session_diff

Compare the state of the application between two points in a recorded session. Use this to understand what changed between "working" and "broken" states.

Usage

CLI

# Compare by timestamp (ms from session start)
krometrail session diff <session-id> --from 5000 --to 15000

# Compare relative to markers
krometrail session diff <session-id> --from-marker "page loaded" --to-marker "error appeared"

MCP: session_diff

// By timestamp
{
	"session_id": "abc123",
	"from_ms": 5000,
	"to_ms": 15000
}

// By marker names
{
	"session_id": "abc123",
	"from_marker": "page loaded",
	"to_marker": "error appeared"
}

What Gets Compared

URL changes — navigation events between the two timestamps. Shows the sequence of pages visited.

Storage diffs — localStorage and sessionStorage changes. Keys added, removed, or modified with old/new values.

Network summary — requests made in the window, grouped by status code. Highlights new failures that appeared in the to window but not the from window.

Console changes — new console errors or warnings that appeared between the timestamps.

Framework state changes — component mount/unmount events and state changes. Shows which React or Vue components changed state during the window.

Screenshot comparison — nearest screenshots at each timestamp for visual reference.

Parameters

Parameter	Type	Description
session_id	string	The recording session
from_ms	number	Start timestamp (ms from session start)
to_ms	number	End timestamp (ms from session start)
from_marker	string	Use a named marker as the start point
to_marker	string	Use a named marker as the end point

Use marker names or timestamps — not both. Get timestamps and marker names from session_overview.

Example: Diagnosing a Form Submission Bug

# Record a session with markers
krometrail browser start http://localhost:3000
# ... fill out the form in Chrome ...
krometrail browser mark "form submitted"
# ... observe the error ...
krometrail browser mark "error displayed"
krometrail browser stop

# Diff between the two markers
krometrail session diff <session-id> \
	--from-marker "form submitted" \
	--to-marker "error displayed"

The diff output shows:

• Which network request failed (and its response body)
• Whether localStorage was modified unexpectedly
• Which React components re-rendered and with what state changes
• Console errors logged in that window

This is often enough to identify the root cause without inspecting individual events.