Tool/Function Calling Design

Status

Accepted (2024-04)

Context

Modern LLMs support tool/function calling for:

• External data retrieval.

• Action execution.

• Structured output generation.

Decision

Support OpenAI-compatible tool format:

Tool calls returned in CompletionResponse::$toolCalls:

• A typed list<ToolCall> (nullable) of ToolCall value objects — each with the tool id, name and its arguments as an already JSON-decoded associative array (not an encoded string). A full tool-execution runtime was added later in ADR-038.

Consequences

Positive:

• ●● Industry-standard format.

• ●● Cross-provider compatibility.

• ● Flexible tool definitions.

• ● Type-safe parameters.

Negative:

• ◑ Complex nested structure.

• ◑ Provider translation needed.

• ✕ No automatic execution.

• ◑ Testing complexity.

Net Score: +5.0 (Positive impact - OpenAI-compatible format ensures broad compatibility)