[Bugfix]: preserve array-formatted system message content (#201)

[wuhang2014]

Purpose

Fixes #201: system messages with OpenAI-style array-formatted content (e.g. [{"type":"text","text":"..."}]) were silently forwarded to upstream vLLM as an empty string "", so the system prompt was completely lost. String-format system content and array-format user content both worked correctly.

Root cause: ChatMessage's custom Deserialize "system" branch coerced the content field with Value::as_str(), which returns None for arrays, falling back to "".

Fix: model ChatMessage::System.content with a role-specific SystemMessageContent (untagged Text | Parts) whose parts are text-only (SystemContentPart), and deserialize it via serde_json::from_value, so both content forms round-trip through the router's parse-and-reserialize forwarding path. Serialization of the untagged enum already handles string vs array.

Strict OpenAI compatibility: system message arrays may only contain text parts. A present but unparseable system content (e.g. image_url parts) is rejected at deserialization time and surfaces as a 4xx (422 via axum's Json extractor) instead of silently degrading to an empty string; missing/null content still defaults to empty text for backward compatibility. User messages remain permissive (UserMessageContent with image_url support).

Test Plan

cargo fmt --check
cargo clippy --all-targets --all-features -- -D warnings
cargo test --lib --bins
cargo test --test '*'

Added unit tests in src/protocols/spec.rs:

• test_chat_message_system_array_content_roundtrip — deserialize → re-serialize → reparse of array-formatted system content
• test_chat_message_system_rejects_non_text_parts — image_url (and other non-text) parts are rejected with a deserialization error; missing/null content still defaults to empty text

End-to-end verification: router built with this change, mock upstream worker echoing the received body, POST /v1/chat/completions.

Test Result

Rust unit/integration tests and CI-grade clippy all pass (only pre-existing failure: test_openai_router_health returns 503 on macOS, also fails on pristine main).

End-to-end forwarded content, before → after:

Message content	Before	After
system array (text parts)	""	[{"type":"text","text":"You are GLM-5.2."}]
system string	"You are GLM-5.2."	"You are GLM-5.2."
system array (image_url part)	""	HTTP 422, rejected
user array	[{"type":"text","text":"hello array user"}]	[{"type":"text","text":"hello array user"}]
user array (image_url part)	forwarded	forwarded (unchanged, permissive)

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[Copilot]

Pull request overview

Fixes a forwarding bug in the OpenAI Chat Completions protocol layer where system messages using OpenAI-style array content parts were deserialized as an empty string and therefore lost when re-serialized and forwarded upstream.

Changes:

• Change ChatMessage::System.content from String to UserMessageContent (same polymorphic representation already used for User messages).
• Update the custom Deserialize implementation for ChatMessage to parse system content via serde_json::from_value.
• Add a regression unit test ensuring system array content survives deserialize → serialize → reparse roundtrips; update benchmark fixtures accordingly.

Reviewed changes

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

File	Description
src/protocols/spec.rs	Makes system message content polymorphic (string or parts array) and adds a regression test for roundtripping array-formatted system content.
benches/request_processing.rs	Updates benchmark request constructors to use UserMessageContent::Text for system messages after the type change.

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