diff --git a/litellm/litellm_core_utils/llm_cost_calc/utils.py b/litellm/litellm_core_utils/llm_cost_calc/utils.py
index e36d6d68367..cbc0763382c 100644
--- a/litellm/litellm_core_utils/llm_cost_calc/utils.py
+++ b/litellm/litellm_core_utils/llm_cost_calc/utils.py
@@ -604,12 +604,22 @@ def generic_cost_per_token(
         reasoning_tokens = completion_tokens_details["reasoning_tokens"]
         image_tokens = completion_tokens_details["image_tokens"]
 
-    # Only assume all tokens are text if there's NO breakdown at all
-    # If image_tokens, audio_tokens, or reasoning_tokens exist, respect text_tokens=0
+    # Handle text_tokens calculation:
+    # 1. If text_tokens is explicitly provided and > 0, use it
+    # 2. If there's a breakdown (reasoning/audio/image tokens), calculate text_tokens as the remainder
+    # 3. If no breakdown at all, assume all completion_tokens are text_tokens
     has_token_breakdown = image_tokens > 0 or audio_tokens > 0 or reasoning_tokens > 0
-    if text_tokens == 0 and not has_token_breakdown:
-        text_tokens = usage.completion_tokens
-        is_text_tokens_total = True
+    if text_tokens == 0:
+        if has_token_breakdown:
+            # Calculate text tokens as remainder when we have a breakdown
+            # This handles cases like OpenAI's reasoning models where text_tokens isn't provided
+            text_tokens = max(
+                0, usage.completion_tokens - reasoning_tokens - audio_tokens - image_tokens
+            )
+        else:
+            # No breakdown at all, all tokens are text tokens
+            text_tokens = usage.completion_tokens
+            is_text_tokens_total = True
     ## TEXT COST
     completion_cost = float(text_tokens) * completion_base_cost
 
diff --git a/tests/test_litellm/litellm_core_utils/llm_cost_calc/test_llm_cost_calc_utils.py b/tests/test_litellm/litellm_core_utils/llm_cost_calc/test_llm_cost_calc_utils.py
index 65e3dbec8bd..5ba78d9eed1 100644
--- a/tests/test_litellm/litellm_core_utils/llm_cost_calc/test_llm_cost_calc_utils.py
+++ b/tests/test_litellm/litellm_core_utils/llm_cost_calc/test_llm_cost_calc_utils.py
@@ -809,3 +809,54 @@ def test_bedrock_anthropic_prompt_caching():
     assert completion_cost >= 0
     assert round(prompt_cost, 3) == 0.111
     assert round(completion_cost, 5) == 0.00820
+
+
+def test_reasoning_tokens_without_text_tokens_gpt5_nano():
+    """
+    Test fix for GitHub issue #18599:
+    https://github.com/BerriAI/litellm/issues/18599
+
+    When OpenAI models (gpt-5-nano, o1, o3) return reasoning_tokens but don't provide
+    text_tokens, LiteLLM should calculate text_tokens as:
+      text_tokens = completion_tokens - reasoning_tokens - audio_tokens - image_tokens
+
+    This ensures ALL completion tokens are billed, not just reasoning tokens.
+    """
+    model = "gpt-5-nano"
+    custom_llm_provider = "openai"
+
+    # Simulate OpenAI gpt-5-nano response where text_tokens is NOT provided
+    # completion_tokens: 977 total
+    # reasoning_tokens: 768
+    # text_tokens: should be calculated as 977 - 768 = 209
+    usage = Usage(
+        prompt_tokens=17,
+        completion_tokens=977,
+        total_tokens=994,
+        completion_tokens_details=CompletionTokensDetailsWrapper(
+            reasoning_tokens=768,
+            audio_tokens=0,
+            # text_tokens NOT provided - this is the key part of the bug
+        ),
+    )
+
+    prompt_cost, completion_cost = generic_cost_per_token(
+        model=model,
+        usage=usage,
+        custom_llm_provider=custom_llm_provider,
+    )
+
+    # gpt-5-nano pricing: $0.05/1M input, $0.40/1M output
+    expected_prompt_cost = 17 * 0.05 / 1_000_000
+    expected_completion_cost = 977 * 0.40 / 1_000_000  # ALL tokens, not just reasoning
+
+    assert abs(prompt_cost - expected_prompt_cost) < 1e-10, \
+        f"Prompt cost incorrect: {prompt_cost} vs {expected_prompt_cost}"
+
+    assert abs(completion_cost - expected_completion_cost) < 1e-10, \
+        f"Completion cost incorrect: {completion_cost} vs {expected_completion_cost}"
+
+    # Verify it's NOT using only reasoning_tokens (the bug)
+    wrong_cost = 768 * 0.40 / 1_000_000  # Only reasoning tokens
+    assert abs(completion_cost - wrong_cost) > 1e-6, \
+        "Bug detected: Cost calculation is using only reasoning_tokens instead of all completion_tokens!"