I spent 3 months using Tab completion for everything. Then 3 months using Agent mode for everything. Both were wrong. Here is the rule I use now to pick the right mode for each task.
When should you use Tab completion instead of Agent mode?
For small edits under 5 lines. Refactoring a variable name. Adding a parameter. Fixing a typo. Tab completion takes 0.3 seconds. Agent mode takes 15 seconds to plan and write. For a single-line fix, Tab is 50x faster. I use Tab for maybe 60% of my daily work — the small edits that add up.
When does Agent mode justify its overhead?
When the change spans multiple files. Agent mode analyzes the full codebase context and generates consistent changes across related files. I use it for adding a new API endpoint, changing a database schema, or refactoring a shared utility. The overhead pays for itself by preventing cross-file inconsistencies.
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