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The lost stack J flagged was partly already present: Langfuse container has been running 2 days with the staffing project, SDK installed, mcp-server tracing gw:/* routes. What was missing was Rust-side /v1/chat emission — the new Phase 38/39 code bypassed Langfuse entirely. This commit bridges it. Fire-and-forget HTTP POST to http://localhost:3001/api/public/ingestion (batch {trace-create + generation-create}) on every chat call. Non-blocking — spawned tokio task, response latency unaffected. Trace failures log warn and drop, never propagate. Verified end-to-end after restart: - Log line "v1: Langfuse tracing enabled" at startup - /v1/chat local (qwen3.5:latest) → v1.chat:ollama trace appears with lat=0.41s, 24+6 tokens - /v1/chat cloud (gpt-oss:120b) → v1.chat:ollama_cloud trace appears with lat=1.87s, 73+87 tokens - mcp-server's existing gw:/log + gw:/intelligence/* traces continue to flow into the same project unchanged Files: - crates/gateway/src/v1/langfuse_trace.rs (new, 195 LOC) — thin client, no SDK. reqwest Basic Auth. ChatTrace payload + event serializer. from_env_or_defaults() resolver matches mcp-server/tracing.ts conventions (pk-lf-staffing / sk-lf- staffing-secret / localhost:3001) - crates/gateway/src/v1/mod.rs — V1State.langfuse field, emission after successful provider call (post-dispatch, pre-usage-update) - crates/gateway/src/main.rs — resolve + log at startup Tests: 12/12 green (9 prior + 3 for langfuse_trace: ingestion-batch serialization, uuid generator uniqueness, env resolver shape). Recovered piece #1 of 3 from the lost-stack narrative. Still open: - Langfuse → observer :3800 pipe (Phase 40 mid-deliverable) - Gitea MCP reconnect in mcp-server/index.ts (Phase 40 late) Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>