Bedrock provider sends null toolUse.input for no-argument tool calls

[clouatre]

Describe the bug

Bedrock's Converse API rejects tool calls when the model invokes a tool with no arguments:

ValidationException: The value at messages.1.content.0.toolUse.input is empty.

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To Reproduce

1. Use the aws_bedrock provider
2. Resume a session from the session DB where a tool call had null/missing arguments, or switch from Google/Vertex provider to Bedrock mid-session
3. On the next API call, Bedrock returns ValidationException

The primary path: tool_result_serde.rs:40 maps Value::Null => None during session deserialization, feeding None arguments into the Bedrock format handler. In a pure Bedrock session without resume, from_bedrock_content_block always produces Some(map), so None arguments do not originate from Bedrock responses.

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Expected behavior

No-argument tool calls should send {} (empty JSON object) as toolUse.input.

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Root cause

In crates/goose/src/providers/formats/bedrock.rs (lines 75 and 90), both ToolRequest and FrontendToolRequest convert arguments via:

.input(to_bedrock_json(&Value::from(call.arguments.clone())))

When call.arguments (Option<JsonObject>) is None, Value::from(None) yields Value::Null, which to_bedrock_json converts to Document::Null. The AWS SDK builder accepts this, but the Bedrock API rejects it.

tool_result_serde.rs:40 is the primary source of None arguments: it explicitly maps Value::Null => None when deserializing tool calls from the session database.

Suggested fix:

let input_val = call.arguments.clone()
    .map(Value::Object)
    .unwrap_or_else(|| Value::Object(Default::default()));

Both ToolRequest (line 75) and FrontendToolRequest (line 90) need this change.

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Retry behavior (secondary issue)

The ValidationException for empty input is miscategorized in the error handling chain. In crates/goose/src/providers/bedrock.rs (line 301), unmatched ValidationException variants fall through to the catch-all:

err => ProviderError::ServerError(format!("Failed to call Bedrock: {:?}", err))

ServerError is retryable per should_retry() in crates/goose/src/providers/retry.rs. Retries are futile because the same deserialized None arguments are re-sent each attempt. After exhausting retries, the session is unrecoverable.

This ValidationException is a deterministic client-side serialization bug, not a transient server error. It should either be prevented by the primary fix above, or mapped to a non-retryable error variant (e.g., ExecutionError).

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Please provide the following information

• OS & Arch: macOS (Apple Silicon)
• Interface: CLI
• Version: v1.28.0 (also confirmed on current main)
• Extensions enabled: developer, code-analyze, context7, brave_search, aptu
• Provider & Model: aws_bedrock / global.anthropic.claude-sonnet-4-6

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Additional context

• The ToolUseBlock API reference documents input as a required field typed as JSON document
• aws/aws-sdk-php#2962 is the same class of bug in the PHP SDK: empty/null input rejected by the Converse API
• vercel/ai#5527 discusses defaulting to {} for tools with no parameters in the Vercel AI SDK's Bedrock provider
• No existing tests cover the None-arguments case

[clouatre]

I've dug into this and can confirm the root cause. The bug has two parts:

1. bedrock.rs sends Document::Null for no-argument tool calls

call.arguments is Option<JsonObject>. When None, Value::from(None) produces Value::Null, which to_bedrock_json converts to Document::Null. Bedrock rejects this with ValidationException: toolUse.input is empty.

Fix: normalize None to an empty object before calling to_bedrock_json at lines 75 and 90.

2. The resulting ValidationException is retryable

The unmatched ValidationException falls through to the err => catch-all, which maps to ServerError -- a retryable error variant. Since the root cause is deterministic, retries are futile and exhaust the retry budget before failing. A bare ConverseError::ValidationException arm before the catch-all would map these to ExecutionError (non-retryable).

The trigger in practice is opus 4.6, which omits the arguments key entirely for no-argument tool calls (e.g., health checks). The session DB stores this as absent/null, and tool_result_serde.rs correctly maps Value::Null => None -- so the fix belongs in the Bedrock format layer, not in serde.

Happy to submit a PR if that's useful.