Headless Mode

Headless mode allows you to run Gemini CLI programmatically from command line scripts and automation tools without any interactive UI. This is ideal for scripting, automation, CI/CD pipelines, and building AI-powered tools.

Headless Mode

Overview

The headless mode provides a headless interface to Gemini CLI that:

Accepts prompts via command line arguments or stdin
Returns structured output (text or JSON)
Supports file redirection and piping
Enables automation and scripting workflows
Provides consistent exit codes for error handling

Basic Usage

Direct Prompts

Use the --prompt (or -p) flag to run in headless mode:

gemini --prompt "What is machine learning?"

Stdin Input

Pipe input to Gemini CLI from your terminal:

echo "Explain this code" | gemini

Combining with File Input

Read from files and process with Gemini:

cat README.md | gemini --prompt "Summarize this documentation"

Output Formats

Text Output (Default)

Standard human-readable output:

gemini -p "What is the capital of France?"

Response format:

The capital of France is Paris.

JSON Output

Returns structured data including response, statistics, and metadata. This format is ideal for programmatic processing and automation scripts.

Response Schema

The JSON output follows this high-level structure:

{
  "response": "string", // The main AI-generated content answering your prompt
  "stats": {
    // Usage metrics and performance data
    "models": {
      // Per-model API and token usage statistics
      "[model-name]": {
        "api": {
          /* request counts, errors, latency */
        },
        "tokens": {
          /* prompt, response, cached, total counts */
        }
      }
    },
    "tools": {
      // Tool execution statistics
      "totalCalls": "number",
      "totalSuccess": "number",
      "totalFail": "number",
      "totalDurationMs": "number",
      "totalDecisions": {
        /* accept, reject, modify, auto_accept counts */
      },
      "byName": {
        /* per-tool detailed stats */
      }
    },
    "files": {
      // File modification statistics
      "totalLinesAdded": "number",
      "totalLinesRemoved": "number"
    }
  },
  "error": {
    // Present only when an error occurred
    "type": "string", // Error type (e.g., "ApiError", "AuthError")
    "message": "string", // Human-readable error description
    "code": "number" // Optional error code
  }
}

Example Usage

gemini -p "What is the capital of France?" --output-format json

Response:

{
  "response": "The capital of France is Paris.",
  "stats": {
    "models": {
      "gemini-2.5-pro": {
        "api": {
          "totalRequests": 2,
          "totalErrors": 0,
          "totalLatencyMs": 5053
        },
        "tokens": {
          "prompt": 24939,
          "candidates": 20,
          "total": 25113,
          "cached": 21263,
          "thoughts": 154,
          "tool": 0
        }
      },
      "gemini-2.5-flash": {
        "api": {
          "totalRequests": 1,
          "totalErrors": 0,
          "totalLatencyMs": 1879
        },
        "tokens": {
          "prompt": 8965,
          "candidates": 10,
          "total": 9033,
          "cached": 0,
          "thoughts": 30,
          "tool": 28
        }
      }
    },
    "tools": {
      "totalCalls": 1,
      "totalSuccess": 1,
      "totalFail": 0,
      "totalDurationMs": 1881,
      "totalDecisions": {
        "accept": 0,
        "reject": 0,
        "modify": 0,
        "auto_accept": 1
      },
      "byName": {
        "google_web_search": {
          "count": 1,
          "success": 1,
          "fail": 0,
          "durationMs": 1881,
          "decisions": {
            "accept": 0,
            "reject": 0,
            "modify": 0,
            "auto_accept": 1
          }
        }
      }
    },
    "files": {
      "totalLinesAdded": 0,
      "totalLinesRemoved": 0
    }
  }
}

Streaming JSON Output

Returns real-time events as newline-delimited JSON (JSONL). Each significant action (initialization, messages, tool calls, results) emits immediately as it occurs. This format is ideal for monitoring long-running operations, building UIs with live progress, and creating automation pipelines that react to events.

When to Use Streaming JSON

Use --output-format stream-json when you need:

Real-time progress monitoring - See tool calls and responses as they happen
Event-driven automation - React to specific events (e.g., tool failures)
Live UI updates - Build interfaces showing AI agent activity in real-time
Detailed execution logs - Capture complete interaction history with timestamps
Pipeline integration - Stream events to logging/monitoring systems

Event Types

The streaming format emits 6 event types:

init - Session starts (includes session_id, model)
message - User prompts and assistant responses
tool_use - Tool call requests with parameters
tool_result - Tool execution results (success/error)
error - Non-fatal errors and warnings
result - Final session outcome with aggregated stats

Basic Usage

# Stream events to console
gemini --output-format stream-json --prompt "What is 2+2?"

# Save event stream to file
gemini --output-format stream-json --prompt "Analyze this code" > events.jsonl

# Parse with jq
gemini --output-format stream-json --prompt "List files" | jq -r '.type'

Example Output

Each line is a complete JSON event:

{"type":"init","timestamp":"2025-10-10T12:00:00.000Z","session_id":"abc123","model":"gemini-2.0-flash-exp"}
{"type":"message","role":"user","content":"List files in current directory","timestamp":"2025-10-10T12:00:01.000Z"}
{"type":"tool_use","tool_name":"Bash","tool_id":"bash-123","parameters":{"command":"ls -la"},"timestamp":"2025-10-10T12:00:02.000Z"}
{"type":"tool_result","tool_id":"bash-123","status":"success","output":"file1.txt\nfile2.txt","timestamp":"2025-10-10T12:00:03.000Z"}
{"type":"message","role":"assistant","content":"Here are the files...","delta":true,"timestamp":"2025-10-10T12:00:04.000Z"}
{"type":"result","status":"success","stats":{"total_tokens":250,"input_tokens":50,"output_tokens":200,"duration_ms":3000,"tool_calls":1},"timestamp":"2025-10-10T12:00:05.000Z"}

File Redirection

Save output to files or pipe to other commands:

# Save to file
gemini -p "Explain Docker" > docker-explanation.txt
gemini -p "Explain Docker" --output-format json > docker-explanation.json

# Append to file
gemini -p "Add more details" >> docker-explanation.txt

# Pipe to other tools
gemini -p "What is Kubernetes?" --output-format json | jq '.response'
gemini -p "Explain microservices" | wc -w
gemini -p "List programming languages" | grep -i "python"

Configuration Options

Key command-line options for headless usage:

Option	Description	Example
`--prompt`, `-p`	Run in headless mode	`gemini -p "query"`
`--output-format`	Specify output format (text, json)	`gemini -p "query" --output-format json`
`--model`, `-m`	Specify the Gemini model	`gemini -p "query" -m gemini-2.5-flash`
`--debug`, `-d`	Enable debug mode	`gemini -p "query" --debug`
`--include-directories`	Include additional directories	`gemini -p "query" --include-directories src,docs`
`--yolo`, `-y`	Auto-approve all actions	`gemini -p "query" --yolo`
`--approval-mode`	Set approval mode	`gemini -p "query" --approval-mode auto_edit`

For complete details on all available configuration options, settings files, and environment variables, see the Configuration Guide.

Examples

Code review

cat src/auth.py | gemini -p "Review this authentication code for security issues" > security-review.txt

Generate commit messages

result=$(git diff --cached | gemini -p "Write a concise commit message for these changes" --output-format json)
echo "$result" | jq -r '.response'

API documentation

result=$(cat api/routes.js | gemini -p "Generate OpenAPI spec for these routes" --output-format json)
echo "$result" | jq -r '.response' > openapi.json

Batch code analysis

for file in src/*.py; do
    echo "Analyzing $file..."
    result=$(cat "$file" | gemini -p "Find potential bugs and suggest improvements" --output-format json)
    echo "$result" | jq -r '.response' > "reports/$(basename "$file").analysis"
    echo "Completed analysis for $(basename "$file")" >> reports/progress.log
done

Code review

result=$(git diff origin/main...HEAD | gemini -p "Review these changes for bugs, security issues, and code quality" --output-format json)
echo "$result" | jq -r '.response' > pr-review.json

Log analysis

grep "ERROR" /var/log/app.log | tail -20 | gemini -p "Analyze these errors and suggest root cause and fixes" > error-analysis.txt

Release notes generation

result=$(git log --oneline v1.0.0..HEAD | gemini -p "Generate release notes from these commits" --output-format json)
response=$(echo "$result" | jq -r '.response')
echo "$response"
echo "$response" >> CHANGELOG.md

Model and tool usage tracking

result=$(gemini -p "Explain this database schema" --include-directories db --output-format json)
total_tokens=$(echo "$result" | jq -r '.stats.models // {} | to_entries | map(.value.tokens.total) | add // 0')
models_used=$(echo "$result" | jq -r '.stats.models // {} | keys | join(", ") | if . == "" then "none" else . end')
tool_calls=$(echo "$result" | jq -r '.stats.tools.totalCalls // 0')
tools_used=$(echo "$result" | jq -r '.stats.tools.byName // {} | keys | join(", ") | if . == "" then "none" else . end')
echo "$(date): $total_tokens tokens, $tool_calls tool calls ($tools_used) used with models: $models_used" >> usage.log
echo "$result" | jq -r '.response' > schema-docs.md
echo "Recent usage trends:"
tail -5 usage.log

Resources

CLI Configuration - Complete configuration guide
Authentication - Setup authentication
Commands - Interactive commands reference
Tutorials - Step-by-step automation guides