diff --git a/changelog.d/fix-image-media-type-sniff.fixed.md b/changelog.d/fix-image-media-type-sniff.fixed.md
new file mode 100644
index 00000000..c2f6ced6
--- /dev/null
+++ b/changelog.d/fix-image-media-type-sniff.fixed.md
@@ -0,0 +1,4 @@
+- Image blocks whose declared `media_type` disagrees with their bytes (e.g. a
+  PNG labeled `image/webp` by an upstream surface) are corrected from the
+  magic bytes at compile time. The provider rejects any request with such a
+  mismatch, so one mislabeled image used to fail every subsequent turn.
diff --git a/src/agent.ts b/src/agent.ts
index 0855aafd..fc5c1eca 100644
--- a/src/agent.ts
+++ b/src/agent.ts
@@ -1,5 +1,6 @@
 import type { Membrane, NormalizedMessage, NormalizedRequest, ContentBlock, YieldingStream } from '@animalabs/membrane';
 import { isAbortedResponse } from '@animalabs/membrane';
+import { correctImageMediaTypes } from './image-media-type.js';
 import { createHash } from 'node:crypto';
 import type { CacheWireReceipt, KvUnifiedRequestHooks } from './kv-unified-wire.js';
 import {
@@ -525,7 +526,7 @@ export class Agent {
   async compileContext(budget?: TokenBudget): Promise<CompileResult> {
     const result = await this.contextManager.compile(this.resolveBudget(budget));
     if (!budget) this.settleRuntimeSettingsTransition();
-    return result;
+    return { ...result, messages: correctImageMediaTypes(result.messages) };
   }
 
   /**
@@ -542,7 +543,7 @@ export class Agent {
       this.resolveBudget(budget), injections, opts as never,
     );
     if (!budget) this.settleRuntimeSettingsTransition();
-    return result;
+    return { ...result, messages: correctImageMediaTypes(result.messages) };
   }
 
   // ==========================================================================
diff --git a/src/image-media-type.ts b/src/image-media-type.ts
new file mode 100644
index 00000000..ef24ddb8
--- /dev/null
+++ b/src/image-media-type.ts
@@ -0,0 +1,55 @@
+import type { ContentBlock, NormalizedMessage } from '@animalabs/membrane';
+
+/** Detect an image's real format from its leading bytes. */
+export function sniffImageMediaTypeBase64(data: string): string | undefined {
+  const head = Buffer.from(data.slice(0, 32), 'base64');
+  if (head.length >= 4 && head[0] === 0x89 && head[1] === 0x50 && head[2] === 0x4e && head[3] === 0x47) return 'image/png';
+  if (head.length >= 3 && head[0] === 0xff && head[1] === 0xd8 && head[2] === 0xff) return 'image/jpeg';
+  if (head.length >= 3 && head[0] === 0x47 && head[1] === 0x49 && head[2] === 0x46) return 'image/gif';
+  if (
+    head.length >= 12 &&
+    head[0] === 0x52 && head[1] === 0x49 && head[2] === 0x46 && head[3] === 0x46 &&
+    head[8] === 0x57 && head[9] === 0x45 && head[10] === 0x42 && head[11] === 0x50
+  ) return 'image/webp';
+  return undefined;
+}
+
+/**
+ * Correct base64 image blocks whose declared media type doesn't match their
+ * bytes (e.g. Discord serving a PNG for an attachment it labels image/webp).
+ * The provider rejects the WHOLE request on a mismatch, so one mislabelled
+ * image in history would fail every turn. Returns new message/block objects
+ * for anything changed; stored history is never touched.
+ */
+export function correctImageMediaTypes(messages: NormalizedMessage[]): NormalizedMessage[] {
+  let changed = 0;
+  const fix = (blocks: ContentBlock[]): ContentBlock[] => {
+    let touched = false;
+    const out = blocks.map((block) => {
+      const b = block as { type: string; source?: { type?: string; data?: string; mediaType?: string }; content?: unknown };
+      if (b.type === 'image' && b.source?.type === 'base64' && typeof b.source.data === 'string') {
+        const actual = sniffImageMediaTypeBase64(b.source.data);
+        if (actual && actual !== b.source.mediaType) {
+          touched = true;
+          changed++;
+          return { ...block, source: { ...b.source, mediaType: actual } } as ContentBlock;
+        }
+      }
+      if (b.type === 'tool_result' && Array.isArray(b.content)) {
+        const inner = fix(b.content as ContentBlock[]);
+        if (inner !== b.content) {
+          touched = true;
+          return { ...block, content: inner } as ContentBlock;
+        }
+      }
+      return block;
+    });
+    return touched ? out : blocks;
+  };
+  const result = messages.map((m) => {
+    const content = fix(m.content);
+    return content === m.content ? m : { ...m, content };
+  });
+  if (changed > 0) console.error(`[images] corrected media type on ${changed} image block(s) whose bytes disagreed with their label`);
+  return result;
+}
diff --git a/test/image-media-type.test.ts b/test/image-media-type.test.ts
new file mode 100644
index 00000000..cf665b9b
--- /dev/null
+++ b/test/image-media-type.test.ts
@@ -0,0 +1,25 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { correctImageMediaTypes } from '../src/image-media-type.js';
+
+const PNG_B64 = 'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR4nGP4////fwAJ+wP9KobjigAAAABJRU5ErkJggg==';
+
+test('corrects a PNG labelled image/webp, including inside tool_result, without mutating input', () => {
+  const img = { type: 'image', source: { type: 'base64', data: PNG_B64, mediaType: 'image/webp' } };
+  const messages = [
+    { participant: 'user', content: [{ type: 'text', text: 'look' }, img] },
+    { participant: 'user', content: [{ type: 'tool_result', toolUseId: 't', content: [img] }] },
+  ] as never;
+  const out = correctImageMediaTypes(messages) as unknown as Array<{ content: any[] }>;
+  assert.equal(out[0].content[1].source.mediaType, 'image/png');
+  assert.equal(out[1].content[0].content[0].source.mediaType, 'image/png');
+  assert.equal(img.source.mediaType, 'image/webp', 'stored block untouched');
+});
+
+test('leaves correct and unknown images alone (same objects)', () => {
+  const messages = [{ participant: 'user', content: [
+    { type: 'image', source: { type: 'base64', data: PNG_B64, mediaType: 'image/png' } },
+    { type: 'image', source: { type: 'base64', data: 'AAAA', mediaType: 'image/png' } },
+  ] }] as never;
+  assert.equal(correctImageMediaTypes(messages)[0], (messages as any)[0]);
+});