REPL JSON Mode

The -j flag switches ilo output to structured JSON - ideal for agents that need to parse results programmatically.

Interactive REPL

Shell

ilo repl -j

Every response is a single JSON object on one line:

Success:

JSON

{"ok": 10}

Error:

JSON

{"code": "ILO-T004", "message": "undefined variable 'y'", "severity": "error", "suggestion": "did you mean 'x'?"}

Inline expressions

The -j flag also works outside the REPL:

Shell

ilo 'dbl x:n>n;*x 2' -j 5

JSON

{"ok": 10}

Why this matters for agents

Agents can’t reliably parse human-formatted output. With -j:

• No ambiguity - success is {"ok": value}, errors have structured code/message/suggestion fields
• Error recovery - the agent reads the suggestion field and retries automatically
• Stateful sessions - the REPL keeps function definitions across lines, so the agent can build up a program incrementally and test each step

Agent workflow example

A typical agent loop using repl -j:

1. Agent sends:  dbl x:n>n;*x 2
   ilo returns:  {"ok": "defined dbl"}

2. Agent sends:  dbl 5
   ilo returns:  {"ok": 10}

3. Agent sends:  dbl "hello"
   ilo returns:  {"code": "ILO-T003", "message": "expected n, got t", ...}

4. Agent reads error, adjusts, retries

Combining with skills

When using ilo as a Claude Code skill, the skill automatically uses -j for reliable output parsing. The two features are designed to work together.