If you’re running a local model through llama.cpp with OpenClaw and your tools aren’t working — getting HTTP 400 errors, silent tool-call failures, or seeing the agent respond as if no tools exist — there’s a one-line config fix.

Set compat.toolSchemaProfile: "llamacpp" on your custom model entry.

Here’s why this happens and exactly how to fix it.

The Root Cause: GBNF Schema Incompatibility

llama.cpp (llama-server) uses a GBNF (GGML BNF grammar)-based parser to handle tool call arguments. This parser is stricter than what most LLM API consumers expect, and it fails on certain JSON Schema constraints that OpenClaw includes in its rich tool definitions.

The two main culprits that break the GBNF parser:

  • pattern fields — JSON Schema regex patterns (e.g., "pattern": "^[a-z]+")
  • maxLength values ≥ 2000 — large string size limits, like those on the trigger.script field of the cron tool

When the GBNF converter encounters these, it throws errors like:

JSON schema conversion failed: Pattern must start with '^' and end with '$'

Or it may fail silently, causing the agent to not use tools at all.

Why Built-in Providers Work But Your Custom Provider Doesn’t

OpenClaw’s built-in llama-cpp, ollama, and lmstudio providers apply a schema cleaner automatically. They know they’re talking to GBNF-constrained backends and strip incompatible constraints before sending tool schemas.

When you set up a custom openai-completions provider — which is the typical way to connect to a remote llama-server endpoint — that automatic cleaning doesn’t happen. Your custom provider is treated as a generic OpenAI-compatible endpoint, so OpenClaw sends the full, unmodified schema.

That’s where toolSchemaProfile comes in.

The Fix

In your OpenClaw config, add compat.toolSchemaProfile: "llamacpp" to the model entry for your llama.cpp model.

Here’s a complete, working example configuration:

"models": {
  "mode": "merge",
  "providers": {
    "local-llm": {
      "baseUrl": "http://localhost:8989/v1",
      "apiKey": "llamacpp",
      "api": "openai-completions",
      "models": [
        {
          "id": "Qwen3-8B-Q8_0",
          "name": "Qwen3-8B-Q8_0",
          "compat": {
            "supportsTools": true,
            "toolSchemaProfile": "llamacpp"
          }
        }
      ]
    }
  }
}

The toolSchemaProfile: "llamacpp" setting tells OpenClaw to apply the GBNF-compatible schema cleaner before submitting tool definitions to this model endpoint.

What the cleaner actually does:

  • Strips pattern values from JSON Schema properties
  • Removes maxLength values ≥ 2000
  • Leaves the rest of the schema intact

It’s a targeted fix, not a full JSON Schema → GBNF conversion — just removing the specific constraints that break the GBNF parser.

After making this change, restart OpenClaw (or reload the config) and re-test a tool-using agent. The error should disappear and tools should start working.

Confirming the Fix Worked

The simplest verification: ask the agent to do something that requires a tool — like searching the web, reading a file, or executing a shell command. If tools were silently failing before, you’ll see the agent actually invoke tools in the response.

If you were getting explicit errors, they should stop appearing in the OpenClaw gateway logs.

Additional Context: Pending Work and Edge Cases

From the official OpenClaw docs and the community discussion thread that surfaced this issue:

  • There is a pending PR targeting improved GBNF compatibility in llama.cpp itself — if that lands, the pattern constraint issue may be resolved upstream and the profile setting may become unnecessary for newer llama-server builds
  • Some users running a community proxy between llama-server and OpenClaw also address this at the proxy layer by stripping schema constraints before forwarding; this is an alternative if you can’t modify the OpenClaw config directly
  • The toolSchemaProfile setting is model-specific — you can have one provider with multiple model entries and only set "llamacpp" on the models that need it

Where This Config Lives

The compat.toolSchemaProfile setting is documented in the official OpenClaw configuration reference under Tools and custom providers. If you need to see the full list of supported toolSchemaProfile values or other compat keys for your setup, that’s the authoritative source.

For troubleshooting the specific cron tool failure (which uses a trigger.script field with maxLength: 4096), this is currently the correct and documented fix.

Sources

  1. OpenClaw Docs: Configuration — Tools and custom providers — official documentation for toolSchemaProfile and compat.* keys
  2. Reddit r/openclaw — OpenClaw and llama.cpp with tools enabled (Qwen3.8B) — community thread documenting the original issue and workaround
  3. X / @gilkmanz — original thread surfacing the issue — practitioner report with reproduction steps on RTX 4090 + Qwen3.5-28B

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