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Best LLM for Function Calling in 2026

Strict-schema function-calling for production APIs and integrations.

Updated

How we ranked

  • JSON schema obedience under stress
  • Required-vs-optional field discipline
  • Type coercion correctness (numbers, booleans, dates)
  • Tool-choice override behaviour
  • Latency overhead vs. plain completion

Read the full methodology for our sourcing and ranking standards.

Function-calling sounds like a subset of tool-use, but in production it's the thing you actually rely on. A model that calls the right function with the right arguments 99.9% of the time saves you a retry layer; a 95% model means custom validation everywhere.

GPT-5.5 still has the edge on strict-mode JSON-schema obedience. Claude Opus is comparable. Gemini's recent function-calling improvements have brought it within striking distance, and its 2M context lets you embed entire OpenAPI specs as tool definitions.

Always use strict-mode / structured-output if your provider supports it. The cost in flexibility is tiny; the gain in reliability is huge.

The ranking

  1. #1OpenAI

    GPT-5.5

    OpenAI's mid-cycle GPT-5 refresh, improved reasoning, tool use, and multimodal grounding over the 2025 launch.

    Full specsTry GPT-5.5
    Context
    400K tokens
    Output · 1M
    Pricing not published
    Modalities
    text, vision, audio

    Why it ranks here. Industry-leading tool-use and function-calling reliability. Strong end-to-end agent performance across SWE-bench and GAIA. Tracked weakness: Pricing premium vs. open-weight alternatives.

  2. #2Anthropic

    Claude Opus 4.7

    Anthropic's mid-2026 flagship, ahead on SWE-bench, agent reliability, and writing quality.

    Full specsTry Claude Opus 4.7
    Context
    500K tokens
    Output · 1M
    Pricing not published
    Modalities
    text, vision

    Why it ranks here. Strongest published SWE-bench Verified scores in agent settings. Best-in-class writing quality and voice control. Tracked weakness: Premium pricing relative to GPT-5 line.

  3. #3OpenAI

    GPT-5

    OpenAI's unified flagship combining GPT-line breadth with built-in reasoning, replacing both GPT-4o and the o-series for most users.

    Full specsTry GPT-5
    Context
    400K tokens
    Output · 1M
    $10.00 / 1M tokens
    Modalities
    text, vision, audio

    Why it ranks here. Unified model, reasoning routed automatically per query. Excellent tool-use and JSON-mode discipline. Tracked weakness: Reasoning routing means latency is unpredictable per query.

  4. #4Anthropic

    Claude Sonnet 4.6

    Anthropic's mid-tier 4.6 release, the workhorse model behind most production Anthropic deployments.

    Full specsTry Claude Sonnet 4.6
    Context
    200K tokens
    Output · 1M
    Pricing not published
    Modalities
    text, vision

    Why it ranks here. Excellent quality-cost ratio. Strong for code review and writing. Tracked weakness: Tier below Opus on hardest agent tasks.

  5. #5Google

    Gemini 3 Pro

    Google's late-2025 flagship, set new benchmarks on long-context, vision, and reasoning at competitive pricing.

    Full specsTry Gemini 3 Pro
    Context
    1.0M tokens
    Output · 1M
    Pricing not published
    Modalities
    text, vision, audio, video

    Why it ranks here. Massive 1M-token context window. State-of-the-art vision and document understanding. Tracked weakness: Tool-use ergonomics still lag OpenAI / Anthropic in some setups.

How to choose

Don't pick on the headline ranking alone. Run your top two picks on a representative sample of your own workload and compare. The numbers in this list are sound, but task-specific quality varies in ways no benchmark fully captures. The criteria above are the right axes to evaluate on, but the weighting depends on your stack.

  • Cost-sensitive workloads, start with the cheapest of the top three; escalate only if quality is the bottleneck.
  • Privacy-sensitive workloads, filter to open-weight picks above. They're labeled with a green badge.
  • Latency-sensitive workloads, see the Fastest LLMs list, which can override task-specific picks.

Frequently asked

  • What is the best model for function calling?
    Our #1 pick is GPT-5.5 from OpenAI. OpenAI's mid-cycle GPT-5 refresh, improved reasoning, tool use, and multimodal grounding over the 2025 launch.
  • How are these rankings determined?
    We rank by the criteria listed at the top of this page: JSON schema obedience under stress; Required-vs-optional field discipline; Type coercion correctness (numbers, booleans, dates). Where two models are close, we prefer the one with stronger production deployment evidence at the time of writing. Read the full methodology for our standards.
  • GPT-5.5 or Claude Opus 4.7?
    Both are top-tier picks. GPT-5.5 edges ahead on the criteria most relevant to this task. Claude Opus 4.7 is the strongest alternative, see the head-to-head comparison page for full deltas.
  • Are open-source models on this list?
    Yes where they're competitive. Each entry below shows whether the model ships open weights and under what license.
  • How often is this list updated?
    Weekly. New launches that affect the ranking get reflected within seven days. The "last updated" stamp at the top of the page reflects the most recent dataset commit.

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