System prompt silently dropped when sent as a string

[dcrdev]

When using CLIProxyAPI as a proxy, any client that sends the system prompt as a plain string rather than as an array of content blocks will have its entire system prompt silently dropped. The system prompt contains all the instructions that guide how the model behaves. This affects any claude client using the default cloaking mode, since those are the requests that go through the code path where the string format isn't handled.

Problem

The checkSystemInstructionsWithMode function in internal/runtime/executor/claude_executor.go only handles the system field when it is an array of content blocks. When a client sends system as a plain string, which is equally valid per the Anthropic API spec - the entire system prompt is silently discarded.

This affects any client that:

1. Sends "system": "..." (string) instead of "system": [{"type": "text", "text": "..."}] (array)
2. Does not have a claude-cli User-Agent, causing shouldCloak() to return true in the default "auto" cloak mode

In practice this means some tools lose their entire system prompt - including user-configured rules, project context, and tool-use instructions, when routing through CLIProxyAPIPlus.

Root Cause

In checkSystemInstructionsWithMode, the non-strict-mode path builds the new system array by iterating over existing content blocks:

result := "[" + billingBlock + "," + agentBlock
if system.IsArray() {
    system.ForEach(func(_, part gjson.Result) bool {
        // ... appends each block to result
        return true
    })
}
result += "]"

When system is a plain JSON string, system.IsArray() is false, so the user's system prompt is never appended. The field is then overwritten with only [billingBlock, agentBlock].

Fix

Handle the string case by converting it into a content block before appending, consistent with how array elements are already handled:

} else if system.Type == gjson.String && system.String() != "" {
    partJSON := `{"type":"text","cache_control":{"type":"ephemeral"}}`
    partJSON, _ = sjson.Set(partJSON, "text", system.String())
    result += "," + partJSON
}

Array-format requests continue to work identically and are unaffected by this change.

Fixes: #1994

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical issue in CLIProxyAPIPlus where system prompts provided as a simple string were inadvertently discarded. The change ensures that all valid system prompt formats, including plain strings, are correctly processed and passed to the model, thereby preserving essential instructions and maintaining expected model behavior for various client configurations.

Highlights

• System Prompt Handling: Fixed a bug where system prompts sent as a plain string were silently dropped by CLIProxyAPIPlus when shouldCloak() returned true, leading to models losing critical instructions.
• String to Content Block Conversion: Implemented logic to correctly parse string-formatted system prompts by converting them into a {"type":"text"} content block, ensuring they are properly appended to the system instruction array.

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Changelog

• internal/runtime/executor/claude_executor.go
  • Added logic to correctly parse and include system prompts provided as a plain string.

Activity

• No specific activity (comments, reviews, progress) has been recorded for this pull request yet.

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[gemini-code-assist]

Code Review

This pull request fixes a bug where the system prompt is silently dropped if it's provided as a string instead of an array of content blocks. The change correctly handles the string case by converting it into a content block. The fix is correct and addresses the issue described. I've added one comment regarding error handling for improved code robustness.

[xkonjin]

Automated review focusing on bugs, security, and test coverage.
PR: System prompt silently dropped when sent as a string (#1992)

Key observations:

• No test files changed. Consider adding or updating tests for modified code paths.

Suggestions:

• Add or update unit/integration tests for changed code paths.
• Run dependency and static security scans (pip-audit/npm audit/Semgrep) and address findings.
• Double check error handling and input validation on new code paths.