crw-best-practices

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Reference skill for building production-ready crw integrations. Covers verb selection, call surfaces (CLI/MCP/REST), post-filtering strategies, context-window hygiene, Hybrid RAG patterns, common pitfalls, and crw-specific operational considerations (search backend limits, renderer pool, proxy rotation). Load this when writing application code that embeds crw, designing a multi-step agent workflow, or debugging an integration that isn't behaving as expected.

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Sourceus/crw

Added on2026-08-11

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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.

bash

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_search

crw_scrape

crw_map

crw_crawl

crw_parse_file

)

Best inside an MCP-capable agent harness. The MCP server runs the engine either in-process (embedded mode, ~6 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

# 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.

python

# 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.

python

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:

python

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.

python

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:

python

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	500 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

crw-best-practices

NPX Install

Tags

SKILL.md Content

crw Best Practices

1. Choosing the right verb

2. Three call surfaces

CLI (
`crw`
)

MCP tools (
`crw_search`
,
`crw_scrape`
,
`crw_map`
,
`crw_crawl`
,
`crw_parse_file`
)

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

3. Post-filtering strategy stack

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

Layer 2: Regex / keyword density filter (cheap)

Layer 3: LLM verify (expensive — use sparingly)

4. Context-window hygiene

5. Self-hosted Hybrid RAG pattern

6. Common pitfalls

7. crw-specific operational awareness

Search backend rate limits and politeness

Renderer pool sizing

Proxy rotation

Managed vs self-hosted call-surface differences

8. Links

crw-best-practices

NPX Install

Tags

SKILL.md Content

crw Best Practices

1. Choosing the right verb

2. Three call surfaces

CLI (crw)

MCP tools (crw_search, crw_scrape, crw_map, crw_crawl, crw_parse_file)

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

3. Post-filtering strategy stack

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

Layer 2: Regex / keyword density filter (cheap)

Layer 3: LLM verify (expensive — use sparingly)

4. Context-window hygiene

5. Self-hosted Hybrid RAG pattern

6. Common pitfalls

7. crw-specific operational awareness

Search backend rate limits and politeness

Renderer pool sizing

Proxy rotation

Managed vs self-hosted call-surface differences

8. Links

CLI (
`crw`
)

MCP tools (
`crw_search`
,
`crw_scrape`
,
`crw_map`
,
`crw_crawl`
,
`crw_parse_file`
)

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