The body chapters use deliberately tool-agnostic vocabulary — briefing doc, extension surface, headless mode — because convergent concepts deserve convergent names. Readers coming in through a single tool’s documentation know a different vocabulary. This glossary is the translation. Each entry: the book’s name for the concept, a short definition, and the specific surface each tool exposes it as.

Terms

Agent

The runtime loop that consumes a prompt, decides what to do, calls tools, observes their output, and iterates until the task is complete. The agent is distinct from the underlying model (which produces text completions) and from the tool (which executes a specific capability like Read or Bash). Across Claude Code, Gemini CLI, and Codex CLI the agent loop has converged on the same shape; differences are in the primitives exposed to the operator.

Approval mode

A permission model — Codex CLI’s distinctive primitive — that configures agent autonomy along a depth axis: “ask every action” → “ask on request” → “ask on failure” → “never ask.” Contrasts with Claude and Gemini’s allow/deny-list models, which configure permission along a tool-scope axis. The two models describe overlapping territory; they are not semantically equivalent. See Permission policy.

Briefing doc

A convention: a Markdown file at the repo root that stateless agents re-read on every session, encoding project-level context (build commands, conventions, current focus). The convergence point is cross-tool:

• Claude Code: CLAUDE.md
• Gemini CLI: GEMINI.md
• Codex CLI: AGENTS.md

All three tools resolve the briefing doc from the working directory upward. The file’s role is identical; only the filename differs.

Context window

The total token budget the model can attend to in a single turn: the briefing doc + conversation history + tool outputs + system prompt. Budget is finite and non-linear — content past certain positions effectively decays (see Context rot). Claude’s production models ship a 200K-token window; Gemini’s span multi-hundred-K to multi-million tokens; Codex-class models typically range around 272K at time of capture. The size difference matters less than it appears once context rot is accounted for.

Context rot

The observation that attention quality degrades across a long context — not only at the window’s hard edge but well before it. A claim made in turn 3 of a 50-turn session may effectively be invisible by turn 40 even if it technically still fits in the token budget. Ch 2 is the main treatment. The phenomenon is substrate-level; it applies to every CLI agent and every large-window model equally.

Convergence / Divergence

Book vocabulary (Ch 15) for how tool behaviors align or differ. A pattern is convergent when all tracked tools have adopted it (e.g. briefing doc at repo root, headless print mode, MCP client support) — signal that the pattern is architecturally stable. A pattern is divergent when the tools diverge on implementation or are not all present (e.g. hook event granularity, approval-mode-vs-allow-list permissions) — signal of open design space. Convergence is a bet practitioners can safely make; divergence is a bet they cannot.

Extension surface

The parts of a tool that operators can extend without modifying the tool itself — slash commands, skills, hooks, MCP servers. Each tool’s extension surface has a different shape (Ch 8); this is the biggest divergence axis across the three tools.

Hook

A user-defined shell command that fires in response to an agent lifecycle event (pre-tool-use, post-tool-use, session-start, etc.). Primarily a Claude Code primitive; Gemini and Codex have hook-adjacent but less-developed surfaces. See Appendix A for the current Claude event list.

Headless mode

Invocation of the agent with no human present to observe, approve, or interrupt during the run. Ch 12 is the main treatment. Cross-tool naming:

• Claude Code: claude -p "<prompt>" (print mode)
• Gemini CLI: gemini -p "<prompt>" or piped stdin
• Codex CLI: approval-mode flags chosen per invocation

Convergent in shape (one-shot non-interactive run); divergent in permission configuration.

MCP (Model Context Protocol)

A cross-tool protocol for exposing capabilities — tools, data sources, prompts — to an agent through a standardized server interface. MCP is one of the clearest convergence points in the field: all three agents are MCP clients; the same MCP server can be consumed by any of them. For Gemini, MCP is the primary extension model; for Claude and Codex, it is one of several extension surfaces.

Permission policy

The set of rules that constrain what an agent can do without asking. In Claude Code: an allow/deny list at tool-name granularity, with glob and path-scoping support, configured in settings.json. In Gemini CLI: similar allow/deny structure with a dangerous --yolo override that bypasses everything. In Codex CLI: approval modes (see above). All three share the underlying abstraction — what may the agent do unattended? — but express it differently.

Plan mode

A read-only agent phase where the agent analyzes, reads, and proposes a plan but cannot mutate files. A Claude Code primitive with a dedicated keyboard shortcut; on Gemini and Codex, closer to a convention operators impose via instructions rather than a first-class mode. Ch 6 treats the principle (think before acting); plan mode is the Claude-specific implementation.

Session

A single bounded conversation between user and agent. Sessions have state (prompt history, context loaded, tools registered). claude --resume, gemini /chat, and Codex’s equivalent let sessions be reattached across invocations. A session is distinct from a workflow (which may span multiple sessions) and from a project (which persists across all sessions).

Skill

A named, self-contained capability the agent can invoke when a situation matches — a directory containing a SKILL.md describing when to use it plus any scripts or references the skill needs. Claude Code uses the term most explicitly and autonomously; Gemini and Codex have adjacent concepts (slash commands, prompt templates) that overlap but are not identical in intent — skills auto-fire, slash commands must be triggered. See Ch 8.

Slash command

A user-invoked shortcut — a slash-prefixed name the operator types to trigger a prompt template or a built-in action. /compact, /memory refresh, /plan and similar. Conventional across the three tools; storage locations differ:

• Claude Code: .claude/commands/ (project) or ~/.claude/commands/ (user)
• Gemini CLI: .gemini/commands/ (project) or ~/.gemini/commands/ (user)
• Codex CLI: project-level config at the repo root

Source tier

One of four trust levels (T1-official → T4-conjecture) assigned to any cited external source per the methodology of Ch 15. The tier determines the audit cadence for claims resting on the source. See Appendix D for the archive index.

Subagent

A child agent spawned by the main agent with its own context window, tool access, and prompt, which runs a bounded subtask and returns a summary to the parent. Claude Code implements this via the Task tool with configurable agent types (.claude/agents/). Gemini and Codex have partial parallels (MCP-based delegation, async task invocation) but not identical ergonomics. Ch 9 treats the principle.

Volatility class

Book vocabulary (Ch 15) for how fast a claim is likely to date: stable-principle, architectural-pattern, or feature-surface. Every chapter carries a volatility class in frontmatter; audit cadences scale accordingly. See the book’s src/content.config.ts for the canonical list.

Tool-to-concept translation table

A compact reference for readers who know one tool’s vocabulary and want to translate:

Volatility note

This glossary’s tool-to-surface mappings are feature-surface volatility — paths, flag names, and specific commands will shift over time. The concepts (briefing doc, headless mode, permission policy, skill, subagent) are architectural-pattern volatility — the shape of the abstraction is durable even when the implementation moves. If a mapping below is stale, the concept is still the right thing to look up; translating to the current tool’s surface is the quarterly audit task.