Function Calling

Tools are declared via ai.ToolInfo (name, description, JSON schema, optional Parallel flag). Each provider adapter converts ToolInfo to its native function-call format. Tool results return as ai.RoleToolResult messages carrying ToolCallID, ToolName, and an IsError flag.

For provider-hosted tools (web search, code execution, computer use, etc.), see server-tools.md.

Tool Declarations

Provider	API	pi-go	Notes
Anthropic	✅ tools array; strict-by-default	✅	ToolParam (convert.go:169-208)
OpenAI Chat	✅ tools array with strict mode	✅	ChatCompletionToolParam (convert.go:190-214); strict gated by compat flag
OpenAI Responses	✅ tools array	✅	FunctionToolParam (convert.go:208-231)
Google Gemini	✅ tools[].functionDeclarations	✅
Claude CLI	❌ (CLI manages its own tools)	❌	Tools ignored (claude.go:124)

Parallel Tool Calls

Provider	API	pi-go	Notes
Anthropic	✅ multiple tool_use blocks per response; disable_parallel_tool_use flag	⚠️	streaming accumulates blocks; flag not forwarded
OpenAI Chat	✅ parallel_tool_calls: true (default)	⚠️	deltas accumulated by index (openai.go:150-193); flag not exposed
OpenAI Responses	✅ multiple output items	✅	tracked by index (openairesponses.go:158)
Google Gemini	✅ multiple functionCall parts	⚠️	parts iterated; no flag
Claude CLI	❌	—

Tool Result Content

ai.ToolResult carries text, an image, or media. ToolResult.Contents renders it into message blocks: text becomes ai.Text, an image becomes ai.Image, and media becomes the extracted ai.Text plus the raw ai.File. Injected blocks in ToolResult.Extra, such as ai.Reminder, follow.

Provider	Text	Image	File	Notes
Anthropic	✅	✅	✅	tool_result content takes text, image, and document blocks
OpenAI Chat	✅	⚠️	⚠️	a tool message takes text only, so media follows in a user message after the tool-result group
OpenAI Responses	✅	✅	✅	function_call_output takes a content list of input_text, input_image, and input_file
Google Gemini	✅	❌	❌	functionResponse.response is a JSON object with no binary part

An ai.Reminder block reaches every provider as plain text, in the same place the text of the tool goes. The Injected and Origin fields never reach the wire: they route the message, and routing is not content.

Tool Choice

ai.ToolChoice supports auto (default), none, required, or a specific tool name.

Provider	auto	none	required / any	specific	pi-go
Anthropic	✅	✅	✅ any	✅	✅ all four (convert.go:215-231)
OpenAI Chat	✅	✅	✅ required	✅	✅ all four (convert.go:222-241)
OpenAI Responses	✅	✅	✅	✅	✅ all four (convert.go:238-261)
Google Gemini	✅ AUTO	✅ NONE	✅ ANY	⚠️ via allowedFunctionNames	⚠️

Provider Documentation

• Anthropic — Tool use
• OpenAI — Function calling
• OpenAI Responses — Tools
• Google Gemini — Function calling

pi-go Gaps

• Strict schema mode plumbed only on OpenAI Chat via compat flag (convert.go:205-206); not exposed as ToolInfo field, not passed to other providers.
• OutputSchema (tool.go) defined on ToolInfo but not forwarded by any provider.
• ToolInfo.Parallel flag exists but is not forwarded; disable_parallel_tool_use (Anthropic) / parallel_tool_calls: false (OpenAI) cannot be set.
• Specific-tool forcing on Gemini uses allowedFunctionNames (a list, in ANY mode); pi-go’s single-string ToolChoice doesn’t map cleanly.
• Claude CLI drops all tool definitions silently.
• Image tool results on Gemini are dropped. The function-response schema has no place for binary data. This adapter also does not send a follow-up user message, as the OpenAI Chat adapter does.