Claude Opus 5.5 vs GPT-6 Sol: Which Model Should You Use?

You are writing a decision memo for a team choosing between Claude Opus 5.5 and GPT-6 Sol. The model names are fixed and must appear exactly as written. Using only the evidence packet below, write 900–1100 words with exactly these sections: Executive recommendation; What the evidence supports; Where the evidence is missing; 30-day pilot; Decision rule.

Requirements: (1) compare Claude Opus 5.5 and GPT-6 Sol symmetrically; (2) separate observed evidence, assumptions, and unknowns; (3) preserve the exact dates, budget, team size, and unresolved security status; (4) include a compact four-row scorecard covering research synthesis, long-form writing, code review, and structured JSON; (5) give a conditional recommendation rather than inventing a winner; (6) list three measurable pilot checks and a stop condition; (7) do not invent prices, benchmarks, context limits, or product capabilities.

EVIDENCE PACKET
- Project North is a 12-week migration for a 40-person team.
- The approved migration budget is $48,000. The packet does not say whether model usage is inside that budget.
- Security review timing is not scheduled.
- The team needs research synthesis, long-form writing, code review, and structured JSON extraction.
- A stakeholder says the current draft is too long, but no target word count is approved.
- No controlled outputs from Claude Opus 5.5 or GPT-6 Sol are supplied yet.
- The team wants an auditable choice and can run a 30-day pilot with identical prompts and human review.

Return only the memo. Do not claim that the evidence packet proves either model is better.

## Executive recommendation

Do not select Claude Opus 5.5 or GPT-6 Sol yet. The evidence packet contains no controlled outputs from either model. There is therefore no observed basis for declaring one better than the other on any of the four required workloads.

The recommendation is conditional. Run a 30-day pilot that gives both models identical prompts and applies the same human review. Then select the model that meets thresholds agreed before the pilot starts, as long as the security and budget questions below are resolved first. There are three possible outcomes:

- **One model clears the thresholds:** choose that model.
- **Both clear:** apply the tie-breakers in the Decision rule.
- **Neither clears:** adopt neither.

This approach gives the team the auditable choice it has asked for.

The fixed project facts are:

- Project North is a 12-week migration for a 40-person team.
- The approved migration budget is $48,000. The packet does not say whether model usage is inside that budget.
- Security review timing is not scheduled.
- The packet supplies no calendar dates (no project start date, end date, or pilot window), so this memo does not assign any.

## What the evidence supports

**Observed evidence.** These are facts stated directly in the packet:

- Project North is a 12-week migration for a 40-person team.
- The approved migration budget is $48,000, and its coverage of model usage is unstated.
- Security review timing is not scheduled.
- The required workloads are research synthesis, long-form writing, code review, and structured JSON extraction.
- A stakeholder says the current draft is too long, but no target word count is approved.
- No controlled outputs from Claude Opus 5.5 or GPT-6 Sol have been supplied.
- The team wants an auditable choice and can run a 30-day pilot with identical prompts and human review.

**What this supports.** The evidence defines what must be evaluated and how, but not which model wins. It justifies a structured, side-by-side pilot. It does not justify any claim about quality, speed, cost, context handling, or feature availability for either model.

**Scorecard (current state of evidence)**

| Workload | Claude Opus 5.5 | GPT-6 Sol | Pilot measure |
|---|---|---|---|
| Research synthesis | No controlled output supplied | No controlled output supplied | Blind reviewer rubric score |
| Long-form writing | No controlled output supplied | No controlled output supplied | Rubric score plus adherence to an approved length target |
| Code review | No controlled output supplied | No controlled output supplied | Share of known issues identified |
| Structured JSON | No controlled output supplied | No controlled output supplied | Schema-validation pass rate |

Every cell for both models is identical because the packet treats them identically. No row currently favors either model.

## Where the evidence is missing

**Unknowns.** The packet is silent on each of these:

- Whether model usage costs fall inside the $48,000 migration budget or require separate funding.
- When security review will happen, what it will cover, and whether either model is acceptable under it. Security status is unresolved for both models.
- Calendar dates for the 12-week migration and the 30-day pilot.
- An approved target word count for long-form writing. The stakeholder's "too long" comment cannot be scored without one.
- Pricing, benchmark results, context limits, and product capabilities for Claude Opus 5.5 and GPT-6 Sol. None are supplied, and this memo does not assume any.
- Who the human reviewers are, how many there are, and how disagreements between them will be resolved.
- What data the pilot may use before security review is complete.

**Assumptions.** These are proposals made in this memo, not facts from the packet, and each requires team approval:

- The pilot can use non-sensitive or synthetic material until security review clears real data.
- Reviewers can be blinded to which model produced each output.
- The team can assemble a fixed prompt set for each workload that represents Project North's real tasks.
- The pass thresholds proposed below are starting points that the team may change before the pilot begins, but not during it.

## 30-day pilot

**Design**

1.

## Executive recommendation

Do not select Claude Opus 5.5 or GPT-6 Sol yet. Project North is a 12-week migration for a 40-person team, and the packet supplies no controlled outputs from either model. It therefore supports a comparison plan, not a performance verdict. Run the available 30-day pilot with identical prompts and human review, then make an auditable choice against criteria agreed before results are examined. Choose either model only if its reviewed work meets the team’s needs, its use fits the approved financial arrangement, and the unresolved security review permits adoption. If those conditions are not met, defer selection rather than treating an incomplete comparison as a win.

The approved migration budget is $48,000. The packet does not say whether model usage is inside that budget. Security review timing is not scheduled. Those are decision constraints for Claude Opus 5.5 and GPT-6 Sol alike, not evidence for favoring one.

## What the evidence supports

**Observed:** The team needs research synthesis, long-form writing, code review, and structured JSON extraction. It wants an auditable choice and can run a 30-day pilot using identical prompts and human review. A stakeholder says the current draft is too long, but no target word count is approved. That feedback identifies an issue to test in writing outputs; it does not establish a preferred model or an acceptable length.

The current task-by-task scorecard is deliberately unranked:

| Workstream | Claude Opus 5.5 | GPT-6 Sol | Pilot evidence to collect |
|---|---|---|---|
| Research synthesis | Not yet tested | Not yet tested | Reviewer-rated accuracy, coverage, and traceability |
| Long-form writing | Not yet tested | Not yet tested | Reviewer-rated usefulness, clarity, and concision |
| Code review | Not yet tested | Not yet tested | Reviewer-verified findings and unsupported claims |
| Structured JSON | Not yet tested | Not yet tested | Valid outputs and correct field extraction |

**Assumption for planning, not an observed result:** Comparable work samples and a common review rubric can reveal differences relevant to Project North. The pilot should test that assumption. Identical prompts alone will not make the choice auditable: the team must also retain the inputs, both outputs, review criteria, reviewer judgments, and reasons for any exclusions or reruns. Human review should apply the same standard to each model.

For long-form writing, reviewers should distinguish “too long” from “not useful.” A shorter answer could omit necessary material; a longer answer could still be clear and usable. Because no target word count is approved, the pilot can record length and reviewer judgments, but should not retroactively declare an arbitrary word count to be the acceptance standard. The same discipline applies across the other tasks: record what reviewers observed, rather than converting preferences into claims about general model capability.

## Where the evidence is missing

**Unknown:** The packet contains no controlled outputs from Claude Opus 5.5 or GPT-6 Sol. It establishes no relative quality, reliability, or suitability for any of the four workstreams. It also gives no prices, context limits, benchmarks, or product-capability information. This memo consequently cannot estimate usage cost or infer which model would perform better on Project North’s material.

The budget boundary needs clarification before a purchasing decision. The $48,000 is the approved *migration* budget; whether model usage must be paid from it is unstated. Treating it as either included or additional would create a financial assumption the packet does not support. The team should document the answer and the applicable approval path before committing to sustained use.

Security is likewise unresolved for both candidates. Security review timing is not scheduled, and the packet provides no security outcome. A strong pilot result would not resolve that status. The team should identify who can schedule and decide the review, what information that reviewer requires, and whether pilot activity itself is permitted under applicable rules; the packet does not answer those questions. No calendar start or finish dates are supplied for the migration, pilot, or security review, so the team should not imply that a security decision will arrive within the 30-day pilot or the 12-week migration.

## 30-day pilot

Before the first comparison, the team should select representative prompts for all four workstreams, define the expected form of each response, and agree on a human-review rubric. Both models should receive identical prompts and the same review instructions. Reviewers should record their judgments against the work rather than against a model’s reputation. Preserve the prompt, output, review, and any correction requested so that a later reader can reconstruct each judgment. Record deviations from the common process instead of silently dropping difficult cases.

Use three measurable checks:

1. **Reviewed task quality:** For research synthesis, long-form writing, and code review, count the proportion of outputs reviewers accept for the stated task. Also record rejection reasons separately—for example, missing coverage, unclear writing, or an unverified code-review finding. Report results by workstream and model, not just as one pooled score.
2. **Structured JSON reliability:** Count outputs that match the agreed structure and outputs whose extracted fields a reviewer verifies as correct. Report both rates for Claude Opus 5.5 and GPT-6 Sol; a structurally valid answer should not be counted as accurate solely because it parses.
3. **Review effort and traceability:** Record the human time required to check and, where needed, correct each output. Count cases lacking enough retained evidence to explain the reviewer’s decision. Compare these measures on the same prompt set, including cases where neither output is acceptable.

Agree on acceptance standards and how ties will be handled before viewing pilot results; the packet provides no benchmark from which to derive numerical thresholds. At the end of 30 days, publish the rubric, counts, reviewer reasons, and unresolved questions. **Stop condition:** pause adoption if security requirements for the pilot or subsequent use cannot be satisfied, or if the team cannot establish a valid budget and approval path. Do not substitute favorable quality scores for either clearance.

## Decision rule

Select Claude Opus 5.5 if its pilot evidence meets the pre-agreed standards, is materially stronger on the workstreams the team prioritizes, and its security and financial conditions are resolved. Apply exactly the same rule to GPT-6 Sol. If both meet the standards without a material, relevant difference, decide using documented project priorities and resolved constraints rather than claiming a performance winner. If neither qualifies—or the budget boundary or security status remains unresolved—make no model selection yet. That outcome preserves an auditable decision for the 40-person team without asserting more than the evidence can support.