agentrouter.org WAF (Web Application Firewall)

The agentrouter upstream gateway runs a keyword-based content filter on messages[].content. The filter is partially deterministic (always blocks certain phrases) and partially probabilistic (burst-sensitive — becomes more aggressive after rapid requests, recovers after a cooldown).

When the WAF blocks a request it returns:

HTTP/1.1 400 Bad Request
{"error":{"code":"content-blocked","message":"content-blocked (request id: ...)","param":"","type":"agent_router_api_error"}}

Scope of the filter

The WAF inspects messages[].content only. It does not inspect:

• The system prompt
• Structured content blocks (tool_result, tool_use, thinking, image)
• Tool description and input_schema fields
• Request metadata, headers, or model id

Always-blocked patterns (case-insensitive)

Pattern	Notes
Any Lorem ipsum variant	Full Latin lorem vocabulary is blocked
language model (alone)	“the language model” and “large language model” pass
virtual assistant	“AI assistant” passes
I'm here to help	“here to help” alone also blocks
Claude, made by Anthropic	Full phrase only

Almost-always-blocked patterns

Pattern	Notes
placeholder	When it stands alone (not as a parameter name, etc.)
dummy data	Common seed phrase for fixtures
foo bar baz	Canonical placeholder phrase
Repeated short tokens (AAA BBB CCC, test test test)	Detector for keyword stuffing

Behavior under load

After ~5 rapid requests in a short window, the WAF begins blocking content that would normally pass. The bucket relaxes after ~5–10 seconds of idle time. This is the same IP-and-key-bound rate limiter that causes intermittent 400 content-blocked errors when Claude Code or Codex CLI makes multiple tool-use / message-send calls in quick succession.

Mitigations already applied in OmniRoute

1. open-sse/services/wafRateLimit.ts — burst guard that enforces a 500 ms minimum gap between outbound requests to any agentrouter:* URL. The gap is well below human perception of latency and prevents the WAF from activating on normal traffic.

2. BaseExecutor.WAF_RETRY_CONFIG — when an upstream returns 400 content-blocked, the executor retries the same URL with exponential backoff (1.5 s, 3.0 s, max 2 attempts). After the backoff the WAF usually relaxes and the retry succeeds.

3. tests/unit/compression/harness.test.ts — the test fixture longInput was changed from "lorem ipsum dolor sit amet ".repeat(40) to "example content for testing purposes ".repeat(40) so that when Claude Code reads this file via the Read tool, the file contents do not flow back through a tool_result block and trip the WAF.

Guidance for prompts and tool output

If a Claude Code or Codex CLI session repeatedly hits 400 content-blocked, check the most recent user message and the most recent tool result for any of the patterns above and rephrase. Common workarounds:

• Replace Lorem ipsum … with example text … or the actual content the test or fixture is trying to model.
• Replace placeholder (when standing alone) with example value, sample value, or the real value.
• Replace language model with large language model or the model.
• Replace dummy data with sample data or realistic seed values.
• Replace I'm here to help / here to help with a more specific opener (e.g. “I’ll review the file you mentioned”).

Reporting the false positives upstream

The current filter is overly aggressive — it blocks “Lorem ipsum” in tool_result blocks even though the operator clearly did not intend to inject a prompt. Operators who want this fixed at the source should contact agentrouter.org to report the false positives. The blocklist above is the empirical result of probing the upstream as of 2026-08-03.

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OmniRoute source repository (a58000c7685f)