ROS 2 Engineering Skills

Single responsibility: This skill is an API reference & code template guide for ROS 2 development. It tells you how to use ROS 2 APIs correctly and what mistakes to avoid. It does NOT do CI/CD orchestration, incident response, data analysis, or deployment automation — those are separate skill categories.

A progressive-disclosure skill for ROS 2 development — from first workspace to production fleet deployment. Each section below gives you the essential decision framework; detailed patterns, code templates, and anti-patterns live in the references/ directory. Read the relevant reference file before writing code.

How to use this skill

Progressive disclosure — do NOT read everything at once. This skill is structured in layers. Only load what you need for the current task:

1. This file (SKILL.md) — always loaded. Contains decision routing, core principles, pitfalls, and anti-patterns. Sufficient for answering quick questions and making architectural decisions.
2. references/*.md — load on demand. Use the Decision Router below to pick the 1–2 files relevant to the user's current task. Do NOT read all 20 reference files — that wastes context and causes confusion.
3. scripts/ — run only when the user needs code generation, QoS checking, or launch validation. These are tools, not reading material.

Steps:

1. If .skill-runs.log exists in the workspace, read the last few lines to understand what was done and what issues occurred in previous sessions.
2. Identify what the user is building (see Decision Router below).
3. Read only the matching references/*.md file(s) for detailed guidance.
4. Check the AI pitfalls table before generating any code.
5. Apply the Core Engineering Principles in every artifact you produce.
6. When multiple domains intersect (e.g. Nav2 + ros2_control), read both files but favor safety > determinism > simplicity when recommendations conflict.

Execution log: The Stop hook automatically appends a session summary to .skill-runs.log in the workspace. This lets you see what was validated last time and what issues were found — check it to avoid repeating past mistakes.

Decision router

User is doing...	Read
Creating a workspace, package, or build config	references/workspace-build.md
Writing nodes, executors, callback groups	references/nodes-executors.md
Topics, services, actions, custom interfaces, QoS	references/communication.md
Lifecycle nodes, component loading, composition	references/lifecycle-components.md
Launch files, conditional logic, event handlers	references/launch-system.md
tf2, URDF, xacro, robot_state_publisher	references/tf2-urdf.md
ros2_control, hardware interfaces, controllers	references/hardware-interface.md
Real-time constraints, PREEMPT_RT, memory, jitter	references/realtime.md
Nav2, SLAM, costmaps, behavior trees	references/navigation.md
MoveIt 2, planning scene, grasp pipelines	references/manipulation.md
Camera, LiDAR, PCL, cv_bridge, depth processing	references/perception.md
Unit tests, integration tests, launch_testing, CI	references/testing.md
ros2 doctor, tracing, profiling, rosbag2	references/debugging.md
Docker, cross-compile, fleet deployment, OTA	references/deployment.md
Gazebo, Isaac Sim, sim-to-real, use_sim_time	references/simulation.md
SROS2, DDS security, certificates, supply chain	references/security.md
micro-ROS, MCU/RTOS, XRCE-DDS, rclc	references/micro-ros.md
Multi-robot fleet, Open-RMF, DDS discovery scale	references/multi-robot.md
Message types, units, covariance, frame conventions	references/message-types.md
ROS 1 migration, ros1_bridge, hybrid operation	references/migration-ros1.md

Cross-cutting concerns: Security, error handling, and QoS are not isolated to single reference files — apply them whenever the data path crosses a trust boundary, a node owns hardware, or communication reliability matters. Use your judgment about which cross-cutting concerns apply to the user's specific situation.

Core engineering principles

These apply to every ROS 2 artifact you produce, regardless of domain.

1. Distro awareness

Always ask which ROS 2 distribution the user targets. Key differences:

Feature	Foxy (EOL)	Humble (LTS)	Jazzy (LTS)	Kilted (non-LTS)	Rolling
EOL	Jun 2023 (ended)	May 2027	May 2029	Nov 2025	Rolling
Ubuntu	20.04	22.04	24.04	24.04	Latest
Default DDS	Fast DDS	Fast DDS	Fast DDS	Fast DDS	Fast DDS
Zenoh support	—	—	—	Tier 1	Tier 1
Type description support	No	No	Yes	Yes	Yes
Service introspection	No	No	Yes	Yes	Yes
EventsExecutor	No	No	Experimental	Stable (+ rclpy)	Stable (+ rclpy)
Default bag format	sqlite3	sqlite3	MCAP	MCAP	MCAP
ros2_control interface	N/A (separate)	2.x	4.x	4.x	Latest
CMake recommendation	ament_target_deps	ament_target_deps	either	target_link_libs	target_link_libs

When the user does not specify, default to the latest LTS (Jazzy). Pin the exact distro in Dockerfile, CI, and documentation so builds are reproducible.

2. C++ vs Python decision

Choose the language based on the node's role, not personal preference.

Use rclcpp (C++) when:

• The node sits in a control loop running ≥100 Hz
• Deterministic memory allocation matters (real-time path)
• The node is a hardware driver or controller plugin
• Intra-process zero-copy communication is required

Use rclpy (Python) when:

• The node is orchestration, monitoring, or parameter management
• Rapid prototyping with frequent iteration
• Heavy use of ML frameworks (PyTorch, TensorFlow) that are Python-native
• The node does not sit in a latency-critical path

Mixed stacks are normal. A typical robot has C++ drivers/controllers and Python orchestration/monitoring. Note: component_container (composition) only loads C++ components via pluginlib. Python nodes run as separate processes, but can share a launch file and communicate via zero-overhead intra-host DDS.

Intra-process communication works for any nodes sharing a process — not only composable components. Any nodes instantiated in the same process with use_intra_process_comms(true) can use zero-copy transfer.

3. Package structure conventions

Every package should follow this layout. Consistency across a workspace reduces onboarding time and makes CI scripts portable.

my_package/
├── CMakeLists.txt          # or setup.py for pure Python
├── package.xml             # format 3, with <depend> tags
├── config/
│   └── params.yaml         # default parameters
├── launch/
│   └── bringup.launch.py   # Python launch file
├── include/my_package/     # C++ public headers (if library)
├── src/                    # C++ source files
├── my_package/             # Python modules (if ament_python or mixed)
├── test/                   # gtest, pytest, launch_testing
├── urdf/                   # URDF/xacro (if applicable)
├── msg/ srv/ action/       # custom interfaces (dedicated _interfaces package preferred)
└── README.md

Separate interface definitions into a *_interfaces package so downstream packages can depend on interfaces without pulling in implementation.

4. Parameter discipline

• Declare every parameter with a type, description, range, and default in the node constructor — never use undeclared parameters.
• Use ParameterDescriptor with FloatingPointRange or IntegerRange for numeric bounds. The parameter server rejects out-of-range values at set time.
• Group related parameters under a namespace prefix: controller.kp, controller.ki, controller.kd.
• Load defaults from a config/params.yaml; allow launch-time overrides.
• For dynamic reconfiguration, register a set_parameters_callback and validate new values atomically before accepting.

5. Error handling philosophy

• Nodes must not silently swallow errors. Log at the appropriate severity, then take a safe action (stop motion, request help, transition to error state).
• Prefer lifecycle node error transitions over ad-hoc boolean flags.
• When calling a service, always handle the "service not available" and "future timed out" cases explicitly.
• For hardware drivers, distinguish transient errors (retry with backoff) from fatal errors (transition to FINALIZED and alert the operator).

6. Quality of Service defaults

Start from these profiles and adjust per use case:

Use case	Reliability	Durability	History	Depth	Deadline	Lifespan
Sensor stream	BEST_EFFORT	VOLATILE	KEEP_LAST	5	—	—
Command velocity	RELIABLE	VOLATILE	KEEP_LAST	1	100 ms	200 ms
Map (latched)	RELIABLE	TRANSIENT_LOCAL	KEEP_LAST	1	—	—
Diagnostics	RELIABLE	VOLATILE	KEEP_LAST	10	—	—
Parameter events	RELIABLE	VOLATILE	KEEP_LAST	1000	—	—
Action feedback	RELIABLE	VOLATILE	KEEP_LAST	1	—	—
Safety heartbeat	RELIABLE	VOLATILE	KEEP_LAST	1	500 ms	1 s

QoS mismatches are the #1 cause of "I published but nobody receives." Always check compatibility with ros2 topic info -v when debugging.

DEADLINE and LIFESPAN are critical for safety-critical systems. DEADLINE fires an event when no message arrives within the specified period (detect stale data). LIFESPAN discards messages older than the specified duration before delivery (prevent acting on stale data). See references/communication.md section 9 for full API and examples.

7. Naming conventions

Entity	Convention	Example
Package	snake_case	arm_controller
Node	snake_case	joint_state_broadcaster
Topic	/snake_case with ns	/arm/joint_states
Service	/snake_case	/arm/set_mode
Action	/snake_case	/arm/follow_joint_trajectory
Parameter	snake_case with dot ns	controller.publish_rate
Frame	snake_case	base_link, camera_optical
Interface	PascalCase.msg/srv/action	JointState.msg

8. Thread safety and callbacks

• A MutuallyExclusiveCallbackGroup serializes its callbacks — safe for shared state without locks, but limits throughput.
• A ReentrantCallbackGroup allows parallel execution — you must protect shared state with std::mutex (C++) or threading.Lock (Python).
• Calling a service from a callback: The service client must be in a separate MutuallyExclusiveCallbackGroup from the calling callback. Otherwise the executor deadlocks — the callback waits for the response while the executor cannot deliver it. Always use async_send_request with a response callback; never use spin_until_future_complete inside an executor callback.
• Never do blocking work (file I/O, long computation, sleep) inside a timer or subscription callback on the default executor. Offload to a dedicated thread or use a MultiThreadedExecutor with a reentrant group.
• In rclcpp, prefer std::shared_ptr<const MessageT> in subscription callbacks to avoid unnecessary copies and enable zero-copy intra-process.

9. Lifecycle-first design

Default to lifecycle (managed) nodes for anything that owns resources: hardware drivers, sensor pipelines, planners, controllers.

                 ┌──────────────┐
  create() ──►  │  Unconfigured │
                 └──────┬───────┘
            on_configure │
                 ┌──────▼───────┐
                 │   Inactive    │
                 └──────┬───────┘
            on_activate  │
                 ┌──────▼───────┐
                 │    Active     │
                 └──────┬───────┘
           on_deactivate │
                 ┌──────▼───────┐
                 │   Inactive    │
                 └──────┬───────┘
            on_cleanup   │
                 ┌──────▼───────┐
                 │  Unconfigured │
                 └──────┬───────┘
           on_shutdown   │
                 ┌──────▼───────┐
                 │   Finalized   │
                 └───────────────┘

This gives the system manager (launch file, orchestrator, or operator) explicit control over when resources are allocated, when the node starts processing, and how it shuts down. It also makes error recovery predictable.

10. Build and CI hygiene

• Use colcon build --cmake-args -DCMAKE_BUILD_TYPE=RelWithDebInfo for development; Release for deployment.
• Enable -Wall -Wextra -Wpedantic and treat warnings as errors in CI.
• Run colcon test with --event-handlers console_cohesion+ so test output groups by package.
• Pin rosdep keys in rosdep.yaml for reproducible dependency resolution.
• Cache /opt/ros/, .ccache/, and build//install/ in CI to cut build times by 60–80%.

Common anti-patterns

Anti-pattern	Why it hurts	Fix
Global variables for node state	Breaks composition, untestable	Store state as class members
spin() in main() for multi-node processes	Starves other nodes	Use MultiThreadedExecutor or component composition
Hardcoded topic names	Breaks reuse across robots	Use relative names + namespace remapping
KEEP_ALL history with no bound	Memory grows unbounded on slow subscribers	Use KEEP_LAST with explicit depth
Using time.sleep() / std::this_thread::sleep_for	Blocks the executor thread	Use create_wall_timer or a dedicated thread
Monolithic launch file for everything	Unmanageable past 10 nodes	Compose launch files with IncludeLaunchDescription
Skipping package.xml dependencies	Builds locally, breaks CI and Docker	Declare every dependency explicitly
Publishing in constructor	Subscribers may not be ready, messages lost	Publish in on_activate or after a short timer
Ignoring QoS compatibility	Silent communication failure	Match publisher/subscriber QoS or check with ros2 topic info -v
Creating timers/subs in callbacks	Resource leak, unpredictable behavior	Create all entities in constructor or on_configure
Synchronous service call in callback	Deadlocks the executor thread	Use async_send_request with a callback or dedicated thread
Service client in same callback group as caller	Deadlocks even with async in MultiThreadedExecutor	Put service client in a separate MutuallyExclusiveCallbackGroup
No safe command on shutdown	Motors hold last velocity after node exits	Send zero-velocity in on_deactivate AND destructor (see references/hardware-interface.md)
Dynamic subscriptions with StaticSingleThreadedExecutor	New subs are never picked up after spin()	Use SingleThreadedExecutor or MultiThreadedExecutor for dynamic entities
CPU frequency governor left on powersave/ondemand	10-100 ms latency spikes in RT path	Set performance governor, disable turbo boost (see references/realtime.md)

AI pitfalls — traps this skill has learned from

These are mistakes AI agents repeatedly make when generating ROS 2 code. Add a new line here every time a failure is discovered in practice.

#	Pitfall	What goes wrong	Correct approach
1	Using spin_until_future_complete inside a callback	Deadlocks the executor — the callback blocks waiting for a response that can never be delivered	Use async_send_request with a response callback; put the service client in a separate MutuallyExclusiveCallbackGroup
2	Generating Foxy-era API for Jazzy/Kilted	node_executable is deprecated, export_state_interfaces() signature changed in ros2_control 4.x	Always check the distro feature matrix above before generating code
3	Omitting QoS in publisher/subscriber creation	Defaults silently mismatch — publisher sends but subscriber receives nothing	Always specify QoS explicitly; use the QoS defaults table in Principle 6
4	Creating a msg/ directory inside a non-interfaces package	Builds locally but fails in CI — interface packages need rosidl_generate_interfaces	Put messages in a dedicated *_interfaces package
5	Hardcoding /opt/ros/humble/ paths in launch files	Breaks on any other distro or install prefix	Use FindPackageShare, PathJoinSubstitution, or environment substitutions
6	Forgetting <depend> tags in package.xml	colcon build works in overlay but rosdep install and Docker builds fail	Declare every find_package() / import as <depend> in package.xml
7	Using time.sleep() for rate control in rclpy	Blocks the executor thread; timers and subscriptions stop firing	Use create_timer() or Rate with a MultiThreadedExecutor
8	Not sending zero-velocity on deactivate/shutdown	Robot holds last commanded velocity when the node crashes	Send zero-command in both on_deactivate and the destructor
9	Mixing ament_target_dependencies() and target_link_libraries()	Kilted deprecated ament_target_dependencies — mixing causes link errors	Use target_link_libraries() with modern CMake targets for Kilted+; ament_target_dependencies() for Humble/Jazzy
10	Generating rospy / roscpp code instead of rclpy / rclcpp	ROS 1 patterns in a ROS 2 context — nothing compiles	This skill is ROS 2 only — always use rclpy/rclcpp APIs
11	Ignoring use_sim_time parameter in simulation	Real clock diverges from Gazebo clock — tf lookups fail, controllers drift	Set use_sim_time:=true in launch and pass --clock to ros2 bag play
12	Publishing before subscribers connect (no TRANSIENT_LOCAL)	First N messages lost — map, URDF, or initial config never received	Use TRANSIENT_LOCAL durability for latched-style data, or publish in on_activate with a startup delay

Maintenance rule: When you encounter a new AI failure pattern while using this skill, append it to this table with the next sequential number. The pitfall list is the single most valuable section for preventing repeated mistakes.

Distro-specific migration notes

When upgrading between distributions, check these breaking changes first:

Foxy → Humble:

• Complete API overhaul. Foxy packages require significant rework.
• ros2_control was not bundled in Foxy — must be built separately.
• Lifecycle node API stabilized in Humble.
• Action server/client API changed significantly.

Humble → Jazzy:

• ros2_control API changed from 2.x to 4.x — export_state_interfaces() and export_command_interfaces() are now auto-generated by the framework. Manual overrides use on_export_state_interfaces(). See references/hardware-interface.md.
• Handle get_value() deprecated → use get_optional<T>() on LoanedStateInterface / LoanedCommandInterface (controller side). Hardware interfaces use set_state() / get_state() / set_command() / get_command() helpers with fully qualified names.
• All joints in <ros2_control> tag must exist in the URDF.
• Controller parameter loading changed — use --param-file with spawner.
• Default bag format changed from sqlite3 to MCAP. Use storage_id='mcap'.
• Default middleware changed internal config paths. Regenerate DDS profiles.
• nav2_params.yaml schema changes — recoveries_server renamed to behavior_server.
• ROS_AUTOMATIC_DISCOVERY_RANGE replaces ROS_LOCALHOST_ONLY (values: LOCALHOST, SUBNET, OFF, SYSTEM_DEFAULT).
• launch_ros actions have new parameter handling — test launch files explicitly.

Jazzy → Kilted (non-LTS):

• Zenoh promoted to Tier 1 middleware — rmw_zenoh is production-ready. Install: sudo apt install ros-kilted-rmw-zenoh-cpp, set RMW_IMPLEMENTATION=rmw_zenoh_cpp. Supports router/peer/client modes.
• EventsExecutor graduated from experimental — available in rclcpp::executors (no experimental namespace). Also ported to rclpy.
• ament_target_dependencies() deprecated — use target_link_libraries() with modern CMake targets (e.g. rclcpp::rclcpp, std_msgs::std_msgs__rosidl_typesupport_cpp).
• Multi-bag replay support in ros2 bag play.
• Gazebo Ionic is the paired simulator (Harmonic was Jazzy; Ionic is the Kilted pairing).

ROS 1 → ROS 2:

• See references/migration-ros1.md for a step-by-step strategy.

Quick reference — ros2 CLI

# Workspace
colcon build --symlink-install --packages-select my_pkg
colcon test --packages-select my_pkg
colcon graph --dot                       # dependency graph (DOT format)
source install/setup.bash

# Introspection
ros2 node list
ros2 topic list -t
ros2 topic info /topic_name -v          # shows QoS details
ros2 topic hz /topic_name
ros2 topic bw /topic_name
ros2 service list -t
ros2 action list -t
ros2 param list /node_name
ros2 param describe /node_name param
ros2 interface show std_msgs/msg/String

# ros2_control
ros2 control list_controllers
ros2 control list_hardware_interfaces
ros2 control list_hardware_components

# Debugging
ros2 doctor --report                    # alias: ros2 wtf
ros2 run tf2_tools view_frames
ros2 bag record -a -o my_bag
ros2 bag info my_bag
ros2 bag play my_bag --clock

# Lifecycle
ros2 lifecycle list /node_name
ros2 lifecycle set /node_name configure
ros2 lifecycle set /node_name activate