# Best LLM for Tool & Function Calling

> Updated Oct 20, 2026 · evidence through Oct 20, 2026. Synthesized from cited community evidence and open benchmarks, not vendor marketing. Canonical page: https://modelmatchbook.com/best/agents-tool-use

Claude Fable 5.1 is the provisional first pick for Tool & Function Calling as of October 20, 2026, while independent task measurements are still limited. It ranks #1 of 48 on LMArena Agent's tool-hallucination signal (score 0.7), where a higher score means fewer fabricated tool calls. GPT-6 Astra scores within two points, so read the top two as close rather than as a measured gap. GPT-6 Astra is the next-ranked alternative. Ranks #2 of 48 on LMArena's agentic arena (score 15.1) and steerability (score 15.5), tied for #1 on tool-hallucination signal (score 0.7). [1,2,3,4,5]

## 1. Claude Fable 5.1
https://modelmatchbook.com/models/anthropic/claude-fable-5.1-20260831

Claude Fable 5.1 ranks #1 of 48 on LMArena Agent's tool-hallucination signal (score 0.7), where a higher score means fewer fabricated tool calls.

**Best when:** Its strongest result for this task is on LMArena Agent tool hallucination, which measures calling only tools that exist, with real arguments. [1]

## 2. GPT-6 Astra
https://modelmatchbook.com/models/openai/gpt-6-astra-20260903

Ranks #2 of 48 on LMArena's agentic arena (score 15.1) and steerability (score 15.5), tied for #1 on tool-hallucination signal (score 0.7).

**Best when:** Use for low-hallucination tool calling where fabricated calls must be minimized, as it ties for the best tool-hallucination signal score (0.7) in the evaluation. [4]

**More tips:**
- Deploy when mid-task direction following is critical, ranking #2 on steerability (score 15.5) among 48 models. [3]

**Watch out for:**
- Expect sensitivity to AGENTS.md instructions and skills, with tendencies toward detailed formatting, more clarifying questions, and over-testing small changes. [6]

## 3. Claude Opus 5.5
https://modelmatchbook.com/models/anthropic/claude-opus-5.5-20260921

Ranks #1 of 48 on LMArena's agentic arena (score 20.4) for tool-use and multi-step task performance, and #1 on steerability (score 17.3).

**Best when:** Use for complex multi-step agent workflows requiring mid-task direction changes, as it leads all models in steerability (score 17.3) and agentic arena performance (score 20.4). [7,8]

**More tips:**
- Deploy for subagent-heavy applications like document processing pipelines, demonstrated in building a complete 387-label map translation app with subagents in one afternoon. [9]

## 4. GPT-6.1 Sol
https://modelmatchbook.com/models/openai/gpt-6.1-sol-20260929

GPT-6.1 Sol ranks #1 of 48 on LMArena Agent's tool-hallucination signal (score 0.7), where a higher score means fewer fabricated tool calls.

**Best when:** Its strongest result for this task is on LMArena Agent tool hallucination, which measures calling only tools that exist, with real arguments. [10]

## 5. Gemini 4 Argon
https://modelmatchbook.com/models/google/gemini-4-argon-20261013

Ranks #6 of 48 on LMArena's agentic arena (score 12.1) and #3 on steerability (score 14.8).

**Best when:** Use when mid-task direction following is prioritized, as it ranks #3 on steerability (score 14.8). [11]

**Watch out for:**
- Expect frequent fabricated tool calls, ranking #30 of 48 on tool-hallucination signal (score 0.1). [12]

## 6. Claude Sonnet 5.5
https://modelmatchbook.com/models/anthropic/claude-sonnet-5.5-20260928

Claude Sonnet 5.5 ranks #5 of 48 on LMArena's agentic arena (score 12.8), measuring tool-use and multi-step task performance from human preference.

**Best when:** Its strongest result for this task is on LMArena Agent, which measures tool calling and multi-step control across a long task. [13]

## 7. Muse Spark 1.3
https://modelmatchbook.com/models/meta/muse-spark-1.3-20260902

Muse Spark 1.3 ranks #10 of 48 on LMArena Agent's tool-hallucination signal (score 0.6), where a higher score means fewer fabricated tool calls.

**Best when:** Its strongest result for this task is on LMArena Agent tool hallucination, which measures calling only tools that exist, with real arguments. [14]

## 8. Muse Wonder
https://modelmatchbook.com/models/meta/muse-wonder-20261012

Ranks #12 of 48 on LMArena's agentic arena (score 3.8) but shows poor tool-hallucination performance (#28 of 48, score 0.2) and steerability (#17 of 48, score 4.8), with limited available evidence.

**Best when:** Its strongest result for this task is on LMArena Agent, which measures tool calling and multi-step control across a long task. [15]

**Watch out for:**
- Expect higher rates of fabricated tool calls, ranking #28 of 48 on tool-hallucination signal (score 0.2). [16]
- Watch for limited mid-task direction following, ranking #17 of 48 on steerability (score 4.8). [17]

## FAQ

**What is the best model for Tool & Function Calling?**

Claude Fable 5.1 ranks first as of October 20, 2026. Claude Fable 5.1 ranks #1 of 48 on LMArena Agent's tool-hallucination signal (score 0.7), where a higher score means fewer fabricated tool calls. [1]

**What is the best alternative to Claude Fable 5.1?**

GPT-6 Astra ranks second. Ranks #2 of 48 on LMArena's agentic arena (score 15.1) and steerability (score 15.5), tied for #1 on tool-hallucination signal (score 0.7). [2,3,4,5]

## Sources
1. "Within the 48 catalog models matched to this evaluation: Ranks #1 of 48 on LMArena Agent's tool-hallucination signal (score 0.7), where a higher score means fewer fabricated tool calls." (LMArena Agent tool hallucination, https://lmarena.ai/leaderboard)
2. "Within the 48 catalog models matched to this evaluation: Ranks #2 of 48 on LMArena's agentic arena (score 15.1), measuring tool-use and multi-step task performance from human preference." (LMArena agentic arena, https://lmarena.ai/leaderboard)
3. "Within the 48 catalog models matched to this evaluation: Ranks #2 of 48 on LMArena Agent's steerability signal (score 15.5), measuring how well the agent follows direction given mid-task." (LMArena Agent steerability, https://lmarena.ai/leaderboard)
4. "Within the 48 catalog models matched to this evaluation: Ranks #1 of 48 on LMArena Agent's tool-hallucination signal (score 0.7), where a higher score means fewer fabricated tool calls." (LMArena Agent tool hallucination, https://lmarena.ai/leaderboard)
5. "Summary Two distinct tool-calling failures were found using the NEAR AI provider from OpenCode (which uses the OpenAI-compatible /v1/chat/completions endpoint). 1. openai/gpt-6-astra — function tools rejected on /v1/chat/completions OpenAI rejects function tools for this model on /v1/chat/completions; tools only work on /v1/responses. OpenCode's openai-compatible provider uses /v1/chat/completions, so agentic runs error out. Not fixable from OpenCode config. Update to:…" (think-in-universe, https://github.com/nearai/cloud-api/issues/1075)
6. "Context GPT-6 Astra is available, but its token usage currently makes it unsuitable as the default day-to-day model. Use it selectively as a high-capability workflow and prompt auditor. Official guidance notes that Astra is more sensitive to instructions in AGENTS.md and skills, may ask more clarifying questions, tends toward detailed formatting, and can over-test small changes. These behaviors overlap with prior dotfiles friction around unnecessary STOPs, repeated confirmations, verbose w…" (jjshen-eland, https://github.com/jjshen-eland/dotfiles/issues/229)
7. "Within the 48 catalog models matched to this evaluation: Ranks #1 of 48 on LMArena Agent's steerability signal (score 17.3), measuring how well the agent follows direction given mid-task." (LMArena Agent steerability, https://lmarena.ai/leaderboard)
8. "Within the 48 catalog models matched to this evaluation: Ranks #1 of 48 on LMArena's agentic arena (score 20.4), measuring tool-use and multi-step task performance from human preference." (LMArena agentic arena, https://lmarena.ai/leaderboard)
9. "How a map on a café wall turned into an app that reads, maps and translates all 387 labels of an 1819 French map of the Bosphorus, built with Claude Opus 5.5 subagents in an afternoon. I wrote up where this came from and how I built it: https: cahidarda.com articles bosphore-1819" (cahid_arda, https://news.ycombinator.com/item?id=49836368)
10. "Within the 48 catalog models matched to this evaluation: Ranks #1 of 48 on LMArena Agent's tool-hallucination signal (score 0.7), where a higher score means fewer fabricated tool calls." (LMArena Agent tool hallucination, https://lmarena.ai/leaderboard)
11. "Within the 48 catalog models matched to this evaluation: Ranks #3 of 48 on LMArena Agent's steerability signal (score 14.8), measuring how well the agent follows direction given mid-task." (LMArena Agent steerability, https://lmarena.ai/leaderboard)
12. "Within the 48 catalog models matched to this evaluation: Ranks #30 of 48 on LMArena Agent's tool-hallucination signal (score 0.1), where a higher score means fewer fabricated tool calls." (LMArena Agent tool hallucination, https://lmarena.ai/leaderboard)
13. "Within the 48 catalog models matched to this evaluation: Ranks #5 of 48 on LMArena's agentic arena (score 12.8), measuring tool-use and multi-step task performance from human preference." (LMArena agentic arena, https://lmarena.ai/leaderboard)
14. "Within the 48 catalog models matched to this evaluation: Ranks #10 of 48 on LMArena Agent's tool-hallucination signal (score 0.6), where a higher score means fewer fabricated tool calls." (LMArena Agent tool hallucination, https://lmarena.ai/leaderboard)
15. "Within the 48 catalog models matched to this evaluation: Ranks #12 of 48 on LMArena's agentic arena (score 3.8), measuring tool-use and multi-step task performance from human preference." (LMArena agentic arena, https://lmarena.ai/leaderboard)
16. "Within the 48 catalog models matched to this evaluation: Ranks #28 of 48 on LMArena Agent's tool-hallucination signal (score 0.2), where a higher score means fewer fabricated tool calls." (LMArena Agent tool hallucination, https://lmarena.ai/leaderboard)
17. "Within the 48 catalog models matched to this evaluation: Ranks #17 of 48 on LMArena Agent's steerability signal (score 4.8), measuring how well the agent follows direction given mid-task." (LMArena Agent steerability, https://lmarena.ai/leaderboard)