learning-log

name

learning-log

description

Auto-document insights and discoveries during development sessions. Unlike MEMORY.md (corrective rules from the self-correction-loop skill), the learning log captures organic discoveries: non-obvious bugs, undocumented architecture decisions, performance findings, workarounds, and gotchas. Stored at .Codex/learning-log.md. Load this skill when Codex discovers something non-obvious, finds a workaround, uncovers an undocumented decision, or when the user asks about "learnings", "discoveries", "gotchas", "what did we learn", or "document this finding".

Learning Log

Core Principles

1. Log insights, not rules — MEMORY.md stores corrective rules ("always use X instead of Y"). The learning log stores discoveries ("X behaves differently when Y is configured because Z"). Rules prescribe behavior; insights explain the world.

2. Structure enables searchability — Every entry has a date, category, title, description, and affected files. Consistent structure means you can search by category, scan titles, or find entries related to specific files.

3. Log during work, not after — Capture insights the moment they occur. Waiting until the end of a session means half the detail is lost. A 2-line entry written immediately is worth more than a paragraph reconstructed from memory.

4. Periodic review extracts patterns — A monthly scan of the learning log reveals recurring themes. Three "gotcha" entries about the same subsystem suggests a systemic issue worth addressing. Individual entries are useful; patterns across entries are actionable.

5. Distinct from handoff notes — The wrap-up-ritual handoff captures session state (done/pending/learned). The learning log is a persistent, growing knowledge base. Handoffs are overwritten; the log only grows (and is periodically pruned).

Patterns

Log Entry Format

Each entry follows a consistent structure in .Codex/learning-log.md:

# Learning Log

## 2025-07-15 | Bug Root Cause | EF Core SaveChanges Silently Succeeds on Duplicate Keys
`SaveChangesAsync` with a duplicate PK throws on the *next* `SaveChangesAsync` call, not the insert.
**Files:** `src/Orders/Features/CreateOrder.cs:42`
**Resolution:** Call `SaveChangesAsync` immediately after `Add()`, before any other operations.

## 2025-07-12 | Gotcha | MassTransit Consumer Registration Order Matters
Multiple consumers for the same message type run in registration order. If the first throws,
subsequent consumers are skipped. Caused missed audit events.
**Files:** `src/Shared/Extensions/MassTransitConfig.cs:15-30`
**Resolution:** Configure independent consumer endpoints or use the retry filter.

Auto-Logging Triggers

Log entries automatically when these situations occur:

TRIGGER 1: Non-Obvious Bug Root Cause
When debugging reveals the root cause is NOT where the error appeared.
→ Log the misdirection, the actual cause, and how to avoid confusion.

TRIGGER 2: Undocumented Architecture Decision
When you discover WHY something was built a certain way (not just HOW).
→ Log the decision, the alternatives considered, and the rationale.

TRIGGER 3: Workaround for Framework/Library Limitation
When the "correct" approach doesn't work and you need an alternative.
→ Log what didn't work, why, and what works instead.

TRIGGER 4: Performance Finding
When profiling or observation reveals unexpected performance behavior.
→ Log the finding, the measurement, and the optimization applied.

TRIGGER 5: External Service Behavior
When an external API/service behaves differently than documented.
→ Log the expected vs actual behavior and any workaround.

TRIGGER 6: Non-Obvious Configuration
When a setting or configuration has a surprising effect.
→ Log the configuration, the surprising behavior, and the correct setup.

Category System

Use these 6 categories consistently:

Architecture Decision  — WHY something is structured a certain way
Bug Root Cause        — Non-obvious bugs where the error ≠ the cause
Performance Discovery — Unexpected performance behavior or optimization
Pattern Found         — Reusable pattern discovered in the codebase
Gotcha                — Surprising behavior in frameworks, libraries, or APIs
External Service      — Quirks of third-party services and APIs

Practical Logging Workflow

How to log during active development:

DURING WORK:
1. You encounter something non-obvious
2. Spend 30 seconds writing a log entry (2-4 lines)
3. Include the category, a descriptive title, and affected files
4. Continue working — the entry is captured, detail can be added later

ENTRY QUALITY LEVELS:
Quick (during work):  Date | Category | Title + 1-line description + files
Full (if time allows): Date | Category | Title + full description + resolution + files

A quick entry is infinitely better than no entry.

Log vs. Memory vs. Handoff

Distinguish between the three knowledge stores:

MEMORY.md (via self-correction-loop):
- Contains: Prescriptive rules ("always do X", "never do Y")
- Source: User corrections, promoted learning log entries
- Lifespan: Permanent until proven wrong
- Format: Category → bullet point rules

.Codex/learning-log.md (this skill):
- Contains: Descriptive insights ("X happens because Y")
- Source: Organic discoveries during development
- Lifespan: 3-6 months, then archive or promote
- Format: Date | Category | Title | Description | Files

.Codex/handoff.md (via wrap-up-ritual):
- Contains: Session state (done/pending/learned)
- Source: End of each session
- Lifespan: Until next session overwrites it
- Format: Completed / Pending / Learned sections

Anti-patterns

Logging Everything

// BAD — low-value entries that add noise
## 2025-07-15 | Pattern Found | Used Primary Constructors
Used primary constructors for the OrderService class.
**Files:** src/Orders/OrderService.cs

// GOOD — only log when it's non-obvious or surprising
## 2025-07-15 | Gotcha | Primary Constructor Parameters Captured as Fields
Primary constructor parameters in C# 14 are implicitly captured as fields.
If you also declare an explicit field with the same name, you get a compiler
warning but no error — and the two can silently diverge.
**Files:** src/Orders/OrderService.cs:5

No Categorization

// BAD — entries without categories are unsearchable
## 2025-07-15 | Compiled queries don't support Include()
## 2025-07-14 | MassTransit consumer ordering matters
## 2025-07-13 | Orders module uses VSA, Identity uses CA

// GOOD — categories enable filtering and pattern detection
## 2025-07-15 | Performance Discovery | Compiled Queries Don't Support Include()
## 2025-07-14 | Gotcha | MassTransit Consumer Registration Order Matters
## 2025-07-13 | Architecture Decision | Why Orders Uses VSA While Identity Uses CA

Write-Only Log (Never Reviewed)

// BAD — 100 entries, never reviewed
.Codex/learning-log.md grows to 500 lines
Same gotchas keep appearing because no one reads the log
No entries are promoted to MEMORY.md

// GOOD — monthly reviews extract value
Every 20 entries, scan for patterns
Promote recurring findings to MEMORY.md as preventive rules
Archive stale entries
The log stays lean and the rules get stronger

Duplicating MEMORY.md Content

// BAD — restating a MEMORY.md rule as a log entry
MEMORY.md: "Always use TimeProvider instead of DateTime.Now"
learning-log.md: "## Gotcha | DateTime.Now Is Not Testable" ← redundant

// GOOD — log adds a concrete incident the rule doesn't capture
learning-log.md: "## Bug Root Cause | Flaky Test Due to DateTime.Now
  OrderExpiry test failed intermittently — DateTime.Now crossed midnight during run."

Decision Guide

Scenario	Action
Found a non-obvious bug root cause	Log it — category: Bug Root Cause
Discovered why code is structured a certain way	Log it — category: Architecture Decision
Framework behaved unexpectedly	Log it — category: Gotcha
Performance surprise (good or bad)	Log it — category: Performance Discovery
Found a reusable pattern in the codebase	Log it — category: Pattern Found
External API behaved differently than docs say	Log it — category: External Service
User corrected Codex's code	Don't log — use self-correction-loop for MEMORY.md
Routine code change, nothing surprising	Don't log — only log insights
Same gotcha appeared 3+ times in the log	Promote to MEMORY.md as a preventive rule
Learning log exceeds 50 entries	Monthly review — archive old, promote recurring
Starting a new session	Scan recent log entries for context on the working area