memtrace-incident-investigation

name

memtrace-incident-investigation

description

Investigate source-code bugs, incidents, regressions, production issues, and failures to root cause with Memtrace symbol search, impact, call graph, and temporal history. Use when the user asks about root cause analysis, what broke, or what changed when debugging a failure. Do not start with Grep, Glob, rg, find, or manual file search for code causes. For plain what-changed questions without a failure, use memtrace-evolution.

Overview

Root cause investigation workflow for incidents, regressions, and production issues. Uses get_evolution to list changes near the incident time, then traces blast radius and execution flows to identify the likely cause.

Steps

1. Establish the timeline

Determine:

• Incident time — when did the problem start? (This becomes the to parameter)
• Lookback window — how far back to search? Start with 24 hours, expand if needed.
• Repo(s) — which services are affected? Call list_indexed_repositories to get repo_ids.

2. List changes near the incident

Call get_evolution:

{
  "repo_id": "<affected-repo>",
  "from": "<incident_time minus lookback, e.g. 24h before>",
  "to": "<incident_time ISO-8601>",
  "mode": "recent",
  "limit": 100
}

Why recent mode? Returns a chronological per-episode changelog. Focus on episodes whose reference_time is closest to the incident — especially those with high nodes_added + nodes_removed or that touch files in the failure area.

Paginate with cursor if next_cursor is present in the response.

Success criteria: A list of episodes in the window, with touched files and change counts for each.

3. Identify hotspot files and symbols

Call get_evolution again on the same window with mode: "compound":

{
  "repo_id": "<affected-repo>",
  "from": "<same as step 2>",
  "to": "<same as step 2>",
  "mode": "compound"
}

Review top_changed_files and top_touched_symbols. Cross-reference with the failure area (endpoint, module, error stack).

Decision: Prioritize symbols/files that appear in both the recent episode list (step 2) and the compound hotspots (step 3).

4. Check for unexpected changes to stable code

For hotspot symbols from step 3, resolve each with find_symbol:

{ "repo_id": "<affected-repo>", "name": "<symbol>", "limit": 10 }

Then call get_timeline — stable code that suddenly changed is suspect:

{ "repo_id": "<affected-repo>", "scope_path": "<from find_symbol>", "file_path": "<from find_symbol>" }

5. Assess the blast radius

For the top 3–5 symbols, call get_impact:

{ "repo_id": "<affected-repo>", "target": "<symbol>", "direction": "upstream" }

• How many downstream consumers were affected?
• What execution flows pass through this symbol?

Decision: Prioritize symbols where the blast radius overlaps with the reported failure area.

6. Trace execution flows

Use get_symbol_context on the top suspects to see which processes (HTTP handlers, background jobs, etc.) they participate in.

Decision: If the incident is in a specific endpoint/flow, focus on suspects that are members of that process.

7. Build the full timeline for the suspect

Once you have a primary suspect, call get_timeline with repo_id, scope_path, and file_path:

• What changed in each episode?
• When was the last "stable" version?
• Was the change a modification, or was it newly added?

8. Correlate adds vs removes per episode

From the step 2 recent response, inspect each episode's nodes_added and nodes_removed:

• High nodes_added — new code introduced (potential new bugs)
• High nodes_removed — deleted code (potential missing functionality)
• Both moderate — changed behaviour (potential regressions)

9. Check historical coupling (cochange)

For the primary suspect, call get_cochange_context:

{ "repo_id": "<affected-repo>", "target": "<symbol>", "limit": 10 }

• Which symbols historically co-change with this one?
• If the blast radius from get_impact doesn't explain the failure area, check cochange partners — the coupling may be behavioral, not structural.

Decision: If a cochange partner is in the failure area but has no direct call relationship to the suspect, it's a hidden dependency — investigate both.

10. Replay the sub-commit implementation history (if needed)

If the suspect's episode isn't clear, call get_episode_replay:

{
  "repo_id": "<affected-repo>",
  "episode_index": 0,
  "symbol": "<suspect>",
  "mode": "graph_summary"
}

• Look for attempted_and_reverted hints — approaches tried and rolled back within the episode often explain why the committed state looks the way it does.

Report: Root Cause Analysis

1. Incident Timeline — when it started, what was observed
2. Most Likely Cause — episodes and symbols closest to the incident with blast radius confirmation
3. Supporting Evidence — timeline sparsity (stable code suddenly changed?), blast radius overlap, process membership overlap
4. Change History — full timeline of the suspect symbol
5. Affected Scope — all processes and downstream consumers impacted
6. Remediation — revert the change, fix forward, or mitigate

Tool selection guide for incidents

Phase	Tool / mode	Why
Initial triage	get_evolution recent	Per-episode changelog near the incident
Hotspot identification	get_evolution compound	Top changed files and symbols in the window
Scope assessment	get_impact	Blast radius of suspect symbols
Hidden coupling	get_cochange_context	Behavioral coupling not in the call graph
Symbol history	get_timeline	Full version history of a suspect
Sub-commit intent	get_episode_replay	What was tried before the committed state
Quick window check	get_evolution overview	Totals only — use before narrowing the window

Full parameter spec for every Memtrace tool: references/mcp-parameters.md (bundled at the memtrace-skills plugin root).

Output

The deliverable is the RCA report above. Abridged example (3 of 6 sections filled):

• Most Likely Cause — episode 2026-07-01T14:22Z modified AuthService::validateToken; upstream blast radius reaches the failing /api/login process
• Supporting Evidence — symbol was stable for 90 days before this episode; cochange partner token_cache.rs changed in the same window despite no call edge
• Remediation — revert the episode, or fix forward with an expiry guard

Common mistakes

Mistake	Reality
Omitting from or passing days	Always pass from (e.g. "24 hours ago") — never days
Using mode: "novel" or "directional"	Not implemented — use compound + get_timeline + get_cochange_context
Only looking at the most recent commit	The root cause may be from an earlier episode whose effects were delayed
Not checking blast radius overlap	A change is only a suspect if its blast radius reaches the failure area
Stopping at call graph analysis	get_cochange_context finds hidden coupling — symbols that move together without calling each other
Reading only committed code	get_episode_replay reveals tried-and-reverted approaches that explain the current implementation