Claude Opus vs Sonnet for Coding: When to Use Each

Two Models, Different Sweet Spots

Claude Opus and Sonnet are both part of the Claude model family, but they optimize for different trade-offs. Opus: the most capable Claude model. Highest reasoning ability, best at complex multi-step problems, strongest at understanding large codebases and architectural decisions. The trade-off: slower response time (2-5x slower than Sonnet for the same prompt) and higher cost ($15/million input tokens vs $3 for Sonnet). Opus is the thinking model — use it when the quality of reasoning matters more than speed.

Sonnet: the balanced Claude model. Excellent coding capability (consistently top-ranked in coding benchmarks alongside Opus), fast response time (tokens stream quickly, especially in fast mode), and lower cost. Sonnet handles: everyday coding tasks, code generation, bug fixes, refactoring, test writing, and documentation with quality that is indistinguishable from Opus for 80% of tasks. Sonnet is the workhorse — use it for everything that does not specifically need Opus reasoning depth.

The practical question is not which model is better (Opus is objectively more capable) but when the capability difference matters for coding. For most coding tasks: Sonnet is sufficient and the speed advantage makes it more productive. For specific complex tasks: Opus quality is worth the slower speed and higher cost. This article maps which tasks fall into which category.

When to Use Opus: Complex Architecture and Reasoning

Use Opus for tasks that require: understanding the entire codebase architecture before making changes ("refactor the authentication system from session-based to JWT across 20 files"), resolving conflicting requirements ("implement caching that respects both performance SLAs and data consistency requirements"), multi-step planning with conditional logic ("design the database schema, API layer, and frontend for a new feature, considering how it interacts with existing features"), and debugging complex issues that span multiple systems ("why does this request occasionally return stale data? trace through the cache, CDN, database replication, and application layers").

Opus excels when: the first attempt matters (complex tasks where getting it wrong means starting over), the context is large (10,000+ lines of relevant code to understand), the task requires weighing trade-offs (performance vs maintainability, consistency vs availability), or the output needs to be architecturally sound (not just syntactically correct, but the right approach). These tasks are: 10-20% of daily coding for most developers, but they are the highest-impact tasks.

In Claude Code: use Opus when you describe a large task and want the agent to plan correctly on the first attempt. Claude Code with Opus: reads more files, considers more implications, and produces more architecturally sound plans. The time savings from getting it right the first time (no iteration) often exceeds the slower generation speed. For a 30-minute task: Opus takes 5 minutes to generate a correct plan. Sonnet takes 2 minutes to generate a plan that needs 10 minutes of iteration.

• Complex refactors spanning 10+ files with architectural implications
• System design decisions that weigh multiple trade-offs
• Debugging issues that span multiple layers (cache, CDN, DB, app)
• First-attempt accuracy: getting it right avoids iteration time
• 10-20% of daily tasks — but the highest-impact ones

When to Use Sonnet: Everyday Coding

Use Sonnet for: writing new functions and components (the scope is clear, the pattern is established), fixing bugs with known causes ("this function returns null when the input is empty — add a guard clause"), writing tests for existing code (the code exists, the test follows a known pattern), refactoring within a single file (rename, extract function, simplify logic), generating boilerplate (CRUD endpoints, form components, database models), documentation (docstrings, README updates, API documentation), and code review suggestions ("review this PR for issues").

Sonnet is the better choice when: speed matters more than depth (quick edits, inline chat, tab completion), the task is well-defined (clear input, clear expected output), the context is small (one file, one function, one component), the pattern is familiar (CRUD, REST endpoint, React component — not novel architecture), or you are iterating rapidly (try something, see the result, adjust — faster iteration beats deeper first attempt). These tasks are: 80-90% of daily coding.

In Claude Code: use Sonnet (or fast mode) for quick tasks. Claude Code with Sonnet: responds faster, costs less per interaction, and produces identical quality for straightforward tasks. The speed difference is tangible: Sonnet streams tokens 2-3x faster than Opus. For a task that takes 30 seconds of generation: Sonnet finishes in 30 seconds, Opus in 60-90 seconds. Over a full day of coding: the cumulative speed difference is significant.

• New functions, components, tests — well-defined scope, known patterns
• Bug fixes with known causes — the diagnosis is done, apply the fix
• Single-file refactoring — rename, extract, simplify within one file
• Boilerplate generation — CRUD, forms, models, endpoints
• 80-90% of daily coding tasks — speed and cost advantages compound

Cost and Speed: The Numbers

Cost comparison: Opus input $15/million tokens, output $75/million tokens. Sonnet input $3/million tokens, output $15/million tokens. Opus is: 5x more expensive per token. A typical coding session: 50,000 input tokens + 10,000 output tokens. Sonnet cost: $0.30. Opus cost: $1.50. Daily (8 hours): Sonnet $2-5, Opus $10-25. Monthly: Sonnet $40-100, Opus $200-500. The 5x cost multiplier is significant for heavy daily use.

Speed comparison: Sonnet generates tokens at roughly 80-100 tokens/second. Opus generates at roughly 30-50 tokens/second. For a 500-token response: Sonnet = 5-6 seconds. Opus = 10-15 seconds. For a 2000-token response (typical for a multi-file edit plan): Sonnet = 20-25 seconds. Opus = 40-60 seconds. The speed difference: Sonnet feels interactive (responses arrive as you read), Opus feels deliberate (you wait, then read). Claude Code fast mode uses Sonnet for the speed advantage.

The ROI calculation: if Opus saves one iteration cycle per task (getting it right the first time), and each iteration costs 5 minutes of developer time: Opus saves $4-8 per iteration (at $50-100/hour developer cost). Opus costs $1-2 more per task. The ROI is: positive for complex tasks (save 5 minutes, spend $1 more) and negative for simple tasks (no iteration saved, spend $1 more). Use Opus when the iteration saving exceeds the cost premium.

• Opus: $15/$75 per million tokens (input/output). Sonnet: $3/$15 — 5x cost difference
• Daily cost: Sonnet $2-5. Opus $10-25 — monthly Sonnet $40-100, Opus $200-500
• Speed: Sonnet ~80-100 tok/s. Opus ~30-50 tok/s — 2-3x faster responses with Sonnet
• ROI positive: Opus saves iteration time on complex tasks ($4-8 saved > $1-2 cost)
• ROI negative: Opus adds cost on simple tasks (no iteration saved, $1-2 extra spent)

Practical Model Selection Strategy

The two-model strategy: default to Sonnet for everything, switch to Opus for specific tasks. In Claude Code: start with Sonnet (or fast mode). When you hit a task that needs deeper reasoning: switch to Opus with /model opus. After the complex task: switch back to Sonnet. This strategy: minimizes cost (Sonnet for 80% of interactions), maximizes quality when it matters (Opus for the 20% that need it), and lets you feel the difference (you learn when Opus quality matters through experience).

Task-based routing rules: write a new React component → Sonnet. Design the state management architecture for a new feature → Opus. Fix a null pointer bug → Sonnet. Debug a race condition across three services → Opus. Generate unit tests → Sonnet. Review an architectural decision for a system redesign → Opus. Write an API endpoint → Sonnet. Plan a database migration that affects 15 tables → Opus. The pattern: routine tasks = Sonnet, architectural decisions = Opus.

For Claude Code specifically: Sonnet (fast mode) is the default because Claude Code makes many small tool calls (read file, search, edit) where speed matters more than reasoning depth. For the actual code generation and planning: Opus produces better first-attempt plans. The hybrid: fast mode for exploration (reading files, searching, understanding the codebase), Opus for the critical generation step (planning and implementing the change). Claude Code handles the model switching seamlessly.

Quality Difference: When It Matters and When It Does Not

When quality is identical: writing a function that sorts an array, generating a REST endpoint, creating a React form component, writing a unit test for a pure function, adding TypeScript types to existing code, or formatting and documenting code. These tasks have: clear patterns, bounded scope, and well-defined correct answers. Both models produce the same output because the task does not require deep reasoning — it requires pattern application. Quality difference: negligible.

When Opus quality is noticeably better: designing a caching strategy that accounts for consistency, invalidation, and performance SLAs. Refactoring a 500-line function into a clean module structure while preserving behavior. Debugging a subtle race condition by reasoning about concurrent execution paths. Planning a migration from REST to GraphQL for a 20-endpoint API. Writing a complex SQL query with multiple CTEs, window functions, and conditional aggregation. These tasks require: multi-step reasoning, trade-off evaluation, and architectural judgment. Quality difference: significant.

The developer experience: you cannot always predict in advance whether a task needs Opus. The practical approach: start with Sonnet. If the output is not architecturally sound (the code works but the approach is wrong), retry with Opus. The retry overhead is: one additional prompt (30-60 seconds). Over time: you develop intuition for which tasks need Opus from the start. The learning curve is: 1-2 weeks of conscious model switching to build the intuition.

Model Selection Summary

Summary of when to use Claude Opus vs Sonnet for coding.

• Default: Sonnet for everything (80-90% of tasks), switch to Opus for complex tasks (10-20%)
• Opus for: architecture decisions, multi-file refactors, complex debugging, system design
• Sonnet for: new components, bug fixes, tests, boilerplate, documentation, single-file edits
• Cost: Sonnet $2-5/day, Opus $10-25/day — 5x difference compounds over a month
• Speed: Sonnet 2-3x faster responses — the speed difference is tangible over a full day
• ROI: Opus saves iteration time on complex tasks (positive ROI), adds cost on simple tasks (negative ROI)
• Claude Code: fast mode (Sonnet) for exploration, Opus for critical generation and planning
• Learning: 1-2 weeks to develop intuition for when Opus quality makes a meaningful difference