3.4 System Prompt Construction Mechanism

Chapter goal: Understand how Claude Code dynamically constructs the System Prompt before each conversation turn, and what information is included.

System Prompt is not static

Many people assume the System Prompt is fixed text. In Claude Code, however, it is built dynamically in real time at the start of each conversation and injected with real current environment state.

fetchSystemPromptParts()

The entry point for System Prompt construction is fetchSystemPromptParts() in src/utils/queryContext.ts:

  tools,
  mainLoopModel,
  additionalWorkingDirectories,
  mcpClients,
  customSystemPrompt,
}: FetchSystemPromptOptions): Promise<{
  defaultSystemPrompt: string[]  // core system prompt parts
  userContext: { [k: string]: string }  // key/value pairs injected into <user-context>
  systemContext: { [k: string]: string } // key/value pairs injected into <system-context>
}>

Constructed parts

1. Core persona prompt

This is Claude's "base persona":

You are Claude Code, an AI assistant made by Anthropic, focused on software engineering tasks...

Includes:

2. Git status snapshot (getGitStatus())

If current directory is a git repo, Claude Code injects:

This is the git status at the start of the conversation.

Current branch: feature/add-auth
Main branch (you will usually use this for PRs): main
Git user: xuwei
Status:
 M src/auth/login.ts
 M src/auth/session.ts
?? src/auth/oauth.ts

Recent commits:
abc1234 Add OAuth provider scaffold
def5678 Fix session expiry bug

This lets Claude know, from the beginning, which branch you are on and what uncommitted changes exist.

Note the source comment: Note that this status is a snapshot in time, and will not update during the conversation. — this is a snapshot, not live. For latest status, Claude must run git status via BashTool.

3. CLAUDE.md content

// src/utils/claudemd.ts

Claude Code recursively searches CLAUDE.md from:

Contents are injected directly into the System Prompt. This is the main mechanism for user/team customization of Claude behavior.

4. Tool descriptions

Each tool's JSON Schema description is injected into the System Prompt so Claude knows available tools and parameter formats.

// Tool description example (BashTool)
{
  name: 'Bash',
  description: 'Execute shell commands...',
  input_schema: {
    type: 'object',
    properties: {
      command: { type: 'string', description: 'The shell command to execute' },
      timeout: { type: 'number', description: 'Timeout in milliseconds' },
    },
    required: ['command']
  }
}

5. Memory files (MEMORY.md)

If memory feature is enabled (EXTRACT_MEMORIES compile switch), the system injects long-term memory:

// src/memdir/memdir.ts

userContext vs systemContext

These two contexts are injected at the end of System Prompt with different XML tags:

<user-context>
  今天是 2026年04月02日。
  当前工作目录：/Users/xuwei/projects/myapp
  [Coordinator 模式下：Workers 可用的工具列表...]
</user-context>

<system-context>
  [Coordinator 模式下的系统级指令...]
</system-context>

userContext contains dynamic information that may change on every call (date, CWD, config).

systemContext contains architecture-level system instructions (e.g., coordinator dispatch rules).

System Prompt size limits

When building System Prompt, git status length is capped:

const MAX_STATUS_CHARS = 2000

const truncatedStatus =
  status.length > MAX_STATUS_CHARS
    ? status.substring(0, MAX_STATUS_CHARS) +
      '\n... (truncated because it exceeds 2k characters. If you need more information, run "git status" using BashTool)'
    : status

If git status exceeds 2000 chars, it is truncated automatically and Claude is told how to fetch full information.

Special injection in Coordinator mode

In Coordinator mode, getCoordinatorUserContext() additionally injects:

// src/coordinator/coordinatorMode.ts
  mcpClients: ReadonlyArray<{ name: string }>,
  scratchpadDir?: string,
): { [k: string]: string } {
  // Inject the list of tools available to Workers
  return { workerToolsContext: `Workers spawned via the Agent tool have access to these tools: ...` }
}

And getCoordinatorSystemPrompt() returns a full coordinator role definition (400+ lines), defining how to dispatch Workers, handle tasks, and forbidden behavior.

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