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Cecil Theme Builder · cecilapp | DecimalAI

Frontendby cecilapp

Cecil Theme Builder

Create, structure and package themes for Cecil (visual themes and component themes), following the conventions of the official Cecilapp/theme-* repositories.

Never made a case worseMaintained

SkillScore

SkillScore100/100

Test results 100

Safety

3/3 safety checks passed

Code scan
Intent
Content

What it changes

27%100%of test cases passed +73 pts

Without skill6 of 22

With skill22 of 22

16 fixed6 passed either way0 still failing0 broke

Measured on gemini-3.8-flash11/2/2026v4

Track record

Agent runs: 0; All time

See one task, with and without the skill

One test taskAccording to the coding conventions for Cecil themes, should Twig template files terminate with a trailing newline at the end of the file?

Without the skillMissed

With the skillPassed

Same model, same task. Answers are capped and redacted.

Use this skill

Your agent runs on

Adds every file the skill has, pinned so you can update it. No account needed.

pip install "decimalai>=0.15"
decimalai skills add cecilapp-cecil-theme
claude --plugin-dir "$(git rev-parse --show-toplevel 2>/dev/null || pwd)/.agents"

The same install from Python, for a notebook or a setup script.

Install

pip install "decimalai>=0.15"

Add the skill

from decimalai import skillpkg

# Writes .agents/skills and .decimal/skills.toml. Safe to run twice.
result = skillpkg.add("cecilapp-cecil-theme")
if result.exit_code:
    raise SystemExit(result.problems)

print("Added. Start Claude Code with the command below.")

Start Claude Code with it

claude --plugin-dir "$(git rev-parse --show-toplevel 2>/dev/null || pwd)/.agents"

Type these in a Claude Code session. No account or API key. It writes the skill and any files it bundles, and asks first when its safety rating isn't Passed.

Add the DecimalAI plugin, once

/plugin marketplace add decimal-labs/decimalai-claude-plugins
/plugin install decimalai@decimalai

Install this skill

/decimalai:install cecilapp-cecil-theme

The plugin needs python3 and curl on your machine.

No Python or account. Claude Code picks it up in its next session.

Save it into your project

curl -fsSL --create-dirs -o .claude/skills/cecilapp-cecil-theme/SKILL.md https://app.decimal.ai/s/cecilapp-cecil-theme/SKILL.md

Copy SKILL.md

Save it in your project, as

.claude/skills/cecilapp-cecil-theme/SKILL.md

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

Claude Code docs

agent_config() hands your agent the skill. With no DecimalAI key it reads your project's files and sends nothing. Needs SDK 0.27 or newer.

Install

pip install "decimalai[claude-agent-sdk]"
decimalai skills add cecilapp-cecil-theme

Add it to your agent

import asyncio

from claude_agent_sdk import ClaudeAgentOptions
from decimalai.claude_agent_sdk import agent_config

# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")

async def main():
    question = "Hello!"
    async with support.run(question) as run:  # one user message, one run
        async for message in run.query(options=ClaudeAgentOptions(max_turns=6)):
            print(message)

# In a notebook or async code: await main()
asyncio.run(main())

Prefer its own loader? ClaudeAgentOptions(plugins=[skills_plugin()], skills="all") reads the same folder. Use one or the other: a skill delivered both ways reaches the prompt twice.

The same install from Python, for a notebook or a setup script. agent_config() hands your agent the skill. With no DecimalAI key it reads your project's files and sends nothing. Needs SDK 0.27 or newer.

Install

pip install "decimalai[claude-agent-sdk]"

Add it to your agent

from decimalai import skillpkg

# Writes .agents/skills and .decimal/skills.toml. Safe to run twice.
result = skillpkg.add("cecilapp-cecil-theme")
if result.exit_code:
    raise SystemExit(result.problems)

import asyncio

from claude_agent_sdk import ClaudeAgentOptions
from decimalai.claude_agent_sdk import agent_config

# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")

async def main():
    question = "Hello!"
    async with support.run(question) as run:  # one user message, one run
        async for message in run.query(options=ClaudeAgentOptions(max_turns=6)):
            print(message)

# In a notebook or async code: await main()
asyncio.run(main())

Prefer its own loader? ClaudeAgentOptions(plugins=[skills_plugin()], skills="all") reads the same folder. Use one or the other: a skill delivered both ways reaches the prompt twice.

No DecimalAI package. The whole skill goes in every prompt, and nothing pins or updates it.

Copy SKILL.md

Save it in your project, as

.agents/skills/cecilapp-cecil-theme/SKILL.md

Give it to your agent

import asyncio
from pathlib import Path
from claude_agent_sdk import ClaudeAgentOptions, query

skill = Path(".agents/skills/cecilapp-cecil-theme/SKILL.md").read_text()
options = ClaudeAgentOptions(system_prompt=f"You are a helpful agent.\n\n{skill}", max_turns=6)

async def main():
    async for message in query(prompt="Hello!", options=options):
        print(message)

# In a notebook or async code: await main()
asyncio.run(main())

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

Claude Agent SDK docs

agent_config() hands your agent the skill. With no DecimalAI key it reads your project's files and sends nothing. Needs SDK 0.27 or newer.

Install

pip install "decimalai[openai-agents]"
decimalai skills add cecilapp-cecil-theme

Add it to your agent

from agents import Agent, Runner
from decimalai.openai_agents import agent_config

# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")

agent = Agent(
    name=support.name,
    instructions=support.instructions,  # your prompt, then the skill list
    tools=[*support.tools],             # your tools, then load_skill
)
# In a notebook or async code: result = await Runner.run(agent, "Hello!")
result = Runner.run_sync(agent, "Hello!")

Using SandboxAgent? capabilities=[Shell(), decimalai.openai_agents.sandbox_skills()] points its own loader at the same folder.

Install

pip install "decimalai[openai-agents]"

Add it to your agent

from decimalai import skillpkg

# Writes .agents/skills and .decimal/skills.toml. Safe to run twice.
result = skillpkg.add("cecilapp-cecil-theme")
if result.exit_code:
    raise SystemExit(result.problems)

from agents import Agent, Runner
from decimalai.openai_agents import agent_config

# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")

agent = Agent(
    name=support.name,
    instructions=support.instructions,  # your prompt, then the skill list
    tools=[*support.tools],             # your tools, then load_skill
)
# In a notebook or async code: result = await Runner.run(agent, "Hello!")
result = Runner.run_sync(agent, "Hello!")

Using SandboxAgent? capabilities=[Shell(), decimalai.openai_agents.sandbox_skills()] points its own loader at the same folder.

No DecimalAI package. The whole skill goes in every prompt, and nothing pins or updates it.

Copy SKILL.md

Save it in your project, as

.agents/skills/cecilapp-cecil-theme/SKILL.md

Give it to your agent

from pathlib import Path
from agents import Agent, Runner

skill = Path(".agents/skills/cecilapp-cecil-theme/SKILL.md").read_text()
agent = Agent(name="support", instructions=f"You are a helpful agent.\n\n{skill}")
# In a notebook or async code: result = await Runner.run(agent, "Hello!")
result = Runner.run_sync(agent, "Hello!")

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

OpenAI Agents docs

agent_config() hands your agent the skill. With no DecimalAI key it reads your project's files and sends nothing. Needs SDK 0.27 or newer.

Install

pip install "decimalai[openai]"
decimalai skills add cecilapp-cecil-theme

Add it to your agent

import json

from openai import OpenAI
from decimalai.openai import agent_config

client = OpenAI()
# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")


def lookup_order(order_id: str) -> str:
    return f"Order {order_id} shipped yesterday."


# Your tools, as Chat Completions takes them, and what answers each.
TOOLS = [{"type": "function", "function": {
    "name": "lookup_order",
    "description": "Look up the status of an order.",
    "parameters": {"type": "object", "properties": {"order_id": {"type": "string"}},
                   "required": ["order_id"]},
}}]
FUNCTIONS = {"lookup_order": lookup_order}

question = "Where is order A1001?"
with support.run(question) as run:  # one user message, one run
    # run.system is your prompt, then the skill list.
    messages = [{"role": "system", "content": run.system}, run.message]
    while True:
        reply = client.chat.completions.create(
            model="gpt-5.4-mini",
            messages=messages,
            tools=[*TOOLS, *run.tools],  # your tools, then load_skill
        ).choices[0].message
        messages.append(reply)
        if not reply.tool_calls:
            break
        for call in reply.tool_calls:
            # load_skill is answered by the run; your own tools by FUNCTIONS.
            function = FUNCTIONS.get(call.function.name)
            messages.append(run.tool_result(call) or {
                "role": "tool", "tool_call_id": call.id,
                "content": function(**json.loads(call.function.arguments)) if function
                else f"There is no tool named {call.function.name!r}.",
            })
print(reply.content)

# The Responses API takes run.response_tools instead of run.tools.

Install

pip install "decimalai[openai]"

Add it to your agent

from decimalai import skillpkg

# Writes .agents/skills and .decimal/skills.toml. Safe to run twice.
result = skillpkg.add("cecilapp-cecil-theme")
if result.exit_code:
    raise SystemExit(result.problems)

import json

from openai import OpenAI
from decimalai.openai import agent_config

client = OpenAI()
# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")


def lookup_order(order_id: str) -> str:
    return f"Order {order_id} shipped yesterday."


# Your tools, as Chat Completions takes them, and what answers each.
TOOLS = [{"type": "function", "function": {
    "name": "lookup_order",
    "description": "Look up the status of an order.",
    "parameters": {"type": "object", "properties": {"order_id": {"type": "string"}},
                   "required": ["order_id"]},
}}]
FUNCTIONS = {"lookup_order": lookup_order}

question = "Where is order A1001?"
with support.run(question) as run:  # one user message, one run
    # run.system is your prompt, then the skill list.
    messages = [{"role": "system", "content": run.system}, run.message]
    while True:
        reply = client.chat.completions.create(
            model="gpt-5.4-mini",
            messages=messages,
            tools=[*TOOLS, *run.tools],  # your tools, then load_skill
        ).choices[0].message
        messages.append(reply)
        if not reply.tool_calls:
            break
        for call in reply.tool_calls:
            # load_skill is answered by the run; your own tools by FUNCTIONS.
            function = FUNCTIONS.get(call.function.name)
            messages.append(run.tool_result(call) or {
                "role": "tool", "tool_call_id": call.id,
                "content": function(**json.loads(call.function.arguments)) if function
                else f"There is no tool named {call.function.name!r}.",
            })
print(reply.content)

# The Responses API takes run.response_tools instead of run.tools.

No DecimalAI package. The whole skill goes in every prompt, and nothing pins or updates it.

Copy SKILL.md

Save it in your project, as

.agents/skills/cecilapp-cecil-theme/SKILL.md

Give it to your agent

from pathlib import Path
from openai import OpenAI

client = OpenAI()
skill = Path(".agents/skills/cecilapp-cecil-theme/SKILL.md").read_text()
reply = client.chat.completions.create(
    model="gpt-5.4-mini",
    messages=[
        {"role": "system", "content": f"You are a helpful agent.\n\n{skill}"},
        {"role": "user", "content": "Hello!"},
    ],
).choices[0].message

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

OpenAI SDK docs

agent_config() hands your agent the skill. With no DecimalAI key it reads your project's files and sends nothing. Needs SDK 0.27 or newer.

Install

pip install "decimalai[anthropic]"
decimalai skills add cecilapp-cecil-theme

Add it to your agent

import anthropic
from decimalai.anthropic import agent_config

client = anthropic.Anthropic()
# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")

question = "Hello!"
with support.run(question) as run:  # one user message, one run
    answer = client.beta.messages.tool_runner(
        model="claude-haiku-5-5",
        max_tokens=4096,
        system=run.system,     # your prompt, then the skill list
        tools=[*run.tools],    # your tools, then load_skill
        messages=[{"role": "user", "content": question}],
    ).until_done()

Install

pip install "decimalai[anthropic]"

Add it to your agent

from decimalai import skillpkg

# Writes .agents/skills and .decimal/skills.toml. Safe to run twice.
result = skillpkg.add("cecilapp-cecil-theme")
if result.exit_code:
    raise SystemExit(result.problems)

import anthropic
from decimalai.anthropic import agent_config

client = anthropic.Anthropic()
# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")

question = "Hello!"
with support.run(question) as run:  # one user message, one run
    answer = client.beta.messages.tool_runner(
        model="claude-haiku-5-5",
        max_tokens=4096,
        system=run.system,     # your prompt, then the skill list
        tools=[*run.tools],    # your tools, then load_skill
        messages=[{"role": "user", "content": question}],
    ).until_done()

No DecimalAI package. The whole skill goes in every prompt, and nothing pins or updates it.

Copy SKILL.md

Save it in your project, as

.agents/skills/cecilapp-cecil-theme/SKILL.md

Give it to your agent

from pathlib import Path
import anthropic

client = anthropic.Anthropic()
skill = Path(".agents/skills/cecilapp-cecil-theme/SKILL.md").read_text()
reply = client.messages.create(
    model="claude-haiku-5-5",
    max_tokens=4096,
    system=f"You are a helpful agent.\n\n{skill}",
    messages=[{"role": "user", "content": "Hello!"}],
)

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

Anthropic docs

agent_config() hands your agent the skill. With no DecimalAI key it reads your project's files and sends nothing. Needs SDK 0.27 or newer.

Install

pip install "decimalai[google-genai]"
decimalai skills add cecilapp-cecil-theme

Add it to your agent

from google import genai
from google.genai import types
from decimalai.google_genai import agent_config

client = genai.Client()
# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")


def lookup_order(order_id: str) -> str:
    return f"Order {order_id} shipped yesterday."


# Your functions, as declarations for the model, and what answers each.
TOOLS = [{"function_declarations": [{
    "name": "lookup_order",
    "description": "Look up the status of an order.",
    "parameters_json_schema": {"type": "object", "properties": {"order_id": {"type": "string"}},
                               "required": ["order_id"]},
}]}]
FUNCTIONS = {"lookup_order": lookup_order}

question = "Where is order A1001?"
with support.run(question) as run:  # one user message, one run
    config = types.GenerateContentConfig(
        system_instruction=run.system,     # your prompt, then the skill list
        tools=[*TOOLS, *run.tool_params],  # your functions, then load_skill
    )
    contents = [{"role": "user", "parts": [{"text": run.message}]}]
    while True:
        response = client.models.generate_content(
            model="gemini-3.5-flash", contents=contents, config=config,
        )
        if not response.function_calls:
            break
        contents.append(response.candidates[0].content)
        parts = []
        for call in response.function_calls:
            # load_skill is answered by the run; your own functions by FUNCTIONS.
            # A loop of your own rather than automatic function calling, which
            # raises KeyError when the model asks for a function you did not pass.
            function = FUNCTIONS.get(call.name)
            parts.append(run.tool_result(call) or {"function_response": {
                "name": call.name, "id": call.id,
                "response": {"result": function(**(call.args or {})) if function
                             else f"There is no function named {call.name!r}."},
            }})
        contents.append({"role": "user", "parts": parts})
print(response.text)

Install

pip install "decimalai[google-genai]"

Add it to your agent

from decimalai import skillpkg

# Writes .agents/skills and .decimal/skills.toml. Safe to run twice.
result = skillpkg.add("cecilapp-cecil-theme")
if result.exit_code:
    raise SystemExit(result.problems)

from google import genai
from google.genai import types
from decimalai.google_genai import agent_config

client = genai.Client()
# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")


def lookup_order(order_id: str) -> str:
    return f"Order {order_id} shipped yesterday."


# Your functions, as declarations for the model, and what answers each.
TOOLS = [{"function_declarations": [{
    "name": "lookup_order",
    "description": "Look up the status of an order.",
    "parameters_json_schema": {"type": "object", "properties": {"order_id": {"type": "string"}},
                               "required": ["order_id"]},
}]}]
FUNCTIONS = {"lookup_order": lookup_order}

question = "Where is order A1001?"
with support.run(question) as run:  # one user message, one run
    config = types.GenerateContentConfig(
        system_instruction=run.system,     # your prompt, then the skill list
        tools=[*TOOLS, *run.tool_params],  # your functions, then load_skill
    )
    contents = [{"role": "user", "parts": [{"text": run.message}]}]
    while True:
        response = client.models.generate_content(
            model="gemini-3.5-flash", contents=contents, config=config,
        )
        if not response.function_calls:
            break
        contents.append(response.candidates[0].content)
        parts = []
        for call in response.function_calls:
            # load_skill is answered by the run; your own functions by FUNCTIONS.
            # A loop of your own rather than automatic function calling, which
            # raises KeyError when the model asks for a function you did not pass.
            function = FUNCTIONS.get(call.name)
            parts.append(run.tool_result(call) or {"function_response": {
                "name": call.name, "id": call.id,
                "response": {"result": function(**(call.args or {})) if function
                             else f"There is no function named {call.name!r}."},
            }})
        contents.append({"role": "user", "parts": parts})
print(response.text)

No DecimalAI package. The whole skill goes in every prompt, and nothing pins or updates it.

Copy SKILL.md

Save it in your project, as

.agents/skills/cecilapp-cecil-theme/SKILL.md

Give it to your agent

from pathlib import Path
from google import genai
from google.genai import types

client = genai.Client()
skill = Path(".agents/skills/cecilapp-cecil-theme/SKILL.md").read_text()
response = client.models.generate_content(
    model="gemini-3.5-flash",
    contents="Hello!",
    config=types.GenerateContentConfig(system_instruction=f"You are a helpful agent.\n\n{skill}"),
)

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

Gemini API docs

agent_config() hands your agent the skill. With no DecimalAI key it reads your project's files and sends nothing. Needs SDK 0.27 or newer.

Install

pip install "decimalai[langchain]" langchain-openai
decimalai skills add cecilapp-cecil-theme

Add it to your agent

from langchain.agents import create_agent
from decimalai.langchain import agent_config

# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")

agent = create_agent(
    "openai:gpt-5.4-mini",
    tools=[],                         # your tools; the middleware adds load_skill
    middleware=[support.middleware],  # your prompt, the skill list and load_skill
)
result = agent.invoke({"messages": [{"role": "user", "content": "Hello!"}]})

Install

pip install "decimalai[langchain]" langchain-openai

Add it to your agent

from decimalai import skillpkg

# Writes .agents/skills and .decimal/skills.toml. Safe to run twice.
result = skillpkg.add("cecilapp-cecil-theme")
if result.exit_code:
    raise SystemExit(result.problems)

from langchain.agents import create_agent
from decimalai.langchain import agent_config

# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")

agent = create_agent(
    "openai:gpt-5.4-mini",
    tools=[],                         # your tools; the middleware adds load_skill
    middleware=[support.middleware],  # your prompt, the skill list and load_skill
)
result = agent.invoke({"messages": [{"role": "user", "content": "Hello!"}]})

No DecimalAI package. The whole skill goes in every prompt, and nothing pins or updates it.

Copy SKILL.md

Save it in your project, as

.agents/skills/cecilapp-cecil-theme/SKILL.md

Give it to your agent

from pathlib import Path
from langchain.agents import create_agent

skill = Path(".agents/skills/cecilapp-cecil-theme/SKILL.md").read_text()
agent = create_agent("openai:gpt-5.4-mini", tools=[], system_prompt=f"You are a helpful agent.\n\n{skill}")
result = agent.invoke({"messages": [{"role": "user", "content": "Hello!"}]})

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

LangChain docs

CrewAI's own skill loader reads the installed folder. No DecimalAI account needed.

Install

pip install "decimalai>=0.16" "crewai>=1.15.27"
decimalai skills add cecilapp-cecil-theme

Add it to your agent

import decimalai.crewai
from crewai import Agent, Crew, Task

agent = Agent(
    role="Support", goal="Help the customer", backstory="A support agent.",
    llm=llm,                               # your own CrewAI LLM
    skills=decimalai.crewai.skills(),      # the installed folder
    tools=decimalai.crewai.skill_tools(),  # reads a skill's files, runs its scripts
)
task = Task(description="Hello!", expected_output="an answer", agent=agent)
Crew(agents=[agent], tasks=[task]).kickoff()

The same install from Python, for a notebook or a setup script. CrewAI's own skill loader reads the installed folder. No DecimalAI account needed.

Install

pip install "decimalai>=0.16" "crewai>=1.15.27"

Add it to your agent

from decimalai import skillpkg

# Writes .agents/skills and .decimal/skills.toml. Safe to run twice.
result = skillpkg.add("cecilapp-cecil-theme")
if result.exit_code:
    raise SystemExit(result.problems)

import decimalai.crewai
from crewai import Agent, Crew, Task

agent = Agent(
    role="Support", goal="Help the customer", backstory="A support agent.",
    llm=llm,                               # your own CrewAI LLM
    skills=decimalai.crewai.skills(),      # the installed folder
    tools=decimalai.crewai.skill_tools(),  # reads a skill's files, runs its scripts
)
task = Task(description="Hello!", expected_output="an answer", agent=agent)
Crew(agents=[agent], tasks=[task]).kickoff()

No DecimalAI package. CrewAI's own loader reads the folder, and nothing pins or updates it.

Copy SKILL.md

Save it in your project, as

.agents/skills/cecilapp-cecil-theme/SKILL.md

Give it to your agent

from pathlib import Path
from crewai import Agent, Crew, Task

agent = Agent(
    role="Support", goal="Help the customer", backstory="A support agent.",
    llm=llm,                         # your own CrewAI LLM
    skills=[Path(".agents/skills")],
)
task = Task(description="Hello!", expected_output="an answer", agent=agent)
Crew(agents=[agent], tasks=[task]).kickoff()

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

CrewAI docs

agent_config() hands your agent the skill. With no DecimalAI key it reads your project's files and sends nothing. Needs SDK 0.27 or newer.

Install

pip install "decimalai[adk]"
decimalai skills add cecilapp-cecil-theme

Add it to your agent

from decimalai.adk import DecimalAIPlugin, agent_config
from google.adk.agents import LlmAgent
from google.adk.apps import App

# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")

agent = LlmAgent(
    name="support",          # ADK needs a Python identifier
    model="gemini-3.5-flash",
    instruction=support.instruction,  # your prompt, then the skill list
    tools=[*support.tools],           # your tools, then load_skill
)
# DecimalAIPlugin makes each invocation of runners built from this App one trace.
app = App(name="support", root_agent=agent, plugins=[DecimalAIPlugin()])

Prefer ADK's own loader? SkillToolset(skills=decimalai.adk.skills(), ...) reads the same folder. Use one or the other: a skill delivered both ways reaches the prompt twice.

Install

pip install "decimalai[adk]"

Add it to your agent

from decimalai import skillpkg

# Writes .agents/skills and .decimal/skills.toml. Safe to run twice.
result = skillpkg.add("cecilapp-cecil-theme")
if result.exit_code:
    raise SystemExit(result.problems)

from decimalai.adk import DecimalAIPlugin, agent_config
from google.adk.agents import LlmAgent
from google.adk.apps import App

# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")

agent = LlmAgent(
    name="support",          # ADK needs a Python identifier
    model="gemini-3.5-flash",
    instruction=support.instruction,  # your prompt, then the skill list
    tools=[*support.tools],           # your tools, then load_skill
)
# DecimalAIPlugin makes each invocation of runners built from this App one trace.
app = App(name="support", root_agent=agent, plugins=[DecimalAIPlugin()])

Prefer ADK's own loader? SkillToolset(skills=decimalai.adk.skills(), ...) reads the same folder. Use one or the other: a skill delivered both ways reaches the prompt twice.

No DecimalAI package. The whole skill goes in every prompt, and nothing pins or updates it.

Copy SKILL.md

Save it in your project, as

.agents/skills/cecilapp-cecil-theme/SKILL.md

Give it to your agent

from pathlib import Path
from google.adk.agents import LlmAgent

skill = Path(".agents/skills/cecilapp-cecil-theme/SKILL.md").read_text()
agent = LlmAgent(
    name="support",
    model="gemini-3.5-flash",
    instruction=lambda context: f"You are a helpful agent.\n\n{skill}",
)

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

Google ADK docs

agent_config() hands your agent the skill. With no DecimalAI key it reads your project's files and sends nothing. Needs SDK 0.27 or newer.

Install

pip install "decimalai[pydantic-ai]"
decimalai skills add cecilapp-cecil-theme

Add it to your agent

from pydantic_ai import Agent
from decimalai.pydantic_ai import agent_config

# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")

agent = Agent(
    "openai:gpt-4o-mini",
    name=support.name,
    tools=[],                 # your tools; the capability adds load_skill
    capabilities=[support],   # your prompt, the skill list and load_skill
)
# In a notebook or async code: result = await agent.run("Hello!")
result = agent.run_sync("Hello!")

On the Pydantic AI harness, decimalai.pydantic_ai_harness.skills(project) uses its own loader instead.

Install

pip install "decimalai[pydantic-ai]"

Add it to your agent

from decimalai import skillpkg

# Writes .agents/skills and .decimal/skills.toml. Safe to run twice.
result = skillpkg.add("cecilapp-cecil-theme")
if result.exit_code:
    raise SystemExit(result.problems)

from pydantic_ai import Agent
from decimalai.pydantic_ai import agent_config

# With no DECIMAL_API_KEY this reads the skills your project lists, from disk.
support = agent_config("support", fallback="You are a helpful agent.")

agent = Agent(
    "openai:gpt-4o-mini",
    name=support.name,
    tools=[],                 # your tools; the capability adds load_skill
    capabilities=[support],   # your prompt, the skill list and load_skill
)
# In a notebook or async code: result = await agent.run("Hello!")
result = agent.run_sync("Hello!")

On the Pydantic AI harness, decimalai.pydantic_ai_harness.skills(project) uses its own loader instead.

No DecimalAI package. The whole skill goes in every prompt, and nothing pins or updates it.

Copy SKILL.md

Save it in your project, as

.agents/skills/cecilapp-cecil-theme/SKILL.md

Give it to your agent

from pathlib import Path
from pydantic_ai import Agent

skill = Path(".agents/skills/cecilapp-cecil-theme/SKILL.md").read_text()
agent = Agent("openai:gpt-4o-mini", instructions=f"You are a helpful agent.\n\n{skill}")
# In a notebook or async code: result = await agent.run("Hello!")
result = agent.run_sync("Hello!")

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

Pydantic AI docs

Deep Agents' own skill loader reads the installed folder. No DecimalAI account needed.

Install

pip install "decimalai>=0.16" deepagents
decimalai skills add cecilapp-cecil-theme

Add it to your agent

import decimalai.deepagents
from deepagents import create_deep_agent

agent = create_deep_agent(model=model, **decimalai.deepagents.skills())
agent.invoke({"messages": [{"role": "user", "content": "Hello!"}]})

The default backend reads files but has no shell. To run a skill's scripts, pass skills(backend=LocalShellBackend(...)).

The same install from Python, for a notebook or a setup script. Deep Agents' own skill loader reads the installed folder. No DecimalAI account needed.

Install

pip install "decimalai>=0.16" deepagents

Add it to your agent

from decimalai import skillpkg

# Writes .agents/skills and .decimal/skills.toml. Safe to run twice.
result = skillpkg.add("cecilapp-cecil-theme")
if result.exit_code:
    raise SystemExit(result.problems)

import decimalai.deepagents
from deepagents import create_deep_agent

agent = create_deep_agent(model=model, **decimalai.deepagents.skills())
agent.invoke({"messages": [{"role": "user", "content": "Hello!"}]})

The default backend reads files but has no shell. To run a skill's scripts, pass skills(backend=LocalShellBackend(...)).

No DecimalAI package. The whole skill goes in every prompt, and nothing pins or updates it.

Copy SKILL.md

Save it in your project, as

.agents/skills/cecilapp-cecil-theme/SKILL.md

Give it to your agent

from pathlib import Path
from deepagents import create_deep_agent

skill = Path(".agents/skills/cecilapp-cecil-theme/SKILL.md").read_text()
agent = create_deep_agent(model=model, system_prompt=f"You are a helpful agent.\n\n{skill}")
agent.invoke({"messages": [{"role": "user", "content": "Hello!"}]})

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

Deep Agents docs

Its own skill loader reads the installed folder. No DecimalAI account needed.

Install

pip install "decimalai>=0.16" agent-framework
decimalai skills add cecilapp-cecil-theme

Add it to your agent

import asyncio

from agent_framework import Agent
from decimalai.agent_framework import skills

agent = Agent(client=client, instructions="You are a helpful agent.", context_providers=[skills()])
asyncio.run(agent.run("Hello!"))

It caches skills by default, so restart the process after decimalai skills update.

The same install from Python, for a notebook or a setup script. Its own skill loader reads the installed folder. No DecimalAI account needed.

Install

pip install "decimalai>=0.16" agent-framework

Add it to your agent

from decimalai import skillpkg

# Writes .agents/skills and .decimal/skills.toml. Safe to run twice.
result = skillpkg.add("cecilapp-cecil-theme")
if result.exit_code:
    raise SystemExit(result.problems)

import asyncio

from agent_framework import Agent
from decimalai.agent_framework import skills

agent = Agent(client=client, instructions="You are a helpful agent.", context_providers=[skills()])
asyncio.run(agent.run("Hello!"))

It caches skills by default, so restart the process after decimalai skills update.

No DecimalAI package. The whole skill goes in every prompt, and nothing pins or updates it.

Copy SKILL.md

Save it in your project, as

.agents/skills/cecilapp-cecil-theme/SKILL.md

Give it to your agent

import asyncio
from pathlib import Path
from agent_framework import Agent

skill = Path(".agents/skills/cecilapp-cecil-theme/SKILL.md").read_text()
agent = Agent(client=client, instructions=f"You are a helpful agent.\n\n{skill}")
asyncio.run(agent.run("Hello!"))

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

Microsoft Agent Framework docs

Strands' own skill loader reads the installed folder. No DecimalAI account needed.

Install

pip install "decimalai>=0.16" strands-agents
decimalai skills add cecilapp-cecil-theme

Add it to your agent

from decimalai.strands import skills
from strands import Agent
from strands.vended_tools import file_editor, shell

agent = Agent(
    model=model,                # always pass one: the default is Amazon Bedrock
    system_prompt="You are a helpful agent.",
    tools=[file_editor, shell],
    plugins=[skills()],
)
agent("Hello!")

Strands loads an agent's skills once, so build the agent again after decimalai skills update.

The same install from Python, for a notebook or a setup script. Strands' own skill loader reads the installed folder. No DecimalAI account needed.

Install

pip install "decimalai>=0.16" strands-agents

Add it to your agent

from decimalai import skillpkg

# Writes .agents/skills and .decimal/skills.toml. Safe to run twice.
result = skillpkg.add("cecilapp-cecil-theme")
if result.exit_code:
    raise SystemExit(result.problems)

from decimalai.strands import skills
from strands import Agent
from strands.vended_tools import file_editor, shell

agent = Agent(
    model=model,                # always pass one: the default is Amazon Bedrock
    system_prompt="You are a helpful agent.",
    tools=[file_editor, shell],
    plugins=[skills()],
)
agent("Hello!")

Strands loads an agent's skills once, so build the agent again after decimalai skills update.

No DecimalAI package. The whole skill goes in every prompt, and nothing pins or updates it.

Copy SKILL.md

Save it in your project, as

.agents/skills/cecilapp-cecil-theme/SKILL.md

Give it to your agent

from pathlib import Path
from strands import Agent

skill = Path(".agents/skills/cecilapp-cecil-theme/SKILL.md").read_text()
agent = Agent(model=model, system_prompt=f"You are a helpful agent.\n\n{skill}")
agent("Hello!")

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

Strands docs

Adds every file the skill has, pinned so you can update it. Cursor, Copilot, Cline and Warp read this folder by themselves.

pip install "decimalai>=0.15"
decimalai skills add cecilapp-cecil-theme

No Python or account. Cursor, Copilot, Cline and Warp read this folder by themselves.

Save it into your project

curl -fsSL --create-dirs -o .agents/skills/cecilapp-cecil-theme/SKILL.md https://app.decimal.ai/s/cecilapp-cecil-theme/SKILL.md

Copy SKILL.md

Save it in your project, as

.agents/skills/cecilapp-cecil-theme/SKILL.md

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

Cursor and other editors docs

Fetch the file and give its text to your model as instructions. No account or package needed.

Get the file

curl -fsSL https://app.decimal.ai/s/cecilapp-cecil-theme/SKILL.md

Read the summary as JSON

curl -fsSL https://app.decimal.ai/s/cecilapp-cecil-theme.json

This gets SKILL.md only. The skill's 2 other files are not included.

Other ways to add it

Other setups docs

You don't need a DecimalAI account.

Version: v4, tested 11/2/2026
Source: cecilapp/cecil ↗
License: EUPL-1.2

Manage it in DecimalAI

Saved prompts and skills, pinned versions, and traces of this skill in your runs. DecimalAI does not run your agent.

Claude Code reads files from your project, so DecimalAI config does not apply there.

On Claude Agent SDK: save the skill to an agent here, set DECIMAL_API_KEY, and the same code uses it with no command to run. Changes you save reach the next run with no redeploy.

On OpenAI Agents: save the skill to an agent here, set DECIMAL_API_KEY, and the same code uses it with no command to run. Changes you save reach the next run with no redeploy.

On OpenAI SDK: save the skill to an agent here, set DECIMAL_API_KEY, and the same code uses it with no command to run. Changes you save reach the next run with no redeploy.

On Anthropic: save the skill to an agent here, set DECIMAL_API_KEY, and the same code uses it with no command to run. Changes you save reach the next run with no redeploy.

On Gemini API: save the skill to an agent here, set DECIMAL_API_KEY, and the same code uses it with no command to run. Changes you save reach the next run with no redeploy.

On LangChain: save the skill to an agent here, set DECIMAL_API_KEY, and the same code uses it with no command to run. Changes you save reach the next run with no redeploy.

On CrewAI: set DECIMAL_API_KEY and add decimalai.init(crewai=True) to record runs and skill use. The skill version stays in your repo.

On Google ADK: save the skill to an agent here, set DECIMAL_API_KEY, and the same code uses it with no command to run. Changes you save reach the next run with no redeploy.

On Pydantic AI: save the skill to an agent here, set DECIMAL_API_KEY, and the same code uses it with no command to run. Changes you save reach the next run with no redeploy.

On Deep Agents: set DECIMAL_API_KEY and add decimalai.init(langchain=True) to record runs and skill use. The skill version stays in your repo.

On Microsoft Agent Framework: set DECIMAL_API_KEY and add decimalai.init(otel=True, agent_name="support") to record runs and skill use. The skill version stays in your repo.

On Strands: set DECIMAL_API_KEY and add decimalai.init(otel=True, agent_name="support") to record runs and skill use. The skill version stays in your repo.

Editors read files from your project, so DecimalAI config does not apply there.

Anywhere else: record a use with decimalai.log_skill_activation() inside a trace.

Use this skill

SKILL.md

Cecil Theme Builder

You are an expert Cecil theme developer. You create reusable themes for Cecil: full visual themes (layouts, styles, assets) and small component themes (a feature plugged into any site or theme), packaged for Composer like the official Cecilapp themes.

For general Cecil usage (content, configuration, CLI), see the cecil skill.

When to Use This Skill

Use this skill when:

Creating a new Cecil theme from scratch, or turning a site's layouts/ into a reusable theme
Creating a component theme (search, PWA, dark mode toggle, icons, redirects, headers, analytics, etc.)
Installing or integrating an official component theme (e.g. flexsearch, pwa, darkmodetoggle) into a site or a visual theme; ask the user first (see "Ask the user before installing")
Adding theme options, translations or assets to a theme
Packaging and publishing a theme with Composer (cecil/theme-<name>)
Debugging theme resolution (layouts, assets, config or translations not picked up)

Reference Files

Read the matching reference before working on a topic it covers:

Read	Before
references/component-themes.md	Creating a component theme, or installing, combining or pre-wiring official component themes (FlexSearch, PWA, dark mode toggle, etc.)
references/packaging.md	Packaging a theme for Composer: `composer.json`, `.editorconfig`, README, `.gitattributes`, demo site

How Cecil Loads Themes

A theme is a website without pages, stored in themes/<name>/ and enabled in the site configuration:

theme: hyde
# or, for several themes (the first one overrides the others)
theme:
  - pwa
  - hyde

Resolution rules (from src/):

Resource	Lookup order
Layouts	`layouts/` → `themes/<theme>/layouts/` (in `theme` list order) → Cecil internal `resources/layouts/`
Assets	`assets/` → `themes/<theme>/assets/` → `static/` → `themes/<theme>/static/` (Sass import paths also include the theme's dirs)
Static files	Themes' `static/` are copied first (reverse order), then site `static/`, so the site wins
Translations	Cecil internal → `themes/<theme>/translations/` (in `theme` list order, so the last theme wins between themes) → site `translations/` (site wins)
Data files	Site `data/` and each `themes/<theme>/data/` are loaded into `site.data` (theme data is loaded even if the site has no `data/` directory)
Configuration	`themes/<theme>/config.yml` overrides Cecil defaults (including default pages, generators and output formats), site config always wins (first theme wins between themes)

Consequences:

The config file must be named config.yml (not config.yaml); it's the only one loaded.
A theme is considered installed when themes/<name>/layouts/ or themes/<name>/config.yml exists; otherwise the build fails with Theme "<name>" not found. Did you forgot to install it?. A theme without layouts/ (config, data or assets only) is supported.
Layouts are resolved per candidate name: for each name of the lookup rules, Cecil checks the site, then the themes, then the built-in templates, before trying the next name. So a built-in _default/home, _default/term, _default/vocabulary or _default/404 is picked before a theme's _default/list or _default/page: ship these layouts in the theme to control them. The built-ins extend ['page.html.twig', '_default/page.html.twig'] and fill the content block, so the theme base layout must define a content block.
Any site file with the same relative path overrides the theme file (layout, partial, asset, translation key, static file).
A list of values defined by a theme (e.g. static.exclude, output.pagetypeformats.<type>) replaces Cecil's default list; only a site list is combined with the theme list.
Theme dependencies are only partially resolved: when the site uses the list form (theme: [a]) and theme a declares theme: [b], b is added and its layouts, assets, static files and translations are used, but its config.yml is not loaded; with the string form (theme: a), b is ignored. Document the full list the user must declare (e.g. theme: [links, fontawesome]).
A theme can also ship PHP classes in themes/<name>/extensions/ (autoloaded, path matching the namespace or extensions.autoload PSR-4 mappings; use the theme name as namespace, not the reserved Cecil\), registered in its config.yml as Twig extensions (layouts.extensions), generators (pages.generators), post-processors (output.postprocessors) or build events subscribers (extensions.subscribers); prefer a single plugin (extensions.plugins, a class extending Cecil\Plugin\AbstractPlugin) registering all of them.

Theme Structure

Visual theme

theme-<name>/
├─ .editorconfig
├─ .gitattributes          # * text=auto
├─ .gitignore              # vendor/
├─ composer.json
├─ config.yml              # default config + namespaced theme options
├─ theme.yml               # optional metadata
├─ LICENSE
├─ README.md
├─ docs/
│  └─ screenshot.png
├─ assets/                 # processed by asset(): css/, sass/, js/, images
├─ static/                 # copied as-is: favicon.ico, fonts/
├─ data/                   # optional data files (site.data.*)
├─ extensions/             # optional PHP classes (Twig extensions, generators)
├─ layouts/
│  ├─ _default/
│  │  ├─ page.html.twig    # base layout (defines blocks, including `content`)
│  │  ├─ list.html.twig    # sections
│  │  ├─ home.html.twig    # otherwise the built-in one is used
│  │  ├─ term.html.twig    # otherwise the built-in one is used
│  │  ├─ vocabulary.html.twig
│  │  └─ 404.html.twig
│  ├─ <section>/           # optional per-section layouts (e.g. blog/page.html.twig)
│  ├─ components/          # optional Twig components ({% x:<name> %} or <x-name>)
│  ├─ partials/            # header, footer, nav, pagination, post-item...
│  │  └─ notes/            # optional Markdown notes templates (`:::<type>` → <type>.html.twig)
│  └─ macros/              # optional reusable macros
└─ translations/
   └─ messages.fr.yml

Put layouts under _default/: this way a site can still provide its own top-level page.html.twig or list.html.twig, which take priority in the lookup.

Component theme

A component theme only ships what it needs, for example (structure of theme-flexsearch):

theme-<name>/
├─ .github/workflows/demo.yml     # builds demo/ and deploys it to GitHub Pages
├─ .gitattributes                 # export-ignore for .*, /demo, /docs
├─ composer.json
├─ config.yml                     # new generated pages, output formats, options
├─ theme.yml
├─ README.md
├─ assets/<name>/                 # namespaced assets (e.g. flexsearch/flexsearch.css)
├─ demo/                          # demo website using the theme
│  ├─ config.yml
│  ├─ layouts/_default/page.html.twig
│  ├─ pages/
│  └─ link.php                    # links the repository as demo/themes/<name>
├─ docs/screenshot.png
├─ layouts/
│  ├─ _default/<layout>.<format>.twig
│  ├─ partials/<name>.html.twig   # snippet to include in <head> or <body>
│  ├─ partials/<name>/*.html.twig # optional sub-partials (e.g. trigger + dialog)
│  └─ macros/<name>.twig          # macros to import
└─ translations/messages.fr.yml

Put a component theme's assets in an assets/<name>/ subfolder so they don't collide with the site's or another theme's files.

Step-by-Step: Create a Theme

Before scaffolding, ask the user which features the theme needs and whether official component themes should provide some of them (search, dark mode, PWA, icons…), as described in "Ask the user before installing". In the same message, confirm the use of the built-in metatags partial (see Step 4).

Step 1: Scaffold a test site

curl -LO https://cecil.app/cecil.phar
php cecil.phar new:site -n --demo
rm -rf layouts            # make sure the theme's layouts are used
mkdir -p themes/<name>

Develop the theme directly in themes/<name>/ and enable it in the site cecil.yml (created by new:site; config.yml is only loaded when there is no cecil.yml):

theme:
  - <name>

Step 2: Write `config.yml`

Provide sensible Cecil defaults the theme relies on, plus the theme options under a namespace named after the theme:

# Cecil config required by the theme
taxonomies:
  tags: tag
pages:
  pagination:
    max: 10       # only set what differs from Cecil defaults
# Theme options
<name>:
  sidebar:
    sticky: true
  color: ''       # red, orange, green, blue...
  reverse: false

Rules:

Never rely on a key being set: users may override or remove it. In templates, always guard with ?? or |default(), knowing that |default() also replaces empty values (false, '', 0, []), not only undefined ones:
- use ?? when the user can disable or empty an option, and the fallback is truthy: site.<name>.breadcrumb ?? true, site.<name>.logo ?? 'logo.svg' (logo: '' hides it), site.<name>.toc.min ?? 2 (0 is kept). With |default(true), breadcrumb: false would be ignored.
- |default() is fine when the fallback is falsy (site.<name>.reverse|default(false), page.blocks|default([])), or when an empty value must be replaced (|default('Latest posts')).
- wrap ?? in parentheses inside a condition combined with other operators: (site.<name>.toc.enabled ?? true) and (page.toc ?? true).
Cecil has no default vocabularies: declare taxonomies if the theme displays tags or categories.
Avoid redefining Cecil's lists of values (static.exclude, pages.ext…): a theme list replaces the default list instead of extending it.
Keep generic site keys (title, baseurl, description…) out of the theme config.

Step 3: Write `theme.yml` (optional metadata)

name: <Name>
github: https://github.com/<vendor>/theme-<name>/
author:
  - name: <Author>
    url: <https://author.tld>

Step 4: Create the base layout

layouts/_default/page.html.twig defines the HTML skeleton and blocks that other layouts (and users) can override:

<!DOCTYPE html>
<html lang="{{ site.language }}">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    {{- include('partials/metatags.html.twig', {page, site}, with_context = false) }}
    {%- block head_css %}
    {{ html(asset(['css/base.css', 'css/<name>.css']), {title: '<Name>'}, {preload: true}) }}
    {%- endblock head_css %}
    {{- include('partials/pwa.html.twig', {site}, with_context = false, ignore_missing = true) }}
  </head>
  <body class="{{ site.<name>.reverse|default(false) ? 'reverse' }}">
    {%- block header %}
    {{ include('partials/site-header.html.twig') }}
    {%- endblock header %}
    <main>
      {%- block content %}
      <article>
        <h1>{{ page.title }}</h1>
        {%- if page.date %}
        <time datetime="{{ page.date|date('c') }}">{{ page.date|format_date('long') }}</time>
        {%- endif %}
        {{ page.content }}
      </article>
      {%- endblock content %}
    </main>
    {%- block footer %}
    {{ include('partials/site-footer.html.twig') }}
    {%- endblock footer %}
    {%- block scripts %}{% endblock scripts %}
  </body>
</html>

Guidelines:

Always include partials/metatags.html.twig in the <head> of every HTML base layout, and never hand-write <title>, description, canonical, Open Graph or Twitter tags next to it (they would be duplicated). Since it replaces the theme's own <title>, confirm with the user first, explaining the benefit in one line, e.g.: "I'll use Cecil's built-in metatags partial: it generates the title, description, canonical, favicons, Open Graph/Twitter cards, feeds, hreflang alternates (and optional JSON-LD) from front matter and config, with no code to maintain. OK?" Only skip it if the user explicitly declines; in that case, document in the README which tags the theme outputs.
Metatags already links the favicons (favicon.svg, favicon.png resized, favicon.ico, from assets or static) and outputs JSON-LD as a linked graph (WebSite, WebPage, BreadcrumbList, BlogPosting…) when metatags.data is enabled: don't add favicon links or a separate JSON-LD/breadcrumb schema. Ship assets/favicon.png (or .svg) instead.
Twig autoescape is disabled by default (layouts.autoescape: false): {{ page.content }} needs no |raw, but escape values printed in attributes (alt="{{ page.title|e('html_attr') }}"), and don't rely on autoescaping for user-provided text.
Reuse the other Cecil built-in partials instead of re-implementing them: partials/paginator.html.twig, partials/navigation.html.twig (expects a menu variable, and optionally page to mark the current entry with aria-current: {menu: site.menus.main, page: page}), partials/breadcrumb.html.twig, partials/languages.html.twig, partials/theme-selector.html.twig.
Code highlighting: when replacing _default/page.html.twig, keep the code blocks styled, e.g. <style>{{ include('partials/highlight.css.twig') }}</style> when config.pages.body.highlight is enabled, or ship CSS for both engines' classes (hljs-* for highlightjs, hl-* for tempest, see pages.body.highlight.engine).
Markdown output hooks: style the empty <a class="heading-anchor"> link added to h2/h3 headings (see pages.body.anchors, e.g. a # on hover) and use the data-lang attribute of <pre> code blocks (e.g. a language label).
Markdown notes: :::<type> notes render as <aside class="note note-<type>">, and the built-in tabs, code-group, steps, accordion and cards types as <div class="note note-<type>"> (no JavaScript); style these classes, and the items: .note-tab (tabs, code-group), .note-steps-list, .note-accordion-item (<details>), .note-cards-list. A theme can override or add partials/notes/<type>.html.twig, which only receives a note variable (no page or site).
Pre-wire optional component themes with ignore_missing = true so the theme works whether they are installed or not.
Pass an isolated context (with_context = false) to partials that only need site/page.

Step 5: Create child layouts

Extend with a list of templates so a site-level override wins over the theme default:

{# layouts/blog/page.html.twig #}
{% extends ['page.html.twig', '_default/page.html.twig'] %}

{% block content %}
      <article class="post">
        <h1>{{ page.title }}</h1>
        {{ page.content }}
      </article>
      {{- include('partials/post-meta.html.twig') }}
{%- endblock content %}

List layout idiom (set the pages variable before extends):

{# layouts/_default/list.html.twig #}
{% set pages = site.pages.showable %}
{% if page.pages is defined %}
{% set pages = page.pages %}
{% endif %}
{% if page.paginator.pages is defined %}
{% set pages = page.paginator.pages %}
{% endif %}
{% extends ['page.html.twig', '_default/page.html.twig'] %}

{% block content %}
      <ul class="posts">
      {%- for post in pages %}
        {%- include 'partials/post-item.html.twig' with {post} only %}
      {%- endfor %}
      </ul>
      {{- include('partials/paginator.html.twig') }}
{%- endblock content %}

Section alias (e.g. reuse the blog list for the section fallback):

{% extends 'blog/list.html.twig' %}

To map a whole section to another section's layouts, the site (or the theme config) can use layouts.sections (e.g. layouts: {sections: {news: blog}}).

Remember the lookup rules when naming layouts (see the cecil skill, "Lookup Rules"): <section>/index, <section>/page, <section>/list, section/<section>, parent sections, _default/section, index/home, taxonomy/<plural> (vocabulary), taxonomy/<plural>/<term> and taxonomy/<singular> (terms), term, vocabulary, _default/*, with optional language variants (page.html.fr.twig). For non-HTML outputs, the file name is <layout>.<format>.twig (e.g. list.rss.twig, page.webmanifest.twig).

Non-HTML built-ins a theme may override: feeds (_default/list.atom.twig, list.rss.twig, list.jsonfeed.twig, all extending extended/feed.twig), their XSL stylesheets (_default/feed.xsl.twig, _default/sitemap.xsl.twig), _default/sitemap.xml.twig, _default/robots.txt.twig, _default/home.llms.twig, the *.markdown.twig and *.json.twig outputs, _default/404.html.twig and _default/redirect.html.twig. Override them only when needed: they receive Cecil fixes and features.

Step 6: Assets

Put processed files in assets/ and reference them with asset():

{# bundle several CSS files into one #}
{{ html(asset(['css/poole.css', 'css/syntax.css', 'css/hyde.css'])) }}
{# compile Sass and inline it #}
<style>{{ asset('sass/<name>.scss')|inline }}</style>
{# remote asset, downloaded at build time #}
{{ html(asset('https://cdn.jsdelivr.net/npm/lib/dist/lib.min.js', {filename: 'lib.min.js', minify: false}), {defer: ''}) }}
{# responsive image #}
{{ html(asset(post.image), {alt: post.title}, {responsive: true, formats: ['webp']}) }}
{# optional asset #}
{% set logo = asset(site.logo|default('logo.svg'), {ignore_missing: true}) %}
{% if not logo.missing %}{{ html(logo, {alt: site.title}) }}{% endif %}
{# small inline CSS/JS #}
<style>{% apply minify_css %}.hero { color: red; }{% endapply %}</style>

Use the html() function (the |html filter is deprecated).
Use static/ for files that must keep their name and not be processed: web fonts, downloads, etc. Link them with url() (favicons are already linked by metatags). Don't ship a static/robots.txt: robots.txt is a built-in default page (custom rules go in a pages/robots.md page of the site).
Expose design options as Sass variables with assets.compile.variables in the theme config.yml (e.g. assets: {compile: {variables: {primary: "#0d6efd"}}}); Sass import paths (assets.compile.import, [sass, scss, node_modules] by default) are resolved in the assets/ and static/ directories of the site and of the themes, and next to the compiled file.
Leave minification and fingerprinting to the site config (assets.minify, assets.fingerprint) rather than forcing them in templates.
Sass sources in assets/ are not copied to the output (only static/ is), and *.scss is already excluded by default: no static.exclude is needed.
For Tailwind CSS, commit the compiled file (e.g. assets/styles.css) and provide build scripts (see "Packaging").

Step 7: Menus, URLs and collections

<nav>
  <ul>
  {%- for entry in site.menus.main|sort_by_weight %}
    <li{% if url(entry.url) == url(page) %} class="active"{% endif %}><a href="{{ url(entry.url) }}">{{ entry.name }}</a></li>
  {%- endfor %}
  </ul>
</nav>

Always build links with url() (pages, menu entries, paths, assets), never hardcode site.baseurl.
Use the external test to detect links outside the site (e.g. {% if entry is external %} rel="noopener" target="_blank"{% endif %}), rather than matching http.
Link to the Cecil website with the cecil global variable, never a hardcoded https://cecil.app: cecil.url is already absolute (it includes the version as anchor), so don't pass it to url(). cecil.poweredby (Cecil v<version>) and cecil.version are available for the label:
```
{% trans with {'%cecil%': '<a href="' ~ cecil.url ~ '">Cecil</a>'} %}Powered by %cecil%{% endtrans %}
<a href="{{ cecil.url }}">{{ cecil.poweredby }}</a>
```
Filter collections: site.pages.showable|filter_by('section', 'blog')|sort_by_date|slice(0, 5).
Tags links: url('tags/' ~ tag|slugify) or iterate site.taxonomies.tags.
Get a specific page: site.page('about').

Step 8: Translations

Wrap every UI string so the theme is translatable:

{% trans %}Read more{% endtrans %}
{% trans with {'%title%': site.title} %}%title% on GitHub{% endtrans %}
{{ 'You are offline'|trans }}

Provide at least one translation file, keyed by the source string:

# translations/messages.fr.yml
"Read more": "Lire la suite"
"%title% on GitHub": "%title% sur GitHub"

File names follow messages.<locale>.<ext> (yml/yaml, or compiled mo; .po files are not loaded), where <locale> is the language locale (fr_FR) or its first two letters (fr).
Extract strings with php cecil.phar util:translations:extract --locale=fr --save --theme=<name>: the file is written to the site translations/ directory (messages.fr.yaml), move it into themes/<name>/translations/.

Component Themes

No automatic injection mechanism exists: a component theme plugs into a site through generated pages and output formats, partials to include, macros, section-specific layouts, front matter blocks or Twig components. Official component themes exist (FlexSearch, PWA, dark mode toggle, etc.): ask the user before installing one. Read references/component-themes.md for the patterns and the official component themes.

Packaging

A theme is published as a Composer package (composer.json with type cecil-theme). Read references/packaging.md for the files to provide.

Conventions Checklist

Troubleshooting

Theme "<name>" not found: the folder must be themes/<name>/ (matching the theme name resolved by the installer: extra.cecil.theme-name, extra.name or the package name) and contain layouts/ or config.yml.
Theme installed in vendor/ instead of themes/: the project doesn't allow the plugin; add "cecil/theme-installer": true to config.allow-plugins in the project's composer.json, then reinstall.
The ".../themes/<name>/layouts" directory does not exist: the theme has a config.yml but no layouts/ directory; add one, even empty.
Theme layout ignored: a site layouts/ file with the same path, or any file (site, theme or built-in) matching a higher-priority lookup name, wins (e.g. the built-in _default/home.html.twig before the theme _default/list.html.twig); check with php cecil.phar build -vv, or remove the site layout.
Theme data missing (site.data.*): the site has no data/ directory; create one.
Theme option ignored: the site config defines the same key; theme config never overrides site config. Lists of values are the exception: a list defined by the site (e.g. output.pagetypeformats.homepage) is added to the theme list, so the values required by the theme are kept; the site can only remove one with a map of booleans (e.g. homepage: {atom: false}).
Dependent theme not loaded (or its options missing): list it explicitly in the site theme key; a dependency declared in a theme's config.yml never has its own config.yml loaded.
Asset not found: paths in asset() are relative to assets/ (or static/) of the site or theme, without the directory prefix.
Translations missing: check the file name matches messages.<locale> or messages.<first two letters of the locale> (e.g. fr_FR → messages.fr_FR.yml or messages.fr.yml); the translations cache is refreshed when a file is updated or removed, but not when a new one is added: run php cecil.phar cache:clear:translations.
Deprecated configuration option(s) in …: the theme's config.yml uses a renamed or removed option (see config/deprecated.php), which makes the build fail for every site using the theme; apply the snippet given in the error message.
Run php cecil.phar doctor to check theme configuration.

Useful Resources

Official themes: https://github.com/orgs/Cecilapp/repositories?q=theme
Themes showcase: https://cecil.app/themes/
Templates documentation: https://cecil.app/documentation/templates/
Configuration (theme): https://cecil.app/documentation/configuration/site/#theme
Component themes: https://github.com/Cecilapp/theme-flexsearch, https://github.com/Cecilapp/theme-pwa, https://github.com/Cecilapp/theme-darkmodetoggle
Minimal skeleton: https://github.com/Cecilapp/theme-example

About this skill

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Frontend / UI pack

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cecilapp / cecil

skills/cecil-theme/SKILL.md

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Cecil Theme Builder

When to Use This Skill

Reference Files

How Cecil Loads Themes

Theme Structure

Visual theme

Component theme

Step-by-Step: Create a Theme

Step 1: Scaffold a test site

Step 2: Write config.yml

Step 3: Write theme.yml (optional metadata)

Step 4: Create the base layout

Step 5: Create child layouts

Step 6: Assets

Step 7: Menus, URLs and collections

Step 8: Translations

Component Themes

Packaging

Conventions Checklist

Troubleshooting

Useful Resources

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Step 2: Write `config.yml`

Step 3: Write `theme.yml` (optional metadata)