diff --git a/routstr/upstream/base.py b/routstr/upstream/base.py
index 1a080eaf..0b6be7f1 100644
--- a/routstr/upstream/base.py
+++ b/routstr/upstream/base.py
@@ -433,6 +433,14 @@ class BaseUpstreamProvider:
         ``cache_creation_input_tokens`` fields are left in place for clients
         that want the breakdown.
 
+        Which field may be folded mirrors ``normalize_usage`` exactly:
+        Anthropic-native ``input_tokens`` *excludes* the cached portion and
+        needs the roll-up, while a ``prompt_tokens`` grand total (OpenAI
+        family, DeepSeek, OpenRouter, litellm) *already includes* it —
+        folding there double-counts the cache in the visible prompt total
+        (Venice showed 27997 prompt tokens for a 14075-token prompt after a
+        13922-token cache read).
+
         For Anthropic-shaped responses (``input_tokens`` present), the cache
         fields are forced to ``0`` when the upstream omitted them, so the
         client always sees a consistent shape.
@@ -459,11 +467,10 @@ class BaseUpstreamProvider:
                 usage["input_tokens"] = int(usage.get("input_tokens") or 0) + extra
             except (TypeError, ValueError):
                 pass
-        if "prompt_tokens" in usage:
-            try:
-                usage["prompt_tokens"] = int(usage.get("prompt_tokens") or 0) + extra
-            except (TypeError, ValueError):
-                pass
+        # ``prompt_tokens`` is deliberately left untouched: in every dialect
+        # that reports it, it is an inclusive grand total that already
+        # contains the cached portion — the same assumption
+        # ``normalize_usage`` subtracts against when billing.
 
     def _apply_provider_field(self, response_json: object) -> None:
         """Stamp the routstr ``provider`` field onto an upstream response payload.
diff --git a/tests/unit/test_coverage_base2.py b/tests/unit/test_coverage_base2.py
index 1c6a7e46..96bcd7e2 100644
--- a/tests/unit/test_coverage_base2.py
+++ b/tests/unit/test_coverage_base2.py
@@ -116,6 +116,54 @@ def test_fold_cache_preserves_total() -> None:
     assert usage.prompt_tokens == 100
 
 
+def test_fold_cache_openai_dialect_prompt_tokens_untouched() -> None:
+    """Venice/OpenAI shape: prompt_tokens already includes cached tokens.
+
+    Regression: folding cache_read into prompt_tokens double-counted the
+    cache (14075 real prompt shown as 27997 after a 13922-token cache read).
+    """
+    from routstr.upstream.base import BaseUpstreamProvider
+
+    usage = {
+        "prompt_tokens": 14075,
+        "completion_tokens": 24,
+        "total_tokens": 14099,
+        "prompt_tokens_details": {"cached_tokens": 13922},
+        "cache_read_input_tokens": 13922,
+    }
+    BaseUpstreamProvider._fold_cache_into_input_tokens(usage)
+    assert usage["prompt_tokens"] == 14075
+    assert usage["cache_read_input_tokens"] == 13922
+
+
+def test_fold_cache_anthropic_dialect_folds_input_tokens() -> None:
+    """Anthropic-native shape: input_tokens excludes cache, so it is folded."""
+    from routstr.upstream.base import BaseUpstreamProvider
+
+    usage = {
+        "input_tokens": 153,
+        "output_tokens": 24,
+        "cache_read_input_tokens": 13922,
+        "cache_creation_input_tokens": 0,
+    }
+    BaseUpstreamProvider._fold_cache_into_input_tokens(usage)
+    assert usage["input_tokens"] == 153 + 13922
+
+
+def test_fold_cache_litellm_mirror_folds_only_input_tokens() -> None:
+    """Both fields present (litellm mirror): fold input_tokens only."""
+    from routstr.upstream.base import BaseUpstreamProvider
+
+    usage = {
+        "prompt_tokens": 14075,  # inclusive grand total
+        "input_tokens": 153,  # additive Anthropic mirror
+        "cache_read_input_tokens": 13922,
+    }
+    BaseUpstreamProvider._fold_cache_into_input_tokens(usage)
+    assert usage["prompt_tokens"] == 14075
+    assert usage["input_tokens"] == 153 + 13922
+
+
 # ===========================================================================
 # get_cached_models / get_cached_model_by_id
 # ===========================================================================