Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add estuary/agent-skills --skill derivation-windowinggit clone --depth 1 https://github.com/estuary/agent-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/estuary/agent-skills/derivation-windowing)<a href="https://agentmods.dev/skills/estuary/agent-skills/derivation-windowing"><img src="https://agentmods.dev/badge/skills/estuary/agent-skills/derivation-windowing/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/estuary/agent-skills/derivation-windowing"><img src="https://agentmods.dev/badge/skills/estuary/agent-skills/derivation-windowing.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00129 | $0.03766 |
| Opus 5.5 | $0.00052 | $0.01506 |
| Sonnet 5.5 | $0.00026 | $0.00753 |
| Haiku 4.5 | $0.00013 | $0.00377 |
Grade A, and why
derivation-windowing scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 260 lines — stays where its author put it; the contents beside it link to each section on GitHub.
derivation-windowing
Estuary derivation that maintains a sliding time window of events per key — events enter the window on arrival and leave it after a fixed delay. Typically used for fraud detection, rate limiting, or "recent activity" signals.
Prereq: read derivation-basics first. This skill is the most complex of the set — it combines internal SQLite state (like derivation-stateful-logic) with a second transform using readDelay to age events out of the window.
Docs:
- https://docs.estuary.dev/getting-started/tutorials/derivations_acmebank/#grouped-windows-of-transfers — the canonical walkthrough for this exact pattern
- https://docs.estuary.dev/concepts/derivations/#internal-state — internal state mechanics
When to use this over alternatives
- Sliding / rolling windows: "last 24h of high-value transfers", "last 5min of failed logins"
- Time-bounded rate limits: "more than 10 requests per user in the last minute"
- Fraud / anomaly signals: "user did X more than N times within a window"
- Real-time dashboards showing "activity in the last hour"
Reach for other skills when:
- Fixed tumbling windows (hourly buckets, daily totals, no overlap) →
derivation-aggregate-metricswith the hour/date as key. Much simpler. - State that doesn't age out on a time basis →
derivation-stateful-logic - Latest N events (not time-bounded) → aggregate with
append+ truncation at query time
Before you build this — cost caveat
This is the rarest derivation pattern in practice and the most expensive. Every event lives in per-shard SQLite state for the full readDelay, so a high-volume real-time stream holds a large working set in RocksDB on the reactor — the state cost scales with event volume times window length. In most cases a simpler tool fits:
- A destination-side window function (
OVER (... RANGE ...)), which most warehouses run natively and cheaply against already-materialized data. This is usually the right answer. - A fixed tumbling window via
derivation-aggregate-metricskeyed on the hour/date, if you only need non-overlapping buckets.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 2mo ago First seen · 260 lines · 129 tokens per session scan A 14313a59f64e
derivation-windowing is a skill published in the GitHub repository estuary/agent-skills (7 stars, last pushed 4d ago), licensed Apache-2.0. It adds 129 tokens to every session and 3,766 once invoked, about $0.0005 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.
derivation-windowing compared with skills from other repositories
| Mod | Type | Stars | Last push | Licence | Scan | Tokens | Works with |
|---|---|---|---|---|---|---|---|
| derivation-windowingestuary/agent-skills | Skill | 7 | 4d ago | Apache-2.0 | A | 129 | Claude Code |
| change-data-capture-adminBanibrataChatterjee/AwesomeSalesforceSkills | Skill | 4 | 6mo ago | Apache-2.0 | B | 50 | Claude Code, Codex |
| stream-processing-designerbookforge-ai/bookforge-skills | Skill | 42 | 6mo ago | CC-BY-SA-4.0 | A | 240 | Claude Code |
| cocoindexcocoindex-io/cocoindex | Skill | 12k | today | Apache-2.0 | A | 88 | Claude Code, Codex |
| leaky-dataconfluentinc/agent-skills | Skill | 64 | 12d ago | Apache-2.0 | A | 60 | Claude Code |
| confluent-cloud-flink-sqlconfluentinc/agent-skills | Skill | 64 | 12d ago | Apache-2.0 | A | 234 | Claude Code |
| flink-udfconfluentinc/agent-skills | Skill | 64 | 12d ago | Apache-2.0 | A | 189 | Claude Code |