Sentinel — an autonomous crypto & web3 watcher agent

Sentinel wakes itself up every 3 hours, checks crypto prices for big moves, checks Devpost for new "web3" hackathons, and (when Google Search grounding is available) checks the live web for newly listed coins — then emails me only when something actually happened. No dashboard to check, no button to click. Built entirely on AWS Free Tier services, deployed by hand through the AWS Console.

What it watches, and how it decides what "matters"

Check	Trigger condition	Data source
Crypto prices	Tracked coin moves ≥5% since the last alert	CoinGecko (free, keyless)
Web3 challenges	A new "web3" hackathon appears that wasn't seen before	Devpost's public listings API (free, keyless)
New coin listings	A coin was listed on a major exchange in the last ~48h	Gemini + Google Search grounding

If none of the three conditions are true on a given run, Sentinel sends nothing — that's the point of a Watcher.

Architecture

 EventBridge Scheduler "MorningScheduler" (rate: every 3 hours)
              │
              ▼
      AWS Lambda "Agentlambda" (Python)
        ├─ CoinGecko API ──────────► crypto prices (no key needed)
        ├─ Devpost API ─────────────► web3 hackathon listings (no key needed)
        ├─ SSM Parameter Store ─────► Gemini API key ("/agent/geminiapi", SecureString)
        ├─ Gemini API (Google Search grounding) ─► new coin listings
        ├─ DynamoDB "Agenttable" ───► price baselines + seen-item tracking
        └─ Amazon SES ──────────────► emails the alert, only if triggered

IAM role Agent grants exactly: ses:SendEmail, dynamodb:GetItem/PutItem on Agenttable, ssm:GetParameter on /agent/*, and CloudWatch Logs write access.

A known, accepted tradeoff

Google Search grounding is not included in Gemini's unbilled free tier — it requires a Google Cloud project with billing enabled to receive its free monthly grounding allowance. On an unbilled project, that specific check returns a 429 and is silently skipped (the code wraps it in a try/except) without affecting the crypto or web3-challenge checks. This was a deliberate choice: rather than let one optional check take the whole agent down, it degrades gracefully. Enabling billing on the Google Cloud project would activate it fully.

How it was deployed (AWS Console, no CLI)

1. SES — verified sender/recipient email identity
2. SSM Parameter Store — stored the Gemini API key as a SecureString at /agent/geminiapi
3. DynamoDB — table Agenttable, partition key date (String), on-demand billing — reused as a general key-value store with prefixed keys (price#, web3challenge#, listing#) rather than one table per feature
4. IAM — role Agent, inline policy scoped to exactly the permissions above
5. Lambda — function Agentlambda (Python), this repo's lambda_function.py, role Agent attached, environment variables set (below), timeout raised to 1 minute
6. EventBridge Scheduler — schedule MorningScheduler, rate-based every 3 hours, targeting Agentlambda

This project started as a daily 6 AM "morning brief" agent (Daybreak) and was iteratively rebuilt into a Watcher through a series of live edits — including debugging real deployment errors (env var typos, a wrong Gemini model name, malformed lat/long values, and the grounding billing requirement above) directly against a running AWS account.

Environment variables

Key	Example value
TABLE_NAME	Agenttable
RECIPIENT_EMAIL	you@example.com
SENDER_EMAIL	you@example.com
CRYPTO_IDS	bitcoin,ethereum,solana
PRICE_CHANGE_THRESHOLD_PCT	5
GEMINI_API_KEY_PARAM	/agent/geminiapi
GEMINI_MODEL	gemini-3-flash

Cost

At one invocation every 3 hours (8/day), this stays comfortably within AWS Free Tier limits for Lambda, DynamoDB on-demand, SES, and EventBridge Scheduler. CoinGecko and Devpost's public endpoints are free and keyless. Gemini usage is minimal since alerts (and thus tokens) are only generated when something's actually worth reporting.