Add --log-for human|machine flag for LLM/agent-friendly output

[waldekmastykarz]

Summary

Add a new CLI flag --log-for human|machine to make Dev Proxy output consumable by coding agents and LLMs.

Problem

The current console output is optimized for human readability but creates problems when consumed by LLMs/coding agents:

Problem	Description
ANSI escape codes	Color codes like \x1B[31m pollute text and waste tokens
Unicode box characters	╭ ╰ │ are visual, not semantic — hard to parse programmatically
No structured output	Console is human-readable prose; LLMs need JSON to reliably extract fields
Buffered output	Messages held until request completes; agents cannot react in real-time
Terse/informal labels	oops, skip, proc are cute but ambiguous for machines
Missing context	Logs like "URL not matched" lack the actual URL
No correlation IDs	Hard to trace request→response chains
Timestamps hidden	Filtered by default; agents cannot measure timing

Example of current output

 req   ╭ GET https://api.example.com/users
 oops  │ 429 TooManyRequests
 warn  │ Exceeded resource limit when calling https://api.example.com/users
       ╰ 

What an agent needs

{"type":"request","method":"GET","url":"https://api.example.com/users","requestId":"1","timestamp":"2026-01-19T10:30:00.000Z"}
{"type":"chaos","plugin":"RateLimitingPlugin","status":429,"message":"TooManyRequests","requestId":"1","timestamp":"2026-01-19T10:30:00.050Z"}
{"type":"warning","plugin":"RateLimitingPlugin","message":"Exceeded resource limit","url":"https://api.example.com/users","requestId":"1","timestamp":"2026-01-19T10:30:00.051Z"}

Proposed solution

Add a new flag:

devproxy --log-for human   # Current behavior (default)
devproxy --log-for machine # Structured JSON Lines output

Why --log-for instead of --output json or --format json?

We evaluated common CLI conventions:

Tool	Flag
kubectl	-o, --output json|yaml|wide
Azure CLI	-o, --output json|table|tsv
GitHub CLI	--json

However, for LLM consumption, audience-centric naming is more intuitive than format-centric:

Aspect	--log-format json	--log-for machine
Semantics	Describes the how	Describes the who
LLM intuition	"Output in JSON format"	"This output is meant for me"
Discoverability	Need to know JSON is the right format	Obvious: "I am a machine, I use machine"
Future-proof	Locked to specific format	Can evolve what "machine" means internally

An LLM reading --help naturally self-identifies with --log-for machine.

Machine output requirements

When --log-for machine is set:

1. JSON Lines format — one JSON object per line (streamable)
2. No ANSI codes — strip all color/formatting escape sequences
3. No Unicode decorations — no box-drawing characters
4. Include correlation IDs — requestId in every log entry
5. Include timestamps — ISO 8601 format
6. Full context — URL, method, plugin name in each entry
7. Semantic message types — request, response, warning, error, mock, chaos (not oops, skip)

Configuration

Also support in devproxyrc.json:

{
  "logFor": "machine"
}

Related

• Reporters (JsonReporter, PlainTextReporter) handle file output after recording
• This flag handles real-time console/stdout output

[garrytrinder]

Great idea! The current output generates a lot of noise for LLMs so this will make the output more concise and parsable by an LLM.