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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[go-faustino]

Thanks for the review. This one is a false positive — the code compiles and the tests pass as-is.

In a Go composite literal, the element type may be elided, so {} inside []map[string]any{nil, {}} is shorthand for map[string]any{} (Go spec, Composite literals: "Within a composite literal of array, slice, or map type T, elements or map keys that are themselves composite literals may elide the respective literal type if it is identical to the element or key type of T"). nil is also a valid map[string]any element.

Verified on the branch:

$ go vet ./pkg/models/
$ go test ./pkg/models/ -run TestConvertGenaiContentsNoArgToolInput -count=1
ok  github.com/kagent-dev/kagent/go/adk/pkg/models  0.855s

Keeping the elided form as it's idiomatic, but happy to spell out map[string]any{} if maintainers prefer explicitness.