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[Bugfix]:Cached_tokens always returns -1 #229

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[Bugfix]:Cached_tokens always returns -1 #229

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50bcdf6
When VLLM is deployed with PD Disaggregation,in cache_aware mode,cach…
chasted Sep 1, 2026
f89e58d
[Bugfix]:Cached_tokens always returns -1
chasted Sep 1, 2026
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242 changes: 241 additions & 1 deletion src/routers/http/logprobs_merge.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,73 @@
use serde_json::Value;
use tracing::debug;

/// Merge usage metadata from prefill response into decode response.
///
/// Prefer the prefill-side cached_tokens, because vLLM decode-side cached token
/// accounting may temporarily report invalid placeholder values such as -1.
pub fn merge_usage_in_json(prefill_json: &Value, decode_json: &mut Value) -> bool {
let Some(prefill_usage) = prefill_json.get("usage") else {
return false;
};

let prefill_cached = prefill_usage
.get("prompt_tokens_details")
.and_then(|v| v.get("cached_tokens"))
.or_else(|| {
prefill_usage
.get("input_tokens_details")
.and_then(|v| v.get("cached_tokens"))
});

let Some(prefill_cached) = prefill_cached else {
return false;
};

let decode_usage = match decode_json.get_mut("usage") {
Some(v) => v,
None => {
decode_json["usage"] = prefill_usage.clone();
return true;
}
};

let Some(decode_usage_obj) = decode_usage.as_object_mut() else {
return false;
};

let details_target = if decode_usage_obj.contains_key("prompt_tokens_details") {
"prompt_tokens_details"
} else if decode_usage_obj.contains_key("input_tokens_details") {
"input_tokens_details"
} else {
"prompt_tokens_details"
};

let decode_details = decode_usage_obj
.entry(details_target.to_string())
.or_insert(Value::Object(serde_json::Map::new()));

let Some(decode_details_obj) = decode_details.as_object_mut() else {
return false;
};

let should_replace = match decode_details_obj.get("cached_tokens") {
Some(existing) => existing.as_i64().is_none_or(|v| v <= 0),
None => true,
};

if should_replace {
decode_details_obj.insert("cached_tokens".to_string(), prefill_cached.clone());
debug!(
"[USAGE MERGE] Replaced decode cached_tokens with prefill value {}",
prefill_cached
);
return true;
}

false
}

/// Merge prompt_logprobs from prefill response into decode response.
///
/// Handles both Completions API (prompt_logprobs in choices) and
Expand All @@ -25,7 +92,7 @@ use tracing::debug;
/// # Returns
/// * `bool` - Whether any logprobs were merged
pub fn merge_logprobs_in_json(prefill_json: &Value, decode_json: &mut Value) -> bool {
let mut merged = false;
let mut merged = merge_usage_in_json(prefill_json, decode_json);

// 1. Try to merge meta_info/input_token_logprobs (for Generate API)
if let (Some(prefill_meta), Some(decode_meta)) = (
Expand Down Expand Up @@ -337,6 +404,179 @@ mod tests {
assert_eq!(merged_offsets[4].as_i64().unwrap(), 23); // Decode token " token" (7 + 16)
}

#[test]
fn test_merge_usage_prefill_cached_tokens_over_decode_invalid_value() {
let prefill_json = json!({
"usage": {
"prompt_tokens": 100,
"completion_tokens": 1,
"total_tokens": 101,
"prompt_tokens_details": {
"cached_tokens": 50
}
}
});

let mut decode_json = json!({
"usage": {
"prompt_tokens": 100,
"completion_tokens": 10,
"total_tokens": 110,
"prompt_tokens_details": {
"cached_tokens": -1
}
}
});

let merged = merge_usage_in_json(&prefill_json, &mut decode_json);
assert!(merged);
assert_eq!(
decode_json["usage"]["prompt_tokens_details"]["cached_tokens"],
json!(50)
);
}

#[test]
fn test_merge_usage_keeps_valid_decode_cached_tokens() {
let prefill_json = json!({
"usage": {
"prompt_tokens": 100,
"completion_tokens": 1,
"total_tokens": 101,
"prompt_tokens_details": {
"cached_tokens": 50
}
}
});

let mut decode_json = json!({
"usage": {
"prompt_tokens": 100,
"completion_tokens": 10,
"total_tokens": 110,
"prompt_tokens_details": {
"cached_tokens": 12
}
}
});

let merged = merge_usage_in_json(&prefill_json, &mut decode_json);
assert!(!merged);
assert_eq!(
decode_json["usage"]["prompt_tokens_details"]["cached_tokens"],
json!(12)
);
}

#[test]
fn test_merge_usage_fills_missing_cached_tokens() {
let prefill_json = json!({
"usage": {
"prompt_tokens": 100,
"completion_tokens": 1,
"total_tokens": 101,
"prompt_tokens_details": {
"cached_tokens": 50
}
}
});

let mut decode_json = json!({
"usage": {
"prompt_tokens": 100,
"completion_tokens": 10,
"total_tokens": 110,
"prompt_tokens_details": {}
}
});

let merged = merge_usage_in_json(&prefill_json, &mut decode_json);
assert!(merged);
assert_eq!(
decode_json["usage"]["prompt_tokens_details"]["cached_tokens"],
json!(50)
);
}

#[test]
fn test_merge_usage_uses_input_tokens_details_fallback() {
let prefill_json = json!({
"usage": {
"input_tokens": 100,
"output_tokens": 1,
"total_tokens": 101,
"input_tokens_details": {
"cached_tokens": 41
}
}
});

let mut decode_json = json!({
"usage": {
"input_tokens": 100,
"output_tokens": 10,
"total_tokens": 110,
"prompt_tokens_details": {
"cached_tokens": -1
}
}
});

let merged = merge_usage_in_json(&prefill_json, &mut decode_json);
assert!(merged);
assert_eq!(
decode_json["usage"]["prompt_tokens_details"]["cached_tokens"],
json!(41)
);
}

#[test]
fn test_merge_usage_copies_usage_when_decode_missing() {
let prefill_json = json!({
"usage": {
"prompt_tokens": 100,
"completion_tokens": 1,
"total_tokens": 101,
"prompt_tokens_details": {
"cached_tokens": 50
}
}
});

let mut decode_json = json!({
"choices": [{ "message": { "content": "ok" } }]
});

let merged = merge_usage_in_json(&prefill_json, &mut decode_json);
assert!(merged);
assert_eq!(
decode_json["usage"]["prompt_tokens_details"]["cached_tokens"],
json!(50)
);
}

#[test]
fn test_merge_usage_noop_without_prefill_usage() {
let prefill_json = json!({ "choices": [{ "message": { "content": "ok" } }] });
let mut decode_json = json!({
"usage": {
"prompt_tokens": 100,
"completion_tokens": 10,
"total_tokens": 110,
"prompt_tokens_details": {
"cached_tokens": -1
}
}
});

let merged = merge_usage_in_json(&prefill_json, &mut decode_json);
assert!(!merged);
assert_eq!(
decode_json["usage"]["prompt_tokens_details"]["cached_tokens"],
json!(-1)
);
}

#[test]
fn test_merge_chat_completions_api_logprobs() {
let prefill_json = json!({
Expand Down
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