Claude Code

Claude Code

Claude Code (claude binary) is Anthropic's agentic coding CLI. Markus invokes it via invoke_coding_tool({ tool: "claude-code", ... }). Use it for complex, multi-turn coding tasks where deep reasoning and broad file exploration are needed.

Installation

npm install -g @anthropic-ai/claude-code
claude --version

Verify with markus doctor. Requires authentication via one of: ANTHROPIC_API_KEY env var, ANTHROPIC_BASE_URL for custom endpoints, or interactive claude login.

How Markus Invokes Claude Code

Markus runs Claude Code in non-interactive print mode with structured streaming output:

claude --print --output-format stream-json --verbose --max-turns 50 --permission-mode bypassPermissions "<prompt>"

Additional args can be configured per-deployment via CodingToolConfig.defaultArgs.

Stream-JSON Output

Each stdout line is a JSON event. Key event types:

The result event includes:

• result — Final summary text
• input_tokens, output_tokens — Token usage
• cache_read_tokens, cache_write_tokens — Prompt cache stats
• cost_usd — Estimated cost in USD

Markus parses these into progress events (file_edit, progress, completed) and extracts cost reports automatically.

CLAUDE.md Context File

When you pass task_id to invoke_coding_tool, Markus writes a CLAUDE.md file in the repository root before invoking Claude Code. This file contains:

• Task title, description, status, and priority
• Subtasks, notes, and deliverables
• Requirement and project context
• Upstream/downstream dependency summaries
• Markus CLI commands for reporting progress

Claude Code reads CLAUDE.md automatically as project context. Do not delete or overwrite it during a session — it is regenerated each invocation.

If the repo already has a permanent CLAUDE.md, Markus overwrites it for the session. Consider restoring project-level content after the task if needed.

Usage Patterns

Complex Refactor

invoke_coding_tool({
  tool: "claude-code",
  prompt: "Refactor the auth module to use dependency injection. Move AuthService to src/services/, update all imports, keep existing test behavior. Run the test suite when done.",
  workdir: "/path/to/repo",
  task_id: "task-123"
})

Debug and Fix

invoke_coding_tool({
  tool: "claude-code",
  prompt: "Tests in packages/core/test/auth.test.ts are failing with 'token expired'. Find the root cause in src/auth/ and fix without changing the public API. Show which tests pass after the fix.",
  workdir: "/path/to/repo",
  task_id: "task-123"
})

Explore Then Implement

For unfamiliar codebases, ask Claude Code to explore first:

"Read the codebase structure under src/coding-tools/. Then implement the feature described in the task context. Start by listing the files you plan to modify."

Retry Strategies for Long Tasks

Claude Code supports up to 50 turns, but long tasks can still stall or partially complete.

If the session completes but work is incomplete

Re-invoke with explicit remaining scope:

"Previous session modified src/foo.ts and src/bar.ts but did not update tests. Complete the test coverage for the changes in src/foo.ts. Do not re-modify files that are already correct."

If the session fails or times out

1. Check git status in workdir for partial changes
2. Either apply partial work with coding_tool_apply or discard with git checkout -- .
3. Retry with a smaller scope — one module or one feature at a time

If Claude Code loops or over-edits

Add constraints to the prompt:

"Modify ONLY files under src/handlers/. Do not touch tests, config, or unrelated packages."

Escalation after 2 retries

Switch to codex for a targeted fix, or edit directly with file_edit.

Model and Effort Selection

Claude Code supports per-invocation model and effort overrides:

invoke_coding_tool({
  tool: "claude-code",
  prompt: "...",
  model: "sonnet",     // default: tool's own default (usually sonnet)
  effort: "medium",    // low | medium | high | xhigh | max
})

Model guidance

Strategy: Start with sonnet (or the user's defaultModel). Only use opus when:

• Sonnet attempt failed or produced poor results
• The task involves complex reasoning across 10+ files
• Architecture decisions or trade-off analysis is required

Warning: Opus is approximately 15x more expensive than Sonnet per token. A task that costs $0.30 with Sonnet could cost $4.50 with Opus. Voluntarily call request_user_approval before using Opus for any task expected to run more than a few turns.

Effort guidance

• low — Simple edits, typo fixes, config changes
• medium — Standard development tasks (default)
• high — Complex reasoning, multi-step problem solving

Budget cap

If the user has set maxBudgetPerSessionUsd, Markus passes it as --max-budget-usd to Claude Code. This is a hard limit enforced by Claude Code — the session terminates if the budget is reached. Claude Code is the only tool with this enforced budget mechanism.

When working under a budget cap:

• Prefer sonnet over opus to stay within budget
• Split large tasks so each invocation stays within the per-session limit
• Monitor cost.estimatedCostUsd in results to gauge remaining budget capacity

Cost Awareness

Claude Code is the most capable but potentially most expensive coding tool. It provides the best cost visibility — every result includes token counts and USD estimates:

{
  "cost": {
    "estimatedCostUsd": 0.45,
    "inputTokens": 85000,
    "outputTokens": 12000,
    "cacheReadTokens": 40000,
    "source": "tool_output"
  }
}

Cost-saving practices:

• Scope prompts tightly — avoid "refactor everything"
• Split large tasks into sequential focused invocations
• Use codex for trivial fixes instead of Claude Code
• Leverage prompt caching (repeated context in CLAUDE.md is cache-friendly)
• Review cost.estimatedCostUsd before chaining multiple invocations
• Use effort: "low" for simple edits, effort: "high" only for complex reasoning
• Prefer sonnet — only escalate to opus when justified

Report unusually high costs (> $1 per invocation) in a task note for visibility.

Best Practices

• Write prompts with explicit file boundaries and test commands
• Always pass task_id so CLAUDE.md carries full task context
• Watch progress output for file_edit events to track what's changing
• Verify result.testResult before calling coding_tool_apply
• Prefer Claude Code when the task requires reading 5+ files to understand context
• Check cost.estimatedCostUsd after each invocation and factor it into your next decision

Quality Verification Loop

For complex tasks, chain Claude Code invocations:

1. Plan: invoke_coding_tool({ tool: "claude-code", mode: "plan", prompt: "Analyze and plan..." })
2. Implement: invoke_coding_tool({ tool: "claude-code", prompt: "Implement the plan..." })
3. Verify: Check result.testResult. If failures exist, re-invoke with: "Fix these test failures: "
4. Apply: Only after tests pass — coding_tool_apply({ session_id, commit_message })

Never apply changes from a session where tests failed.

Cost Management

• Start with the default model. Only escalate to more expensive models for tasks that demonstrably need deeper reasoning.
• Check cost.estimatedCostUsd after each invocation and note it in task progress.
• For exploratory work, set a mental budget — if cost exceeds expectations, split into smaller focused invocations.

Rules

• DO use for multi-file refactors and exploratory coding
• DO monitor token/cost data in the response
• DO break very large tasks into sequential invocations
• DO start with sonnet and only escalate to opus when needed
• DO request user approval before using opus for long tasks
• DO NOT use for one-line fixes — use codex or direct edit instead
• DO NOT ignore failed tests in the result — iterate until they pass
• DO NOT use opus by default — its cost can surprise users