System Prompt & CLAUDE.md

The system prompt is assembled dynamically at startup by SystemPromptBuilder. It combines hardcoded agent instructions with project context, discovered instruction files (CLAUDE.md), and runtime configuration.

Prompt Structure

The system prompt is built as a Vec<String> of sections, joined with double newlines:

The Dynamic Boundary

The __SYSTEM_PROMPT_DYNAMIC_BOUNDARY__ marker separates static (cacheable) sections from dynamic (per-session) sections. Everything above the boundary is the same across sessions; everything below changes based on the project context.

CLAUDE.md Discovery

Instruction files are discovered by collecting directories from the working directory up to the filesystem root, then processing them in root-first order:

For each directory (from root down to cwd), these files are checked in order:

1. CLAUDE.md
2. CLAUDE.local.md
3. .claude/CLAUDE.md
4. .claude/instructions.md

Deduplication

Files with identical content (after normalizing blank lines and trimming) are deduplicated using a hash:

fn dedupe_instruction_files(files: Vec<ContextFile>) -> Vec<ContextFile> {
    let mut seen_hashes = Vec::new();
    files.into_iter().filter(|file| {
        let hash = stable_content_hash(&normalize(file.content));
        if seen_hashes.contains(&hash) { false }
        else { seen_hashes.push(hash); true }
    }).collect()
}

Content Budgeting

Instruction files are subject to character limits:

• Per file: 4,000 characters max
• Total: 12,000 characters max across all files

Files exceeding the per-file limit are truncated with a [truncated] marker. Once the total budget is exhausted, remaining files show: "Additional instruction content omitted after reaching the prompt budget."

Environment Section

# Environment context
 - Model family: Claude Opus 4.6
 - Working directory: /home/user/project
 - Date: 2026-04-01
 - Platform: linux 6.8

Trivia: Hardcoded Frontier Model

The frontier model name is hardcoded as "Claude Opus 4.6" in a constant called FRONTIER_MODEL_NAME. This is embedded in every system prompt regardless of which model is actually being used.

Git Context

When using ProjectContext::discover_with_git(), the prompt includes:

1. Git status — output of git --no-optional-locks status --short --branch
2. Git diff — both staged (git diff --cached) and unstaged (git diff) changes

These give the agent awareness of the current repository state.

Runtime Config Section

If a settings file is loaded, its contents are rendered into the prompt:

# Runtime config
 - Loaded User: /home/user/.claude/settings.json

{"permissionMode":"acceptEdits","hooks":{...}}

This allows the agent to see its own configuration and adjust behavior accordingly.