diff --git a/agent/image_routing.py b/agent/image_routing.py
index 49eaed2f9d..0b6687787a 100644
--- a/agent/image_routing.py
+++ b/agent/image_routing.py
@@ -144,7 +144,51 @@ def decide_image_input_mode(
 # it fires, which is cheaper than permanent quality loss.
 
 
-def _guess_mime(path: Path) -> str:
+def _sniff_mime_from_bytes(raw: bytes) -> Optional[str]:
+    """Detect image MIME from magic bytes. Returns None if unrecognised.
+
+    Filename-based detection (``mimetypes.guess_type``) is unreliable when
+    upstream platforms lie about content-type. Discord, for example, can
+    serve a PNG with ``content_type=image/webp`` for proxied/animated
+    stickers, custom emoji previews, or images uploaded via certain bots.
+    Anthropic strictly validates that declared media_type matches the
+    actual bytes and returns HTTP 400 on mismatch, so we sniff to be safe.
+    """
+    if not raw:
+        return None
+    # PNG: 89 50 4E 47 0D 0A 1A 0A
+    if raw.startswith(b"\x89PNG\r\n\x1a\n"):
+        return "image/png"
+    # JPEG: FF D8 FF
+    if raw.startswith(b"\xff\xd8\xff"):
+        return "image/jpeg"
+    # GIF87a / GIF89a
+    if raw[:6] in (b"GIF87a", b"GIF89a"):
+        return "image/gif"
+    # WEBP: "RIFF" .... "WEBP"
+    if len(raw) >= 12 and raw[:4] == b"RIFF" and raw[8:12] == b"WEBP":
+        return "image/webp"
+    # BMP: "BM"
+    if raw.startswith(b"BM"):
+        return "image/bmp"
+    # HEIC/HEIF: ftypheic / ftypheix / ftypmif1 / ftypmsf1 etc.
+    if len(raw) >= 12 and raw[4:8] == b"ftyp" and raw[8:12] in (
+        b"heic", b"heix", b"hevc", b"hevx", b"mif1", b"msf1", b"heim", b"heis",
+    ):
+        return "image/heic"
+    return None
+
+
+def _guess_mime(path: Path, raw: Optional[bytes] = None) -> str:
+    """Return image MIME type for *path*.
+
+    If *raw* bytes are provided, magic-byte sniffing wins (authoritative).
+    Otherwise we fall back to ``mimetypes`` then suffix-based defaults.
+    """
+    if raw is not None:
+        sniffed = _sniff_mime_from_bytes(raw)
+        if sniffed:
+            return sniffed
     mime, _ = mimetypes.guess_type(str(path))
     if mime and mime.startswith("image/"):
         return mime
@@ -178,7 +222,7 @@ def _file_to_data_url(path: Path) -> Optional[str]:
     except Exception as exc:
         logger.warning("image_routing: failed to read %s — %s", path, exc)
         return None
-    mime = _guess_mime(path)
+    mime = _guess_mime(path, raw=raw)
     b64 = base64.b64encode(raw).decode("ascii")
     return f"data:{mime};base64,{b64}"
 
diff --git a/tests/agent/test_image_routing.py b/tests/agent/test_image_routing.py
index 89b19a6d42..75f842b471 100644
--- a/tests/agent/test_image_routing.py
+++ b/tests/agent/test_image_routing.py
@@ -217,19 +217,34 @@ class TestBuildNativeContentParts:
         assert str(img2) in text_part["text"]
 
     def test_mime_inference_jpg(self, tmp_path: Path):
+        # Real JPEG bytes (SOI marker FF D8 FF): sniffing now wins over suffix.
         img = tmp_path / "photo.jpg"
-        img.write_bytes(_png_bytes())  # bytes are PNG but extension is jpg
+        img.write_bytes(b"\xff\xd8\xff\xe0\x00\x10JFIF\x00\x01" + b"\x00" * 32)
         parts, _ = build_native_content_parts("x", [str(img)])
         url = parts[1]["image_url"]["url"]
         assert url.startswith("data:image/jpeg;base64,")
 
     def test_mime_inference_webp(self, tmp_path: Path):
+        # Real WEBP bytes (RIFF....WEBP): sniffing now wins over suffix.
         img = tmp_path / "pic.webp"
-        img.write_bytes(_png_bytes())
+        img.write_bytes(b"RIFF\x24\x00\x00\x00WEBPVP8 " + b"\x00" * 32)
         parts, _ = build_native_content_parts("", [str(img)])
         url = parts[1]["image_url"]["url"]
         assert url.startswith("data:image/webp;base64,")
 
+    def test_mime_sniff_overrides_misleading_extension(self, tmp_path: Path):
+        """Discord-style bug: file is named .webp but contains PNG bytes.
+        Anthropic rejects on MIME mismatch (HTTP 400) so we MUST sniff.
+        Regression guard for the user-reported Discord PNG-as-WEBP failure.
+        """
+        img = tmp_path / "discord_cached.webp"
+        img.write_bytes(_png_bytes())  # bytes are PNG, suffix lies
+        parts, _ = build_native_content_parts("", [str(img)])
+        url = parts[1]["image_url"]["url"]
+        assert url.startswith("data:image/png;base64,"), (
+            f"Expected MIME sniffing to detect PNG bytes regardless of .webp suffix, got: {url[:60]}"
+        )
+
 
 # ─── Oversize handling ───────────────────────────────────────────────────────