fix: bound uncached remainder by prompt-max(cached,write) and forward compact prompt_cache_key

service/relayconvert/internal/oai_chat/to_claude_messages_resp.go

@@ -44,7 +44,8 @@ func buildClaudeUsageFromOpenAIUsage(oaiUsage *dto.Usage) *dto.ClaudeUsage {
 	if oaiUsage.PromptTokensDetails.CacheWriteTokens > 0 {
 		// OpenAI native cache-write usage counts cached and cache-write tokens
 		// inside prompt_tokens, while Claude semantics reports input_tokens
-		// excluding both; the uncached remainder clamps at zero.
+		// excluding both. Both counts are unadjusted prefixes and may overlap,
+		// so clamp a negative remainder at zero.
 		inputTokens = oaiUsage.PromptTokens - oaiUsage.PromptTokensDetails.CachedTokens - cacheCreationTokens
 		if inputTokens < 0 {
 			inputTokens = 0

service/relayconvert/internal/oai_chat/to_claude_messages_resp_test.go

@@ -92,7 +92,7 @@ func TestBuildClaudeUsageFromOpenAICacheWriteUsage(t *testing.T) {
 
 	require.NotNil(t, usage)
 	// Claude semantics reports input_tokens excluding cache read/write; the
-	// remainder 3619-2921-3616 clamps to 0.
+	// overlapping unadjusted prefixes drive the remainder negative, clamp to 0.
 	assert.Equal(t, 0, usage.InputTokens)
 	assert.Equal(t, 2921, usage.CacheReadInputTokens)
 	assert.Equal(t, 3616, usage.CacheCreationInputTokens)

service/text_quota.go

@@ -294,9 +294,10 @@ func calculateTextQuotaSummary(ctx *gin.Context, relayInfo *relaycommon.RelayInf
 			}
 		}
 
-		// OpenAI cache-write usage can report cached_tokens + cache_write_tokens
-		// exceeding prompt_tokens; the uncached remainder must clamp at zero so
-		// billing never subtracts more than the reported input.
+		// OpenAI cache-write usage reports unadjusted prefix counts, so
+		// cached_tokens + cache_write_tokens can exceed prompt_tokens and the
+		// remainder can go negative. Clamp at zero so overlap never turns into
+		// a negative base charge.
 		if baseTokens.IsNegative() {
 			baseTokens = decimal.Zero
 		}

service/text_quota_test.go

@@ -411,8 +411,8 @@ func TestCalculateTextQuotaSummaryBillsOpenAICacheWriteTokens(t *testing.T) {
 
 	t.Run("uncached remainder clamps to zero", func(t *testing.T) {
 		// Real OpenAI payload shape: cached_tokens + cache_write_tokens exceeds
-		// prompt_tokens, so the uncached remainder must clamp to 0 instead of
-		// producing a negative charge component.
+		// prompt_tokens because both are unadjusted prefix counts. The negative
+		// remainder must clamp to zero, never turn into a negative base charge.
 		usage := &dto.Usage{
 			PromptTokens:     3619,
 			CompletionTokens: 36,

service/tiered_settle.go

@@ -67,6 +67,8 @@ func BuildTieredTokenParams(usage *dto.Usage, isClaudeUsageSemantic bool, usedVa
 		}
 	}
 
+	// OpenAI cache-write usage reports unadjusted prefix counts, so cr + cc can
+	// exceed the prompt and drive the remainder negative. Clamp at zero.
 	if p < 0 {
 		p = 0
 	}