diff --git a/src/agents/tools/image-tool.test.ts b/src/agents/tools/image-tool.test.ts
index 1dbf63d9630c..d8c7e1a5d183 100644
--- a/src/agents/tools/image-tool.test.ts
+++ b/src/agents/tools/image-tool.test.ts
@@ -1696,8 +1696,8 @@ describe("image tool implicit imageModel config", () => {
             items: { type: "string" },
           },
           model: { type: "string" },
-          maxBytesMb: { type: "number" },
-          maxImages: { type: "number" },
+          maxBytesMb: { type: "number", exclusiveMinimum: 0 },
+          maxImages: { type: "integer", minimum: 1 },
         },
       });
     });
@@ -2343,6 +2343,40 @@ describe("image tool MiniMax VLM routing", () => {
     expect(tooManyDetails?.max).toBe(1);
   });
 
+  it("rejects invalid image cap values before loading images", async () => {
+    const { fetch, tool } = await createMinimaxVlmFixture({ status_code: 0, status_msg: "" });
+
+    await expect(
+      tool.execute("t1", {
+        prompt: "Compare these images.",
+        image: `data:image/png;base64,${ONE_PIXEL_PNG_B64}`,
+        maxImages: 1.5,
+      }),
+    ).rejects.toThrow("maxImages must be a positive integer");
+
+    await expect(
+      tool.execute("t2", {
+        prompt: "Compare these images.",
+        image: `data:image/png;base64,${ONE_PIXEL_PNG_B64}`,
+        maxBytesMb: 0,
+      }),
+    ).rejects.toThrow("maxBytesMb must be greater than 0");
+    expect(fetch).not.toHaveBeenCalled();
+  });
+
+  it("accepts string image caps through shared numeric readers", async () => {
+    const { fetch, tool } = await createMinimaxVlmFixture({ status_code: 0, status_msg: "" });
+
+    await tool.execute("t1", {
+      prompt: "Describe this image.",
+      image: `data:image/png;base64,${ONE_PIXEL_PNG_B64}`,
+      maxImages: "1",
+      maxBytesMb: "1",
+    });
+
+    expect(fetch).toHaveBeenCalledTimes(1);
+  });
+
   it("surfaces MiniMax API errors from /v1/coding_plan/vlm", async () => {
     const { tool } = await createMinimaxVlmFixture({ status_code: 1004, status_msg: "bad key" });
 
diff --git a/src/agents/tools/image-tool.ts b/src/agents/tools/image-tool.ts
index f4fafb3f2634..e001263d022f 100644
--- a/src/agents/tools/image-tool.ts
+++ b/src/agents/tools/image-tool.ts
@@ -45,6 +45,8 @@ import {
   resolveImageFallbackCandidates,
   resolveImageFallbackDefaultProvider,
 } from "../model-fallback.js";
+import { optionalFiniteNumberSchema, optionalPositiveIntegerSchema } from "../schema/typebox.js";
+import { readFiniteNumberParam, readPositiveIntegerParam } from "./common.js";
 import {
   coerceImageAssistantText,
   coerceImageModelConfig,
@@ -761,8 +763,8 @@ export function createImageTool(options?: {
         }),
       ),
       model: Type.Optional(Type.String()),
-      maxBytesMb: Type.Optional(Type.Number()),
-      maxImages: Type.Optional(Type.Number()),
+      maxBytesMb: optionalFiniteNumberSchema({ exclusiveMinimum: 0 }),
+      maxImages: optionalPositiveIntegerSchema(),
     }),
     execute: async (_toolCallId, args) => {
       const record = args && typeof args === "object" ? (args as Record<string, unknown>) : {};
@@ -794,11 +796,7 @@ export function createImageTool(options?: {
       }
 
       // MARK: - Enforce max images cap
-      const maxImagesRaw = typeof record.maxImages === "number" ? record.maxImages : undefined;
-      const maxImages =
-        typeof maxImagesRaw === "number" && Number.isFinite(maxImagesRaw) && maxImagesRaw > 0
-          ? Math.floor(maxImagesRaw)
-          : DEFAULT_MAX_IMAGES;
+      const maxImages = readPositiveIntegerParam(record, "maxImages") ?? DEFAULT_MAX_IMAGES;
       if (imageInputs.length > maxImages) {
         return {
           content: [
@@ -815,7 +813,11 @@ export function createImageTool(options?: {
         record,
         DEFAULT_PROMPT,
       );
-      const maxBytesMb = typeof record.maxBytesMb === "number" ? record.maxBytesMb : undefined;
+      const maxBytesMb = readFiniteNumberParam(record, "maxBytesMb", {
+        min: 0,
+        minExclusive: true,
+        message: "maxBytesMb must be greater than 0",
+      });
       const maxBytes = pickMaxBytes(options?.config, maxBytesMb);
       const imageModelConfig =
         resolvedImageModelConfig ??