Reject invalid numeric CLI flags

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+## [Unreleased]
+
+### Fixed
+
+- Invalid or negative numeric CLI flags now fail clearly instead of producing
+  `NaN` costs or silently skipping token-budget compaction.
+
 ## [0.1.1] - 2026-08-06
 
 ### Changed

README.md

@@ -74,6 +74,8 @@ tokencut <payload.json> --compact    cut, print savings
   --json           machine-readable output
 ```
 
+Numeric flags must be finite and non-negative. A zero token budget is valid.
+
 ## License
 
 [MIT](LICENSE)

bin/tokencut.mjs

@@ -23,7 +23,20 @@ Payload: an array of messages, or { system, messages } (Anthropic or OpenAI styl
   process.exit(file ? 0 : 1);
 }
 
-const price = Number(flag("--price", 3));
+const numericFlag = (name, def) => {
+  const raw = flag(name, def);
+  const value = Number(raw);
+  if (typeof raw === "boolean" || !Number.isFinite(value) || value < 0) {
+    console.error(
+      `${name} must be a finite, non-negative number; received ${JSON.stringify(raw)}`,
+    );
+    process.exit(1);
+  }
+  return value;
+};
+const price = numericFlag("--price", 3);
+const maxTokens = has("--max") ? numericFlag("--max", null) : null;
+const maxToolResultTokens = numericFlag("--max-tool", 500);
 let payload;
 try { payload = JSON.parse(readFileSync(file, "utf8")); }
 catch (e) { console.error(`could not read ${file}: ${e.message}`); process.exit(1); }
@@ -33,8 +46,8 @@ const usd = (n) => "$" + n.toFixed(n < 0.01 ? 5 : 4);
 
 if (has("--compact")) {
   const res = compact(payload, {
-    maxTokens: flag("--max", null) ? Number(flag("--max", null)) : null,
-    maxToolResultTokens: Number(flag("--max-tool", 500)),
+    maxTokens,
+    maxToolResultTokens,
     dropDuplicates: !has("--no-dedupe"),
   });
   if (flag("--out", null)) { writeFileSync(String(flag("--out", null)), JSON.stringify(res.payload, null, 2)); }

test/cli.test.mjs

@@ -0,0 +1,58 @@
+import assert from "node:assert/strict";
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { spawnSync } from "node:child_process";
+import test, { after } from "node:test";
+
+const fixtureDir = mkdtempSync(join(tmpdir(), "tokencut-cli-"));
+const fixture = join(fixtureDir, "payload.json");
+writeFileSync(fixture, JSON.stringify([{ role: "user", content: "hello" }]));
+after(() => rmSync(fixtureDir, { recursive: true, force: true }));
+
+function run(...args) {
+  return spawnSync(process.execPath, ["bin/tokencut.mjs", fixture, ...args], {
+    cwd: new URL("..", import.meta.url),
+    encoding: "utf8",
+  });
+}
+
+test("rejects invalid numeric flags with the flag and received value", async (t) => {
+  const cases = [
+    {
+      args: ["--compact", "--max", "abc"],
+      flag: "--max",
+      received: '"abc"',
+    },
+    {
+      args: ["--compact", "--max-tool", "-1"],
+      flag: "--max-tool",
+      received: '"-1"',
+    },
+    { args: ["--price", "NaN"], flag: "--price", received: '"NaN"' },
+    {
+      args: ["--price", "Infinity"],
+      flag: "--price",
+      received: '"Infinity"',
+    },
+    { args: ["--price"], flag: "--price", received: "true" },
+  ];
+
+  for (const scenario of cases) {
+    await t.test(scenario.flag + " " + scenario.received, () => {
+      const result = run(...scenario.args);
+      assert.notEqual(result.status, 0);
+      assert.equal(
+        result.stderr,
+        `${scenario.flag} must be a finite, non-negative number; received ${scenario.received}\n`,
+      );
+      assert.equal(result.stdout, "");
+    });
+  }
+});
+
+test("accepts zero as a compact token budget", () => {
+  const result = run("--compact", "--max", "0", "--json");
+  assert.equal(result.status, 0, result.stderr);
+  assert.doesNotThrow(() => JSON.parse(result.stdout));
+});