Claude Sonnet 4.6 1M vs Composer 2.5 Fast: A Practical LLM Comparison

TL;DR

• Composer 2.5 Fast looks like the better default inside Cursor for day-to-day agentic coding, especially when the work is edit-heavy, iterative, and tied to IDE context. On CursorBench 3.1, Cursor reports Composer 2.5 at 63.2% and $0.55/task, versus Sonnet 4.6 Max at 49.0% and $3.09/task.
• Claude Sonnet 4.6 1M is the broader platform model: 1M-token context at standard pricing, adaptive/extended thinking, strong long-context reasoning, computer use, tool use, and wider API/cloud availability.
• The headline price for Sonnet 4.6 and Composer 2.5 Fast is identical: $3/M input and $15/M output. Composer 2.5 Standard is much cheaper at $0.50/M input and $2.50/M output, but the user asked about Composer 2.5 Fast.
• For speed, Composer 2.5 Fast is explicitly positioned as the default fast Cursor experience. I did not find a public, reproducible tokens-per-second benchmark for Composer 2.5 Fast, so treat speed claims as product-positioning unless you measure your own workload.
• For intelligence, the answer depends on the arena. Composer 2.5 wins hard on Cursor’s public coding benchmark. Sonnet 4.6 has stronger public evidence for long-context retrieval, agent planning, computer use, and general professional work.
• My recommendation: use Composer 2.5 Fast as the “inner loop coding model” in Cursor; route big-repo reasoning, long-document synthesis, cross-tool agents, and API products to Sonnet 4.6 1M when the context and platform features matter.

What You Will Learn Here

• How Claude Sonnet 4.6 1M and Cursor Composer 2.5 Fast differ in practice
• Which public metrics are useful, and which are marketing-shaped
• Where each model fits for coding, agents, PM workflows, and long-context work
• How to run a small internal eval before standardizing on either model
• A simple routing pattern for teams that want both speed and deeper reasoning

The Short Version

These two models are not trying to be exactly the same product.

Claude Sonnet 4.6 1M is a general-purpose frontier model for agents, coding, computer use, long-context reasoning, knowledge work, and design. Anthropic launched it on February 17, 2026, then made the full 1M-token context window generally available at standard pricing on March 13, 2026.

Composer 2.5 Fast is Cursor’s fast coding-agent model, released on May 18, 2026. It is built for interactive software engineering inside Cursor: planning, editing, debugging, codebase understanding, and long-running IDE sessions.

That difference matters.

If you ask, “Which model is smarter?”, the honest answer is:

For Cursor-native coding loops?
    Composer 2.5 Fast is probably the first model to test.

For long-context agent work across documents, tools, browsers, PDFs, and APIs?
    Sonnet 4.6 1M is probably the first model to test.

Quick Comparison

The table has one important caveat: CursorBench is Cursor’s benchmark, and Composer is Cursor’s model. That does not make the result useless. It does mean you should treat it as strong evidence for Cursor-like coding sessions, not as universal proof that Composer 2.5 is better than Sonnet 4.6 at every coding task.

Speed: What We Can Actually Say

The speed story is partly clear and partly under-measured.

Cursor says Composer 2.5 has a faster variant with the same intelligence, priced at $3/M input and $15/M output, and that Fast is the default option. That tells us how Cursor wants the model to feel: quick enough for interactive coding.

Anthropic says Sonnet 4.6 works across thinking-effort settings, including strong performance with extended thinking off. That gives teams a way to trade quality, latency, and cost. But when you enable deeper thinking or feed hundreds of thousands of tokens, the wall-clock experience will naturally change.

The missing piece: I did not find a public, apples-to-apples, reproducible tokens-per-second / time-to-first-token / full-task-latency comparison between Sonnet 4.6 1M and Composer 2.5 Fast.

So use this practical interpretation:

Interactive code edit in Cursor
    |
    +--> Prefer Composer 2.5 Fast first

Huge context load, cross-doc reasoning, tool-heavy agent
    |
    +--> Prefer Sonnet 4.6 1M first

Need a real speed answer for your team
    |
    +--> Measure time-to-first-token, total task time, total tokens, and retry rate

For Engineers, speed is not just tokens per second. It is also:

• How often the model needs clarification
• How often it edits the wrong files
• How many tool calls it needs
• How many retries it takes after failing tests
• How often a human has to stop and repair the plan

For PMs, the speed question is simpler:

Which model gets the user to a correct result with the least waiting and rework?

That is exactly why local evals matter.

Intelligence: Different Arenas, Different Winner

Composer 2.5 has the strongest public head-to-head coding signal here.

CursorBench 3.1 evaluates agents on ambiguous, multi-file tasks from real Cursor sessions, including codebase understanding, bugfinding, planning, and code review. Cursor reports:

That is a meaningful gap if your workflow looks like CursorBench.

But Sonnet 4.6 has a different kind of public evidence:

• Anthropic describes it as an upgrade across coding, computer use, long-context reasoning, agent planning, knowledge work, and design.
• It supports adaptive thinking and extended thinking on the Claude Platform.
• It has the full 1M context window at standard pricing.
• Anthropic’s Sonnet 4.6 system card reports strong long-context results, including 65.1 on OpenAI MRCR v2 1M 8-needles with 64K extended thinking and 65.8 with max effort.
• On GraphWalks BFS 1M, the system card reports 68.4 with 64K extended thinking and 73.8 with max effort.

My read:

Composer 2.5 Fast intelligence = "excellent at Cursor-shaped software work"
Sonnet 4.6 1M intelligence = "strong general reasoning plus unusually useful long context"

Those are both valuable. They are just not interchangeable.

Coding: Inner Loop vs Big Context

For coding, I would split the world into two loops.

The inner loop is what happens dozens of times per day:

• “Refactor this component.”
• “Fix this failing test.”
• “Add the missing validation.”
• “Explain this stack trace.”
• “Update these files to match the new API.”

Composer 2.5 Fast is built for this. Cursor says 2.5 improves sustained work on long-running tasks, complex instruction following, communication style, and effort calibration. It is also sitting inside the IDE, where context collection, diffs, terminal output, and file edits are part of the product experience.

The big-context loop is less frequent but often more strategic:

• “Read this repo and propose a migration plan.”
• “Audit these 80 files for security problems.”
• “Compare the implementation against the PRD, tickets, logs, and customer notes.”
• “Review an agent trace with thousands of tool calls.”
• “Analyze a large codebase plus PDFs, spreadsheets, and design notes.”

That is where Sonnet 4.6 1M becomes more attractive. The 1M window lets you keep far more raw material in the model’s working set, and Anthropic’s platform features around compaction, tool use, and thinking effort are directly relevant to long-horizon agent work.

Here is the engineering pattern I would use:

Developer request
    |
    +--> Small/medium edit inside Cursor?
    |       |
    |       +--> Composer 2.5 Fast
    |
    +--> Needs repo-wide context or long docs?
    |       |
    |       +--> Sonnet 4.6 1M
    |
    +--> Ambiguous architecture decision?
    |       |
    |       +--> Sonnet 4.6 for plan, Composer 2.5 Fast for implementation
    |
    +--> High-volume repetitive fixes?
            |
            +--> Benchmark Composer 2.5 Standard too, not only Fast

Agents: The Most Important Difference

Agent quality is not only model IQ. It is model plus environment.

For Composer 2.5 Fast, the environment is Cursor:

• It can work directly with project files.
• It benefits from Cursor’s context-building and pruning.
• It is optimized for code edits, diffs, plans, and developer feedback.
• Its strongest public benchmark is based on real Cursor sessions.

For Sonnet 4.6 1M, the environment is broader:

• Claude API, Claude Code, Claude Cowork, and major cloud providers
• 1M context at standard pricing
• adaptive and extended thinking
• context compaction for long conversations
• tool use, code execution, web search/fetch, memory, and programmatic tool calling availability
• computer-use positioning for UI automation and legacy systems

This means Composer 2.5 Fast may be the better coding coworker inside Cursor, while Sonnet 4.6 1M may be the better agent brain for a product or platform.

For example:

Product support agent
    Reads tickets + docs + CRM + PDFs + tool results
    Needs safe tool use and long context
    -> Sonnet 4.6 1M is the stronger default candidate

IDE coding agent
    Reads files + diffs + tests + terminal output
    Needs fast edit cycles
    -> Composer 2.5 Fast is the stronger default candidate

Cost: Same Fast Price, Different Cost Shape

At the compared tier, both models have the same headline price:

But headline token price is not total cost.

Total cost looks more like this:

total cost =
  input tokens
  + cache write/read behavior
  + output tokens
  + hidden/reasoning/tool-call overhead
  + retries
  + failed edits
  + human review time

Sonnet 4.6 can read a massive 1M context, but if you send 900K tokens repeatedly, you still pay for a huge input. Anthropic’s prompt caching can reduce repeated-input cost, but your app has to be designed for stable cached prefixes.

Composer 2.5 Fast has the same headline token price as Sonnet 4.6, but CursorBench reports dramatically lower cost/task for Composer 2.5 than Sonnet 4.6 inside Cursor’s benchmark. That likely reflects both model behavior and the surrounding Cursor harness.

For a team, the key question is:

Are we paying for tokens, or are we paying for completed tasks?

You should measure the second one.

A Tiny Eval Harness You Can Actually Run

Do not choose a model from one public benchmark. Build a small internal eval with your own tasks.

Start with 10 to 20 tasks:

• 5 small edits
• 5 bugfixes with tests
• 3 refactors
• 3 architecture/planning questions
• 2 long-context tasks that include docs, tickets, or traces
• 2 agent tasks that require tool use

Track the result like this:

type ModelRun = {
  model: "sonnet-4.6-1m" | "composer-2.5-fast";
  taskId: string;
  category: "edit" | "bugfix" | "refactor" | "planning" | "long-context" | "agent";
  passed: boolean;
  humanScore: 1 | 2 | 3 | 4 | 5;
  timeToFirstTokenMs?: number;
  totalWallTimeMs: number;
  inputTokens?: number;
  outputTokens?: number;
  estimatedCostUsd?: number;
  retries: number;
  notes: string;
};

function taskEfficiency(run: ModelRun) {
  if (!run.passed) return 0;

  const minutes = run.totalWallTimeMs / 60_000;
  const cost = run.estimatedCostUsd ?? 0;
  const retryPenalty = run.retries * 0.15;

  return run.humanScore / (1 + minutes + cost + retryPenalty);
}

Then make a simple decision table:

The moment you do this with your own repo, the conversation becomes calmer. You stop arguing about “best model” and start routing work to the best model for the job.

Recommended Routing Strategy

If I were setting this up for an engineering team, I would not pick one model globally.

I would route:

Incoming task
    |
    +--> In Cursor, mostly code edits, short feedback loop
    |       -> Composer 2.5 Fast
    |
    +--> In Cursor, high-volume simple edits
    |       -> Also test Composer 2.5 Standard
    |
    +--> Long context, many files, many docs, agent trace
    |       -> Sonnet 4.6 1M
    |
    +--> Needs browser/computer use or productized API agent
    |       -> Sonnet 4.6 1M
    |
    +--> Needs careful architecture plan before implementation
    |       -> Sonnet 4.6 1M for plan, Composer 2.5 Fast for patch

For PMs, the product version is:

• Use Composer 2.5 Fast when the user is already in the coding environment and wants momentum.
• Use Sonnet 4.6 1M when the user needs the model to hold a bigger map of the work.

For Engineers, the system version is:

• Use Composer 2.5 Fast as an IDE agent.
• Use Sonnet 4.6 1M as a platform agent and long-context reasoning layer.

Gaps and Cautions

There are a few gaps I would not hide from stakeholders:

• No public independent speed benchmark: I found product claims and pricing, but not a reproducible latency comparison for Sonnet 4.6 1M vs Composer 2.5 Fast.
• CursorBench is not neutral: It is useful, but it is owned by Cursor and based on Cursor-like tasks.
• Composer 2.5 Fast context details are less transparent: I found clear pricing and benchmark positioning, but not a comparable public 1M-context spec.
• Sonnet 4.6 can become expensive in long-context mode: 1M context is standard-priced, not free. Sending too much context too often is still a product design problem.
• Thinking effort changes the comparison: Sonnet 4.6 can trade speed and cost for deeper reasoning. Composer 2.5 Fast is positioned as same intelligence but faster, yet task-level latency still needs local measurement.
• Agents need evals, not vibes: A model that feels better in chat can still fail more often in a real tool loop.

Final Recommendation

If you are choosing a default for Cursor-based coding, start with Composer 2.5 Fast. The public CursorBench result is too strong to ignore, and the product is tuned for exactly that workflow.

If you are choosing a model for long-context agents, product APIs, research workflows, computer use, or mixed document/code reasoning, start with Claude Sonnet 4.6 1M. The context window, platform features, and long-context evidence matter more there.

The best team setup is probably not “Sonnet or Composer.”

It is:

Composer 2.5 Fast for fast implementation loops
Sonnet 4.6 1M for broad reasoning, planning, and long-context agents
Internal evals to decide the handoff point

That is less dramatic than a winner-takes-all leaderboard, but it is much closer to how real engineering work gets done.

Source List

• Anthropic, “Introducing Claude Sonnet 4.6”, February 17, 2026.
• Claude, “1M context is now generally available for Opus 4.6 and Sonnet 4.6”, March 13, 2026.
• Anthropic Docs, “Pricing”, accessed May 27, 2026.
• Anthropic, Claude Sonnet 4.6 System Card, accessed May 27, 2026.
• Cursor, “Introducing Composer 2.5”, May 18, 2026.
• Cursor, “Composer 2.5” changelog, May 18, 2026.
• Cursor, “CursorBench 3.1”, accessed May 27, 2026.