Sonnet vs Opus for Code Generation: Model Routing in Agent

Quick answer

Claude Sonnet (3.5 and 3.7) delivers faster, cheaper code generation for most everyday tasks—refactors, bug fixes, feature additions—while Opus (3.0 and 3.5) excels at complex architectural decisions, deep reasoning, and multi-file refactors where context depth matters more than speed. The right choice depends on task complexity, token budget, and whether you’re willing to wait (and pay) for Opus’s deeper analysis.

Key takeaways

• Sonnet 3.7 is the workhorse for routine coding: fast, cost-effective, and strong enough for 80% of agent tasks.
• Opus 3.5 shines in architectural planning, large refactors, and tasks requiring deep multi-file reasoning.
• Token costs differ dramatically: Opus input tokens cost ~5× more than Sonnet 3.7, and output tokens cost ~3× more.
• Model switching mid-conversation is possible in Claude Desktop and CLI (shared native store), but context may need explicit summarization or re-injection.
• Routing strategies include task-type rules (Sonnet for features, Opus for architecture), approval gates (start Sonnet, escalate to Opus if stuck), and parallel lanes (Sonnet for quick wins, Opus for deep work).
• MobileVibe lets you monitor both models from your phone, approve escalations, and switch surfaces without losing the conversation thread.

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Speed vs Depth: When Sonnet Wins the Race

Claude Sonnet coding is optimized for speed and cost. Sonnet 3.7 (the latest as of early 2025) generates code faster than Opus 3.5, with lower latency and smaller token budgets for typical tasks. For routine work—adding a REST endpoint, fixing a type error, writing a test suite, or refactoring a single module—Sonnet delivers comparable quality to Opus at a fraction of the cost and wait time.

When Sonnet is the right choice:

• Feature additions in well-structured codebases where the agent has clear instructions and a narrow scope.
• Bug fixes with obvious root causes or stack traces pointing to a single file.
• Iterative refinement: Sonnet handles “try this, then adjust” loops efficiently.
• High-volume tasks: running multiple agents in parallel across worktrees or lanes, where cost and speed compound.

Real-world example: A developer uses Sonnet 3.7 to add a new API route, write integration tests, and update documentation. The agent completes the task in under two minutes, consuming ~8K input tokens and ~3K output tokens. Total cost: under $0.10. The same task with Opus 3.5 would take longer and cost closer to $0.50—acceptable for one task, but not for ten parallel lanes.

Sonnet speed vs Opus matters most when you’re checking progress from a phone. If an agent needs approval or hits a quota, you want to review the diff, approve, and move on—not wait 30 seconds for Opus to finish thinking. Sonnet’s faster response loop makes mobile-driven workflows practical.

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Complex Reasoning: Where Opus Earns Its Cost

Opus code quality becomes visible in tasks requiring deep reasoning, multi-file context, or architectural judgment. Opus 3.5 (the current flagship) excels at:

• Architectural decisions: choosing between design patterns, refactoring a module structure, or planning a migration path.
• Large refactors: touching 10+ files, renaming abstractions, or consolidating duplicated logic across a codebase.
• Ambiguous requirements: when the task description is vague or the agent must infer intent from incomplete context.
• Debugging gnarly issues: race conditions, subtle type mismatches, or performance bottlenecks requiring careful analysis.

When Opus is worth the cost:

• The task is high-stakes (production code, public API changes, security-sensitive logic).
• You’ve already tried Sonnet and it produced a shallow or incorrect solution.
• The codebase is unfamiliar or poorly documented, and the agent needs to “figure things out.”
• You’re planning a multi-step refactor and want a coherent strategy before committing changes.

Real-world example: A developer asks an agent to refactor a legacy authentication module spread across six files, consolidating logic into a single service. Sonnet 3.7 produces a working solution but misses edge cases around session expiry and token refresh. Opus 3.5, given the same prompt, identifies the edge cases, proposes a cleaner abstraction, and writes comprehensive tests. The extra cost (~$2 for the full session) is justified by avoiding a production bug.

Opus’s reasoning depth also shows up in approval gates. When an agent running Opus asks for approval, the explanation is often more thorough—“I’m refactoring this because X, Y, and Z are tightly coupled, and splitting them will simplify future changes.” Sonnet’s explanations are shorter and more mechanical.

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Token Budget and Quota Management Across Models

Token costs are the hidden variable in sonnet vs opus coding. As of early 2025, approximate pricing (check current Anthropic rates):

• Sonnet 3.7: ~$3 per million input tokens, ~$15 per million output tokens.
• Opus 3.5: ~$15 per million input tokens, ~$75 per million output tokens.

For a typical agent session (50K input tokens, 10K output tokens):

• Sonnet 3.7: ~$0.30 total.
• Opus 3.5: ~$1.50 total.

Over a day of active development (10 sessions), Sonnet costs ~$3, Opus costs ~$15. Over a month, the difference compounds: ~$90 for Sonnet, ~$450 for Opus.

Quota management strategies:

1. Default to Sonnet, escalate to Opus. Start every task with Sonnet 3.7. If the agent gets stuck, produces low-quality output, or you anticipate complexity, switch to Opus mid-conversation.
2. Per-lane budgets. In multi-agent setups, assign Sonnet to “quick win” lanes (bug fixes, small features) and Opus to “deep work” lanes (architecture, refactors).
3. Approval gates as cost checkpoints. Configure agents to ask for approval before large token spends. Review the plan, decide if Opus is justified, and approve or downgrade to Sonnet.
4. Monitor from mobile. MobileVibe’s dashboard shows which conversations are consuming tokens and which models they’re using. If an Opus agent is spinning on a trivial task, you can intervene from your phone and switch it to Sonnet.

Quota limits (per Anthropic’s current tiers) also differ: Sonnet quotas are generally higher for the same subscription level. If you’re running multiple agents in parallel, hitting Opus quotas is more likely. Sonnet gives you more runway.

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Routing Strategies for Multi-Agent Codebases

Model routing becomes critical when you’re running several agents across related folders or worktrees. Common patterns:

Task-type routing

• Sonnet for features and fixes. Agents working on isolated features, bug fixes, or test coverage default to Sonnet 3.7.
• Opus for architecture and planning. Agents tasked with “design a new module,” “refactor the auth layer,” or “plan a database migration” use Opus 3.5.

Approval-based escalation

• Start every agent with Sonnet.
• If the agent asks for approval and the diff looks shallow or incorrect, switch to Opus and re-run the task.
• If the agent completes successfully, stick with Sonnet for follow-up tasks.

Parallel lanes by complexity

• Lane 1 (Sonnet): Quick wins—documentation updates, linting fixes, small refactors.
• Lane 2 (Opus): Deep work—architectural changes, multi-file refactors, performance optimization.
• Run both lanes simultaneously. Sonnet lanes ship faster; Opus lanes take longer but produce higher-quality output.

Hybrid handoff

• Use Sonnet to generate a first draft or proof-of-concept.
• Switch to Opus for review, refinement, and edge-case handling.
• Example: Sonnet writes a new API endpoint; Opus reviews it for security, error handling, and test coverage.

MobileVibe’s role: The dashboard shows all active conversations, their models, and their status (working, needs approval, blocked). From your phone, you can see which lanes are Sonnet (fast, cheap) and which are Opus (slow, expensive), and decide whether to approve, escalate, or pause based on progress and cost.

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Real Workflows: Approval Gates and Model Switching

Approval gates are where sonnet vs opus coding decisions happen in real time. Here’s how it works in practice:

Scenario 1: Sonnet hits a wall

• You start a Sonnet 3.7 agent to refactor a module.
• The agent produces a diff, but it’s incomplete—missing edge cases or breaking tests.
• You get a notification on your phone: “Agent needs approval.”
• You review the diff in MobileVibe, see the issue, and decide to escalate.
• You switch the conversation to Opus 3.5 (in Claude Desktop or CLI, this is a model-selection change; context carries over via the native store).
• Opus re-runs the task with deeper reasoning, produces a better diff, and you approve.

Scenario 2: Opus is overkill

• You start an Opus 3.5 agent for what you thought was a complex task.
• The agent finishes quickly with a simple, obvious solution.
• You realize Sonnet would have been sufficient.
• For the next similar task, you default to Sonnet and save the cost.

Scenario 3: Parallel lanes with different models

• You run three agents in parallel worktrees:
  • Lane A (Sonnet): Add a new feature to the API.
  • Lane B (Sonnet): Fix a bug in the frontend.
  • Lane C (Opus): Refactor the database layer.
• Lanes A and B finish in minutes; Lane C takes longer but produces a cleaner architecture.
• You approve all three from your phone, merge them sequentially, and ship.

Model switching mid-conversation is straightforward in Claude Desktop and CLI (both use the native Claude store, so context persists). In Codex or other surfaces, switching models may require forking the conversation or re-injecting context. MobileVibe handles this by letting you open the conversation in the right surface and continue from there.

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Cost-Per-Task Calculations for Long-Running Agents

Long-running agents amplify cost differences between Sonnet and Opus. Here’s a breakdown:

Short task (single feature, ~10 minutes)

• Sonnet 3.7: 20K input tokens, 5K output tokens → ~$0.15.
• Opus 3.5: 20K input tokens, 5K output tokens → ~$0.68.

Medium task (multi-file refactor, ~1 hour)

• Sonnet 3.7: 100K input tokens, 20K output tokens → ~$0.60.
• Opus 3.5: 100K input tokens, 20K output tokens → ~$2.25.

Long task (architectural overhaul, ~4 hours)

• Sonnet 3.7: 400K input tokens, 80K output tokens → ~$2.40.
• Opus 3.5: 400K input tokens, 80K output tokens → ~$12.00.

For a single long task, the $10 difference may be acceptable if opus code quality prevents a production bug or saves hours of manual debugging. But if you’re running multiple long tasks daily, the cost compounds quickly.

Cost-per-task heuristic:

• Sonnet for tasks where “good enough” is acceptable and iteration is cheap.
• Opus for tasks where “right the first time” matters and rework is expensive.

MobileVibe’s dashboard helps you track cost over time by showing which conversations are consuming the most tokens and which models they’re using. If Opus costs are climbing, you can shift more tasks to Sonnet or set stricter approval gates.

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Hybrid Setups: Running Both Models in Parallel Lanes

Parallel lanes let you run Sonnet and Opus simultaneously, each in its own folder or worktree. This is the most flexible approach for large codebases or multi-agent workflows.

Example setup

• Main branch (Sonnet): Routine features, bug fixes, and tests. Fast iteration, low cost.
• Refactor branch (Opus): Architectural changes, deep refactors, and performance work. Slower, higher quality.
• Experimental branch (Sonnet): Proof-of-concept features, throwaway code. Speed matters more than polish.

Workflow

1. Start three agents in three worktrees, each tied to a different branch and model.
2. Monitor all three from MobileVibe’s dashboard.
3. Approve Sonnet agents quickly (they’re low-risk, fast).
4. Review Opus agents more carefully (they’re high-stakes, expensive).
5. Merge branches sequentially as agents complete.

Benefits

• Sonnet lanes ship quickly, keeping momentum high.
• Opus lanes produce high-quality work without blocking fast iteration.
• Cost is optimized: you pay for Opus only where it adds value.

MobileVibe’s role: The dashboard shows all lanes, their models, and their status. From your phone, you can see which lanes need attention, which are safe to let run, and which are blocked. Notifications alert you when any agent needs approval, regardless of model.

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FAQ

Is Sonnet or Opus better for routine code generation tasks?

Sonnet 3.7 is better for routine tasks—bug fixes, feature additions, test writing, and small refactors. It’s faster, cheaper, and produces quality comparable to Opus for well-scoped work. Reserve Opus for tasks requiring deep reasoning or architectural judgment.

When should I route complex architectural decisions to Opus instead of Sonnet?

Route to Opus 3.5 when the task involves multi-file refactors, ambiguous requirements, high-stakes production code, or decisions requiring deep context (e.g., “Should we split this module into three services?”). If Sonnet produces a shallow or incorrect solution, escalate to Opus.

How do token costs differ between Sonnet and Opus over a full agent session?

Sonnet 3.7 costs ~$3 per million input tokens and ~$15 per million output tokens. Opus 3.5 costs ~$15 per million input tokens and ~$75 per million output tokens. For a typical session (50K input, 10K output), Sonnet costs ~$0.30, Opus costs ~$1.50—a 5× difference. Over many sessions, this compounds significantly.

Can I switch models mid-conversation without losing context?

Yes, in Claude Desktop and CLI, which share the native Claude store. Context persists when you switch models. In other surfaces (Codex, Cursor, Windsurf), switching may require forking the conversation or re-injecting context. MobileVibe lets you open the conversation in the right surface and continue from there.

What’s the practical difference in reasoning quality for refactoring tasks?

Opus produces deeper, more coherent refactors—identifying edge cases, proposing cleaner abstractions, and writing thorough tests. Sonnet handles straightforward refactors well but may miss subtle issues or produce mechanical solutions. For high-stakes refactors, Opus’s reasoning quality justifies the extra cost.

How do I set up automatic model routing based on task complexity?

Most surfaces don’t offer automatic routing yet. Instead, use manual routing strategies: default to Sonnet, escalate to Opus when stuck; assign models per lane (Sonnet for features, Opus for architecture); or use approval gates to decide mid-task. MobileVibe’s dashboard helps you monitor and switch models from your phone as tasks evolve.

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Choosing between Sonnet and Opus isn’t about picking a “better” model—it’s about matching the model to the task. Sonnet 3.7 handles the bulk of everyday coding faster and cheaper; Opus 3.5 delivers deeper reasoning when complexity demands it. The best setups use both: Sonnet for speed, Opus for depth, and approval gates to decide in real time. Try MobileVibe free to monitor both models from your phone, approve escalations on the go, and keep multi-agent workflows moving—whether you’re at your desk or away from it.