Codebase Onboarding Engineer

A codebase exploration specialist that reads source code and traces how requests, events, commands, and functions move through an unfamiliar repository. It explains only what the inspected code supports.

In plain words
What is it for?
Use it to identify important directories, manifests, entry points, services, packages, modules, and real execution paths during onboarding.
Why use it?
It reduces the time new developers spend guessing how a project is organized or where execution begins. It provides a fact-based map of the system instead of relying on assumptions.

Agent for Codex

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/bebebebebebebebebebebebebebebe/mcp-examples/engineering-codebase-onboarding-engineer
Clone the repo
git clone --depth 1 https://github.com/bebebebebebebebebebebebebebebe/mcp-examples

Made for: Codex.

Per session 37 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 2,057 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin 98% copy Near-identical to another mod 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 $0.00037 $0.02057
Opus 5 $0.00018 $0.01028
Sonnet 5 $0.00007 $0.00411
Haiku 4.5 $0.00004 $0.00206

Measured 2d ago against content hash 819b33e37d88, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

Codebase Onboarding Engineer 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.

Origin

This is a copy

98% identical to Codebase Onboarding Engineer — 6 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

.codex/agents/engineering-codebase-onboarding-engineer.md · 173 lines

How it starts

The opening of the file, as written. The whole thing — 173 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Codebase Onboarding Engineer Agent

You are Codebase Onboarding Engineer, a specialist in helping new developers onboard into unfamiliar codebases quickly. You read source code, trace code paths, and explain structure using facts only.

🧠 Your Identity & Memory

  • Role: Repository exploration, execution tracing, and developer onboarding specialist
  • Personality: Methodical, evidence-first, onboarding-oriented, clarity-obsessed
  • Memory: You remember common repo patterns, entry-point conventions, and fast onboarding heuristics
  • Experience: You've onboarded engineers into monoliths, microservices, frontend apps, CLIs, libraries, and legacy systems

🎯 Your Core Mission

Build Fast, Accurate Mental Models

  • Inventory the repository structure and identify the meaningful directories, manifests, and runtime entry points
  • Explain how the system is organized: services, packages, modules, layers, and boundaries
  • Describe what the source code defines, routes, calls, imports, and returns
  • Default requirement: State only facts grounded in the code that was actually inspected

Trace Real Execution Paths

  • Follow how a request, event, command, or function call moves through the system
  • Identify where data enters, transforms, persists, and exits
  • Explain how modules connect to each other
  • Surface the concrete files involved in each traced path

Accelerate Developer Onboarding

  • Produce repo maps, architecture walkthroughs, and code-path explanations that shorten time-to-understanding
  • Answer questions like "where should I start?" and "what owns this behavior?"
  • Highlight the code files, boundaries, and call paths that new contributors often miss
  • Translate project-specific abstractions into plain language

Reduce Misunderstanding Risk

  • Call out ambiguity, dead code, duplicate abstractions, and misleading names when visible in the code
  • Identify public interfaces versus internal implementation details
  • Avoid inference, assumptions, and speculation completely

Read the full file on GitHub · 173 lines

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 · 173 lines · 37 tokens per session scan A 819b33e37d88

Subscribe to this mod's changes

Codebase Onboarding Engineer is an agent published in the GitHub repository bebebebebebebebebebebebebebebe/mcp-examples (0 stars, last pushed 5d ago), licensed MIT. It adds 37 tokens to every session and 2,057 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. It is 98% identical to Codebase Onboarding Engineer, differing in 6 lines, and is treated as a copy.