Custom Engine Implementation

Use this skill when adding a new agentic engine definition or extending the behavior-defined engine infrastructure.

Choose the smallest implementation

Prefer a shared Markdown engine definition. Installation, execution, MCP configuration, provider routing, caching, manifests, plugins, network defaults, harness scripts, and log parsing can already be declared through engine.behaviors.

Only change Go code when the engine needs a reusable behavior that the declarative model cannot express. Add a dedicated Go engine only when the shared behavior-defined runtime cannot run the engine at all.

Need	Implementation
Existing behaviors are sufficient	Add an imported shared engine definition
A reusable declarative behavior is missing	Extend the definition model and behavior-defined runtime
The shared runtime is fundamentally unsuitable	Add a dedicated Go engine and register it

The relevant implementation is in:

• pkg/workflow/engine_definition.go for the definition model and catalog
• pkg/workflow/behavior_defined_engine.go for the shared runtime
• pkg/workflow/engine_definition_loader.go for embedded built-in definitions
• pkg/workflow/agentic_engine.go for dedicated Go engine registration
• pkg/parser/schemas/main_workflow_schema.json for the workflow schema

Study representative engines

Read the closest examples before making changes:

• .github/workflows/shared/opencode.md: npm installation, merged configuration, MCP, provider routing, and log parsing
• .github/workflows/shared/aider.md: Python installation and a custom harness without native MCP
• .github/workflows/shared/crush.md: native MCP configuration adapter and harness
• .github/workflows/shared/cursor.md: plugin support
• .github/workflows/shared/deepseek-harness.md: provider endpoint discovery and a headless profile

Use the examples to identify a pattern, not as a reason to copy optional behaviors.

Add a shared engine definition

1. Create .github/workflows/shared/<engine>.md.
2. Declare engine.id, display-name, description, experimental, provider metadata, authentication, and only the required behaviors.
3. Pin a default CLI version. Make installation deterministic and provide a verification command.
4. Document setup, authentication, model syntax, MCP support, and limitations in the Markdown body. Keep in-repository integrations clearly identified as unsupported samples.
5. Add the engine ID and shared definition path to .github/aw/engines.json. Imported behavior-defined engines are registered dynamically; do not add them to NewEngineRegistry().
6. Add .github/workflows/smoke-<engine>.md, following the closest existing smoke workflow. Import the shared definition and exercise the capabilities the engine claims to support.
7. Update the unsupported sample table in docs/src/content/docs/reference/engines.md.
8. Add a changeset when the engine is a user-visible repository sample. New experimental engines have precedent as a minor change.
9. Run make recompile and include the generated smoke workflow .lock.yml.

TestKnownEngineImportsFile_MatchesSharedEngineFiles in pkg/workflow/engine_definition_test.go enforces catalog coverage for shared external engines.

Design the definition

Keep the definition declarative and minimal:

• Select the correct package manager, package name, binary name, version, and verification command under behaviors.installation.
• Put invocation arguments and non-secret environment variables under behaviors.execution.
• Use an existing secret strategy and provider environment mode where possible.
• Declare native MCP support only when the CLI can consume the generated configuration. Otherwise use a harness or the gh-aw CLI proxy pattern.
• Add only required default domains and map provider-specific domains under behaviors.network.provider-domains.
• Declare manifest files, cache paths, and plugin support only when the CLI consumes them.
• Add a log parser only when its emitted event format can be tested.

Treat inline JavaScript in harnesses, configuration adapters, and log parsers as production code. Avoid shell interpolation, validate paths and child-process arguments, preserve nonzero exit codes, and never print secret values. Enabling package lifecycle scripts requires a pinned version and an explicit reason.

For log and error regular expressions, also load .github/skills/error-pattern-safety/SKILL.md.

Extend declarative infrastructure

When an engine exposes a generally useful capability that existing behaviors cannot express:

1. Add the smallest field to the types in pkg/workflow/engine_definition.go.
2. Validate and render it in the focused behavior-defined engine files.
3. Update pkg/parser/schemas/main_workflow_schema.json.
4. Add focused tests for parsing, validation, and rendered execution behavior.
5. Run make generate-schema-docs when generated schema documentation changes.

Do not add a schema field for a behavior that can be represented by existing installation, execution, environment, harness, or adapter fields.

For a dedicated Go runtime, implement the engine interfaces by composing the existing helpers, keep engine-specific code in its own files, and register the engine in pkg/workflow/agentic_engine.go. Cover configuration, commands, environment, authentication, tools, logs, and failure handling with focused tests.

Test the right layers

Use tests that match the changed behavior:

Area	Tests and examples
Harness, configuration, MCP, and environment	behavior_defined_engine_harness_test.go
Log parsing	behavior_defined_engine_log_parser_test.go
Cache behavior	behavior_defined_engine_cache_test.go
Definitions, catalog, and known imports	engine_definition_test.go, engine_catalog_test.go
Embedded definitions	engine_definition_loader_test.go
Real shared workflow harness	aider_workflow_test.go
Sandboxed CLI visibility	cloud_hypervisor_test.go
Generated smoke workflows	compiled_lock_files_test.go

Test both definition parsing and the generated installation/execution steps. Include negative cases for unsafe paths, invalid configuration, missing credentials, or malformed logs when relevant.

Validate

After Go changes:

make build
make fmt

After workflow Markdown changes:

make recompile

Run focused go test commands for the changed behavior while iterating. Before an intermediate progress report, run:

make agent-report-progress-no-test

Before the final progress report, run:

make agent-report-progress

Use make agent-finish for the final repository validation when time allows. Do not trigger a smoke workflow from a Copilot cloud agent run.

Completion checklist

• The declarative path was preferred unless its limitations are documented.
• The engine definition uses a pinned, verified installation.
• Authentication, provider/model handling, MCP, and network access are covered.
• Harnesses and adapters do not expose secrets or interpolate untrusted input.
• .github/aw/engines.json, smoke workflow, generated lock file, documentation, and changeset are updated when applicable.
• Focused tests cover parsing and rendered runtime behavior.
• Repository validation passes.