fix: always use array content for system messages with block-level cache control

[robert-j-y]

Description

Fixes #389

System messages now always use array content format, with cache_control placed exclusively on content blocks (never at the message level). This eliminates the string | Array union and prevents any possibility of cache_control duplication between message and block levels.

Before:

{ "role": "system", "content": "...", "cache_control": { "type": "ephemeral" } }

After (with cache control):

{ "role": "system", "content": [{ "type": "text", "text": "...", "cache_control": { "type": "ephemeral" } }] }

After (without cache control):

{ "role": "system", "content": [{ "type": "text", "text": "..." }] }

Key changes:

1. Type change (src/types/openrouter-chat-completions-input.ts): ChatCompletionSystemMessageParam.content is now Array<ChatCompletionContentPartText> (no string union). Removed message-level cache_control field entirely.
2. Logic change (src/chat/convert-to-openrouter-chat-messages.ts): System content always converts to array format. cache_control is conditionally spread onto the block only when present.
3. E2E test (e2e/issues/issue-389-system-cache-control.test.ts): Verifies cache write/read behavior with system message cache control.

Cache control audit (no duplication)

Message type	cache_control location	Duplication?
system	Block-level only	No
user (single text)	Block-level only	No
user (multi-part)	Block-level only (last text part)	No
assistant	Message-level only (string content)	No
tool	Message-level only (string content)	No

Validation

• Verified via live API tests that always-array format works across Anthropic, OpenAI, and Google Gemini
• Confirmed block-level cache_control is the canonical Anthropic pattern per their docs
• ChatCompletionSystemMessageParam is not exported publicly — type change is internal only

Human review checklist

• Verify always-array system content doesn't break any downstream consumers you're aware of
• Confirm the spread pattern ...(cacheControl && { cache_control: cacheControl }) correctly omits cache_control when undefined
• Check that no cache_control appears at both message and block level for any message type

Checklist

• I have run pnpm stylecheck and pnpm typecheck
• I have run pnpm test and all tests pass
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)

Changeset

• I have run pnpm changeset to create a changeset file

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Link to Devin run: https://app.devin.ai/sessions/ac3d2be5447d40518a2b0ecac907e315
Requested by: @robert-j-y

[louisgv]

I'd argue to rid this union and just use an array for this

[louisgv]

Overall lgtm 👍

Would double check that the cache_control is not duplicated between top level and child level part