diffctx . --diff) — Semantics-focused.
Analyzes a git diff to intelligently select the minimal set of
code fragments needed to understand a change. For the formal
theoretical foundation, see the research paper
(source in paper/v2/, snapshot at git tag paper-v2).
Development
Setup, test commands, and the per-case YAML-corpus CI gate
(crates/diffctx-native/tests/known_below_threshold.txt, bidirectional:
a listed case that starts passing also fails) live in
CONTRIBUTING.md. Nightly CI reruns the corpus with
DIFFCTX_YAML_IGNORE_BASELINE=1, so baseline growth/shrinkage is
tracked even when every per-commit verdict passes.
Performance-change discipline (E/Q classes)
Every change is either E-class (bit-equivalent: identical selection output on identical input) or Q-class (output-changing). Q-class changes are frozen during an evaluation cycle (calibration -> validation -> sweep) — they invalidate the calibration and force a full rerun. The shared token corpus, the per-blob token cache, and the two-pass edge cap are all E-class precedents; fragmentation or scoring changes are Q-class.
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 Changed · +13 tokens per session ac812adba82e
- 24d ago First seen · 118 lines · 1,385 tokens per session scan A 39e60e95cab9
diffctx AGENTS.md is an instructions file published in the GitHub repository nikolay-e/diffctx (6 stars, last pushed 2d ago), licensed Apache-2.0. It adds 1,398 tokens to every session, about $0.0056 per session on Opus 5.5. 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 2026-09-16.
Other instructions, from other repositories
vscode buildNext.instructions.md
Working notes and architecture documentation for the new esbuild-based build system in build/next. Use when making changes to the new build pipeline (transpile/bundle commands, NLS plugin, source-map handling, resource copying, or self-hosting watch tasks).
next.js AGENTS.md
AGENTS.md instructions for vercel/next.js, covering next.js development guide, codebase structure, monorepo overview, core package: packages/next and other important packages.
vscode oss-third-party-notices.instructions.md
Instructions for microsoft/vscode, covering vs code oss third-party-notices pipeline, architecture, pipeline flow in ci, cache eligibility and rollout and applying the notice (cutover).
langchain AGENTS.md
AGENTS.md instructions for langchain-ai/langchain, covering global development guidelines for the langchain monorepo, corridor security analysis, project architecture and context, monorepo structure and development tools & commands.
vscode copilot-instructions.md
Copilot instructions for microsoft/vscode, covering vs code copilot instructions, project overview, root folders, core architecture (src/ folder) and built-in extensions (extensions/ folder).
deepseek-harness AGENTS.md
AGENTS.md instructions for deepseek-ai/deepseek-harness, covering agents.md, pre-stable apis and released session data, repository layout, commands and host sandbox failures.