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negative-test-generator

Generates negative / error-path test cases that mirror happy-path tests - for each happy-path test, produces companions exercising input validation rejection, missing required fields, type mismatches, authorization failures, rate-limit errors, and adversarial payloads from the malicious-payload-bank. Emits cases as parameterized tests in the project's runner format. Use when a feature has happy-path coverage but the rejection / error / unauthorized paths are untested.

negative-test-generator

Overview

A typical test suite has a happy-path test for every endpoint: "valid input → 200 / created object / etc." What's missing is the rejection path - what happens with malformed input, missing fields, wrong type, unauthorized user, rate-limited request, adversarial payload?

This skill takes a happy-path test and emits its negative companions - one per failure mode. The result is a paired set where every "should accept X" has a sibling "should reject Y" catalog.

When to use

  • A feature has a happy-path test but no error-path coverage.
  • An API endpoint accepts user input without documented rejection testing.
  • A team is migrating from "test only what works" to "test every documented response code."
  • Pairs with acceptance-criteria-extractor output: every Then-clause's positive assertion gets a negative sibling.

Negative-path categories

For each happy-path test of a request <verb> <path> with body fields <f1, f2, ...>, the negative-test set covers:

1. Schema violations (validation rejection)

PatternTest
Missing required fieldSend the request without <f1>; expect 400 with <f1> named.
Wrong typeSend <f1> as wrong type (string where int expected); expect 400.
Out-of-range valueSend <f1> outside [min, max] per boundary-value-generator; expect 400.
Wrong enum valueSend <f1> with a value not in enum_values; expect 400.
Wrong formatString that fails the regex / format constraint; expect 400.
Extra unknown fieldSend field not in the schema; behavior depends on policy (strict reject vs. ignore).
Empty / null where forbiddenSend null or empty string for a non-nullable field.
Excess length / countSend string longer than max-length; collection larger than max-count.

2. Authentication failures

PatternTest
No Authorization headerExpect 401.
Expired tokenExpect 401 with token expired indication.
Malformed tokenExpect 401, no info leak about token shape.
Token from a different tenantExpect 401 / 403.

3. Authorization failures

PatternTest
User without permissionAuthenticated but role lacks required permission; expect 403.
Resource owned by another userRead/write attempt on someone else's resource (IDOR); expect 403 / 404.
Cross-tenant accessAttempt resource from another organization; expect 403 / 404.

4. Rate / quota failures

PatternTest
Burst exceeds rate limitSend N+1 requests in window; expect 429.
Plan quota exceededFree-tier user exceeds Free quota; expect 403 / 429 with quota info.

5. Conflict / state errors

PatternTest
Duplicate creationCreate same resource twice; expect 409.
Optimistic-lock conflictStale If-Match; expect 412.
State machine violationInvalid status transition (e.g. placed → completed skipping shipped); expect 422 / 409.

6. Adversarial inputs

Pull from malicious-payload-bank:

Field typePayload classes
Free-text inputXSS, SQLi, Unicode confusables
URL fieldSSRF, open-redirect
File upload namePath traversal
Search fieldReDoS, SQLi

7. Server errors (where applicable)

PatternTest
Upstream dependency unavailableMock the dependency to fail (per api-chaos-runner); expect 502 / 503 with retry hint.
TimeoutMock slow upstream; expect 504 / circuit-breaker fallback.

Worked example

Given a happy-path test:

def test_create_order_succeeds():
    response = post('/api/orders',
                    headers={'Authorization': f'Bearer {token}'},
                    json={'sku': 'SKU-1', 'qty': 2})
    assert response.status_code == 201
    assert response.json()['order_id']

The skill emits negative companions:

import pytest
from tests.fixtures import token, expired_token, free_tier_token
from tests.payloads import XSS, SQLI

# 1. Schema violations
@pytest.mark.parametrize('body,expected_status,expected_field', [
    ({'qty': 2},                  400, 'sku'),         # missing required
    ({'sku': 'SKU-1'},            400, 'qty'),         # missing required
    ({'sku': 'SKU-1', 'qty': 'two'}, 400, 'qty'),    # wrong type
    ({'sku': 'SKU-1', 'qty': 0},  400, 'qty'),         # below min
    ({'sku': 'SKU-1', 'qty': -1}, 400, 'qty'),         # negative
    ({'sku': 'SKU-1', 'qty': 1000000}, 400, 'qty'),    # above max
    ({'sku': 'INVALID-SKU', 'qty': 2}, 400, 'sku'),     # unknown enum
])
def test_create_order_schema_rejects(body, expected_status, expected_field):
    response = post('/api/orders',
                    headers={'Authorization': f'Bearer {token}'},
                    json=body)
    assert response.status_code == expected_status
    assert expected_field in response.json().get('errors', {})

# 2. Auth failures
def test_create_order_no_token():
    response = post('/api/orders', json={'sku': 'SKU-1', 'qty': 2})
    assert response.status_code == 401

def test_create_order_expired_token():
    response = post('/api/orders',
                    headers={'Authorization': f'Bearer {expired_token}'},
                    json={'sku': 'SKU-1', 'qty': 2})
    assert response.status_code == 401

# 3. Authorization failures
def test_create_order_free_tier_blocked_for_premium_sku():
    response = post('/api/orders',
                    headers={'Authorization': f'Bearer {free_tier_token}'},
                    json={'sku': 'PREMIUM-1', 'qty': 1})
    assert response.status_code == 403

# 6. Adversarial payloads
@pytest.mark.parametrize('payload', XSS + SQLI)
def test_create_order_handles_adversarial_sku(payload):
    response = post('/api/orders',
                    headers={'Authorization': f'Bearer {token}'},
                    json={'sku': payload, 'qty': 1})
    # Either reject (preferred) or escape — never pass through with execution
    assert response.status_code in (400, 404, 422)
    assert '<script>' not in response.text

Output format

## Negative tests for `<endpoint>``<verb> <path>`

**Happy path:** `tests/<file>::test_<happy_name>`
**Negative companions generated:** N (across 6 categories)

### Tests by category

| Category               | Count | File                                                |
|------------------------|------:|-----------------------------------------------------|
| Schema violations      |    7  | `tests/<file>::test_<endpoint>_schema_rejects`     |
| Auth failures          |    2  | `tests/<file>::test_<endpoint>_no_token` etc.       |
| Authorization failures |    1  | `tests/<file>::test_<endpoint>_unauthorized`        |
| Rate/quota failures    |    0  | (skipped; rate-limit not yet implemented)           |
| Conflict/state errors  |    1  | `tests/<file>::test_<endpoint>_duplicate`           |
| Adversarial payloads   |    8  | `tests/<file>::test_<endpoint>_handles_adversarial`|

### Skipped categories

- Rate/quota failures: rate limiting not yet implemented; revisit when added.
- Server errors: covered by `api-chaos-runner` in a separate suite.

### Recommended next step

1. Run the new negative tests; expect all to pass given the
   documented behavior.
2. Any failure indicates a real gap — either the validator is
   missing the case OR the assertion is wrong.
3. For passes: commit.

Anti-patterns

Anti-patternWhy it failsFix
Negative tests that assert vague status codes ("not 200")The test passes if the server returns 500 (a bug); should fail.Always assert specific status: 400 for validation, 401 for auth, 403 for authz, 429 for rate, etc.
Skipping the "wrong type" casesThe most common validator bug class.Always include type-mismatch cases for every field.
One mega-test that covers all negativesFailure attribution unclear; one failure cascades.Parameterize per category; one test function per category.
Negative-path coverage but no _synthetic dataReal-looking PII flows through; compliance issue if logs leak.Use synthetic-pii-generator for any field that could be PII.
Asserting 'error' in response.bodyBrittle; error-message text changes; locale-specific bugs.Assert structured field: response.json()['errors']['<field>'] or response.headers['X-Error-Code'].

Limitations

  • Doesn't auto-detect input domain. The team must declare required fields, types, ranges, and enum values for the skill to generate the matching cases. Use acceptance-criteria-extractor
  • Server-error simulation requires mocking. Categories 5 and 7 need api-chaos-runner or wiremock-stubs / msw-handlers for the upstream failure simulation.
  • Per-language output styling. Test scaffolding adapts to pytest / Jest / xUnit / JUnit; the skill emits the project's preferred runner format.

References