fix(adk/bedrock): coerce nil tool-call args to empty object

[go-faustino]

Summary

The Bedrock converter serializes a no-argument tool call as toolUse.input: null, which Bedrock Converse rejects on the follow-up (tool-result) turn with:

ValidationException: Malformed input request
(The value at messages.N.content.M.toolUse.input is empty)

In convertGenaiContentsToBedrockMessages (go/adk/pkg/models/bedrock.go), a tool call's input is built as document.NewLazyDocument(part.FunctionCall.Args). A no-argument call reaches this point with a nil Args map, because genai's FunctionCall.Args is tagged json:"args,omitempty": an empty map[string]any{} is dropped when the event is persisted to the session store and reloaded as nil. document.NewLazyDocument(nil) then serializes to JSON null, and Bedrock requires toolUse.input to be a JSON object ({}).

This is reproducible against us.anthropic.claude-opus-4-8 in any multi-turn conversation where the model calls a tool that takes no arguments (e.g. datetime_get_current_time) and the history is reloaded before the next turn.

Fix

Coerce a nil Args map to an empty map[string]any{} before building the ToolUseBlock, so no-argument tool calls round-trip as {} instead of null.

Test

Adds TestConvertGenaiContentsNoArgToolInput covering both nil and empty ({}) Args, asserting the produced toolUse.input is a non-null, empty JSON object. Without the fix, the nil case serializes to null and the test fails.

go test ./pkg/models/ -run TestConvertGenaiContents
ok  github.com/kagent-dev/kagent/go/adk/pkg/models

[Copilot]

Pull request overview

Fixes Bedrock Converse tool-call serialization by ensuring no-argument tool calls produce toolUse.input as an empty JSON object ({}) instead of null, which Bedrock rejects on follow-up turns.

Changes:

• Coerces nil FunctionCall.Args to an empty map[string]any{} before creating the Bedrock ToolUseBlock.
• Adds a unit test to cover both nil and empty-map tool arguments, asserting toolUse.input is a non-null empty object.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
go/adk/pkg/models/bedrock.go	Coerces nil tool-call args to an empty object before building toolUse.input.
go/adk/pkg/models/bedrock_test.go	Adds coverage for no-arg tool calls to ensure toolUse.input is {} not null.

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