MCAP is a container file format and logging library for storing serialized data, especially messages exchanged in robotics systems. It is used across several programming languages and workloads to record, inspect, merge, and split data files. The catalogue instruction relates to developing with or using MCAP and its command-line tools.
Borrowing it
Nothing to install: this file belongs to foxglove/mcap. 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/foxglove/mcap/main/AGENTS.mdgit clone --depth 1 https://github.com/foxglove/mcapWrote 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/foxglove/mcap/agents-md)<a href="https://agentmods.dev/instructions/foxglove/mcap/agents-md"><img src="https://agentmods.dev/badge/instructions/foxglove/mcap/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/foxglove/mcap/agents-md"><img src="https://agentmods.dev/badge/instructions/foxglove/mcap/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 | $0.01480 | $0.01480 |
| Opus 5.5 | $0.00592 | $0.00592 |
| Sonnet 5.5 | $0.00296 | $0.00296 |
| Haiku 4.5 | $0.00148 | $0.00148 |
Grade A, and why
mcap 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 2mo 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 — 109 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Development Guide
Overview
This is a polyglot library monorepo for the MCAP log file format, with implementations in TypeScript, Python, Go, Rust, C++, and Swift. There are no running services — this is purely a library/SDK project. The core functionality is writing and reading MCAP files.
MCAP is a modular container file format for recording timestamped pub/sub messages with arbitrary serialization formats. It is designed to work well under various workloads, resource constraints, and durability requirements. The format specification lives in website/docs/spec/index.md, with the well-known registry in website/docs/spec/registry.md, and feature notes in website/docs/spec/notes.md.
Design principles
Bounded memory when reading. Neither the language libraries nor the CLI may read (or force a consumer to read) an entire MCAP file into memory — files can be many GB. Reader memory should scale with the record or chunk being processed, not with the file length: holding one record, chunk, or attachment at a time is fine, but buffering the whole file, or all of its messages, is an out-of-memory foot-gun.
- Memory-map (
mmap) seekable local files where the language supports it, or have the API consumer supply the bytes (e.g. via their own mmap); use seek + bounded range reads for random access and streaming for sequential scans. - When input isn't seekable (e.g. a stdin pipe) or an operation needs random access over a stream (e.g. sorting), spool to a temporary file and mmap it rather than buffering in memory. A tmpfs temp dir keeps the spool resident, though that is typically swap-backed.
General prerequisites
- Git LFS — test data under
tests/conformance/data/andrust/mcap/tests/data/is stored in Git LFS. Tests will fail withInvalidMagicerrors if LFS pointers haven't been pulled. Rungit lfs pullbefore running tests. - Conformance tests orchestrate cross-language testing via the TypeScript harness:
yarn test:conformance:generate-inputs && yarn test:conformance. They require Git LFS data and pre-built binaries for each target language. - Releasing — see RELEASING.md for the release process for each language.
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.
- 2mo ago First seen · 109 lines · 1,480 tokens per session scan A a74122b5705f
mcap AGENTS.md is an instructions file published in the GitHub repository foxglove/mcap (1,134 stars, last pushed 3d ago), licensed MIT. It adds 1,480 tokens to every session, about $0.0059 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 2026-08-30.
mcap AGENTS.md compared with instructions from other repositories
| Mod | Type | Stars | Last push | Licence | Scan | Tokens | Works with |
|---|---|---|---|---|---|---|---|
| mcap AGENTS.mdfoxglove/mcap | Instructions file | 1.1k | 3d ago | MIT | A | 1,480 | Codex, OpenCode |
| vscode buildNext.instructions.mdmicrosoft/vscode | Instructions file | 195k | yesterday | MIT | A | 9,719 | GitHub Copilot |
| next.js AGENTS.mdvercel/next.js | Instructions file | 143k | today | MIT | A | 7,461 | Codex, OpenCode |
| langchain AGENTS.mdlangchain-ai/langchain | Instructions file | 149k | yesterday | MIT | A | 4,680 | Codex, OpenCode |
| vscode oss-third-party-notices.instructions.mdmicrosoft/vscode | Instructions file | 195k | yesterday | MIT | A | 5,516 | GitHub Copilot |
| vscode copilot-instructions.mdmicrosoft/vscode | Instructions file | 195k | yesterday | MIT | A | 3,093 | GitHub Copilot |
| deepseek-harness AGENTS.mddeepseek-ai/deepseek-harness | Instructions file | 274k | 2d ago | MIT | A | 4,313 | Codex, OpenCode |