Plan Mode and Debugging Strategies

Plan Before You Build

The most common mistake developers make with Claude Code is jumping straight to implementation. "Build me a payment service" produces mediocre results. "Let's plan the payment service first, then build it" produces excellent results.

Plan mode gives you a structured way to think before you code.

/plan Mode

Type /plan during a session to enter plan mode:

/plan

In plan mode, Claude:

• Can: Read files, analyze code, search the codebase, run read-only commands
• Cannot: Write files, modify code, run destructive commands

This is deliberately limiting. The constraint forces Claude to think, not act.

Using Plan Mode

/plan

"I need to add order cancellation with refund processing to our API.
Read the existing order service, payment service, and database schema.
Then create a detailed plan covering:
1. Database changes needed
2. New service methods
3. API endpoint design
4. Integration with existing payment processing
5. Test strategy
6. Migration plan"

Claude reads the codebase, understands the existing architecture, and produces a thorough plan — without touching any files.

Reviewing the Plan

Claude's plan might look like:

## Order Cancellation Plan

Database Changes
Add canceled_at and cancellation_reason columns to orders table
Add refund_id column to link cancellation to refund
New migration: 20260404_add_order_cancellation.sql

Service Layer
OrderService.cancelOrder(orderId, reason): 
  - Validates order is in cancellable state
  - Creates refund via PaymentService
  - Updates order status to 'canceled'
  - Emits OrderCanceled event

API Endpoint
POST /api/v1/orders/:id/cancel
Body: { reason: string }
Response: { order: Order, refund: Refund }
Auth: requires order owner or admin

Test Strategy
Unit: cancelOrder service method (happy path, already canceled, 
  non-cancellable status, refund failure)
Integration: full endpoint test with database
Edge: concurrent cancellation attempts

You can review, ask questions, and refine the plan before any code is written.

Executing the Plan

When you're satisfied with the plan:

/execute

Claude switches to full mode and implements the plan. It has the complete plan in context, so it follows the architecture, naming, and patterns you agreed on.

Starting in Plan Mode

You can start a session directly in plan mode:

claude --permission-mode plan

This is useful when you know you want to analyze before acting — like reviewing a new codebase or planning a large refactoring.

The /debug Skill

Claude Code has a built-in debugging skill that provides guided troubleshooting:

/debug

This activates a structured debugging workflow where Claude:

1. Asks you to describe the symptoms

2. Reads relevant code and logs

3. Forms hypotheses

4. Tests each hypothesis systematically

5. Identifies the root cause

6. Proposes and implements a fix

Parallel Debugging with /fork

When you're debugging and have multiple theories about the root cause, use /fork to test them in parallel:

# In a debugging session
"I think the timeout is either in the database query or the Redis connection"

# Fork to test database theory
/fork theory-database
"Let's check the database query. Read the order-repo.ts query 
and check for slow operations..."

# Go back to the original session
/resume
# Select the pre-fork session

# Fork to test Redis theory  
/fork theory-redis
"Let's check the Redis connection. Read the cache service 
and check for connection timeouts..."

Each fork explores independently. When you find the answer, continue in the successful fork.

The Systematic Debugging Approach

When debugging with Claude, follow this structure:

Step 1: Reproduce

"The bug: users report seeing 'null' for their order total. 
Here's the error log: @logs/api-errors-2026-04-04.log
Can you reproduce this by finding the code path?"

Claude traces the code path to understand how the bug manifests.

Step 2: Isolate

"Narrow it down. Is the null coming from:
a) The database (stored as null)
b) The service layer (calculation error)
c) The API serialization (formatting issue)"

Claude reads each layer and isolates where the null originates.

Step 3: Diagnose

"We've isolated it to the service layer. The calculateTotal function
returns null when the order has no items. Read the function and explain
why — what's the root cause?"

Claude analyzes the specific function and identifies the root cause.

Step 4: Fix

"Fix it. The calculateTotal function should return 0 for empty orders, 
not null. Also add a test that catches this case."

Claude implements the fix and writes the test.

Step 5: Verify

"Run the tests. Also check if there are other places in the codebase 
where we make the same mistake — returning null instead of a default 
value for calculations."

Claude runs tests and searches for similar issues.

Combining Strategies

The most effective debugging sessions combine multiple strategies:

# Start in plan mode to understand the problem
/plan
"Analyze the payment processing code and identify all places where
a timeout could occur. Don't fix anything yet — just map the risks."

# Review the analysis
# ...

# Switch to execution with a clear target
/execute
"Fix the timeout in the webhook handler — add a 30-second timeout 
with retry logic"

# Fork to test the fix safely
/fork test-fix
"Apply the fix and run the payment service test suite"

# If tests pass, continue in the fork
# If tests fail, go back and try a different approach

Plan → Execute → Fork → Verify. Each tool serves a specific purpose in the workflow.

Key Takeaway

Plan mode (/plan) gives Claude read-only access for analysis and planning before making changes. Use /execute to apply the plan. Use /fork to test multiple debugging hypotheses in parallel. The systematic debugging approach — reproduce, isolate, diagnose, fix, verify — gives Claude the structure it needs to find root causes efficiently. Always plan complex changes before implementing them, and fork for risky experiments.