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feat: add per-tool CEL expression rail - #2430
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📝 WalkthroughWalkthroughThis change adds per-tool CEL checks for tool calls and tool results. It adds configuration and evaluation actions, registers the checks as IORails surfaces, adds unit and integration tests, and documents configuration and engine support. ChangesPer-tool CEL rails
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~25 minutes Change: Feature Sequence Diagram(s)sequenceDiagram
participant IORails
participant ModelTransport
participant evaluate_tool_output_cel
participant Tool
participant evaluate_tool_input_cel
ModelTransport->>IORails: tool call
IORails->>evaluate_tool_output_cel: tool call and tool definition
evaluate_tool_output_cel-->>IORails: RailOutcome
IORails->>Tool: allowed tool call
Tool-->>IORails: tool result
IORails->>evaluate_tool_input_cel: tool result and tool call
evaluate_tool_input_cel-->>IORails: RailOutcome
IORails->>ModelTransport: allowed tool result
Suggested reviewers: Merge Risk: 🟡 Moderate · up to The example cannot be relied on as written: its stated protections do not match its rules, and it omits the documented tool setup. An enabled rail with a null CEL section also returns an error. Correct these issues before merging. 🚥 Pre-merge checks | ✅ 4 | ❌ 2❌ Failed checks (2 warnings)
✅ Passed checks (4 passed)
Full details: Docstring CoverageExplanation Docstring coverage is 12.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 50 functions across 8 files. (6 skipped: 6 unsupported.) Full details: Test Results For Major ChangesExplanation The PR adds a major feature: 15 files change with about 1,159 insertions, including a new CEL rail, optional dependency, and integration tests. The full PR description has a Resolution Populate the PR description's
✨ Finishing Touches 💡 1📝 Generate docstrings 💡
🧪 Generate unit tests (beta)
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Actionable comments posted: 4
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Inline comments:
Review comments at @docs/configure-rails/guardrail-catalog/cel.mdx:
- Around line 106-132: Add a `run_shell` tool declaration to the complete
configuration example so the `cel check tool output` flow can reach CEL
evaluation, or include the required per-request `options.llm_params.tools` setup
alongside it. Keep the existing expressions and checks intact.
- Line 104: Update the description preceding the configuration block to say it
blocks run_shell calls whose working directory is under /etc; do not describe it
as blocking writes to /etc, since the rule checks only args.cwd.
- Line 119: Update the `args.command.matches` rule in the CEL guardrail to match
recursive forced-delete flags with either `r` before `f` or `f` before `r`,
while preserving the existing case-insensitive `rm` matching.
Review comments at @nemoguardrails/library/cel/actions.py:
- Line 97: Guard CEL configuration access in both CEL action paths, including
evaluate_tool_input_cel, so a null config.rails.config.cel yields no options
instead of calling getattr on None; preserve the existing no-options behavior
before evaluation.
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docs/configure-rails/guardrail-catalog/cel.mdxdocs/index.ymldocs/reference/rail-engine-support.mdxdocs/telemetry.mdxnemoguardrails/library/cel/__init__.pynemoguardrails/library/cel/actions.pynemoguardrails/library/cel/rail.pynemoguardrails/library/cel/rail_config.pypyproject.tomlschemas/rails_config.snapshot.jsontests/guardrails/test_per_tool_cel_rails_iorails.pytests/rails/llm/test_config.pytests/test_cel_actions.pytests/test_runtime_flow_gate_equivalence.py
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Looks good ! A few updates needed relating to the CEL compilation and where they're stored, syntax checking on the config, and doc updates.
| compiled.append(_compile(expression)) | ||
| except ValueError as e: | ||
| raise ValueError(f"Invalid CEL expression at index {i} ({expression!r}): {e}") from e | ||
| object.__setattr__(self, "_compiled_expressions", compiled) |
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Storing compiled Programs in the RailsConfig complicates serializing and deserializing, and also breaks the synchronous generate() and check() because they do a deepcopy(). Could you validate the expressions and store them as text rather than Program? They could then be cached and used as long as it's thread-safe
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fixed in the new commit, also add a unit test for the synchronous check. ensured cashing and using the cached expression is thread safe as well
| return cel.compile(expression) | ||
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| class CelOptions(RailConfigBaseModel): |
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Unknown keys under rails.config.cel are ignored, and a tool with no expressions is treated as "nothing-to-check". So if you have a typo mismatch this turns the CEL into a fail-open with no warning.
Recommend setting model_config = ConfigDict(extra="forbid") on CelOptions and CelConfig and require at least one expression when a tool entry is present (with Field(min_length=1). Raise an exception to catch these corner cases
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addressed in the new commit
| if not isinstance(result, bool): | ||
| raise TypeError(f"returned {type(result).__name__}, not bool") | ||
| except Exception as e: | ||
| log.warning("CEL expression %r for tool %r failed: %s: %s", expression, tool_name, type(e).__name__, e) |
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This exception might include the text being checked, which we don't want to leak through an exception. Recommend raising using type(error).__name__ and drop the str(error). We can't separate out the debugging info from the text that caused the exception.
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addressed in the new commit, removed the warning and the runtime error now only shows the error type
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| ## Complete Example | ||
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| The following configuration blocks recursive forced deletes in `run_shell`, writes to `/etc` or `/root` and appends to `.bashrc` through `write_file`, and any `run_shell` result that contains a private key: |
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Could you add a note to say it's not production-level and is intended to illustrate the concept simply rather than being comprehensive? You could bypass these with rm -r -f, rm -f -r, etc
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fixed in the new commit
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Description
Adds
cel, a deterministic per-tool rail that blocks a tool call (tool_outputdirection) or a tool result (tool_inputdirection) when any of its Common Expression Language (CEL) expressions evaluates to true. Unlike regex, which matches text, an expression reads the structured fields of a call directly, so a single rule can combine several arguments (has(args.cwd) && args.cwd.startsWith("/etc")). It calls no model and builds on the per-tool rail infrastructure from #2377Key design points:
rails.config.cel.tool_output/tool_input, keyed by tool name, and are scoped throughrails.tool_output.per_tool/rails.tool_input.per_tool.cel check tool outputruns behind@tool_output_validation, so a call to an undeclared tool, or one with schema-invalid arguments, is blocked before any expression runs.args(the call's arguments) andtool(the tool name). Tool results getresult(exactly as the tool returned it: a string, a list of content blocks, ornull), plusargsandtoolfrom the call the result answers. The cel-go extension functions (lowerAscii,split,lists.range, ...) are registered so Numbat-style rules work.$argument=scoping. Expressions already address fields directly, so the flag would be redundant. The existing unknown-$param=check rejects it at compile time.matched_expressionsin the metadata.sourceand the author-written expressions, never the arguments or result, because metadata reaches logs and traces.common-expression-language(Rust-backed, imported ascel) is added as a newcelextra, pinned>=0.10.0,<0.11.0and limited to Python 3.11+, because the package has no 3.10 release. It is imported lazily, so configs that don't use the rail never load it. The manifest declares it inoptional_dependencies, so a config that uses the rail without the extra fails to compile with an error naming the extra, the same waysensitive_data_detectionhandles Presidio on 3.13.Related Issue(s)
Verification
make test: 7507 passed, 214 skipped.nemoguardrails/library/cel/: 100% of lines and branches (94 statements, 20 branches).Ran a real
nemoguardrails server(IORails engine) against the hosted NIM endpoint(
nvidia/nemotron-3-ultra-550b-a55b,engine: nim) with no mocked transport:--
cel check tool output:args.command.matches("\\brm\\s+-rf")blocked a realrun_shellcallfor
rm -rf /tmp/scratchand passedls -la--
cel check tool input:result.contains("BEGIN RSA PRIVATE KEY")blocked a tool resultcontaining a private key and passed a clean one
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