Lesson 3: Read and Transform

Source Code

agent-client-tutorial/02-read-and-transform

What You’ll Build

A Spring Boot application that asks an agent to read log files, count severity levels (ERROR, WARNING, INFO), and produce a summary CSV. This demonstrates agents working with existing files and producing structured output.

Step 1: Set Up Sample Data

Clone the tutorial repository and navigate to the lesson:

git clone https://github.com/markpollack/agent-client-tutorial.git
cd agent-client-tutorial/02-read-and-transform

The application creates sample log files automatically if they don’t exist. Three service logs (auth.log, api.log, worker.log) with mixed severity levels.

Step 2: Understand the Goal

The goal is a multi-line prompt that specifies the input, the transformation, and the expected output format:

String goal = """
    In the logs/ directory there are multiple .log files from different services.
    Scan all .log files and count how many lines contain "ERROR", "WARNING", and "INFO".

    Output your results to summary.csv with the following structure:
    severity,count
    ERROR,<total ERROR lines>
    WARNING,<total WARNING lines>
    INFO,<total INFO lines>

    The output should be a valid CSV file with exactly four lines.""";

Notice the specificity: file format, column names, exact line count. Agents perform better with precise output specifications.

Step 3: Run It

# With Claude (default)
./mvnw spring-boot:run

# Or with a different provider
./mvnw spring-boot:run -Dspring.profiles.active=codex

Step 4: Verify

Check the output CSV:

cat summary.csv
# severity,count
# ERROR,4
# WARNING,4
# INFO,7

What’s Different from Lesson 1

Agent reads existing files — not just creating from scratch
Multi-file input — the agent scans a directory of logs
Structured output — the result is a well-defined CSV, not free text
Data transformation — counting, aggregating, formatting

This pattern — read existing state, transform it, write structured output — is the core of most agent tasks in production.

Lesson 4: Git Operations → — Use an agent to inspect git history and recover lost changes.

Source Code

​What You’ll Build

​Step 1: Set Up Sample Data

​Step 2: Understand the Goal

​Step 3: Run It

​Step 4: Verify

​What’s Different from Lesson 1

​Next