Skills in the API

MessagesSkills

Using Agent Skills with the API

Learn how to use Agent Skills to extend Claude's capabilities through the API.

Agent Skills extend Claude's capabilities through organized folders of instructions, scripts, and resources. This guide shows you how to use both pre-built and custom Skills with the Claude API.

For complete API reference including request/response schemas and all parameters, see:

Skill Management API Reference - CRUD operations for Skills
Skill Versions API Reference - Version management

This feature is not eligible for Zero Data Retention (ZDR). Data is retained according to the feature's standard retention policy.

Quick links

Get started with Agent Skills

Create your first Skill

Create custom Skills

Best practices for authoring Skills

Overview

For a deep dive into the architecture and real-world applications of Agent Skills, read the engineering blog post: Equipping agents for the real world with Agent Skills.

Skills integrate with the Messages API through the code execution tool. Whether using pre-built Skills managed by Anthropic or custom Skills you've uploaded, the integration shape is identical: both require code execution and use the same container structure.

Using Skills

Skills integrate identically in the Messages API regardless of source. You specify Skills in the container parameter with a skill_id, type, and optional version, and they execute in the code execution environment.

You can use Skills from two sources:

Aspect	Anthropic Skills	Custom Skills
Type value	`anthropic`	`custom`
Skill IDs	Short names: `pptx`, `xlsx`, `docx`, `pdf`	Generated: `skill_01AbCdEfGhIjKlMnOpQrStUv`
Version format	Date-based: `20251013` or `latest`	Epoch timestamp: `1759178010641129` or `latest`

Both skill sources are returned by the List Skills endpoint (use the source parameter to filter). The integration shape and execution environment are identical. The only difference is where the Skills come from and how they're managed.

Prerequisites

To use Skills, you need:

Claude API key from the Console
Beta headers:
- code-execution-2025-08-25 - Enables code execution (required for Skills)
- skills-2025-10-02 - Enables Skills API
- files-api-2025-04-14 - For uploading/downloading files to/from container
Code execution tool enabled in your requests

Using Skills in Messages

Container parameter

Skills are specified using the container parameter in the Messages API. You can include up to 8 Skills per request.

The structure is identical for both Anthropic and custom Skills. Specify the required type and skill_id, and optionally include version to pin to a specific version:

Downloading generated files

When Skills create documents (Excel, PowerPoint, PDF, Word), they return file_id attributes in the response. You must use the Files API to download these files.

How it works:

Skills create files during code execution
Response includes file_id for each created file
Use Files API to download the actual file content
Save locally or process as needed

Example: Creating and downloading an Excel file

Additional Files API operations:

For complete details on the Files API, see the Files API documentation.

Multi-turn conversations

Reuse the same container across multiple messages by specifying the container ID:

Long-running operations

Skills may perform operations that require multiple turns. Handle pause_turn stop reasons:

The response may include a pause_turn stop reason, which indicates that the API paused a long-running Skill operation. You can provide the response back as-is in a subsequent request to let Claude continue its turn, or modify the content if you wish to interrupt the conversation and provide additional guidance.

Using Multiple Skills

Combine multiple Skills in a single request to handle complex workflows:

Managing Custom Skills

Creating a Skill

A Skill bundle is a directory containing a SKILL.md file at the top level with name and description YAML frontmatter, plus any supporting scripts or resources. See Get started with Agent Skills in the API to author one, and the Requirements list following the examples for the full constraints.

Upload your custom Skill to make it available in your workspace. You can upload a zip archive or individual file objects; the Python SDK additionally provides a files_from_dir helper that accepts a directory path.

Requirements:

Must include a SKILL.md file at the top level
All files must specify a common root directory in their paths
Total upload size must be under 30 MB
YAML frontmatter requirements:
- name: Maximum 64 characters, lowercase letters/numbers/hyphens only, no XML tags, no reserved words ("anthropic", "claude")
- description: Maximum 1024 characters, non-empty, no XML tags

For complete request/response schemas, see the Create Skill API reference.

Listing Skills

Retrieve all Skills available to your workspace, including both Anthropic pre-built Skills and your custom Skills. Use the source parameter to filter by skill type:

See the List Skills API reference for pagination and filtering options.

Retrieving a Skill

Get details about a specific Skill:

Deleting a Skill

To delete a Skill, you must first delete all its versions:

Attempting to delete a Skill with existing versions returns a 400 error.

Versioning

Skills support versioning to manage updates safely:

Anthropic Skills:

Versions use date format: 20251013
New versions released as updates are made
Specify exact versions for stability

Custom Skills:

Auto-generated epoch timestamps: 1759178010641129
Use "latest" to always get the most recent version
Create new versions when updating Skill files

See the Create Skill Version API reference for complete details.

How Skills are loaded

When you specify Skills in a container:

Metadata Discovery: Claude sees metadata for each Skill (name, description) in the system prompt
File Loading: Skill files are copied into the container at /skills/{directory}/
Automatic Use: Claude automatically loads and uses Skills when relevant to your request
Composition: Multiple Skills compose together for complex workflows

The progressive disclosure architecture ensures efficient context usage: Claude only loads full Skill instructions when needed.

Use cases

Organizational Skills

Brand & Communications

Apply company-specific formatting (colors, fonts, layouts) to documents
Generate communications following organizational templates
Ensure consistent brand guidelines across all outputs

Project Management

Structure notes with company-specific formats (OKRs, decision logs)
Generate tasks following team conventions
Create standardized meeting recaps and status updates

Business Operations

Create company-standard reports, proposals, and analyses
Execute company-specific analytical procedures
Generate financial models following organizational templates

Personal Skills

Content Creation

Custom document templates
Specialized formatting and styling
Domain-specific content generation

Data Analysis

Custom data processing pipelines
Specialized visualization templates
Industry-specific analytical methods

Development & Automation

Code generation templates
Testing frameworks
Deployment workflows

Example: financial modeling

Combine Excel and custom DCF analysis Skills:

Limits and constraints

Request limits

Maximum Skills per request: 8
Maximum Skill upload size: 30 MB (all files combined)
YAML frontmatter requirements:
- name: Maximum 64 characters, lowercase letters/numbers/hyphens only, no XML tags, no reserved words ("anthropic", "claude")
- description: Maximum 1024 characters, non-empty, no XML tags

Environment constraints

Skills run in the code execution container with these limitations:

No network access: Cannot make external API calls
No runtime package installation: Only pre-installed packages available
Isolated environment: Containers are isolated; a fresh container is created unless you specify an existing container ID

See Code execution tool for available packages.

Best practices

When to use multiple Skills

Combine Skills when tasks involve multiple document types or domains:

Good use cases:

Data analysis (Excel) + presentation creation (PowerPoint)
Report generation (Word) + export to PDF
Custom domain logic + document generation

Avoid:

Including unused Skills (impacts performance)

Version management strategy

For production:

# Pin to specific versions for stability
container = {
    "skills": [
        {
            "type": "custom",
            "skill_id": "skill_01AbCdEfGhIjKlMnOpQrStUv",
            "version": "1759178010641129",  # Specific version
        }
    ]
}

For development:

# Use latest for active development
container = {
    "skills": [
        {
            "type": "custom",
            "skill_id": "skill_01AbCdEfGhIjKlMnOpQrStUv",
            "version": "latest",  # Always get newest
        }
    ]
}

Prompt caching considerations

When using prompt caching, note that changing the Skills list in your container breaks the cache:

For best caching performance, keep your Skills list consistent across requests.

Error handling

Handle Skill-related errors gracefully:

Data retention

Agent Skills are not covered by ZDR arrangements. Skill definitions and execution data are retained according to Anthropic's standard data retention policy.

For ZDR eligibility across all features, see API and data retention.

Next steps

API Reference

Complete API reference with all endpoints

Authoring Guide

Best practices for writing effective Skills

Was this page helpful?

MessagesSkills

Using Agent Skills with the API

Learn how to use Agent Skills to extend Claude's capabilities through the API.

Agent Skills extend Claude's capabilities through organized folders of instructions, scripts, and resources. This guide shows you how to use both pre-built and custom Skills with the Claude API.

For complete API reference including request/response schemas and all parameters, see:

Skill Management API Reference - CRUD operations for Skills
Skill Versions API Reference - Version management

This feature is not eligible for Zero Data Retention (ZDR). Data is retained according to the feature's standard retention policy.

Quick links

Get started with Agent Skills

Create your first Skill

Create custom Skills

Best practices for authoring Skills

Overview

For a deep dive into the architecture and real-world applications of Agent Skills, read the engineering blog post: Equipping agents for the real world with Agent Skills.

Using Skills

You can use Skills from two sources:

Aspect	Anthropic Skills	Custom Skills
Type value	`anthropic`	`custom`
Skill IDs	Short names: `pptx`, `xlsx`, `docx`, `pdf`	Generated: `skill_01AbCdEfGhIjKlMnOpQrStUv`
Version format	Date-based: `20251013` or `latest`	Epoch timestamp: `1759178010641129` or `latest`

Prerequisites

To use Skills, you need:

Claude API key from the Console
Beta headers:
- code-execution-2025-08-25 - Enables code execution (required for Skills)
- skills-2025-10-02 - Enables Skills API
- files-api-2025-04-14 - For uploading/downloading files to/from container
Code execution tool enabled in your requests

Using Skills in Messages

Container parameter

Skills are specified using the container parameter in the Messages API. You can include up to 8 Skills per request.

The structure is identical for both Anthropic and custom Skills. Specify the required type and skill_id, and optionally include version to pin to a specific version:

Downloading generated files

When Skills create documents (Excel, PowerPoint, PDF, Word), they return file_id attributes in the response. You must use the Files API to download these files.

How it works:

Skills create files during code execution
Response includes file_id for each created file
Use Files API to download the actual file content
Save locally or process as needed

Example: Creating and downloading an Excel file

Additional Files API operations:

For complete details on the Files API, see the Files API documentation.

Multi-turn conversations

Reuse the same container across multiple messages by specifying the container ID:

Long-running operations

Skills may perform operations that require multiple turns. Handle pause_turn stop reasons:

Using Multiple Skills

Combine multiple Skills in a single request to handle complex workflows:

Managing Custom Skills

Creating a Skill

Requirements:

Must include a SKILL.md file at the top level
All files must specify a common root directory in their paths
Total upload size must be under 30 MB
YAML frontmatter requirements:
- name: Maximum 64 characters, lowercase letters/numbers/hyphens only, no XML tags, no reserved words ("anthropic", "claude")
- description: Maximum 1024 characters, non-empty, no XML tags

For complete request/response schemas, see the Create Skill API reference.

Listing Skills

Retrieve all Skills available to your workspace, including both Anthropic pre-built Skills and your custom Skills. Use the source parameter to filter by skill type:

See the List Skills API reference for pagination and filtering options.

Retrieving a Skill

Get details about a specific Skill:

Deleting a Skill

To delete a Skill, you must first delete all its versions:

Attempting to delete a Skill with existing versions returns a 400 error.

Versioning

Skills support versioning to manage updates safely:

Anthropic Skills:

Versions use date format: 20251013
New versions released as updates are made
Specify exact versions for stability

Custom Skills:

Auto-generated epoch timestamps: 1759178010641129
Use "latest" to always get the most recent version
Create new versions when updating Skill files

See the Create Skill Version API reference for complete details.

How Skills are loaded

When you specify Skills in a container:

Metadata Discovery: Claude sees metadata for each Skill (name, description) in the system prompt
File Loading: Skill files are copied into the container at /skills/{directory}/
Automatic Use: Claude automatically loads and uses Skills when relevant to your request
Composition: Multiple Skills compose together for complex workflows

The progressive disclosure architecture ensures efficient context usage: Claude only loads full Skill instructions when needed.

Use cases

Organizational Skills

Brand & Communications

Apply company-specific formatting (colors, fonts, layouts) to documents
Generate communications following organizational templates
Ensure consistent brand guidelines across all outputs

Project Management

Structure notes with company-specific formats (OKRs, decision logs)
Generate tasks following team conventions
Create standardized meeting recaps and status updates

Business Operations

Create company-standard reports, proposals, and analyses
Execute company-specific analytical procedures
Generate financial models following organizational templates

Personal Skills

Content Creation

Custom document templates
Specialized formatting and styling
Domain-specific content generation

Data Analysis

Custom data processing pipelines
Specialized visualization templates
Industry-specific analytical methods

Development & Automation

Code generation templates
Testing frameworks
Deployment workflows

Example: financial modeling

Combine Excel and custom DCF analysis Skills:

Limits and constraints

Request limits

Maximum Skills per request: 8
Maximum Skill upload size: 30 MB (all files combined)
YAML frontmatter requirements:
- name: Maximum 64 characters, lowercase letters/numbers/hyphens only, no XML tags, no reserved words ("anthropic", "claude")
- description: Maximum 1024 characters, non-empty, no XML tags

Environment constraints

Skills run in the code execution container with these limitations:

No network access: Cannot make external API calls
No runtime package installation: Only pre-installed packages available
Isolated environment: Containers are isolated; a fresh container is created unless you specify an existing container ID

See Code execution tool for available packages.

Best practices

When to use multiple Skills

Combine Skills when tasks involve multiple document types or domains:

Good use cases:

Data analysis (Excel) + presentation creation (PowerPoint)
Report generation (Word) + export to PDF
Custom domain logic + document generation

Avoid:

Including unused Skills (impacts performance)

Version management strategy

For production:

# Pin to specific versions for stability
container = {
    "skills": [
        {
            "type": "custom",
            "skill_id": "skill_01AbCdEfGhIjKlMnOpQrStUv",
            "version": "1759178010641129",  # Specific version
        }
    ]
}

For development:

# Use latest for active development
container = {
    "skills": [
        {
            "type": "custom",
            "skill_id": "skill_01AbCdEfGhIjKlMnOpQrStUv",
            "version": "latest",  # Always get newest
        }
    ]
}

Prompt caching considerations

When using prompt caching, note that changing the Skills list in your container breaks the cache:

For best caching performance, keep your Skills list consistent across requests.

Error handling

Handle Skill-related errors gracefully:

Data retention

Agent Skills are not covered by ZDR arrangements. Skill definitions and execution data are retained according to Anthropic's standard data retention policy.

For ZDR eligibility across all features, see API and data retention.

Next steps

API Reference

Complete API reference with all endpoints

Authoring Guide

Best practices for writing effective Skills

Was this page helpful?

client = anthropic.Anthropic()

response = client.beta.messages.create(
    model="claude-opus-4-7",
    max_tokens=4096,
    betas=["code-execution-2025-08-25", "skills-2025-10-02"],
    container={
        "skills": [{"type": "anthropic", "skill_id": "pptx", "version": "latest"}]
    },
    messages=[
        {"role": "user", "content": "Create a presentation about renewable energy"}
    ],
    tools=[{"type": "code_execution_20250825", "name": "code_execution"}],
)

client = anthropic.Anthropic()

# Step 1: Use a Skill to create a file
response = client.beta.messages.create(
    model="claude-opus-4-7",
    max_tokens=4096,
    betas=["code-execution-2025-08-25", "skills-2025-10-02"],
    container={
        "skills": [{"type": "anthropic", "skill_id": "xlsx", "version": "latest"}]
    },
    messages=[
        {
            "role": "user",
            "content": "Create an Excel file with a simple budget spreadsheet",
        }
    ],
    tools=[{"type": "code_execution_20250825", "name": "code_execution"}],
)


# Step 2: Extract file IDs from the response
def extract_file_ids(response):
    file_ids = []
    for item in response.content:
        if item.type == "bash_code_execution_tool_result":
            content_item = item.content
            if content_item.type == "bash_code_execution_result":
                # concrete-typed list: List[BashCodeExecutionOutputBlock]
                for file in content_item.content:
                    file_ids.append(file.file_id)
    return file_ids


# Step 3: Download the file using Files API
for file_id in extract_file_ids(response):
    file_metadata = client.beta.files.retrieve_metadata(file_id=file_id)
    file_content = client.beta.files.download(file_id=file_id)

    # Step 4: Save to disk
    file_content.write_to_file(file_metadata.filename)
    print(f"Downloaded: {file_metadata.filename}")

client = anthropic.Anthropic()
file_id = "file_abc123"
# Get file metadata
file_info = client.beta.files.retrieve_metadata(file_id=file_id)
print(f"Filename: {file_info.filename}, Size: {file_info.size_bytes} bytes")

# List all files
files = client.beta.files.list()
for file in files.data:
    print(f"{file.filename} - {file.created_at}")

# Delete a file
client.beta.files.delete(file_id=file_id)

# First request creates container
response1 = client.beta.messages.create(
    model="claude-opus-4-7",
    max_tokens=4096,
    betas=["code-execution-2025-08-25", "skills-2025-10-02"],
    container={
        "skills": [{"type": "anthropic", "skill_id": "xlsx", "version": "latest"}]
    },
    messages=[{"role": "user", "content": "Analyze this sales data"}],
    tools=[{"type": "code_execution_20250825", "name": "code_execution"}],
)

# Continue conversation with same container
messages = [
    {"role": "user", "content": "Analyze this sales data"},
    {"role": "assistant", "content": response1.content},
    {"role": "user", "content": "What was the total revenue?"},
]

response2 = client.beta.messages.create(
    model="claude-opus-4-7",
    max_tokens=4096,
    betas=["code-execution-2025-08-25", "skills-2025-10-02"],
    container={
        "id": response1.container.id,  # Reuse container
        "skills": [{"type": "anthropic", "skill_id": "xlsx", "version": "latest"}],
    },
    messages=messages,
    tools=[{"type": "code_execution_20250825", "name": "code_execution"}],
)

messages = [{"role": "user", "content": "Process this large dataset"}]
max_retries = 10

response = client.beta.messages.create(
    model="claude-opus-4-7",
    max_tokens=4096,
    betas=["code-execution-2025-08-25", "skills-2025-10-02"],
    container={
        "skills": [
            {
                "type": "custom",
                "skill_id": "skill_01AbCdEfGhIjKlMnOpQrStUv",
                "version": "latest",
            }
        ]
    },
    messages=messages,
    tools=[{"type": "code_execution_20250825", "name": "code_execution"}],
)

# Handle pause_turn for long operations
for i in range(max_retries):
    if response.stop_reason != "pause_turn":
        break

    messages.append({"role": "assistant", "content": response.content})
    response = client.beta.messages.create(
        model="claude-opus-4-7",
        max_tokens=4096,
        betas=["code-execution-2025-08-25", "skills-2025-10-02"],
        container={
            "id": response.container.id,
            "skills": [
                {
                    "type": "custom",
                    "skill_id": "skill_01AbCdEfGhIjKlMnOpQrStUv",
                    "version": "latest",
                }
            ],
        },
        messages=messages,
        tools=[{"type": "code_execution_20250825", "name": "code_execution"}],
    )

response = client.beta.messages.create(
    model="claude-opus-4-7",
    max_tokens=4096,
    betas=["code-execution-2025-08-25", "skills-2025-10-02"],
    container={
        "skills": [
            {"type": "anthropic", "skill_id": "xlsx", "version": "latest"},
            {"type": "anthropic", "skill_id": "pptx", "version": "latest"},
            {
                "type": "custom",
                "skill_id": "skill_01AbCdEfGhIjKlMnOpQrStUv",
                "version": "latest",
            },
        ]
    },
    messages=[
        {"role": "user", "content": "Analyze sales data and create a presentation"}
    ],
    tools=[{"type": "code_execution_20250825", "name": "code_execution"}],
)

# Option 1: Upload individual files (one --file flag per file)
ant beta:skills create \
  --display-title "Financial Analysis" \
  --file financial_skill/SKILL.md \
  --file financial_skill/analyze.py \
  --beta skills-2025-10-02

# Option 2: Upload a zip archive
ant beta:skills create \
  --display-title "Financial Analysis" \
  --file financial_analysis_skill.zip \
  --beta skills-2025-10-02

# Step 1: Delete all versions
ant beta:skills:versions list \
  --skill-id skill_01AbCdEfGhIjKlMnOpQrStUv \
  --transform version --raw-output \
  | while read -r VERSION; do
      ant beta:skills:versions delete \
        --skill-id skill_01AbCdEfGhIjKlMnOpQrStUv \
        --version "$VERSION" >/dev/null
    done

# Step 2: Delete the Skill
ant beta:skills delete \
  --skill-id skill_01AbCdEfGhIjKlMnOpQrStUv >/dev/null

# Create a new version
VERSION_NUMBER=$(ant beta:skills:versions create \
  --skill-id skill_01AbCdEfGhIjKlMnOpQrStUv \
  --file updated_skill/SKILL.md \
  --transform version --raw-output)

# Use specific version
ant beta:messages create \
  --beta code-execution-2025-08-25 \
  --beta skills-2025-10-02 <<YAML
model: claude-opus-4-7
max_tokens: 4096
container:
  skills:
    - type: custom
      skill_id: skill_01AbCdEfGhIjKlMnOpQrStUv
      version: $VERSION_NUMBER
messages:
  - role: user
    content: Use updated Skill
tools:
  - type: code_execution_20250825
    name: code_execution
YAML

# Use latest version
ant beta:messages create \
  --beta code-execution-2025-08-25 \
  --beta skills-2025-10-02 <<'YAML'
model: claude-opus-4-7
max_tokens: 4096
container:
  skills:
    - type: custom
      skill_id: skill_01AbCdEfGhIjKlMnOpQrStUv
      version: latest
messages:
  - role: user
    content: Use latest Skill version
tools:
  - type: code_execution_20250825
    name: code_execution
YAML

# Create custom DCF analysis Skill
from anthropic.lib import files_from_dir

dcf_skill = client.beta.skills.create(
    display_title="DCF Analysis",
    files=files_from_dir("/path/to/dcf_skill"),
)

# Use with Excel to create financial model
response = client.beta.messages.create(
    model="claude-opus-4-7",
    max_tokens=4096,
    betas=["code-execution-2025-08-25", "skills-2025-10-02"],
    container={
        "skills": [
            {"type": "anthropic", "skill_id": "xlsx", "version": "latest"},
            {"type": "custom", "skill_id": dcf_skill.id, "version": "latest"},
        ]
    },
    messages=[
        {
            "role": "user",
            "content": "Build a DCF valuation model for a SaaS company with the attached financials",
        }
    ],
    tools=[{"type": "code_execution_20250825", "name": "code_execution"}],
)
print(response)

# First request creates cache
response1 = client.beta.messages.create(
    model="claude-opus-4-7",
    max_tokens=4096,
    betas=[
        "code-execution-2025-08-25",
        "skills-2025-10-02",
        "prompt-caching-2024-07-31",
    ],
    container={
        "skills": [{"type": "anthropic", "skill_id": "xlsx", "version": "latest"}]
    },
    messages=[{"role": "user", "content": "Analyze sales data"}],
    tools=[{"type": "code_execution_20250825", "name": "code_execution"}],
)

# Adding/removing Skills breaks cache
response2 = client.beta.messages.create(
    model="claude-opus-4-7",
    max_tokens=4096,
    betas=[
        "code-execution-2025-08-25",
        "skills-2025-10-02",
        "prompt-caching-2024-07-31",
    ],
    container={
        "skills": [
            {"type": "anthropic", "skill_id": "xlsx", "version": "latest"},
            {
                "type": "anthropic",
                "skill_id": "pptx",
                "version": "latest",
            },  # Cache miss
        ]
    },
    messages=[{"role": "user", "content": "Create a presentation"}],
    tools=[{"type": "code_execution_20250825", "name": "code_execution"}],
)

client = anthropic.Anthropic()

try:
    response = client.beta.messages.create(
        model="claude-opus-4-7",
        max_tokens=4096,
        betas=["code-execution-2025-08-25", "skills-2025-10-02"],
        container={
            "skills": [
                {
                    "type": "custom",
                    "skill_id": "skill_01AbCdEfGhIjKlMnOpQrStUv",
                    "version": "latest",
                }
            ]
        },
        messages=[{"role": "user", "content": "Process data"}],
        tools=[{"type": "code_execution_20250825", "name": "code_execution"}],
    )
except anthropic.BadRequestError as e:
    if "skill" in str(e):
        print(f"Skill error: {e}")
        # Handle skill-specific errors
    else:
        raise