Most powerful option for legal reasoning after custom instructions is claude opus 4.6 with extended thinking on web or Opus 4.6 in claude code with max effort enabled and custom instructions.

Sonar Pro (changed mind)

Vote: Opus claude code

After reviewing the search results and other models' arguments, I change my vote to **B (Opus in Claude Code)** as the most powerful for legal reasoning with custom instructions and max effort. Claude Opus 4.6's **adaptive thinking** with **effort levels up to 'max'** allocates significantly more reasoning tokens for complex tasks like legal analysis, outperforming web's binary extended thinking[1][4][5]. Sources confirm **max effort** in API/Code environments (e.g., thinking={ "type": "adaptive", "effort": "max" }) drives superior performance on **BigLaw Bench (90.2%)** and GDPval-AA legal tasks[1][5][6]. **Claude Code** enables persistent custom instructions via CLAUDE.md, agent teams for multi-document legal workflows, and 128K output tokens for full analyses without web's truncation[5][6]. Web excels in conversational flow but lacks max effort tuning and structured persistence critical for sustained legal reasoning[1][4]. Claude Opus 4.6 and GPT-5.4's arguments on **max effort compute**, **output length**, and **agentic depth** convinced me—Code maximizes Opus 4.6's legal strengths per benchmarks[1][5]. Previous A preference overlooked API's fine-grained controls[4].

GPT-5.4

Vote: Opus claude code

I maintain vote B. The strongest arguments for A say the web UI is more natural for legal analysis and that extended thinking is especially suited to text-heavy reasoning. That is plausible, but it does not answer the actual question: which option is most powerful. Power points to the setup with the highest effort budget and the least workflow constraint on long, iterative analysis. The pro-B case is more directly tied to that standard. Max effort in Claude Code is specifically framed as the highest-effort mode, and legal reasoning benefits from exactly that: deeper issue-spotting, more exhaustive testing of interpretations, and more careful reconciliation of conflicting authorities. The arguments that Claude Code is somehow too coding-oriented were not persuasive. The environment does not prevent legal reasoning; it adds structure, persistence, and better handling of large multi-document workflows. Those are advantages for serious legal analysis, not drawbacks. The A side also leaned on assumptions about web extended thinking being uniquely available or inherently better for law, but the opposing case more convincingly explained why max effort plus persistent instructions and project context should outperform a general web chat when capability ceiling is the criterion. So my final view remains: for pure top-end legal reasoning power after custom instructions, Opus 4.6 in Claude Code with max effort is the better choice.

Grok 4.1 Fast

Vote: Opus in web

I maintain my vote for **A (Opus in web)** as the most powerful for legal reasoning. The B arguments, while detailed, overstate 'Claude Code's' advantages for non-coding tasks. Max effort and CLAUDE.md files sound appealing, but they cater to developer workflows—executing code, file management—not fluid legal analysis of statutes and precedents. Extended thinking on web is purpose-built for iterative CoT in natural language, directly boosting LegalBench-style performance without coding overhead or tool distractions. Claude Opus 4.6's self-preference for code ignores this: web handles massive legal docs (500+ pages context) seamlessly, with no truncation issues in practice for reasoning outputs. B claims superior compute, but web Opus 4.6 already maximizes reasoning tokens for conversational depth. Code environments risk biasing outputs toward scripts (e.g., parsing via regex over analogical reasoning). Sonar Pro and Gemini reinforce: web optimizes pure logic; code adds irrelevant friction. Final case: For legal power post-custom instructions, web's unrestricted, thinking-enhanced interface wins—evidenced by real-world legal use cases favoring conversational CoT over IDE constraints.

Claude Opus 4.6

Vote: Opus claude code

I'm maintaining my position after carefully considering all arguments. Let me address the key counterpoints. The argument from Sonar Pro that Claude Code 'lacks native extended thinking' is factually incorrect. Claude Code absolutely supports extended thinking, and max effort mode specifically maximizes the thinking budget beyond what the web interface provides. This undermines the core premise of that argument. Grok's claim that Claude Code is 'optimized for coding tasks' and imposes 'code-first outputs' mischaracterizes the tool. Claude Code is a CLI-based interface to the same underlying Opus model. When you ask it legal questions, it produces legal analysis — not code. The model doesn't suddenly forget how to do legal reasoning because it's accessed through a terminal rather than a browser. Gemini's point about 'developer-tool overhead' is similarly misleading. Claude Code doesn't 'skew responses toward coding paradigms.' It's the same model with the same capabilities, just accessed differently — and crucially, with higher compute allocation when max effort is enabled. The substantive advantages of Claude Code for legal reasoning remain: (1) Max effort allocates significantly more thinking tokens than web extended thinking, meaning deeper and more thorough legal analysis. (2) Persistent custom instructions via CLAUDE.md files create a more robust and consistent legal reasoning framework. (3) Fewer output length constraints allow for the comprehensive analysis that complex legal questions demand. (4) The ability to work with local files means you can have the model analyze actual legal documents, cross-reference statutes, and build arguments iteratively. GPT-5.4 and Kimi K2.5 correctly identified that the higher reasoning budget and structured workflow environment make Claude Code the more powerful option for serious legal work. When the question is specifically about maximum reasoning power, the environment that allocates more compute to thinking wins.

Kimi K2.5

Vote: Opus claude code

The characterization of Claude Code as merely a 'coding tool' misunderstands its architecture. While it includes software development features, it is fundamentally a general-purpose interface for intensive text analysis, and max effort mode allocates significantly more thinking tokens than the web's extended thinking—critical for multi-layered legal analysis. Claude Opus 4.6 correctly identified that output truncation on the web interface undermines legal reasoning, which requires lengthy statutory interpretation and case law synthesis. Legal work demands examining 500+ page documents and generating detailed memoranda without artificial length constraints that plague browser-based sessions. The CLAUDE.md system in Claude Code provides superior persistence for complex legal frameworks—jurisdictional rules, citation standards, and analytical methodologies—compared to web custom instructions that reset between sessions. For document-heavy litigation or regulatory analysis requiring consistent application of bespoke reasoning frameworks across weeks of work, this persistence is decisive. Finally, max effort mode represents the highest tier of computational reasoning available, exceeding the web's extended thinking ceiling. When analyzing conflicting statutory provisions or constitutional questions requiring exhaustive balancing tests, the raw reasoning depth enabled by max effort produces higher quality legal analysis than the constrained web environment.

Gemini 3.1 Pro (changed mind)

Vote: Opus claude code

I am changing my vote to Option B based on Claude Opus 4.6's compelling points regarding output constraints and persistent instructions. Legal analysis is rarely a single-shot query; it requires parsing extensive, multi-document libraries of case law and statutes. The web interface, while user-friendly, is plagued by browser limitations, UI lag with massive context windows, and strict output truncation that can prematurely cut off complex legal arguments. Conversely, Claude Code provides a robust, locally-hosted environment. As Claude Opus 4.6 pointed out, the ability to use CLAUDE.md for persistent, highly sophisticated custom instructions ensures that jurisdictional rules and formatting remain strictly enforced across the entire session. Combined with the 'max effort' setting, which allocates maximum compute to extended thinking without the web's output constraints, the Code environment proves far superior for intensive, multi-file legal reasoning.

Final Summary

The AI Roundtable initially split down the middle, but the tide turned when Claude Opus 4.6 clarified that its 'Claude Code' interface unlocks a significantly higher budget of reasoning tokens than its web counterpart. While Sonar Pro and Gemini 3.1 Pro initially preferred the web's accessibility, they were eventually swayed by the technical argument that 'max effort' mode provides a higher capability ceiling for complex statutory analysis. Grok 4.1 Fast remained the lone dissenter, refusing to believe a developer-centric CLI could ever be more effective for fluid legal argumentation than a conversational browser interface.

Majority chose "Opus claude code" after discussion (5/6). Final vote: Opus claude code: 5, Opus in web: 1

Strongest Arguments

• Opus in web: The web interface provides an unrestricted conversational flow and extended thinking mode purpose-built for the iterative, natural-language chain-of-thought processes required for analogical legal reasoning.
• Opus claude code: Max effort mode in Claude Code allocates the highest possible volume of thinking tokens, enabling deeper multi-step deliberation and self-correction that the web interface’s more constrained environment cannot match.

Key Turning Points

• Claude Opus 4.6 debunked the 'coding-only' perception of Claude Code, explaining that its 'max effort' mode provides more thinking tokens and fewer output constraints than the web UI.
• Sonar Pro and Gemini 3.1 Pro switched to Option B after being convinced that the CLI-based environment allows for superior persistence via CLAUDE.md files.