Debugging methodology

Skill thoughtbot/thoughtbot-dev/skills/debugging-methodology

Use when encountering bugs, test failures, or unexpected behavior. Provides a systematic debugging process based on surfacing assumptions, building decision tables, and validating one assumption at a time.From its SKILL.md

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npx -y skills add thoughtbot/thoughtbot-dev --skill debugging-methodology

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SKILL.md

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Debugging Methodology

When you hit a bug or unexpected behavior, follow this process. Don't guess — be systematic.

Step 1: List Your Assumptions

Write down everything you believe is true about the system. Making implicit assumptions explicit often reveals the bug immediately.

Common false assumptions:

"This value is never nil"
"This callback always runs"
"These records are always in this order"
"This association is always present"
"Time only moves forwards"

Write them down. Then validate them.

Step 2: Build a Decision Table

List all input combinations and their expected vs actual outputs. Include boundary values.

| Input          | Expected Output | Actual Output | Match? |
|----------------|----------------|---------------|--------|
| "hello"        | "HELLO"        | "HELLO"       | ✓      |
| ""             | ""             | ""            | ✓      |
| nil            | nil            | NoMethodError | ✗      |
| "hello world"  | "HELLO WORLD"  | "HELLO WORLD" | ✓      |

This reveals both implementation bugs (nil handling) AND spec flaws (what SHOULD nil do?).

Step 3: Validate Assumptions One at a Time

Don't trust your assumptions — verify them:

Read the code (don't assume you know what it does)
Check the database (don't assume the data looks right)
Add logging or use a debugger (don't assume the flow is what you think)
Run the failing case in isolation

When an assumption is false, you've likely found the bug.

Step 4: Diagram Relationships

When the bug involves data flow or associations:

Draw the entity relationships
Trace the data path from input to output
Mark where the actual behavior diverges from expected

Switching from code to visual representation reveals hidden connections — missing joins, unexpected associations, circular dependencies.

Step 5: Know When to Stop

If you're trying the same approach repeatedly:

Document your current state (what you know, what you've tried)
Step back and reconsider your assumptions from Step 1
Ask for help with a clear summary: "Here's the bug, here's what I've tried and ruled out"

Don't keep pulling the same lever expecting different results.

References

For detailed techniques, read:

references/listing-assumptions.md
references/decision-tables.md

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