Anthropic /v1/messages silently drops system field when passed as content-block array

[mikemolinet]

What happened?

POST /v1/messages silently drops the system field when it is passed as an array of content blocks. The Anthropic Messages API spec (https://docs.anthropic.com/en/api/messages) allows system as either a string or an array of content blocks (currently only {type: "text", text: "..."} blocks are defined). Clients that follow the spec — or SDKs that serialize system as an array — lose their system prompt at the proxy boundary without any error.

The relevant code at index.js:1044-1047:

const allMessages =
  typeof body.system === "string" && body.system.trim()
    ? [{ role: "system", content: body.system.trim() }, ...messages]
    : messages

typeof body.system === "string" is false for arrays, so the system content never reaches buildSystemPrompt and the request proceeds without a system prompt.

Steps to reproduce

1. Start the proxy with npm start (or via opencode with the plugin loaded).
2. Send a request to POST /v1/messages with system as an array of text content blocks.
3. Observe that the model's response ignores the system prompt (vs. the same content sent as a string works correctly).

Expected behaviour

The system field, whether sent as a string OR as an array of {type: "text", text: "..."} content blocks, should reach the underlying model as a system prompt.

Request / response (if applicable)

Array form (currently dropped):

curl -X POST http://127.0.0.1:4010/v1/messages \
  -H 'content-type: application/json' \
  -d '{
    "model": "anthropic/claude-3-5-sonnet",
    "system": [
      { "type": "text", "text": "You are a pirate. Every reply must start with Arr." }
    ],
    "messages": [
      { "role": "user", "content": "Hello." }
    ]
  }'
# → 200 OK, reply has no pirate voice (system prompt silently ignored)

String form (works today — same intent):

curl -X POST http://127.0.0.1:4010/v1/messages \
  -H 'content-type: application/json' \
  -d '{
    "model": "anthropic/claude-3-5-sonnet",
    "system": "You are a pirate. Every reply must start with Arr.",
    "messages": [
      { "role": "user", "content": "Hello." }
    ]
  }'
# → 200 OK, reply starts with "Arr"

opencode-llm-proxy version

1.6.1

Runtime and OS

Reproduces on Node.js >= 20 and Bun >= 1.0, any OS — it's a pure JS logic bug.

Provider / model

Any Anthropic-model provider. Verified against anthropic/claude-3-5-sonnet conceptually; the bug is at the proxy boundary and is independent of the downstream provider.

---

I have a fix ready — add an exported normalizeAnthropicSystem helper that handles both forms, mirroring the existing normalizeAnthropicMessages array-of-content-blocks pattern (filter to text blocks, trim, join with \n\n). Happy to open a PR once you confirm the approach.