ToolResult — skyl

A ToolResult answers a ToolCall. Its IsError flag is one of skyl's genuinely lossy fields — it reaches the model faithfully on exactly one adapter.

Reference#

type ToolResult struct { CallID string Content string IsError bool }

Parameters

CallID — must match the ToolCall.ID this result answers. Required.
Content — the tool's output, rendered as text.
IsError — reports that the tool failed.

Caveats

Validate requires CallID.
Every provider rejects a result that does not follow the call it answers. Append the assistant's turn first.
IsError arrives intact only on Anthropic, as a real is_error boolean.
The OpenAI-format adapters are lossy: they prefix "error: " to the content, and drop the flag entirely when the content is empty.
Gemini drops it completely — the signal never reaches the wire.
Never return an empty Content for a failure; it erases the signal on two adapters at once.

Usage#

The portable way to report a failure

goCompiles

// The prose is the signal that works everywhere. IsError is a bonus where
// it survives.
skyl.ToolErrorMessage(call.ID,
	fmt.Sprintf("ERROR: %s failed: %v. Do not retry; tell the user the data "+
		"is unavailable.", call.Name, err))

// The prose is the signal that works everywhere. IsError is a bonus where
// it survives.
skyl.ToolErrorMessage(call.ID,
	fmt.Sprintf("ERROR: %s failed: %v. Do not retry; tell the user the data "+
		"is unavailable.", call.Name, err))

Three things that text does which the boolean cannot: it names the tool, it says what to do next, and it works on all four adapters.

A successful result

goCompiles

skyl.ToolResultMessage(call.ID, "22C and sunny in Kampala")

skyl.ToolResultMessage(call.ID, "22C and sunny in Kampala")

Constructing one directly

skyl.Message{
	Role: skyl.RoleTool,
	Parts: []skyl.Part{skyl.ToolResult{
		CallID:  call.ID,
		Content: "22C and sunny",
		IsError: false,
	}},
}

skyl.Message{
	Role: skyl.RoleTool,
	Parts: []skyl.Part{skyl.ToolResult{
		CallID:  call.ID,
		Content: "22C and sunny",
		IsError: false,
	}},
}

Troubleshooting#

The model kept calling a tool that had failed

On Gemini and, when content is empty, on the OpenAI family, the model was never told it failed. Put the failure in the text and say explicitly not to retry.

A 400 after appending my tool result

Almost always ordering: the assistant turn containing the call must be appended before the result. Every provider enforces this.

A call I skipped caused the whole turn to be rejected

Every call must get a result. If a tool panicked, send an error message rather than nothing.