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Glossary — Claude API terms in plain English

You do not need to memorize this page. Use it when a term interrupts your understanding; every definition explains what it means in a program, not just what the acronym expands to.

API​

An application programming interface is a way for one program to ask another program to do something. Here, your code sends a request to Anthropic and receives Claude's response.

Anthropic​

Anthropic is the company that develops Claude and operates the API this project calls.

Claude​

Claude is Anthropic's family of AI models. Your application does not talk to a chatbot window; it calls a Claude model through the API and decides what to do with the result.

Model​

A model is the particular Claude engine that generates a response. Models differ in cost, speed, context window, and performance on your own tasks. Choose the cheapest one that passes your evaluation—not the one with the most impressive name.

SDK​

An SDK is a library that makes an API pleasant to call from a programming language. @anthropic-ai/sdk is the TypeScript SDK used in this repository. It turns a Messages API request into client.messages.create(...).

Messages API​

The Messages API is the main Claude API surface used here. You send a list of messages plus options such as a model and token limit; Claude returns a message containing content blocks, a stop reason, and usage.

Prompt​

A prompt is the information sent to Claude to guide a response. It can include system instructions, user messages, tool definitions, and previous conversation. A prompt is input to the model, not a program that it executes.

System prompt​

The system prompt gives a model its role and rules—for example, Northwind's support policy. Keep repeated, unchanging system content stable when using prompt caching.

Token​

A token is a small piece of text that models read and generate. API pricing and max_tokens use tokens, not words. Input tokens are what you send; output tokens are what Claude generates.

Context window​

A context window is the maximum amount of information a model can consider in one request. System prompts, messages, tool definitions, and conversation history all take up part of it.

max_tokens​

max_tokens is an enforced maximum for generated output. If Claude reaches it, the response can be cut off even though the API call succeeded. Check stop_reason instead of guessing from punctuation.

Content block​

Claude responses contain an array of content blocks, not always one string. A block can be text, thinking, tool use, and more. In TypeScript, check a block's type before reading properties specific to text.

Structured outputs / schema​

A schema defines the data shape your program accepts: allowed categories, required fields, and value types. Structured outputs constrain the model to that shape and validate the result, so code does not have to gamble on a prompt that says “return JSON.”

Tool use​

Tool use lets Claude ask your program to run a named function, such as looking up an order. Your program runs the tool, returns its result, and Claude uses that result in the next turn. Tools retrieve or compute facts; they are not permission to let the model perform unrestricted actions.

Tool trace​

A tool trace records every tool call and result in order. It lets a person audit what facts informed an answer and lets server-side guardrails recompute what the model is allowed to recommend.

Guardrail​

A guardrail is deterministic code around the model that enforces a rule. For example, Northwind recalculates refund authority from verified tool results. A guardrail is stronger than a sentence asking the model to obey a rule.

Streaming / SSE​

Streaming sends a response as it is generated rather than waiting for all of it. This project uses Server-Sent Events (SSE), a simple HTTP stream of named events. A streaming client must handle both the final done event and an in-band error event.

Prompt caching​

Prompt caching reuses a large, unchanged beginning of a request at a lower input cost. It is a prefix match: changing text before the cache breakpoint, including a timestamp, prevents a cache hit.

Rate limit​

A rate limit is a provider-enforced limit on how quickly an API can accept requests or tokens. A well-behaved client reads the response headers, backs off, and retries only when the error is retryable.

Usage and cost​

Usage reports how many tokens a request consumed. For cached work, total input is fresh input plus cache writes plus cache reads. For a tool loop, sum usage across every turn—one final response does not represent the full bill.

Evaluation / eval​

An eval is a repeatable test set for an AI feature. It measures whether the system behaves correctly on representative, difficult, adversarial, and benign inputs. An eval makes model or prompt changes observable instead of intuitive.

Prompt injection / untrusted input​

Prompt injection is text in user-controlled data that tries to change the model's instructions. Treat public text as untrusted data, delimit and escape it structurally, redact sensitive information, and enforce important rules in server code.