[cunninghamcard-bit]

Summary

Before: a non-streaming Hugging Face function_call item with an output gave only the tool-call; the output was dropped. An mcp_call whose output was an empty string still gave a tool-result.

After: both follow upstream's if (part.output): a non-empty output string adds a tool-result holding that string, an empty or missing one adds nothing. The stream's function_call uses the same check.

Found while porting the upstream Hugging Face tests (#303, case tool calls › should handle function call tool responses). Part of the #276 list. Stacked on #305 (CHANGELOG lines touch); the stream-error PR follows.

How it works today

HuggingFaceResponsesModel::do_generate                 aimux-providers/src/huggingface/responses.rs:163
└─ build_generate_content(&response)                   responses.rs:933                     // upstream: huggingface-responses-language-model.ts:220
   ├─ "function_call"                                   responses.rs:995                     // upstream: case 'function_call' :304
   │  └─ RawToolCall only                                                                     //   if (part.output) push tool-result { result: part.output } :312
   └─ "mcp_call"                                        responses.rs:1013                    // upstream: case 'mcp_call' :264
      └─ if part.get("output") is not null → ToolResult { result: output.clone() }   :1030   //   if (part.output) :273   ('' is false)
HuggingFaceResponsesModel::do_stream → transform_event  responses.rs:306
└─ output_item.done › "function_call"                   responses.rs:422
   └─ if output is a string → ToolResult               :450                                 // upstream: if (value.item.output) :489

What this PR changes

build_generate_content                                 responses.rs:931
├─ "function_call"   if let Some(output) = non_empty_output(part)    :1010   ← new ToolResult { result: Value::String(output) }
└─ "mcp_call"        if let Some(output) = non_empty_output(part)    :1040   ← empty string gives no tool-result
transform_event › output_item.done › "function_call"
   if let Some(output) = non_empty_output(item)        :449
non_empty_output(part) -> Option<&str>                 :1087   ← `part.output` when JavaScript reads it as true: a non-empty string

Tests (tests/huggingface_responses_test.rs): should_handle_function_call_tool_responses asserted the dropped output; it now expects tool-call, tool-result, text. mcp_call_with_empty_output_has_no_tool_result is new. The full upstream case is in #303.

Docs: CHANGELOG Fixed.

Type of change

• Bug fix (non-breaking)
• New feature (non-breaking)
• Breaking change
• New provider
• New / updated binding
• Docs / RFC

Checklist

• cargo +1.99 fmt -p aimux-providers -- --check passes
• cargo +1.99 clippy -p aimux-providers --lib --tests -- -D warnings passes
• cargo +1.99 test -p aimux-providers --test huggingface_responses_test passes (33)
• scripts/check_provider_boundaries.sh passes
• Updated relevant docs (CHANGELOG)
• Commit messages follow Conventional Commits

Notes for reviewers

The mcp_call result is now always the output string (upstream's result: part.output); before, a non-string JSON output was passed through as is.

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Generated by Claude Code

[cunninghamcard-bit]

Folded into #307 together with the other Hugging Face fixes, so each vendor has one fix PR. This change is commit fcdc9c0a there. Closing this one; the branch stays as it is.

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Generated by Claude Code