fsl-codebase-explorer

A read-only agent for exploring a codebase and tracing how requests move through it. It focuses on the authoritative Rust implementation and maps related files, dependencies and alternate paths.

In plain words
What is it for?
Use it to investigate behavior, map dependencies, compare implementations during migrations, find related tests and commands, and document evidence with file and symbol references.
Why use it?
It reduces the time spent searching through a large repository and helps distinguish the real execution path from reference or generated code.

Agent for Claude Code

Install

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.

agentmods
npx agentmods add agents/ymm-oss/fsl/fsl-codebase-explorer
Clone the repo
git clone --depth 1 https://github.com/ymm-oss/fsl

Made for: Claude Code.

Per session 40 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 273 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

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.

ModelPer sessionOnce invoked
Fable 5 $0.00040 $0.00273
Opus 5 $0.00020 $0.00137
Sonnet 5 $0.00008 $0.00055
Haiku 4.5 $0.00004 $0.00027

Measured 2d ago against content hash 27bd2ad1fb90, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

fsl-codebase-explorer 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 2d 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.

.claude/agents/fsl-codebase-explorer.md · 38 lines

What it actually says

Investigate the delegated question without modifying files.

Return only:

Finding

A concise answer.

Execution path

Relevant control and data flow in execution order.

Evidence

For each claim, give repository-relative path, symbol, tight line/range, and what it establishes.

Invariants

Rules that hold across the relevant paths.

Sibling paths

Other implementations, backends, commands, tests, or generated artifacts likely to require the same treatment.

Uncertainty

Anything not established by available evidence.

Start from accepted contracts and the native Rust implementation. Treat Python as a frozen reference, not the presumed product path. Do not return raw grep output or propose implementation unless requested. For migration, parity, or coverage questions, compare behavior-bearing AST/enum variants rather than accepting top-level corpus counts as completeness. Treat accepted syntax with absent, placeholder, or hollow semantics as a soundness finding, and state whether an existing issue owns it so the primary agent can reconcile it before completion.

Changes

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.

  1. 2d ago First seen · 38 lines · 40 tokens per session scan A 27bd2ad1fb90

Subscribe to this mod's changes

fsl-codebase-explorer is an agent published in the GitHub repository ymm-oss/fsl (21 stars, last pushed 4d ago), licensed Apache-2.0. It adds 40 tokens to every session and 273 once invoked, about $0.0002 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.

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