Antigravity Planning Mode vs Fast Mode: A Practical Guide to Maximizing Development Speed

One of the first questions developers ask when getting started with Antigravity is: "Should I use Planning Mode or Fast Mode?"

This isn't just a speed question. The right answer depends on cost, accuracy requirements, and the nature of the task. This guide explains how both modes work and gives you practical rules for choosing between them.

The Fundamental Difference

The two modes use different underlying AI models.

Planning Mode uses Gemini 2.5 Pro (or Claude Opus 4.6). It reasons through complex problems in multiple steps, building a "plan" before execution.

Fast Mode uses Gemini 2.5 Flash (or Claude Haiku 4.5). It minimizes reasoning and responds quickly.

Planning Mode
─────────────────────────────────────────────
Input → Analyze → Plan → Execute → Verify
           ↑ Multi-step reasoning
           ↑ Full context understanding
           ↑ Side-effect prediction

Fast Mode
─────────────────────────────────────────────
Input → Execute directly
      (minimal reasoning, fast response)

Here's a practical comparison of speed, cost, and quality:

Metric              Planning Mode    Fast Mode
──────────────────────────────────────────────
Response time        5–15 seconds    0.5–3 seconds
API cost             ★★★★★           ★★
Quality (complex)    ★★★★★           ★★★
Quality (simple)     ★★★★            ★★★★★

When to Use Planning Mode

Planning Mode shines on tasks where "thinking before acting" makes a real difference.

1. Designing and Implementing New Features

# When to use Planning Mode
"Implement a user authentication system with:
- JWT + Refresh Token
- Social login (Google, GitHub)
- Rate limiting
- Audit logging"

# What Planning Mode does:
# 1. Analyzes requirements and maps dependencies
# 2. Designs the file structure
# 3. Determines implementation order
# 4. Implements each step in sequence
# 5. Adds integration tests

2. Large-Scale Refactoring

Multi-file changes are where Planning Mode has the biggest advantage:

antigravity --mode=planning \
  "Migrate all API client code in src/ from fetch() to axios.
   Standardize error handling patterns across the codebase."
 
# Planning Mode:
# - Scans all files first
# - Analyzes the scope of impact
# - Creates a migration plan
# - Makes changes safely in batches
# - Runs tests to verify

3. Root Cause Analysis for Bugs

"Users are reporting login failures in production.
Error log: [JWT_INVALID_SIGNATURE at /api/auth/verify]
Investigate the cause and fix it."

# Planning Mode:
# 1. Analyzes error log patterns
# 2. Traces related code paths
# 3. Checks environment variables and config files
# 4. Forms and tests hypotheses
# 5. Proposes the minimal fix

When to Use Fast Mode

Fast Mode is ideal for tasks where "the answer is obvious and thinking isn't needed."

1. Repetitive Mechanical Changes

# Fast Mode handles this instantly
# "Replace all console.log calls with logger.debug"
# → Fast Mode executes immediately (no analysis needed)
 
# Before
console.log("User logged in:", userId);
console.log("Request received:", req.method, req.path);
 
# After (Fast Mode processes this instantly)
logger.debug("User logged in:", userId);
logger.debug("Request received:", req.method, req.path);

2. Formatting and Lint Fixes

# Fast Mode tasks
antigravity --mode=fast "Fix all ESLint errors"
antigravity --mode=fast "Organize TypeScript imports"
antigravity --mode=fast "Standardize code indentation"

3. Mechanical Test Generation

When adding unit tests to existing functions, Fast Mode is sufficient:

// Fast Mode: "Write unit tests for this function"
function add(a: number, b: number): number {
  return a + b;
}
 
// Fast Mode generates instantly:
describe("add", () => {
  it("adds positive numbers", () => {
    expect(add(1, 2)).toBe(3);
  });
  it("adds negative numbers", () => {
    expect(add(-1, -2)).toBe(-3);
  });
  it("adds zero", () => {
    expect(add(0, 5)).toBe(5);
  });
});

4. Documentation and Comments

antigravity --mode=fast "Add JSDoc comments to all functions in src/utils/"

Setting Default Modes in AGENTS.md

To avoid specifying the mode every time, configure per-task defaults in your AGENTS.md file:

# AGENTS.md
 
## Antigravity Configuration
 
### Default Mode Settings
 
- **New feature development**: Always use Planning Mode
  - Reason: Architecture decisions require careful consideration
 
- **Bug fixes**: Planning Mode for unknown bugs, Fast Mode for obvious typos
 
- **Refactoring**: Planning Mode for multi-file, Fast Mode for single-file
 
- **Test generation**: Fast Mode (mechanical task)
 
- **Documentation**: Fast Mode (straightforward generation)
 
### Cost Management
 
- Daily Planning Mode budget: 50 requests
- Fast Mode: Unlimited
- Alert when Planning Mode usage exceeds 30/day

Switching Modes Mid-Workflow

A practical pattern for switching modes during development:

# Start with Planning Mode for architecture
antigravity --mode=planning "Design and implement a shopping cart feature"
# → Planning Mode creates the architecture
 
# Switch to Fast Mode for detail work
antigravity --mode=fast "Add loading styles to the cart component"
antigravity --mode=fast "Translate error messages to English"

Cost-Conscious Workflows for Teams

As covered in Antigravity Pricing Guide, Planning Mode costs roughly 5–10x more than Fast Mode.

# Cost estimation (rough reference)
tasks_per_day = {
    "planning_mode": 20,   # Design, debugging, complex implementation
    "fast_mode": 100       # Formatting, tests, documentation
}
 
# Monthly costs (estimated)
planning_cost = 20 * 22 * 0.05  # ~$22/month
fast_cost = 100 * 22 * 0.005    # ~$11/month
total = planning_cost + fast_cost  # ~$33/month

Combined with AgentKit 2.0, using Planning Mode as an orchestrator while delegating execution to Fast Mode sub-agents is a highly effective cost architecture.

Common Failure Patterns

❌ Mistake: Using Planning Mode for everything
→ Problem: Costs spiral, simple tasks become slow
→ Fix: Identify mechanical tasks and route them to Fast Mode

❌ Mistake: Using Fast Mode for everything
→ Problem: Multi-file refactoring produces inconsistent changes
→ Fix: Always use Planning Mode for changes spanning multiple files

❌ Mistake: Using Fast Mode for bug investigation
→ Problem: Surface-level fixes leave root causes unresolved
→ Fix: Unknown bugs always go to Planning Mode

Wrapping up

Knowing when to use Planning Mode vs Fast Mode is one of the most impactful skills for getting the most out of Antigravity.

The simple rule: tasks that are multi-file, have an unknown cause, or require design thinking → Planning Mode. Single-file, clear answer, or repetitive → Fast Mode.

Apply this consistently and you'll see both better results and significantly lower costs.