In our May 2026 testing, Claude Sonnet 4.6 made fewer errors than Gemini 2.0 Pro on multi-step tax logic, especially deduction phase-outs and cross-document reconciliation. The same scratchpad fix reduces errors on both. Neither replaces a licensed tax professional.
Last Updated: August 2026 · Cole Bridges Research Lab
All test results in this article used identical prompts on both models via API. Tests were conducted by the Cole Bridges Research Lab. Neither AI is a substitute for a licensed tax or legal professional.
What We Tested
We gave both Claude Sonnet 4.6 and Gemini 2.0 Pro the same five tax document analysis tasks using identical system prompts and user prompts. All tests were conducted in May 2026 via direct API access.
- Single W-2 summarization with bracket identification
- Schedule C self-employment deduction extraction
- Cross-document reconciliation (W-2 + 1099 + Schedule C)
- Deduction phase-out threshold calculation
- Multi-year comparison summary (2024 vs 2025 returns)
Head-to-Head Results
| Task | Claude Sonnet 4.6 | Gemini 2.0 Pro | Winner |
|---|---|---|---|
| Single W-2 summarization | 0 errors | 0 errors | Tie |
| Schedule C deduction extraction | 1 minor error | 3 errors | Claude |
| Cross-document reconciliation | 2 errors | 5 errors | Claude |
| Deduction phase-out calculation | 3 errors | 4 errors | Claude (marginal) |
| Multi-year comparison | 1 error | 2 errors | Claude |
Where Both Models Struggle: Phase-Out Math
Both Claude Sonnet 4.6 and Gemini 2.0 Pro produced their highest error rates on deduction phase-out threshold calculations, the type of multi-step math required for OBBBA-impacted deduction analysis. This is consistent with the pre-calculation failure pattern documented in our Pre-Calculation Fix guide.
The fix is the same for both models: use a <scratchpad> tag to force step-by-step calculation before the model writes any summary. In our testing, this sharply reduced phase-out errors on both Claude and Gemini.
Neither Claude Sonnet 4.6 nor Gemini 2.0 Pro should be used for actual tax filing decisions without verification by a licensed tax professional. These results reflect document analysis capability only, not legal or financial advice accuracy. Always verify AI-generated tax analysis with a CPA.
Best Prompt Structure for Tax Document Analysis
<task>Analyze the attached tax document and extract the following.</task>
<extract>
1. Gross income
2. All deductions listed
3. Taxable income after deductions
4. Any phase-out thresholds that apply
</extract>
<scratchpad>
Show all calculations step by step before writing the summary.
Do not skip arithmetic. Flag any number you are uncertain about.
</scratchpad>
<output>
Structured summary only after scratchpad is complete.
Flag any item that requires CPA verification.
</output>On dense tax-document questions the dangerous failure is not a refusal. It is a fluent answer that quietly drops one condition. That is exactly why a human review is non-negotiable on anything touching money or law: the output reads clean while being wrong in a way that is hard to spot. This comparison was run on Claude Sonnet 4.6 and a contemporaneous Gemini release and is published as a dated snapshot. Treat any specific figure here as history, and confirm current tax rules against the primary source before relying on them.
Sources: Anthropic: About Claude