fix(checks): reject unsatisfiable rail_types with 422 instead of silent pass

[m-misiura]

Description

Related Issue(s)

This PR deals with the following GH issue

Verification

1. All local tests pass, e.g.

make test TEST=tests/server/test_guardrail_checks.py

.....................                                                                                          [100%]

══════════════════════════════════════════════════ inline-snapshot ═══════════════════════════════════════════════════
INFO: inline-snapshot was disabled because you used xdist. This means that tests with snapshots will continue to run, 
but snapshot(x) will only return x and inline-snapshot will not be able to fix snapshots or generate reports.


================================================= 21 passed in 6.18s =================================================

2. Tests against the live local server yields the following results:

Case 1: define only input rail

rails:
  config:
    hf_classifier:
      hap:
        engine: local
        model: ibm-granite/granite-guardian-hap-125m
        threshold: 0.5
        blocked_labels:
          - LABEL_1

  input:
    flows:
      - hf classifier check input $classifier=hap

successful request (200)

curl -s localhost:8000/v1/checks \
  -H 'Content-Type: application/json' \
  -d '{
    "model": "checks_input_only",
    "messages": [{"role": "user", "content": "hello"}],
    "guardrails": {"config_id": "checks_input_only", "rail_types": ["input"]}
  }'
{"status":"passed","content":"hello"}%

unsuccessful request (422 error)

curl -s localhost:8000/v1/checks \
  -H 'Content-Type: application/json' \
  -d '{
    "model": "checks_input_only",
    "messages": [{"role": "user", "content": "hello"}],
    "guardrails": {"config_id": "checks_input_only", "rail_types": ["output"]}
  }'
{"error":{"message":"Requested rail type 'output' has no configured rails.","type":"invalid_request_error","param":null,"code":null}}

AI Assistance

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• AI tools were used; a human reviewed and can explain every change (tool: Claude Code).

Checklist

• I've read the CONTRIBUTING guidelines.
• This PR links to a triaged issue assigned to me.
• My PR title follows the project commit convention.
• I've updated the documentation if applicable.
• I've added tests if applicable.
• I've noted any verification beyond CI and any checks I couldn't run.
• I did not update generated changelog files manually.
• I addressed all CodeRabbit, Greptile, and other review comments, or replied with why no change is needed.
• @mentions of the person or team responsible for reviewing proposed changes.

Summary by CodeRabbit

• New Features

  • Added clear validation when explicitly requested rail types are not configured.
  • Exposed a dedicated error for unsatisfiable rail requests.
  • API responses now return HTTP 422 with a descriptive error message.

• Bug Fixes

  • Prevented generation or checks from running when requested rails cannot be satisfied.
  • Automatic rail detection continues to work without unnecessary validation errors.

• Tests

  • Added coverage for configured, unconfigured, and automatically detected rail types.

[m-misiura]

cc @tgasser-nv @Pouyanpi

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Actionable comments posted: 1

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Inline comments:
In `@nemoguardrails/guardrails/iorails.py`:
- Around line 1357-1359: Document the new unsatisfiable rail_types behavior: in
nemoguardrails/guardrails/iorails.py lines 1357-1359 and
nemoguardrails/rails/llm/llmrails.py lines 1667-1669, update the public
check_async and check documentation to state that requesting a rail type with no
configured flows raises ValueError; in nemoguardrails/server/api.py lines
802-805, document the corresponding HTTP 422 response for explicitly requested
rail types without configured flows.

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---

ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 6e53c30 and 1ae02cb.

📒 Files selected for processing (6)

• nemoguardrails/guardrails/iorails.py
• nemoguardrails/rails/llm/llmrails.py
• nemoguardrails/server/api.py
• tests/guardrails/test_iorails_check.py
• tests/server/test_guardrail_checks.py
• tests/test_llmrails_check_async.py

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[greptile-apps]

Greptile Summary

The PR rejects explicitly requested rail types when the selected configuration has no corresponding flows, while preserving automatic rail detection.

• Adds RailTypeNotConfiguredError across both LLMRails and IORails check paths.
• Maps that dedicated exception to an OpenAI-shaped HTTP 422 response.
• Documents the new behavior and adds engine-level and server-level coverage.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

Important Files Changed

Filename	Overview
nemoguardrails/exceptions.py	Defines and exports the dedicated configuration exception used for unsatisfiable explicit rail requests.
nemoguardrails/guardrails/iorails.py	Validates explicit rail types before running the IORails check pipeline.
nemoguardrails/rails/llm/llmrails.py	Applies equivalent explicit rail-type validation to the LLMRails check path.
nemoguardrails/server/api.py	Registers the dedicated exception handler without reintroducing the previously reported broad runtime-error catch.
nemoguardrails/server/exception_handlers.py	Returns a 422 error envelope specifically for unconfigured requested rail types.
tests/guardrails/test_iorails_check.py	Covers satisfiable, unsatisfiable, and automatically detected IORails checks.
tests/server/test_guardrail_checks.py	Verifies that the checks endpoint maps the dedicated exception to HTTP 422.
tests/test_llmrails_check_async.py	Covers explicit and automatically selected rail types for LLMRails.

_{Reviews (4): Last reviewed commit: ":memo: rename to RailTypeNotConfiguredEr..." | Re-trigger Greptile}

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📥 Commits

Reviewing files that changed from the base of the PR and between 6e53c30 and a359aaf.

📒 Files selected for processing (8)

• nemoguardrails/exceptions.py
• nemoguardrails/guardrails/iorails.py
• nemoguardrails/rails/llm/llmrails.py
• nemoguardrails/server/api.py
• nemoguardrails/server/exception_handlers.py
• tests/guardrails/test_iorails_check.py
• tests/server/test_guardrail_checks.py
• tests/test_llmrails_check_async.py

Included review availability: Your plan includes up to 12 reviews per rolling hour; 10 remain after this review.

---

📝 Walkthrough

Walkthrough

The change adds UnsatisfiableRailTypeError and validates explicitly requested rail types in IORails and LLMRails. The checks API returns HTTP 422 for unavailable rail types. Tests cover explicit, configured, and automatic rail selection.

Changes

Rail type validation

Layer / File(s)	Summary
Rail availability validation nemoguardrails/exceptions.py, nemoguardrails/guardrails/iorails.py, nemoguardrails/rails/llm/llmrails.py, tests/guardrails/test_iorails_check.py, tests/test_llmrails_check_async.py	Adds the public UnsatisfiableRailTypeError. Explicit rail types without configured flows now raise this error. Configured input rails and automatic rail detection remain supported.
Checks API error mapping nemoguardrails/server/api.py, nemoguardrails/server/exception_handlers.py, tests/server/test_guardrail_checks.py	Registers an exception handler that returns HTTP 422 and includes the requested rail type in the error response.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🔵 Low · up to a359a

The PR changes the checks API to reject unsupported rail_types with a 422 error, but the new public behavior is not documented. This may leave API users unaware of the changed request and error contract, so the PR is mergeable with explicit owner follow-up to update the relevant documentation.

Suggested reviewers: pouyanpi, tgasser-nv

🚥 Pre-merge checks | ✅ 5 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: rejecting unsatisfiable rail types with a 422 response instead of silently passing.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Test Results For Major Changes	✅ Passed	The diff adds a new rail-validation feature and tests; the PR description documents make test with 21 passed plus live-server 200/422 results. Numerics and performance are not affected.

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[Pouyanpi]

Thanks @m-misiura Looks good overall 👍🏻

RailTypeNotConfiguredError may also be clearer than UnsatisfiableRailTypeError, what do you think?

a short docs note would be helpful too @ docs/run-rails/using-python-apis/check-messages.mdx

[m-misiura]

RailTypeNotConfiguredError may also be clearer than UnsatisfiableRailTypeError, what do you think?

a short docs note would be helpful too @ docs/run-rails/using-python-apis/check-messages.mdx

Many thanks for the review @Pouyanpi I've renamed to RailTypeNotConfiguredError as that name certainly conveys the message more explicitly than UnsatisfiableRailTypeError. I've also added a short note describing the error behaviour