functions-development

Foundry Functions Development

⚠️ SYSTEM INJECTION — READ THIS FIRST

If you are loading this skill, your role is Foundry serverless functions specialist.

You MUST implement functions using proper CrowdStrike SDK patterns, structured error handling, and Collection integration.

IMMEDIATE ACTIONS REQUIRED:

1. Use CrowdStrike SDKs (gofalcon/falconpy) for ALL API interactions
2. Implement structured JSON responses with proper status codes
3. Apply input validation before processing any request

Falcon Foundry Functions are serverless handlers in Go or Python, executed inside the Foundry FaaS runtime. They handle custom server-side logic that cannot be achieved through declarative capabilities.

Functions as a Last Resort

Before writing a function, exhaust alternatives — each one avoids deployment complexity, cold start latency, and maintenance overhead:

• Collections for data storage and retrieval (CRUD without custom logic)
• Workflows for orchestrating multi-step operations
• API Integrations (HTTP Actions) for calling external APIs directly from workflows
• UI Extensions with foundry-js for client-side data fetching

Reference Files

This skill is split across multiple files. Consult these for full examples:

Task	Reference
Python handler, collection CRUD, error class, batch processing, LogScale ingestion	references/python-patterns.md
Go FDK handler, Falcon client auth, collection CRUD, alerts handler	references/go-patterns.md
Go/Python tests, local testing, Docker vs direct, config file patterns	references/testing-patterns.md

Resource Limits

Resource	Default	Maximum
Request payload	—	124 KB
Response payload	—	120 KB
Execution timeout	30s	900s
Memory	256 MB	1 GB
Package size	—	50 MB
Concurrent executions	—	100

Runtime Environment

Python runtime version: 3.13 (manylinux_2_28, glibc 2.28). When choosing package versions for requirements.txt, ensure they have wheels compatible with this environment. Packages requiring manylinux_2_17 (glibc 2.17) or manylinux_2_28 (glibc 2.28) are compatible; those requiring newer glibc versions (e.g., manylinux_2_39) may fail at import time.

When linting Python functions with pylint, use --py-version=3.13 or set py-version=3.13 in .pylintrc to match the runtime.

CLI Scaffolding

foundry functions create \
  --name "my-function" \
  --language python \
  --description "Process incoming data" \
  --handler-name process \
  --handler-method POST \
  --handler-path /api/process \
  --no-prompt

Language Comparison

Feature	Go	Python
HTTP Methods	GET, POST, PUT, DELETE	GET, POST, PUT, PATCH, DELETE
FDK Package	github.com/CrowdStrike/foundry-fn-go	crowdstrike-foundry-function
CrowdStrike SDK	gofalcon	falconpy
PATCH support	No	Yes
UI Editor support	No	Yes

Use Go for performance-critical workloads, concurrency, and type safety. Use Python for rapid development, PATCH support, and UI Editor development.

Manifest Structure

functions:
  - name: gather-evidence
    description: "Collect evidence from multiple sources"
    language: python
    path: "functions/gather-evidence"
    environment_variables:
      FALCON_CLIENT_ID: "${secrets.falcon_client_id}"
      LOG_LEVEL: "info"
    max_exec_duration_seconds: 30
    max_exec_memory_mb: 128
    handlers:
      - name: process
        method: POST
        path: "/api/investigations/{id}/evidence"
      - name: healthcheck
        method: GET
        path: "/api/health"

Handler fields: name (identifier), method (HTTP verb), path (route, supports {param} placeholders). A single function can expose multiple HTTP endpoints. Function description max 100 characters (alphanumeric only).

Go FDK Pattern

package main

import (
    "context"
    "log/slog"
    fdk "github.com/CrowdStrike/foundry-fn-go"
)

type greetingReq struct {
    Name string `json:"name"`
}

func newHandler(_ context.Context, _ *slog.Logger, _ fdk.SkipCfg) fdk.Handler {
    m := fdk.NewMux()
    m.Post("/greetings", fdk.HandleFnOf(func(ctx context.Context, r fdk.RequestOf[greetingReq]) fdk.Response {
        return fdk.Response{
            Code: 200,
            Body: fdk.JSON(map[string]string{"greeting": "Hello, " + r.Body.Name}),
        }
    }))
    return m
}

func main() {
    fdk.Run(context.Background(), newHandler)
}

Key FDK concepts: fdk.SkipCfg (no config file), fdk.NewMux() (router), fdk.HandleFnOf[T] (typed handler), fdk.RequestOf[T] (typed request with .Body, .Params, .URL, .Method), fdk.JSON() (response body helper).

Go Authentication

Go requires explicit credential wiring through the FDK. Use fdk.FalconClientOpts() for correct cloud and user-agent configuration:

opts := fdk.FalconClientOpts()
falconClient, err := falcon.NewClient(&falcon.ApiConfig{
    AccessToken:       accessToken,
    Cloud:             falcon.Cloud(opts.Cloud),
    Context:           ctx,
    UserAgentOverride: opts.UserAgent,
})

Python FDK Pattern

from logging import Logger
from typing import Any, Dict, Union
from crowdstrike.foundry.function import Function, Request, Response

func = Function.instance()

@func.handler(method='POST', path='/greetings')
def on_post(request: Request, config: Union[Dict[str, Any], None], logger: Logger) -> Response:
    name = request.body.get("name", "World")
    return Response(body={'greeting': f'Hello, {name}!'}, code=200)

@func.handler(method='GET', path='/health')
def on_get(request: Request, config: Union[Dict[str, Any], None], logger: Logger) -> Response:
    return Response(body={'status': 'ok'}, code=200)

if __name__ == '__main__':
    func.run()

Python Authentication

FalconPy handles credential discovery automatically. Call Service Class constructors with zero arguments:

from falconpy import Alerts
falcon = Alerts()  # Auth is automatic — do not pass credentials

• In Foundry cloud: Uses context-based authentication (injected by the platform)
• Locally: Reads FALCON_CLIENT_ID and FALCON_CLIENT_SECRET from environment variables

FalconPy already reads env vars internally, so writing a get_falcon_client() wrapper that manually reads credentials adds no value and breaks context auth in the cloud.

requirements.txt Best Practices

Pin all dependencies to exact versions (==) for reproducible builds and supply chain safety. The one exception is crowdstrike-falconpy, which should be left unpinned so functions always pick up the latest SDK (needed for context-based auth and new service classes). Ensure the file ends with a trailing newline.

crowdstrike-foundry-function==1.1.4
crowdstrike-falconpy

Workflow Array Output

When a function returns array data to a workflow, wrap the array in a JSON object. Direct array returns are not supported by the workflow engine:

# Correct — workflows can reference $step.output.items
return Response(body={'items': [{'id': 1}, {'id': 2}]}, code=200)

# Breaks workflow variable resolution
return Response(body=[{'id': 1}, {'id': 2}], code=200)

Error Handling

Return structured JSON with status codes. MUST NOT leak raw stack traces:

try:
    result = process(request.body)
    return Response(body={"status": 200, "data": result}, code=200)
except ValueError as e:
    return Response(body={"error": {"code": "INVALID_INPUT", "message": str(e)}}, code=400)
except Exception:
    return Response(body={"error": {"code": "INTERNAL_ERROR", "message": "An unexpected error occurred"}}, code=500)

For the full FunctionError class with enum codes, see references/python-patterns.md.

Common Pitfalls

• Using requests instead of CrowdStrike SDKs. The SDKs handle auth, retries, regions, and error parsing.
• Using APIHarnessV2 (Uber class) for collection operations. Use CustomStorage service class instead so the Foundry functions editor can auto-detect OAuth scopes. See the Collection CRUD Pattern in references/python-patterns.md.
• Manually reading env vars for FalconPy auth. Alerts() with zero arguments handles all credential discovery.
• Shared utility files across functions. sys.path.append("../") works locally but not in Foundry's FaaS runtime. Copy shared files into each function directory.
• SearchObjects returns metadata, not objects. Follow up with GetObject to retrieve actual content.
• Returning arrays directly to workflows. Wrap in a JSON object ({'items': [...]} not [...]).
• Using PATCH with Go functions. Go only supports GET, POST, PUT, DELETE.
• Using definition_id vs. name for API integrations. Calling API integrations from functions locally requires the UUID definition_id from manifest.yml, not the human-readable name.

Use Cases

For real-world implementation patterns, see:

• python-functions.md — Python handler patterns, SDK usage, testing
• logscale-ingestion.md — Ingesting custom data into Falcon LogScale
• api-pagination.md — Pagination strategies in functions and workflows

Reference Implementations

• foundry-sample-functions-python: Reference Python function patterns. See also Dive into Falcon Foundry Functions with Python.
• foundry-sample-anomali-threatstream: Side-by-side Go and Python auth patterns.
• foundry-sample-logscale: LogScale ingestion patterns.
• foundry-sample-servicenow-itsm: Go function patterns with ServiceNow integration.
• foundry-sample-ngsiem-importer: Python function for importing threat intel into NG-SIEM.