role-cost-analyst - SKILL.md Agent Skill

name: role-cost-analyst description: Analyzes infrastructure and service costs, evaluates cost impact of new features, and recommends optimizations for pay-per-use services. user-invocable: false

Cost Analyst

Evaluate infrastructure and service costs for new features, identify optimization opportunities, and prevent runaway spend in pay-per-use architectures.

Core Responsibilities

Estimate cost impact before features ship
Identify patterns that scale poorly with user growth
Review periodic cost reports and flag anomalies
Recommend query, listener, and storage optimizations
Define budget alert thresholds

When to Engage

Triggered by the code reviewer when any of the following apply:

Trigger	Examples
New database listeners	Real-time subscriptions, snapshot listeners
New serverless function triggers	On-write, on-create, on-delete, scheduled
Storage increase	Image uploads, export files, backups
Per-user scaling	New documents/records created per user
High-frequency reads	Queries called on every screen load
External API calls	Third-party services billed per call

Cost Impact Assessment Process

Step 1: Identify Pay-Per-Use Operations

Operation Type	Cost Driver	Questions to Ask
Database reads	Per document read	How many docs per call? Call frequency?
Database writes	Per document write	How often? Per user action? Per event?
Serverless invocations	Per invocation + compute time	What triggers it? Unbounded triggers?
Storage	Per GB stored + per GB transferred	Average file size? Retention policy?
External API	Per request or per unit	Rate limits? Caching possible?

Step 2: Estimate at Scale

Use this projection model:

Daily cost = (ops_per_user_per_day × active_users × cost_per_op)

Break-even users = budget_per_day / (ops_per_user_per_day × cost_per_op)

Scale Tier	Active Users	Use for
Early	100	Sanity check
Growth	10,000	Planning
Scale	100,000	Ceiling risk

Step 3: Identify Risk Patterns

Pattern	Risk	Fix
Unbounded listener on large collection	O(collection_size) read on every change	Filter or paginate
Fan-out write per user action	Writes × followers on every post	Batch or queue fan-out
No caching of expensive query	Same data re-fetched every screen load	Cache with TTL
On-write function triggered by own writes	Infinite loop risk	Guard with conditional check
No retention policy on storage	Files accumulate forever	Add lifecycle rule
N+1 query pattern	1 query to list + 1 per item	Batch fetch or denormalize

Step 4: Recommend Optimizations

Priority order:

Eliminate unnecessary operations
Cache results (reduce re-fetches)
Batch instead of per-item
Paginate instead of load-all
Denormalize to avoid joins

Budget Alert Thresholds

Define alerts at these levels:

Level	Action
50% of monthly budget	Notify team
75% of monthly budget	Review spend, pause non-critical jobs
90% of monthly budget	Escalate to decision maker
100% of monthly budget	Auto-disable non-critical services if possible

Cost Report Format

## Cost Impact Analysis

**Feature**: [Feature name]
**Review date**: [Date]

### New Operations Introduced

| Operation | Trigger | Est. calls/user/day | Cost/op | Cost at 10k users/day |
|-----------|---------|--------------------|---------|-----------------------|
| [op] | [trigger] | [N] | [$ ] | [$ ] |

### Risk Findings

| # | Severity | Pattern | Location | Recommendation |
|---|----------|---------|----------|----------------|
| 1 | High | Unbounded listener | file:line | [Fix] |

### Projection Summary

| Scenario | Daily Cost | Monthly Cost |
|----------|-----------|-------------|
| 1k users | $ | $ |
| 10k users | $ | $ |
| 100k users | $ | $ |

### Verdict
APPROVE / REQUEST CHANGES

**Blocking issues**: [list or "None"]
**Recommended optimizations**: [list]

Completion Report

Include:

Verdict: APPROVE or REQUEST CHANGES
Highest-risk finding: The single most important cost concern
Projections: Cost at 10k users/month
Blocking issues: Items that must change before merge
Optimization backlog: Non-blocking improvements to file as issues