diff --git a/packages/engine/src/utils/ffprobe.test.ts b/packages/engine/src/utils/ffprobe.test.ts
index 899ffc90b..ab1cf211f 100644
--- a/packages/engine/src/utils/ffprobe.test.ts
+++ b/packages/engine/src/utils/ffprobe.test.ts
@@ -6,6 +6,7 @@ import { afterEach, describe, expect, it, vi } from "vitest";
 import {
   extractMediaMetadata,
   extractPngMetadataFromBuffer,
+  parseFrameRate,
   pixelFormatHasAlpha,
 } from "./ffprobe.js";
 
@@ -578,47 +579,58 @@ describe("ffprobe option separator", () => {
   });
 });
 
-describe("ffprobe frame rate parsing", () => {
-  afterEach(() => {
-    vi.resetModules();
-    vi.doUnmock("child_process");
+describe("parseFrameRate", () => {
+  // Direct against the exported function. The previous table drove this
+  // through extractMediaMetadata behind a spawn mock, which cost a
+  // vi.resetModules() plus a dynamic re-import of core's 238-file barrel per
+  // row (74.9 ms vs 0.094 ms) — and 4 of its 7 rows produced identical values
+  // against the pre-fix implementation, so it could not fail for the bugs it
+  // was written to catch.
+  it.each([
+    ["30/1", 30],
+    ["30000/1001", 29.97],
+    ["24000/1001", 23.98],
+    ["60", 60],
+    ["25.5", 25.5],
+  ])("parses %s as %s", (input, expected) => {
+    expect(parseFrameRate(input)).toBe(expected);
   });
 
   it.each([
-    { r: "30/1", avg: "30/1", expected: 30 },
-    { r: "30000/1001", avg: "30000/1001", expected: 29.97 },
-    { r: "30/", avg: undefined, expected: 0 },
-    { r: "30/0", avg: undefined, expected: 0 },
-    { r: "0/0", avg: undefined, expected: 0 },
-    { r: "abc/def", avg: undefined, expected: 0 },
-    { r: "60", avg: undefined, expected: 60 },
-  ])("parses r=$r avg=$avg as fps=$expected", async ({ r, avg, expected }) => {
-    const { spawn } = createSpawnSpy([
-      {
-        kind: "exit",
-        code: 0,
-        stdout: JSON.stringify({
-          streams: [
-            {
-              codec_type: "video",
-              codec_name: "h264",
-              width: 320,
-              height: 180,
-              r_frame_rate: r,
-              avg_frame_rate: avg,
-            },
-          ],
-          format: { duration: "1.5" },
-        }),
-      },
-    ]);
-    vi.resetModules();
-    vi.doMock("child_process", () => ({ spawn }));
+    ["30/", 0],
+    ["30/0", 0],
+    ["0/0", 0],
+    ["abc/def", 0],
+    ["", 0],
+    [undefined, 0],
+  ])("returns 0 for unusable input %s", (input, expected) => {
+    expect(parseFrameRate(input)).toBe(expected);
+  });
 
-    const { extractMediaMetadata } = await import("./ffprobe.js");
-    const meta = await extractMediaMetadata("/tmp/frame-rate.mp4");
+  // Finite operands, infinite quotient — the operand-only guard missed these.
+  it.each(["1e308/1e-10", "2/1e-320"])("returns 0 for overflowing quotient %s", (input) => {
+    expect(parseFrameRate(input)).toBe(0);
+  });
 
-    expect(meta.fps).toBe(expected);
+  // Negatives were truthy, so `meta.fps || 30` did not rescue them and
+  // buildEncoderArgs emitted `-r -30`.
+  it.each(["-30/1", "30/-1", "-60"])("returns 0 for negative rate %s", (input) => {
+    expect(parseFrameRate(input)).toBe(0);
+  });
+
+  // Fell through to a bare parseFloat that stops at trailing garbage.
+  it.each(["30/1/2", "60fps"])("returns 0 for malformed input %s", (input) => {
+    expect(parseFrameRate(input)).toBe(0);
+  });
+
+  // 2dp rounding collapsed these to 0, and the caller's `|| 30` then
+  // re-encoded a 300-second timelapse as a ~1/30-second clip.
+  it.each([
+    ["1/300", 0.01],
+    ["1/1000", 0.01],
+    ["1/200", 0.01],
+  ])("floors sub-0.005 rate %s to %s rather than 0", (input, expected) => {
+    expect(parseFrameRate(input)).toBe(expected);
   });
 });
 
diff --git a/packages/engine/src/utils/ffprobe.ts b/packages/engine/src/utils/ffprobe.ts
index 08e07a597..217f0807e 100644
--- a/packages/engine/src/utils/ffprobe.ts
+++ b/packages/engine/src/utils/ffprobe.ts
@@ -326,19 +326,54 @@ function readTagCI(tags: Record<string, string | undefined> | undefined, name: s
   return "";
 }
 
-function parseFrameRate(frameRateStr: string | undefined): number {
+/**
+ * Parse an ffprobe rational frame rate ("30000/1001") or plain number.
+ *
+ * Returns 0 for anything not a usable positive rate. Exported so tests
+ * exercise the shipped function directly instead of re-importing the module
+ * behind a spawn mock.
+ *
+ * Every guard here is load-bearing, because a bad value is NOT caught
+ * downstream: callers use `meta.fps || 30`, which only rescues 0 and NaN.
+ * Infinity and negatives are truthy and flow into buildEncoderArgs as
+ * `-r Infinity` / `-r -30`, which ffmpeg rejects mid-render, and into
+ * frameCount arithmetic that then goes negative or non-finite.
+ *
+ *  - the QUOTIENT is checked, not just the operands: "1e308/1e-10" and
+ *    "2/1e-320" have finite parts and an infinite result;
+ *  - the sign is checked: "-30/1", "30/-1" and "-60" all parsed clean;
+ *  - more than two parts is rejected: "30/1/2" used to fall through to the
+ *    bare parseFloat below and return 30, as did "60fps", because parseFloat
+ *    stops at trailing garbage;
+ *  - sub-0.005 rates round to 0 at 2dp and would be replaced by the caller's
+ *    30fps default, re-encoding a 300-second 1/300-fps timelapse as a
+ *    ~1/30-second clip. Kept as 0 is wrong too, so they are floored to the
+ *    smallest representable 2dp rate instead.
+ */
+export function parseFrameRate(frameRateStr: string | undefined): number {
   if (!frameRateStr) return 0;
+
   const parts = frameRateStr.split("/");
-  if (parts.length === 2) {
-    const num = parseFloat(parts[0] ?? "");
-    const den = parseFloat(parts[1] ?? "");
-    if (Number.isFinite(num) && Number.isFinite(den) && den !== 0) {
-      return Math.round((num / den) * 100) / 100;
-    }
-    return 0;
-  }
-  const parsed = parseFloat(frameRateStr);
-  return Number.isFinite(parsed) ? parsed : 0;
+  if (parts.length > 2) return 0;
+
+  const raw =
+    parts.length === 2
+      ? (() => {
+          const num = parseFloat(parts[0] ?? "");
+          const den = parseFloat(parts[1] ?? "");
+          if (!Number.isFinite(num) || !Number.isFinite(den) || den === 0) return NaN;
+          return num / den;
+        })()
+      : // Not parseFloat: it stops at trailing garbage, so "60fps" parsed as
+        // 60. Number() rejects the whole string.
+        Number(frameRateStr.trim());
+
+  if (!Number.isFinite(raw) || raw <= 0) return 0;
+
+  const rounded = Math.round(raw * 100) / 100;
+  // A real but very slow rate must not collapse to 0 and inherit the
+  // caller's 30fps default.
+  return rounded > 0 ? rounded : 0.01;
 }
 
 /**