LanguageModel: contextUsage property
Limited availability
This feature is not Baseline because it does not work in some of the most widely-used browsers.
Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.
The contextUsage read-only property of the LanguageModel interface returns the number of context window tokens currently consumed by the session that calls it, including initial prompts and all subsequent turns.
This value increases every time you call prompt(), promptStreaming(), or append().
Compare contextUsage with LanguageModel.contextWindow to determine how many tokens remain. When contextUsage would exceed contextWindow, subsequent method calls throw a QuotaExceededError and the contextoverflow event fires.
To estimate how many tokens a new prompt would use before sending it, call LanguageModel.measureContextUsage().
Value
A number representing the current context window usage in tokens.
Examples
Monitoring context usage during a conversation
This example writes context usage to the console after a session prompt completes.
const session = await LanguageModel.create();
await session.prompt("Tell me about the history of the internet.");
console.log(
`Context used: ${session.contextUsage} / ${session.contextWindow} tokens`,
);
Warning when the context is nearly full
The following example uses a function to verify that context is available before calling LanguageModel.prompt(). It first calculates the remaining context and passes that value to measureContextUsage(). If needed is less than or equal to remaining, it returns true and the session continues.
const promptText = "Let me ask you an interesting question...";
async function contextAvailable(promptText) {
const remaining = session.contextWindow - session.contextUsage;
const needed = await session.measureContextUsage(promptText);
return needed <= remaining;
}
const session = await LanguageModel.create();
if (await contextAvailable(promptText)) {
const response = await session.prompt(promptText);
console.log(response);
} else {
console.warn("Prompt skipped: Not enough context window remaining.");
}
Specifications
| Specification |
|---|
| Prompt API # dom-languagemodel-contextusage |