diff --git a/docs/technical/architecture.md b/docs/technical/architecture.md
index c03e5e0..cca2b78 100644
--- a/docs/technical/architecture.md
+++ b/docs/technical/architecture.md
@@ -79,6 +79,9 @@ failure at any step ─► retry with backoff ─► dead letter ─► requests
 | Vertex AI (`eu` endpoint, gemini-3.5-flash) | extraction | showcase + customer; local dev may use the Gemini free tier with synthetic data |
 | ERP | export target | pilot: `erp-mock`; contract `contracts/erp-export.openapi.yaml` |
 
+AI cost per document, monthly limits (showcase 10 EUR, pilot 50 EUR, proposed) and what happens at the limit:
+[operations.md → AI cost](operations.md#ai-cost) (#109).
+
 No secrets in this document; every variable is documented in `.env.example`.
 
 ## Database roles
diff --git a/docs/technical/operations.md b/docs/technical/operations.md
index 8b03204..010607d 100644
--- a/docs/technical/operations.md
+++ b/docs/technical/operations.md
@@ -160,6 +160,63 @@ endpoints are disabled except `set-active` (404) – every change goes through t
   confirm the notification; reset the variable afterwards.
 - Out of scope: error-rate alerts from log aggregation, monitoring of a customer environment.
 
+## AI cost
+
+Issue #109. Every processed document is **one** model call per attempt (`process-request.ts`: one call
+per document of a request; `.msg` attachments travel inside their document). A failed attempt repeats
+the calls: an invalid or failed model answer (`model_output_invalid`, `model_error`) is retryable, and
+the processing job then runs again – up to **6 attempts** (`retryLimit: 5`, `queues.ts`) – calling the
+model again for **every** document of the request, including those that already succeeded. A paid but
+unusable answer can thus be billed up to 6 times per document, plus each manual reprocess ("Erneut verarbeiten"). Tokens are recorded per run
+(`extraction_runs.total_tokens`, per document in `extraction_runs.documents`); the query for cost per
+request is signal S4 of the pilot metrics page (`docs/product/metrics.md`, pending in #121). Tokens are
+stored only for a successful run, so calls of failed attempts are **not** in these figures.
+
+**Cost per document (estimate, 2026-10-16).** Formula: `input_tokens × input price + (total_tokens −
+input_tokens) × output price` – the difference is output **plus thinking** tokens, which Google bills at
+the output price. Token source: the two sample recordings (`src/features/samples/data/*.recording.json`,
+`gemini-3.5-flash`, prompt `extract_v2`). Prices per 1M tokens, Gemini API standard (paid) tier:
+
+| Price basis | Input / output (USD) | "Pumpe P204" (1,280 in / 3,242 out) | "Werk Ost" (1,281 in / 2,277 out) |
+|---|---|---|---|
+| `gemini-3.5-flash` – **unconfirmed**: Google's pricing page (updated 2026-10-15) no longer lists it; figure from third-party price lists | 1.50 / 9.00 | 0.031 | 0.022 |
+| `gemini-3.6-flash` (Google's recommended successor), until 2026-12-31 | 0.75 / 3.75 | 0.013 | 0.009 |
+| `gemini-3.6-flash` from 2027-01-01 | 1.50 / 7.50 | 0.026 | 0.019 |
+
+Assumptions: the same token counts for 3.6 (not measured – a model change runs the eval gate and a new
+recording first); Vertex AI list prices are not confirmed here. On the free tier (showcase until the D11
+exception ends, at the latest 2026-10-31) the cost is 0 – only the quota counts.
+
+**What bounds the cost – and what does not.** The upload caps bound the number of calls, not their size:
+`UPLOAD_MAX_PER_HOUR` (runbook example for the showcase: 20 per person) × `UPLOAD_MAX_FILES` (showcase: 3) = at most 60 documents per
+person and hour – about 1.90 USD per hour at the worst figure above (10 EUR after roughly 5 hours of
+uploading without pause), and up to 6 times that while answers keep failing (360 calls per hour). The model call itself has **no output limit and no input-size limit** (no
+`max_output_tokens`; the document text goes in whole, up to the 20 MB file cap), so one large document can
+cost far more than the samples – follow-up #123 (cap output tokens and document text per call).
+
+**Monthly limits** (proposed – the orchestrator confirms them when setting up the budget):
+
+| Environment | Limit / month | ≈ document calls at the worst figure (1 EUR counted as 1 USD – conservative) | Basis |
+|---|---|---|---|
+| Showcase (invite-only, one person) | 10 EUR | ~320 | brief → Risks ("10 €/month GCP budget alert") |
+| Pilot (3–5 test users) | 50 EUR | ~1,600 | assumption: 5 users × ~15 documents per working day × 21 days ≈ 1,575; reviewed after the first month |
+
+**Budget alert (account action, orchestrator).** A Cloud Billing budget on the project that pays for the
+model (paid Gemini API key or Vertex), amount = the limit above, alerts at 50 %, 90 % and 100 % of actual
+spend, recipients: billing admins (the orchestrator) – no address in the repo. Status: **not yet
+configured** (the showcase still runs on the free tier). A budget alert only notifies; it never stops
+spending.
+
+**At the limit** (any 100 % alert, or 90 % before mid-month):
+
+1. Stop new model calls: pause the Vercel project `requestflow-ai` (uploads then end in a visible
+   processing error once their retries are used up), or – to stop all processing – remove `CRON_SECRET`
+   and set `JOB_DRAIN_INLINE=false` (`deployment-vercel.md` §9).
+2. Find the cause with the S4 query (tokens per request) and the `jobs.drain_run` logs: one large
+   document, a retry loop, or simply more use.
+3. Resume only after the orchestrator raises the limit or the cause is fixed; reprocess requests that went
+   to `ERROR` in between.
+
 ## Frequent failures
 
 | Symptom | Cause | Action |