diff --git a/crates/proxy/src/integrations/langfuse.rs b/crates/proxy/src/integrations/langfuse.rs
index 567fa2c..8838fc5 100644
--- a/crates/proxy/src/integrations/langfuse.rs
+++ b/crates/proxy/src/integrations/langfuse.rs
@@ -1,13 +1,351 @@
-// Langfuse integration stub (implementation in Task 3).
+// Langfuse named callback integration.
+//
+// Sends LLM generation events to Langfuse's batch ingestion API.
+// Activated when LANGFUSE_PUBLIC_KEY and LANGFUSE_SECRET_KEY are set,
+// either explicitly via env vars or when "langfuse" appears in
+// litellm_settings.callbacks.
+//
+// Fire-and-forget: send() spawns a tokio task and returns immediately.
 
+use crate::admin::state::RequestLogEntry;
 use std::sync::Arc;
 
-pub struct LangfuseClient;
+pub struct LangfuseClient {
+    pub(crate) public_key: String,
+    pub(crate) secret_key: String,
+    pub(crate) host: String,
+    client: reqwest::Client,
+}
 
 impl LangfuseClient {
+    /// Construct from env vars: LANGFUSE_PUBLIC_KEY, LANGFUSE_SECRET_KEY, LANGFUSE_HOST.
+    /// Returns None when either required key is absent or empty.
     pub fn from_env() -> Option<Arc<Self>> {
-        None
+        let public_key = std::env::var("LANGFUSE_PUBLIC_KEY").ok()?;
+        let secret_key = std::env::var("LANGFUSE_SECRET_KEY").ok()?;
+        if public_key.is_empty() || secret_key.is_empty() {
+            return None;
+        }
+        let host = std::env::var("LANGFUSE_HOST")
+            .unwrap_or_else(|_| "https://cloud.langfuse.com".to_string());
+        let client = reqwest::Client::builder()
+            .timeout(std::time::Duration::from_secs(5))
+            .build()
+            .expect("langfuse http client");
+        Some(Arc::new(Self {
+            public_key,
+            secret_key,
+            host,
+            client,
+        }))
     }
 
-    pub fn send(&self, _entry: &crate::admin::state::RequestLogEntry) {}
+    /// Fire-and-forget: POST generation event to Langfuse ingestion API.
+    /// Returns immediately; the HTTP call happens in a spawned tokio task.
+    pub fn send(&self, entry: &RequestLogEntry) {
+        let payload = build_generation_payload(entry);
+        let url = format!("{}/api/public/ingestion", self.host.trim_end_matches('/'));
+        let auth = format!(
+            "Basic {}",
+            base64_encode(format!("{}:{}", self.public_key, self.secret_key).as_bytes())
+        );
+        let client = self.client.clone();
+        tokio::spawn(async move {
+            match client
+                .post(&url)
+                .header("Authorization", &auth)
+                .json(&payload)
+                .send()
+                .await
+            {
+                Ok(resp) => {
+                    if !resp.status().is_success() {
+                        tracing::debug!(
+                            url = %url,
+                            status = %resp.status(),
+                            "langfuse ingestion returned non-2xx"
+                        );
+                    }
+                }
+                Err(e) => {
+                    tracing::debug!(error = %e, "langfuse ingestion request failed");
+                }
+            }
+        });
+    }
+}
+
+/// Build the Langfuse batch ingestion payload for a single generation.
+pub(crate) fn build_generation_payload(entry: &RequestLogEntry) -> serde_json::Value {
+    let end_time = &entry.timestamp;
+    let start_time = iso8601_to_epoch(end_time)
+        .map(|epoch| {
+            crate::admin::db::epoch_to_iso8601(epoch.saturating_sub(entry.latency_ms / 1000))
+        })
+        .unwrap_or_else(|| end_time.clone());
+
+    let model = entry
+        .model_mapped
+        .as_deref()
+        .or(entry.model_requested.as_deref())
+        .unwrap_or("unknown");
+
+    let mut usage = serde_json::json!({ "unit": "TOKENS" });
+    if let Some(input) = entry.input_tokens {
+        usage["input"] = serde_json::json!(input);
+    }
+    if let Some(output) = entry.output_tokens {
+        usage["output"] = serde_json::json!(output);
+    }
+    if let Some(cost) = entry.cost_usd {
+        usage["totalCost"] = serde_json::json!(cost);
+    }
+
+    let level = if entry.error_message.is_some() { "ERROR" } else { "DEFAULT" };
+
+    let mut metadata = serde_json::json!({
+        "backend": entry.backend,
+        "status_code": entry.status_code,
+        "latency_ms": entry.latency_ms,
+    });
+    if let Some(ref msg) = entry.error_message {
+        metadata["error"] = serde_json::json!(msg);
+    }
+
+    serde_json::json!({
+        "batch": [{
+            "id": entry.request_id,
+            "type": "generation-create",
+            "timestamp": end_time,
+            "body": {
+                "id": entry.request_id,
+                "name": model,
+                "startTime": start_time,
+                "endTime": end_time,
+                "model": model,
+                "usage": usage,
+                "level": level,
+                "metadata": metadata,
+            }
+        }]
+    })
+}
+
+/// Encode bytes as standard base64 (RFC 4648). No external dependencies.
+pub(crate) fn base64_encode(input: &[u8]) -> String {
+    const CHARS: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+    let mut out = String::with_capacity(input.len().div_ceil(3) * 4);
+    let mut i = 0;
+    while i + 2 < input.len() {
+        let b = ((input[i] as u32) << 16)
+            | ((input[i + 1] as u32) << 8)
+            | (input[i + 2] as u32);
+        out.push(CHARS[((b >> 18) & 0x3f) as usize] as char);
+        out.push(CHARS[((b >> 12) & 0x3f) as usize] as char);
+        out.push(CHARS[((b >> 6) & 0x3f) as usize] as char);
+        out.push(CHARS[(b & 0x3f) as usize] as char);
+        i += 3;
+    }
+    let rem = input.len() - i;
+    if rem == 1 {
+        let b = (input[i] as u32) << 16;
+        out.push(CHARS[((b >> 18) & 0x3f) as usize] as char);
+        out.push(CHARS[((b >> 12) & 0x3f) as usize] as char);
+        out.push('=');
+        out.push('=');
+    } else if rem == 2 {
+        let b = ((input[i] as u32) << 16) | ((input[i + 1] as u32) << 8);
+        out.push(CHARS[((b >> 18) & 0x3f) as usize] as char);
+        out.push(CHARS[((b >> 12) & 0x3f) as usize] as char);
+        out.push(CHARS[((b >> 6) & 0x3f) as usize] as char);
+        out.push('=');
+    }
+    out
+}
+
+/// Parse ISO 8601 UTC timestamp ("2026-03-27T10:15:30Z") to Unix epoch seconds.
+/// Returns None on parse failure.
+pub(crate) fn iso8601_to_epoch(s: &str) -> Option<u64> {
+    if s.len() < 20 {
+        return None;
+    }
+    let year: i64 = s[0..4].parse().ok()?;
+    let month: i64 = s[5..7].parse().ok()?;
+    let day: i64 = s[8..10].parse().ok()?;
+    let hour: i64 = s[11..13].parse().ok()?;
+    let min: i64 = s[14..16].parse().ok()?;
+    let sec: i64 = s[17..19].parse().ok()?;
+    if !(1..=12).contains(&month) || !(1..=31).contains(&day) {
+        return None;
+    }
+    let days = days_from_civil(year, month, day);
+    let total = days * 86400 + hour * 3600 + min * 60 + sec;
+    u64::try_from(total).ok()
+}
+
+/// Days from 1970-01-01 to the given date (may be negative for dates before epoch).
+/// Algorithm: http://howardhinnant.github.io/date_algorithms.html
+fn days_from_civil(year: i64, month: i64, day: i64) -> i64 {
+    // Shift March 1 to start of year so Feb (with leap day) is last month.
+    let y = if month <= 2 { year - 1 } else { year };
+    let m = month;
+    let d = day;
+    let era = y.div_euclid(400);
+    let yoe = y - era * 400; // [0, 399]
+    let doy = (153 * (m + if m > 2 { -3 } else { 9 }) / 5) + d - 1; // [0, 365]
+    let doe = yoe * 365 + yoe / 4 - yoe / 100 + doy; // [0, 146096]
+    era * 146097 + doe - 719468
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::sync::Mutex;
+
+    // Serialize env-var tests to avoid races with parallel test runner.
+    static ENV_LOCK: Mutex<()> = Mutex::new(());
+
+    #[test]
+    fn from_env_returns_none_when_keys_absent() {
+        let _guard = ENV_LOCK.lock().unwrap_or_else(|e| e.into_inner());
+        unsafe {
+            std::env::remove_var("LANGFUSE_PUBLIC_KEY");
+            std::env::remove_var("LANGFUSE_SECRET_KEY");
+        }
+        assert!(LangfuseClient::from_env().is_none());
+    }
+
+    #[test]
+    fn from_env_returns_some_when_keys_present() {
+        let _guard = ENV_LOCK.lock().unwrap_or_else(|e| e.into_inner());
+        unsafe {
+            std::env::set_var("LANGFUSE_PUBLIC_KEY", "pk-test");
+            std::env::set_var("LANGFUSE_SECRET_KEY", "sk-test");
+        }
+        let client = LangfuseClient::from_env();
+        unsafe {
+            std::env::remove_var("LANGFUSE_PUBLIC_KEY");
+            std::env::remove_var("LANGFUSE_SECRET_KEY");
+        }
+        assert!(client.is_some());
+    }
+
+    #[test]
+    fn from_env_uses_custom_host() {
+        let _guard = ENV_LOCK.lock().unwrap_or_else(|e| e.into_inner());
+        unsafe {
+            std::env::set_var("LANGFUSE_PUBLIC_KEY", "pk-test");
+            std::env::set_var("LANGFUSE_SECRET_KEY", "sk-test");
+            std::env::set_var("LANGFUSE_HOST", "https://my-langfuse.example.com");
+        }
+        let client = LangfuseClient::from_env();
+        unsafe {
+            std::env::remove_var("LANGFUSE_PUBLIC_KEY");
+            std::env::remove_var("LANGFUSE_SECRET_KEY");
+            std::env::remove_var("LANGFUSE_HOST");
+        }
+        assert_eq!(client.unwrap().host, "https://my-langfuse.example.com");
+    }
+
+    #[test]
+    fn base64_encode_empty() {
+        assert_eq!(base64_encode(b""), "");
+    }
+
+    #[test]
+    fn base64_encode_produces_expected_output() {
+        let encoded = base64_encode(b"pk-test:sk-test");
+        assert_eq!(encoded, "cGstdGVzdDpzay10ZXN0");
+    }
+
+    #[test]
+    fn base64_encode_one_byte_padding() {
+        assert_eq!(base64_encode(b"A"), "QQ==");
+    }
+
+    #[test]
+    fn base64_encode_two_byte_padding() {
+        assert_eq!(base64_encode(b"AB"), "QUI=");
+    }
+
+    #[test]
+    fn base64_encode_three_bytes_no_padding() {
+        assert_eq!(base64_encode(b"ABC"), "QUJD");
+    }
+
+    #[test]
+    fn iso8601_to_epoch_unix_epoch() {
+        let epoch = iso8601_to_epoch("1970-01-01T00:00:00Z").unwrap();
+        assert_eq!(epoch, 0);
+    }
+
+    #[test]
+    fn iso8601_to_epoch_parses_2026() {
+        let epoch = iso8601_to_epoch("2026-03-27T10:00:00Z").unwrap();
+        assert!(epoch > 1_735_689_600); // > 2025-01-01
+        assert!(epoch < 1_900_000_000); // < 2030
+    }
+
+    #[test]
+    fn iso8601_to_epoch_returns_none_on_short_string() {
+        assert!(iso8601_to_epoch("not-a-date").is_none());
+        assert!(iso8601_to_epoch("").is_none());
+    }
+
+    #[test]
+    fn build_payload_structure() {
+        let entry = RequestLogEntry {
+            request_id: "req-123".to_string(),
+            timestamp: "2026-03-27T10:00:01Z".to_string(),
+            backend: "openai".to_string(),
+            model_requested: Some("claude-3-haiku-20240307".to_string()),
+            model_mapped: Some("gpt-4o-mini".to_string()),
+            status_code: 200,
+            latency_ms: 500,
+            input_tokens: Some(100),
+            output_tokens: Some(50),
+            is_streaming: false,
+            error_message: None,
+            key_id: None,
+            cost_usd: Some(0.001),
+        };
+        let payload = build_generation_payload(&entry);
+        let batch = payload["batch"].as_array().unwrap();
+        assert_eq!(batch.len(), 1);
+        let event = &batch[0];
+        assert_eq!(event["type"], "generation-create");
+        let body = &event["body"];
+        assert_eq!(body["model"], "gpt-4o-mini");
+        assert_eq!(body["usage"]["input"], 100);
+        assert_eq!(body["usage"]["output"], 50);
+        assert_eq!(body["usage"]["unit"], "TOKENS");
+        assert!(body["startTime"].as_str().is_some());
+        assert!(body["endTime"].as_str().is_some());
+        assert_eq!(body["level"], "DEFAULT");
+        assert_eq!(body["metadata"]["backend"], "openai");
+        assert_eq!(body["metadata"]["latency_ms"], 500);
+    }
+
+    #[test]
+    fn build_payload_error_entry() {
+        let entry = RequestLogEntry {
+            request_id: "req-err".to_string(),
+            timestamp: "2026-03-27T10:00:01Z".to_string(),
+            backend: "openai".to_string(),
+            model_requested: Some("gpt-4o".to_string()),
+            model_mapped: None,
+            status_code: 500,
+            latency_ms: 100,
+            input_tokens: None,
+            output_tokens: None,
+            is_streaming: false,
+            error_message: Some("internal error".to_string()),
+            key_id: None,
+            cost_usd: None,
+        };
+        let payload = build_generation_payload(&entry);
+        let body = &payload["batch"][0]["body"];
+        assert_eq!(body["level"], "ERROR");
+        assert_eq!(body["metadata"]["error"], "internal error");
+    }
 }