diff --git a/src/vision/describe.ts b/src/vision/describe.ts
index 68744fd90..4bbc198ee 100644
--- a/src/vision/describe.ts
+++ b/src/vision/describe.ts
@@ -16,6 +16,8 @@ export interface VisionSettings {
 export type DescribeOutcome = { text: string; error?: string };
 
 const ALLOWED_IMAGE_MIME = new Set(["image/png", "image/jpeg", "image/jpg", "image/webp", "image/gif"]);
+const MAX_MIME_CHARS = 127;
+const MIME_PATTERN = /^[a-z0-9][a-z0-9!#$&^_.+-]*\/[a-z0-9][a-z0-9!#$&^_.+-]*$/i;
 /** ~20 MB — generous enough for screenshots; rejects pathological payloads before forwarding. */
 const MAX_IMAGE_BYTES = 20 * 1024 * 1024;
 
@@ -29,6 +31,7 @@ function validateImageUrl(url: string): string | null {
   if (url.startsWith("data:")) {
     const m = /^data:([^;,]+?)(;base64)?,(.*)$/s.exec(url);
     if (!m) return "malformed data URL";
+    if (m[1].length > MAX_MIME_CHARS || !MIME_PATTERN.test(m[1])) return "invalid image media type";
     const mime = m[1].toLowerCase();
     if (!ALLOWED_IMAGE_MIME.has(mime)) return `unsupported image type "${mime}"`;
     if (m[2]) {
diff --git a/src/vision/index.ts b/src/vision/index.ts
index f44b713ec..c6d607274 100644
--- a/src/vision/index.ts
+++ b/src/vision/index.ts
@@ -160,6 +160,10 @@ function clamp(s: string, max: number): string {
   return s.length <= max ? s : `${s.slice(0, max)}\n…[description truncated]`;
 }
 
+function boundedError(error: string): string {
+  return clamp(error.replace(/[\u0000-\u001f\u007f]/g, " ").trim(), DESC_MAX_CHARS);
+}
+
 export interface AnthropicVisionProvider {
   providerName: string;
   provider: OcxProviderConfig;
@@ -270,7 +274,7 @@ function renderDescription(out: { text: string; error?: string }): OcxTextConten
   return {
     type: "text",
     text: out.error
-      ? `[An image was attached but could not be processed: ${out.error}]`
+      ? `[An image was attached but could not be processed: ${boundedError(out.error)}]`
       : `[Image content — described by a vision model because you cannot see images directly:\n${clamp(out.text.trim(), DESC_MAX_CHARS)}]`,
   };
 }
diff --git a/tests/vision-cache.test.ts b/tests/vision-cache.test.ts
index da8f4278f..7c0cdb827 100644
--- a/tests/vision-cache.test.ts
+++ b/tests/vision-cache.test.ts
@@ -283,6 +283,22 @@ describe("vision description cache and per-turn cap", () => {
     expect(textParts(request).join("\n")).toContain(expected);
   });
 
+  test("bounds invalid media types and sidecar errors before inserting prompt text", async () => {
+    const invalidMime = parsed([{ type: "input_image", image_url: `data:${"x".repeat(10_000)},payload` }]);
+    await describeImagesInPlace(invalidMime, plan(), new Headers({ authorization: "Bearer test" }));
+    const invalidMarker = textParts(invalidMime).join("\n");
+    expect(invalidMarker).toContain("invalid image media type");
+    expect(invalidMarker).not.toContain("x".repeat(128));
+
+    globalThis.fetch = (async () => { throw new Error(`bad\u0000\n${"y".repeat(2_100)}`); }) as typeof fetch;
+    const failed = parsed([{ type: "input_image", image_url: DATA_A }]);
+    await describeImagesInPlace(failed, plan(), new Headers({ authorization: "Bearer test" }));
+    const failedMarker = textParts(failed).join("\n");
+    expect(failedMarker).not.toContain("\u0000");
+    expect(failedMarker).toContain("…[description truncated]");
+    expect(failedMarker.length).toBeLessThan(2_100);
+  });
+
   test("cache hit returns the same clamped description without a sidecar call", async () => {
     let calls = 0;
     globalThis.fetch = (async () => { calls++; return openaiSse("y".repeat(2_100)); }) as typeof fetch;