diff --git a/CHANGES/1302.bugfix b/CHANGES/1302.bugfix
new file mode 100644
index 000000000..66b0f43cf
--- /dev/null
+++ b/CHANGES/1302.bugfix
@@ -0,0 +1 @@
+Fixed Simple API cache incorrectly serving JSON responses for HTML requests when clients used `?format=json`.
diff --git a/pulp_python/app/cache.py b/pulp_python/app/cache.py
index f67c5c9ef..4639262ee 100644
--- a/pulp_python/app/cache.py
+++ b/pulp_python/app/cache.py
@@ -1,25 +1,26 @@
 from pulpcore.plugin.cache import CacheKeys, SyncContentCache
 from pulpcore.plugin.util import cache_key
 
-ACCEPT_HEADER_KEY = "accept_header"
+MEDIA_TYPE_KEY = "media_type"
 
 
 class PythonApiCache(SyncContentCache):
     """
     Cache for the Simple API.
 
-    Adds Accept header to the cache key so HTML and JSON responses are cached separately.
+    Keys on the negotiated media type so HTML and JSON are cached separately,
+    including when the same Accept header is used with `?format=json`.
     """
 
     def __init__(self, base_key=None):
-        keys = (CacheKeys.path, CacheKeys.method, ACCEPT_HEADER_KEY)
+        keys = (CacheKeys.path, CacheKeys.method, MEDIA_TYPE_KEY)
         super().__init__(base_key=base_key, keys=keys)
 
     def make_key(self, request):
         all_keys = {
             CacheKeys.path: request.path,
             CacheKeys.method: request.method,
-            ACCEPT_HEADER_KEY: request.headers.get("accept", ""),
+            MEDIA_TYPE_KEY: getattr(request, "accepted_media_type", "") or "",
         }
         return ":".join(all_keys[k] for k in self.keys)
 
diff --git a/pulp_python/tests/functional/api/test_simple_cache.py b/pulp_python/tests/functional/api/test_simple_cache.py
index f2123e610..448902656 100644
--- a/pulp_python/tests/functional/api/test_simple_cache.py
+++ b/pulp_python/tests/functional/api/test_simple_cache.py
@@ -6,6 +6,7 @@
 from pulp_python.tests.functional.constants import (
     PYPI_SIMPLE_V1_HTML,
     PYPI_SIMPLE_V1_JSON,
+    PYPI_TEXT_HTML,
     PYTHON_SM_PROJECT_SPECIFIER,
 )
 
@@ -59,7 +60,7 @@ def test_simple_cache_hit_miss_and_headers(synced_distro):
 @pytest.mark.parallel
 def test_simple_cache_separate_accept_headers(synced_distro):
     """
-    HTML and JSON responses are cached separately.
+    HTML and JSON responses are cached separately by negotiated media type.
     """
     url = urljoin(synced_distro.base_url, "simple/")
 
@@ -74,6 +75,45 @@ def test_simple_cache_separate_accept_headers(synced_distro):
         assert r.headers["X-PULP-CACHE"] == "HIT"
 
 
+@pytest.mark.parallel
+def test_simple_cache_format_json_does_not_poison_html(synced_distro):
+    """
+    A ?format=json response must not poison a later request with the same Accept.
+
+    Clients like uv/pip send an Accept that allows both JSON and HTML. DRF's
+    ?format=json overrides negotiation to JSON, while the same Accept without
+    that query param selects HTML. Caching must key on the negotiated type so
+    the JSON entry is not served (and re-rendered) for the HTML request.
+    """
+    url = f"{urljoin(synced_distro.base_url, 'simple/')}aiohttp"
+    # pip/uv-style Accept: JSON preferred, HTML still acceptable
+    headers = {
+        "Accept": (f"{PYPI_SIMPLE_V1_JSON}, {PYPI_SIMPLE_V1_HTML};q=0.1, {PYPI_TEXT_HTML};q=0.01")
+    }
+
+    r_json = requests.get(url, headers=headers, params={"format": "json"})
+    assert r_json.status_code == 200
+    assert PYPI_SIMPLE_V1_JSON in r_json.headers["Content-Type"]
+    assert r_json.headers["X-PULP-CACHE"] == "MISS"
+    assert r_json.json()["name"] == "aiohttp"
+
+    r_html = requests.get(url, headers=headers)
+    assert r_html.status_code == 200
+    assert PYPI_TEXT_HTML in r_html.headers["Content-Type"]
+    assert r_html.headers["X-PULP-CACHE"] == "MISS"
+    assert b"<a href=" in r_html.content
+
+    r_html_hit = requests.get(url, headers=headers)
+    assert r_html_hit.status_code == 200
+    assert r_html_hit.headers["X-PULP-CACHE"] == "HIT"
+    assert PYPI_TEXT_HTML in r_html_hit.headers["Content-Type"]
+
+    r_json_hit = requests.get(url, headers=headers, params={"format": "json"})
+    assert r_json_hit.status_code == 200
+    assert r_json_hit.headers["X-PULP-CACHE"] == "HIT"
+    assert PYPI_SIMPLE_V1_JSON in r_json_hit.headers["Content-Type"]
+
+
 @pytest.mark.parallel
 def test_simple_cache_etag_conditional_request(synced_distro):
     """