Tool Use and Function Calling

Learning Objectives

• Explain Toolformer's self-supervised training signal: keep tool annotations only when execution reduces next-token loss.
• Name BFCL V4's five evaluation categories and what each measures.
• Implement a stdlib tool registry with schema validation, argument coercion, and execution sandboxing.
• Diagnose the three 2026 open problems: long-horizon tool chaining, dynamic decision-making, and memory.

The Problem

Early tool use asked: can the model predict a correct function call? Modern tool use asks: can the model chain tools across 40 steps, with memory, with partial observability, with recovery from tool failures, without hallucinating tools that do not exist?

Toolformer established the baseline: models can learn when to call tools with self-supervision. BFCL V4 defines the 2026 evaluation target. The gap between them is the space production agents live in.

The Concept

Toolformer (Schick et al., NeurIPS 2023)

Idea: let the model annotate its own pretraining corpus with candidate API calls. For each candidate, execute it. Keep the annotation only if including the tool result reduces loss on the next token. Fine-tune on the filtered corpus.

Tools covered: calculator, QA system, search engines, translator, calendar. The self-supervision signal is purely about whether the tool helps predict text — no human labels.

Scale result: tool use emerges at scale. Smaller models hurt from tool annotations; larger models gain. This is why 2026 frontier models have strong tool use baked in while most 7B models need explicit tool-use fine-tuning to be reliable.

Berkeley Function Calling Leaderboard V4 (Patil et al., ICML 2025)

BFCL is the 2026 de facto evaluation. V4 composition:

• Agentic (40%) — full agent trajectories: memory, multi-turn, dynamic decisions.
• Multi-Turn (30%) — interactive conversations with tool chains.
• Live (10%) — user-submitted real prompts (harder distribution).
• Non-Live (10%) — synthetic test cases.
• Hallucination (10%) — detect when no tool should be called.

V3 introduced state-based evaluation: after a tool sequence, check the API's actual state (e.g. "is the file created?") rather than match the AST of the tool calls. V4 added web search, memory, and format sensitivity categories.

Key 2026 finding: single-turn function calling is near-solved. Failures concentrate in memory (carrying context across turns), dynamic decision-making (choosing tools based on prior results), long-horizon chains (drift after 20+ steps), and hallucination detection (refusing to call when no tool fits).

Tool schema

Every provider has a schema. They differ in details but share the same shape:

name: string
description: string (what it does, when to use it)
input_schema: JSON Schema (properties, required, types, enums)

Anthropic uses input_schema directly. OpenAI uses function.parameters. Both accept JSON Schema. Descriptions are load-bearing — the model reads them to pick the right tool. Bad tool descriptions are the #1 root cause of wrong-tool-picked failures.

Argument validation

Trust no tool call. Validate:

1. Type coercion. Model may return a string "5" where the schema says int. Coerce if unambiguous; reject if not.
2. Enum validation. If the schema says status in {"open", "closed"} and model emits "in_progress", reject with a descriptive error.
3. Required fields. Missing required field -> immediate error observation back to the model, not a crash.
4. Format validation. Dates, emails, URLs — validate with concrete parsers, not regex.

Every validation failure should return a structured observation so the model can retry with the correct shape.

Parallel tool calls

Modern providers support parallel tool calls in one assistant turn. The loop:

1. Model emits 3 tool calls with distinct tool_use_ids.
2. Runtime executes them (in parallel if independent).
3. Each result goes back as a tool_result block correlated by tool_use_id.

Engineering rule: treat correlation IDs as load-bearing. Swap them and you get wrong-tool-to-wrong-result routing.

Sandboxing

Tool execution is the sandbox boundary. See Phase 13 · 26 for detail. Short version: every tool should specify read/write surface, network access, timeout, memory cap. Generic run_shell(cmd) is a red flag; specific git_status() is safer.

tool-routing

Build It

code/main.py implements a production-shape tool registry:

• JSON Schema subset validator (stdlib only).
• Tool registration with description, input schema, timeout, and executor.
• Argument coercion and enum validation.
• Parallel tool dispatch with correlation IDs.
• Error observations as structured strings.

Run it:

python3 code/main.py

The trace shows a mini agent calling three tools in one turn, with one deliberately malformed call that is rejected with a descriptive error the model can act on.

Use It

Every provider has its own tool schema — Anthropic, OpenAI, Gemini, Bedrock. Use a translation layer (OpenAI Agents SDK, Vercel AI SDK, LangChain tool adapter) if you need multi-provider. BFCL is the reference benchmark — run it against your agent before shipping if tool use is central to the product.

Ship It

outputs/skill-tool-registry.md generates a tool catalog, schema, and registry for a given task domain. Includes description-quality checks (does each tool's description tell the model when to use it?).

Exercises

1. Add a "no-op" tool that lets the model explicitly refuse to use any other tool. Measure on a BFCL-like hallucination test.
2. Implement argument coercion for int-as-string and float-as-string. Where does coercion start to hide real bugs?
3. Add a per-tool timeout and a circuit breaker (refuse the tool for 60s after 3 consecutive failures). What does this change about how the model recovers?
4. Read BFCL V4 description. Pick one category (e.g. "multi-turn") and run 10 example prompts through your agent. Report pass rate.
5. Port the stdlib validator to Pydantic or Zod. What did Pydantic/Zod catch that the toy missed?

Key Terms

Further Reading

• Schick et al., Toolformer (arXiv:2302.04761) — self-supervised tool annotation
• Berkeley Function Calling Leaderboard (V4) — 2026 eval benchmark
• Anthropic, Tool use documentation — production tool schema in the Claude Agent SDK
• OpenAI Agents SDK docs — function tool type and Guardrails