[codex] add embedded Langfuse client

Cargo.toml

@@ -25,13 +25,14 @@ thiserror = "2"
 # multi-threaded runtime stays a dev-dependency for examples and tests.
 tokio = { version = "1", default-features = false, features = ["sync", "time", "macros"] }
 
-# Optional HTTP client, pulled in only by hosted-provider features. The `stream`
-# feature enables `Response::bytes_stream` for Server-Sent-Events streaming.
+# HTTP client used by hosted providers and the embedded Langfuse exporter. The
+# `stream` feature enables `Response::bytes_stream` for Server-Sent-Events
+# streaming.
 reqwest = { version = "0.12", default-features = false, features = [
     "json",
     "rustls-tls",
     "stream",
-], optional = true }
+] }
 
 # Optional embedded SQLite checkpointer backend (`graph::checkpoint::sqlite`).
 # `bundled` compiles SQLite from vendored C source so no system library is
@@ -47,7 +48,7 @@ rhai = { version = "1", features = ["sync"], optional = true }
 [features]
 default = []
 # Hosted OpenAI Chat Completions provider (`harness::providers::openai`).
-openai = ["dep:reqwest"]
+openai = []
 # Embedded SQLite-backed checkpointer (`graph::checkpoint::SqliteCheckpointer`).
 sqlite = ["dep:rusqlite"]
 # Embedded Rhai-backed `.ragsh` REPL session runtime (`repl::session`).

README.md

@@ -183,6 +183,29 @@ export OPENAI_API_KEY=...
 cargo run --features openai --example openai_chat
 ```
 
+Export durable harness observations to Langfuse with the embedded client:
+
+```rust
+use tinyagents::{LangfuseClient, LangfuseTraceConfig};
+
+let client = LangfuseClient::proxy("https://api.tinyhumans.ai", backend_jwt)?;
+client
+    .send_observations(
+        LangfuseTraceConfig {
+            user_id: Some("user_123".to_string()),
+            session_id: Some("thread_abc".to_string()),
+            ..Default::default()
+        },
+        &observations,
+    )
+    .await?;
+```
+
+`LangfuseClient::proxy` sends to the backend
+`/telemetry/langfuse/ingestion` endpoint with bearer auth. Use
+`LangfuseClient::direct(langfuse_url, public_key, secret_key)` when an
+application is allowed to talk to Langfuse directly.
+
 ## Examples to explore
 
 All live in [`examples/`](examples/):