crw Best Practices

Reference documentation for developers and AI agents using crw (fastCRW) in production. Covers decision-making, integration patterns, and crw-specific operational details.

1. Choosing the right verb

Stop at the cheapest rung that answers the need. Don't reach for a heavier verb than the task requires.

Need	Verb	Notes
You have a question/topic, not a URL	search	Own search backend, no API key required. Returns titles + URLs + snippets. Add scrapeOptions to get markdown inline.
You have one (or a few) known URLs	scrape	Returns markdown, HTML, links, or structured JSON. JS auto-detected.
You need to discover which URLs exist on a site	map	Fast URL discovery via sitemap + BFS. No content fetched. Use before committing to a crawl.
You need content from many pages under a site	crawl	Async BFS job. Poll with crw_check_crawl_status. Always map first to estimate size.
The source is a local file (PDF)	parse	crw_parse_file (MCP) or crw scrape path/to/file.pdf (CLI). No network call.
You need a typed JSON object from a page	extract	--extract '<schema>' (CLI) or extract: {schema: {...}} (MCP/REST). Runs an LLM; costs tokens.
You want to detect what changed on a page	watch / diff	POST /v1/change-tracking/diff. Stateless diff primitive; no stored state needed.

Common chains:

• search → pick URLs → scrape the best ones
• search --json (or crw_search) → filter in Python subprocess → crw scrape chosen URLs
• map → estimate page count → crawl a bounded section → stream results
• map "https://docs.example.com" → find URLs → filter for /docs/api/auth → scrape that one URL

2. Three call surfaces

crw runs identically in three modes. Pick the one available in your environment.

CLI (crw)

Best for scripting, one-shot queries, and agent bash calls. Binary must be on PATH.

crw search "query" --json --limit 5
crw scrape "https://example.com" --format json
crw map "https://docs.example.com"
crw scrape "report.pdf"                              # local PDF auto-detected

Use CLI when: the binary is on PATH and you're in a Bash context. Especially good for the dynamic-search pattern (pipe into Python subprocess).

MCP tools (crw_scrape, crw_crawl, crw_check_crawl_status, crw_map, crw_extract, crw_check_extract_status, crw_cancel_extract, crw_search, crw_parse_file)

Best inside an MCP-capable agent harness. The MCP server runs the engine either in-process (embedded mode, ~14 MB RAM, no server) or as a proxy to a REST endpoint.

crw_scrape(url="https://example.com", formats=["markdown"], onlyMainContent=true)
crw_search(query="query", limit=5)
crw_map(url="https://docs.example.com", limit=200)

MCP output bounds (defaults): content truncated to ~15,000 chars per call; crw_map returns ≤ 100 URLs. Both carry truncated: true when clipped. Pass maxLength: 0 / limit: 0 to opt out.

Use MCP when: you're inside Claude Code, Cursor, Windsurf, or any harness that manages MCP connections. Lower per-call overhead than REST for agent loops.

REST API (/v1/scrape, /v1/search, etc.)

Best for application code, cross-language clients (Go, Java, Ruby), or when you need a shared microservice. Firecrawl-compatible — SDK swap is one api_url change.

# Python SDK (pip install crw)
from crw import CrwClient
client = CrwClient(api_url="https://api.fastcrw.com", api_key="crw_live_...")
result = client.scrape("https://example.com", formats=["markdown"])
results = client.search("AI news", limit=10)

# Drop-in for Firecrawl SDK
from firecrawl import FirecrawlApp
app = FirecrawlApp(api_url="https://api.fastcrw.com", api_key="crw_live_...")

Use REST when: writing application code, needing async crawl jobs with polling, or integrating with frameworks like LangChain / CrewAI / LlamaIndex.

3. Post-filtering strategy stack

Raw web results carry noise. Apply these in order, stopping when you have enough signal.

Layer 1: Rank/order-based triage (free)

The search backend's raw score is unreliable (engine-dependent, often null). Position is the reliable signal — it reflects the aggregator's Reciprocal Rank Fusion over N engines. Default: trust the top 3-5 results unless they're obviously off-topic.

# Rely on position, not score
top = [r for r in results if r['position'] <= 5]

Layer 2: Regex / keyword density filter (cheap)

Before fetching full pages, filter descriptions for relevance. Drop results whose description doesn't contain any query-adjacent term.

keywords = {'commercializ', 'battery', 'production', '2025', '2026'}
relevant = [r for r in results
            if any(kw in r['description'].lower() for kw in keywords)]

After scraping full markdown, apply paragraph-level filtering:

for para in markdown.split('\n\n'):
    if len(para) > 60 and any(kw in para.lower() for kw in keywords):
        print(para)

Layer 3: LLM verify (expensive — use sparingly)

When layers 1-2 aren't precise enough, send a small batch of candidate snippets to a cheap model for binary relevance classification.

import anthropic

def is_relevant(snippet: str, query: str) -> dict:
    """Returns {is_match: bool, confidence: float, reasoning: str}"""
    client = anthropic.Anthropic()
    msg = client.messages.create(
        model="claude-haiku-4-5",   # cheap model for classification
        max_tokens=128,
        messages=[{
            "role": "user",
            "content": (
                f"Query: {query}\n\n"
                f"Snippet: {snippet[:500]}\n\n"
                "Does this snippet directly answer or provide evidence for the query? "
                "Reply with JSON only: {\"is_match\": true/false, \"confidence\": 0-1, "
                "\"reasoning\": \"one sentence\"}"
            )
        }]
    )
    import json
    return json.loads(msg.content[0].text)

Gate: only call LLM-verify on snippets that passed layers 1-2. Don't send all 10 results through an LLM — pick the 3-5 most promising first.

4. Context-window hygiene

The single most important practice. See crw-dynamic-search for the full pattern. Summary:

• Never pipe crw search --json or crw scrape --format json bare into context. Always filter in a Python subprocess — only your print() output enters context.
• Write large results to .crw/ or /tmp/, not stdout. Use crw scrape -o .crw/page.json then read selectively with grep or a Python heredoc.
• MCP truncation is your first line of defense (default ~15K chars). But don't rely on it alone — a 15K char page is still 3,500+ tokens.
• Target 150-600 tokens per source in your filtered output. If you're printing more from a single page, you're probably including boilerplate.

5. Self-hosted Hybrid RAG pattern

crw is optimized for the retrieve → filter → embed pipeline. Typical setup:

crw search "query" → top-N results (titles + snippets)
→ scrape top 3-5 full pages → filter to relevant paragraphs
→ embed filtered paragraphs → merge with local vector store
→ retrieve top-K chunks → feed to generation model

Why crw for RAG:

• Search costs $0 per query (no per-call API fees)
• Recurring crawls use VPS cost, not per-page credits
• crw_crawl + jsonSchema can extract typed objects per page directly — skip the embed step for structured data

Python RAG skeleton:

from crw import CrwClient

client = CrwClient()  # embedded mode, no server

def retrieve_and_chunk(query: str, top_n: int = 5) -> list[str]:
    results = client.search(query, limit=top_n)
    chunks = []
    for r in results:
        # Scrape full page if the snippet isn't sufficient
        page = client.scrape(r['url'], formats=['markdown'])
        md = page.get('markdown', '') or ''
        # Split into paragraphs, keep non-trivial ones
        for para in md.split('\n\n'):
            para = para.strip()
            if len(para) > 100:
                chunks.append(para)
    return chunks

For a local vector store (Chroma, Qdrant, pgvector): embed these chunks, upsert with URL + position as metadata, then merge vector-store retrieval results with fresh crw search results at query time (hybrid retrieval).

6. Common pitfalls

Problem	Impact	Solution
Piping raw JSON into context	50K-500K chars enters context; token waste, reasoning degradation	Always filter in a Python subprocess — see crw-dynamic-search
Trusting score for triage	The search backend's scores are engine-dependent, often null; wrong results picked	Triage by position (rank order) + keyword density in description
Crawling without mapping first	Committing to a 500-page crawl when you needed 20 pages	Always crw map first to estimate site size; cap with maxPages
JS rendering on every scrape	Unnecessary browser spawn on plain-HTML pages; slow	crw auto-detects SPAs — don't add --js / renderJs: true unless the page is blank
Blocking on crawl job poll	Agent hangs waiting for async crawl	Set a poll interval (5-10s), set maxPages to bound job size, check status: "completed"
Ignoring truncated: true	Missing content from MCP calls; silent data loss	Check for truncated: true in MCP responses; pass maxLength: 0 if you need full content
Writing one-shot scripts to /tmp/	Wasteful; file left behind	Use heredocs for one-shot filtering; only write data (JSON results) to /tmp/
Scraping robots.txt-blocked pages	403/empty response; wasted call	crw respects robots.txt by default; use --stealth + proxy for legitimate access to blocked pages

7. crw-specific operational awareness

Unlike credit-based APIs (Firecrawl, Tavily), crw's costs are infra-denominated. The right mental model: you're paying for VPS time and renderer pool capacity, not per-page fees.

Search backend rate limits and politeness

• Public instances rate-limit or block JSON requests — always use a local instance (crw setup --local boots one via Docker).
• The self-hosted search backend has no built-in per-client rate limit, but the upstream engines (Google, Bing, DDG) do. Burst too hard and engines start returning 429s or CAPTCHAs to your instance.
• Practical safe rate: 2-4 searches/second burst, < 1/second sustained. Space parallel searches with a short sleep or process them in series.
• --category news and --time-range week bypass the general engine pool — lighter on upstream rate limits.

Renderer pool sizing

crw runs a renderer ladder per request (HTTP → LightPanda → Chrome by default; additional tiers such as playwright and chrome_proxy are available via config).

• HTTP tier is instant and stateless (no pool cost).
• LightPanda is lightweight (~50 MB) but single-process per binary instance. Under load, requests queue behind the LightPanda instance.
• Chrome (optional, docker compose --profile heavy) is the stealth fallback. Each Chrome instance is ~200 MB RAM. Scale by running multiple Chrome instances or pointing at a remote CDP endpoint via [renderer.chrome] ws_url in your server config (the CRW_CDP_URL env var is honored by crw scrape --js in CLI mode only, not by server/MCP mode).
• If you see consistent p90 timeouts, you're likely hitting the renderer queue. Add more Chrome instances or switch to fast mode (LightPanda-only, lower recall but faster tail).

Proxy rotation

Self-hosted crw supports per-request BYOP (bring-your-own-proxy) via --proxy URL (CLI) or proxy / proxyRotation (MCP/REST). Rotation modes: round_robin, random, sticky_per_host.

• LightPanda can't proxy — when a proxy is active, LightPanda is skipped (fail-closed). Only the HTTP and Chrome tiers route through the proxy.
• If using proxies for scraping targets that block cloud IPs, set proxyRotation: "sticky_per_host" so sessions from the same domain always hit the same exit IP (avoids anti-bot CAPTCHA triggers from IP-hopping mid-session).
• Proxy rotation applies to scrape, crawl, and map — not search (which goes to your local search backend, not directly to search engines).

Managed vs self-hosted call-surface differences

Feature	Self-hosted	Managed (api.fastcrw.com)
Search	Requires a local search-backend sidecar	Included (managed backend)
Proxy pool	BYOP via config	Managed proxy network
Rate limiting	Token-bucket (configurable)	Per-plan limits; X-RateLimit-* headers
Credits	N/A	1000 one-time lifetime free credits
AGPL obligation	Applies if you expose to third parties	Carve-out included

8. Links

• Hub skill: crw
• Token-saving subprocess pattern: crw-dynamic-search
• REST API reference: https://docs.fastcrw.com/#rest-api
• Self-host guide: https://docs.fastcrw.com/#self-hosting
• Firecrawl compatibility matrix: COMPATIBILITY-firecrawl.md in the repo
• Benchmarks: https://fastcrw.com/benchmarks