diff --git a/packages/opencode/src/tool/read.ts b/packages/opencode/src/tool/read.ts
index 0b44c7ad5a79..7e14fa59a2a5 100644
--- a/packages/opencode/src/tool/read.ts
+++ b/packages/opencode/src/tool/read.ts
@@ -150,8 +150,20 @@ export const ReadTool = Tool.defineEffect(
       const loaded = yield* instruction.resolve(ctx.messages, filepath, ctx.messageID)
 
       const mime = AppFileSystem.mimeType(filepath)
-      const isImage = mime.startsWith("image/") && mime !== "image/svg+xml" && mime !== "image/vnd.fastbidsheet"
+      const supportedImageMimes = new Set(["image/jpeg", "image/png", "image/gif", "image/webp"])
+      const isImage = supportedImageMimes.has(mime)
+      const isUnsupportedImage =
+        !isImage && mime.startsWith("image/") && mime !== "image/svg+xml" && mime !== "image/vnd.fastbidsheet"
       const isPdf = mime === "application/pdf"
+
+      if (isUnsupportedImage) {
+        return yield* Effect.fail(
+          new Error(
+            `Cannot read image: ${mime} is not a supported format. Supported image formats: JPEG, PNG, GIF, WebP.`,
+          ),
+        )
+      }
+
       if (isImage || isPdf) {
         const msg = `${isImage ? "Image" : "PDF"} read successfully`
         return {
diff --git a/packages/opencode/test/tool/read.test.ts b/packages/opencode/test/tool/read.test.ts
index 12345266b318..f4ba81125845 100644
--- a/packages/opencode/test/tool/read.test.ts
+++ b/packages/opencode/test/tool/read.test.ts
@@ -425,6 +425,39 @@ root_type Monster;`
       expect(result.output).toContain("table Monster")
     }),
   )
+
+  it.live("rejects unsupported image formats like BMP", () =>
+    Effect.gen(function* () {
+      const dir = yield* tmpdirScoped()
+      // minimal BMP file header
+      const bmp = Buffer.from("BM", "ascii")
+      yield* put(path.join(dir, "image.bmp"), bmp)
+
+      const err = yield* fail(dir, { filePath: path.join(dir, "image.bmp") })
+      expect(err.message).toContain("image/bmp is not a supported format")
+      expect(err.message).toContain("JPEG, PNG, GIF, WebP")
+    }),
+  )
+
+  it.live("rejects unsupported image formats like TIFF", () =>
+    Effect.gen(function* () {
+      const dir = yield* tmpdirScoped()
+      yield* put(path.join(dir, "photo.tiff"), Buffer.from("II", "ascii"))
+
+      const err = yield* fail(dir, { filePath: path.join(dir, "photo.tiff") })
+      expect(err.message).toContain("is not a supported format")
+    }),
+  )
+
+  it.live("rejects unsupported image formats like AVIF", () =>
+    Effect.gen(function* () {
+      const dir = yield* tmpdirScoped()
+      yield* put(path.join(dir, "photo.avif"), Buffer.from([0x00, 0x00, 0x00, 0x1c]))
+
+      const err = yield* fail(dir, { filePath: path.join(dir, "photo.avif") })
+      expect(err.message).toContain("is not a supported format")
+    }),
+  )
 })
 
 describe("tool.read loaded instructions", () => {