diff --git a/TODO.md b/TODO.md
index a799e22..21dc34d 100644
--- a/TODO.md
+++ b/TODO.md
@@ -30,8 +30,10 @@ declares one.
   `dateTimeOriginal()`, `offsetTimeOriginal()`, `exposureTime()`, `fNumber()`,
   `iso()`, `focalLength()`, `orientation()`, `gpsLatitude()`, `gpsLongitude()`
   and `gpsAltitude()` (issue 148). Each calls `exif()` and returns its one
-  field, or undefined when the file lacks it. A test checks, on the JPEG and the
-  HEIC, that every field `exif()` returns has a method giving the same value.
+  field, or undefined when the file lacks it. `Photo` implements a type with one
+  method per `PhotoExif` field, so the build's type check fails when a field has
+  no method. A test checks, on the JPEG and the HEIC, that each method gives the
+  same value as `exif()`.
 
 - 2026-10-01: `photo.exif()` and `backup-metadata --exif` read EXIF from
   HEIC/HEIF originals, a live photo's HEIC image included, as well as JPEG and
diff --git a/src/library/read.ts b/src/library/read.ts
index c2d8203..8af2165 100644
--- a/src/library/read.ts
+++ b/src/library/read.ts
@@ -35,9 +35,16 @@ const byNewest = (a: PhotoRecord, b: PhotoRecord): number =>
 const byNewestAlbum = (a: AlbumRecord, b: AlbumRecord): number =>
     b.updationTime - a.updationTime || b.collectionID - a.collectionID;
 
+// A method for each `PhotoExif` field, named after it, taking the options
+// `exif()` takes and giving that field. `Photo` implements it, so the build's
+// type check fails when `PhotoExif` has a field `Photo` has no method for.
+type PhotoExifMethods = {
+    [K in keyof PhotoExif]-?: (opts?: ContentOptions) => Promise<PhotoExif[K]>;
+};
+
 // A single photo. Field access mirrors `PhotoRecord`; `record()` returns the
 // underlying plain record for callers that need the IPC-safe value.
-export class Photo {
+export class Photo implements PhotoExifMethods {
     constructor(
         private readonly rec: PhotoRecord,
         private readonly cache?: PhotoContent,
diff --git a/test/library/content-library.test.ts b/test/library/content-library.test.ts
index 7864483..7485ade 100644
--- a/test/library/content-library.test.ts
+++ b/test/library/content-library.test.ts
@@ -462,6 +462,10 @@ describe("Photo save path, local copy, content and EXIF", () => {
         await lib.close();
     });
 
+    // The build's type check, not this test, makes sure `Photo` has a method
+    // for every `PhotoExif` field, whatever the fixtures hold: `Photo`
+    // implements a type with one method per field. This test checks that each
+    // method gives the same value as exif().
     it.each([
         ["JPEG", JPEG_WITH_EXIF],
         ["HEIC", HEIC_WITH_EXIF],