fix: validate compact budget options

CHANGELOG.md

@@ -6,6 +6,8 @@
 
 - Invalid or negative numeric CLI flags now fail clearly instead of producing
   `NaN` costs or silently skipping token-budget compaction.
+- `compact()` now rejects negative or non-finite token budgets and invalid
+  protected-turn counts before transforming the payload.
 
 ## [0.1.1] - 2026-08-06
 

README.md

@@ -55,6 +55,9 @@ const { payload, report: cut } = compact(messages, {
 // cut -> { beforeTokens, afterTokens, savedTokens, savedPct, actions }
 ```
 
+Token budgets must be finite, non-negative numbers. `keepLastTurns` must also
+be an integer; invalid values throw a `RangeError` before the payload changes.
+
 `payload` is an array of messages, or `{ system, messages }`. Message `content` may be a string or an array of Anthropic-style blocks.
 
 ## Accuracy

src/index.mjs

@@ -57,6 +57,11 @@ export function analyzePayload(payload, { pricePerMTok = 3, counter = estimateTo
 
 function clone(x) { return JSON.parse(JSON.stringify(x)); }
 function tokensToChars(t) { return Math.max(0, Math.round(t * 4)); }
+function requireNonNegativeNumber(name, value, { integer = false } = {}) {
+  if (typeof value !== "number" || !Number.isFinite(value) || value < 0 || (integer && !Number.isInteger(value))) {
+    throw new RangeError(`${name} must be a non-negative${integer ? " integer" : " finite number"}`);
+  }
+}
 
 // Cut a payload's token cost deterministically. Returns { payload, report }.
 // Options:
@@ -66,6 +71,9 @@ function tokensToChars(t) { return Math.max(0, Math.round(t * 4)); }
 //   keepLastTurns        never drop the last N messages when trimming to budget (default 4)
 export function compact(payload, opts = {}) {
   const { maxToolResultTokens = 500, dropDuplicates = true, maxTokens = null, keepLastTurns = 4, counter = estimateTokens } = opts;
+  requireNonNegativeNumber("maxToolResultTokens", maxToolResultTokens);
+  if (maxTokens != null) requireNonNegativeNumber("maxTokens", maxTokens);
+  requireNonNegativeNumber("keepLastTurns", keepLastTurns, { integer: true });
   const before = analyzePayload(payload, { counter }).totalTokens;
   const out = clone(payload);
   const actions = [];

test/basic.test.mjs

@@ -74,3 +74,21 @@ test("compact trims a large payload with linear token counting", () => {
   assert.equal(report.actions.length, 496);
   assert.ok(counterCalls <= messages.length * 3, `counter called ${counterCalls} times`);
 });
+
+test("compact rejects invalid budgets before transforming the payload", () => {
+  const payload = { messages: [{ role: "user", content: "keep me" }] };
+  const invalidOptions = [
+    ["maxToolResultTokens", -1],
+    ["maxToolResultTokens", Number.POSITIVE_INFINITY],
+    ["maxTokens", Number.NaN],
+    ["keepLastTurns", -1],
+    ["keepLastTurns", 1.5],
+  ];
+
+  for (const [name, value] of invalidOptions) {
+    assert.throws(
+      () => compact(payload, { [name]: value }),
+      { name: "RangeError", message: new RegExp(`${name} must be`) },
+    );
+  }
+});