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+# COSTS.md
+
+> **Mandate.** Tracks the infrastructure cost attributed to **stream**
+> (stream.waow.tech) and what we're doing about it. Numbers drift, so this
+> file names the levers and the instruments, never a hardcoded figure.
+
+## current cost — fetch it live, never hardcode
+
+The daily snapshot (collected by my-prefect-server, surfaced at
+https://hub.waow.tech) attributes this repo's resources to the `stream`
+project:
+
+```bash
+curl -s https://hub.waow.tech/api/costs.json | jq '{
+  as_of: .generatedAt,
+  stream_monthly_usd: (
+    [ .lineItems[] | select(.project == "stream") ]
+    | (map(.amount) | add // 0) / 100
+  ),
+  lines: [ .lineItems[] | select(.project == "stream")
+           | {service, provider, usd: (.amount/100), estimated} ]
+}'
+```
+
+Resources: the `stream-cx43` hetzner box (hel1) + its 3 TB volume, plus any
+fly apps whose names contain `stream` (e.g. the loadgen). Attribution lives
+in my-prefect-server `packages/mps/src/mps/costs/projects.py`; fix mappings
+there, not numbers here.
+
+## the shape of the spend (measured 2026-08-09)
+
+- **the volume is the cost.** block storage bills per provisioned GB
+  (€0.0767/GB·mo gross); the sealed archive (~1.75 TB) plus grow-only
+  headroom dwarfs the server line. treemap snapshot of the full breakdown:
+  operator session artifact "stream · where the money goes".
+- **memory is not a lever.** ~3 GB RSS on 16 GB; the rest is page cache over
+  a network-attached volume, which is what makes cold serving tolerable.
+  breakdown: `stream_manifest_resident_bytes` + tail + tombstone gauges
+  (grafana → "Memory contributors").
+- **format density is not a lever.** recompression of real blocks measured
+  ≤ 11% headroom (per-block l19 / whole-file l19 / trained dict) — not worth
+  a 1.7 TB rewrite, and upstream owns the format.
+
+## levers, in order
+
+1. compaction deletes-fold reclaiming segment bytes (throttled workers).
+2. right-sizing the volume — hetzner volumes grow but never shrink, so this
+   is a copy-to-smaller-volume migration, sized after growth settles.
+3. object-storage tiering for cold segments. go/no-go is empirical:
+   `stream_compaction_rewritten_age_seconds` reports whether steady-state
+   rewrites cluster in recent segments (tiering works) or scatter across
+   deep history (it doesn't). decide from the histogram, not from vibes,
+   and not while upstream format work is in flight.
+
+## changelog
+- **2026-08-09** — initial notice. hetzner cost connector taught to price
+  volumes (my-prefect-server 665b2dd); `stream` project mapping added
+  (5c3f8d7). pending: HCLOUD token for the stream hetzner project in the
+  `hetzner-tokens` secret block — until it exists, snapshots show no
+  hetzner lines for this repo.