Gondulf/docs/reports/2025-12-17-bugfix-pkce-optional.md

Implementation Report: PKCE Optional Bug Fix

Date: 2025-12-17 Developer: Claude (Developer Agent) Design Reference: /home/phil/Projects/Gondulf/docs/designs/bugfix-pkce-optional-v1.0.0.md

Summary

Successfully implemented the PKCE optional bug fix in the authorization endpoint. The /authorize endpoint was incorrectly requiring PKCE parameters (code_challenge and code_challenge_method), which contradicted ADR-003 that explicitly defers PKCE to v1.1.0. The fix makes PKCE parameters optional while maintaining validation when they are provided. All tests pass (536 passed, 5 skipped) with overall test coverage at 90.51%.

What Was Implemented

Components Modified

1. /home/phil/Projects/Gondulf/src/gondulf/routers/authorization.py (lines 325-343)

   • Replaced mandatory PKCE validation with optional validation
   • Added default behavior for code_challenge_method (defaults to S256)
   • Added logging for clients not using PKCE

2. /home/phil/Projects/Gondulf/tests/integration/api/test_authorization_flow.py

   • Removed test that incorrectly expected PKCE to be required (test_missing_code_challenge_redirects_with_error)
   • Added comprehensive test suite for optional PKCE behavior (4 new tests)

Key Implementation Details

Authorization Endpoint Changes:

• Removed the error response for missing code_challenge parameter
• Changed validation logic to only check code_challenge_method when code_challenge is provided
• Added default value of "S256" for code_challenge_method when code_challenge is present but method is not specified
• Added info-level logging when clients don't use PKCE for monitoring purposes (per ADR-003)

Test Updates:

• Created new test class TestAuthorizationPKCEOptional with 4 test scenarios
• Tests verify all behaviors from the design's behavior matrix:
  • Authorization without PKCE succeeds (session created with None values)
  • Authorization with PKCE succeeds (session created with PKCE values)
  • Authorization with code_challenge but no method defaults to S256
  • Authorization with invalid method (not S256) is rejected

How It Was Implemented

Approach

1. Read and understood the design document thoroughly before making any changes
2. Reviewed ADR-003 to understand the architectural decision behind PKCE deferral
3. Implemented the exact code replacement specified in the design document
4. Identified and removed the incorrect test that expected PKCE to be mandatory
5. Added comprehensive tests covering all scenarios in the behavior matrix
6. Ran the full test suite to verify no regressions

Implementation Order

1. Modified authorization.py with the exact replacement from the design
2. Removed the test that contradicted ADR-003
3. Added 4 new tests for optional PKCE behavior
4. Verified all tests pass with good coverage

Key Decisions Made

All decisions were made within the bounds of the design:

• Used exact code replacement from design document (lines 325-343)
• Followed the behavior matrix exactly as specified
• Applied testing standards from /home/phil/Projects/Gondulf/docs/standards/testing.md

Deviations from Design

No deviations from design.

Issues Encountered

Challenges

No significant challenges encountered. The design was clear and comprehensive, making implementation straightforward.

Unexpected Discoveries

None - the implementation proceeded exactly as designed.

Test Results

Test Execution

============================= test session starts ==============================
platform linux -- Python 3.11.14, pytest-9.0.1, pluggy-1.6.0
rootdir: /home/phil/Projects/Gondulf
configfile: pyproject.toml
plugins: anyio-4.11.0, asyncio-1.3.0, mock-3.15.1, cov-7.0.0, Faker-38.2.0
asyncio: mode=Mode.AUTO, debug=False

================= 536 passed, 5 skipped, 39 warnings in 18.09s =================

Test Coverage

• Overall Coverage: 90.51%
• Coverage Tool: pytest-cov 7.0.0
• Coverage Target: 80.0% (exceeded)

Authorization Router Coverage: 74.32%

• The coverage drop in authorization.py is due to error handling paths that are difficult to trigger in tests (e.g., database failures, DNS failures)
• The core PKCE logic added/modified is fully covered by the new tests

Test Scenarios

New Tests Added (TestAuthorizationPKCEOptional)

1. test_authorization_without_pkce_succeeds

   • Verifies clients without PKCE can complete authorization
   • Confirms session created with code_challenge=None and code_challenge_method=None
   • Tests the primary bug fix (PKCE is optional)

2. test_authorization_with_pkce_succeeds

   • Verifies clients with PKCE continue to work unchanged
   • Confirms session stores PKCE parameters correctly
   • Ensures backward compatibility for PKCE-using clients

3. test_authorization_with_pkce_defaults_to_s256

   • Verifies code_challenge without method defaults to S256
   • Confirms session stores code_challenge_method="S256" when not provided
   • Tests the graceful defaulting behavior

4. test_authorization_with_invalid_pkce_method_rejected

   • Verifies invalid method (e.g., "plain") is rejected when code_challenge provided
   • Confirms error redirect with proper OAuth error format
   • Tests that we only support S256 method

Modified Tests

• Removed: test_missing_code_challenge_redirects_with_error
  • This test was incorrect - it expected PKCE to be mandatory
  • Removal aligns tests with ADR-003

Integration Tests

All existing integration tests continue to pass:

• End-to-end authorization flows (9 tests)
• Token exchange flows (15 tests)
• Authorization verification flows (10 tests)
• Response type flows (20 tests)

Test Results Analysis

All tests passing: Yes (536 passed, 5 skipped)

Coverage acceptable: Yes (90.51% overall, exceeds 80% requirement)

Test coverage gaps: The authorization router has some uncovered error paths (DNS failures, email failures) which are difficult to trigger in integration tests without extensive mocking. These are acceptable as they are defensive error handling.

Known issues: None

Technical Debt Created

Debt Item: Authorization router error handling paths have lower coverage (74.32%)

Reason: Many error paths involve external service failures (DNS, email) that are difficult to trigger without extensive mocking infrastructure

Suggested Resolution:

• Consider adding unit tests specifically for error handling paths
• Could be addressed in v1.1.0 alongside PKCE validation implementation
• Not blocking for this bug fix as core functionality is well-tested

Next Steps

1. Architect Review: This implementation report is ready for review
2. Deployment: Once approved, this fix can be deployed to production
3. Monitoring: Monitor logs for clients not using PKCE (info-level logging added)
4. v1.1.0 Planning: This fix prepares the codebase for PKCE validation in v1.1.0

Sign-off

Implementation status: Complete

Ready for Architect review: Yes

All acceptance criteria met: Yes

• Clients without PKCE can complete authorization flow ✓
• Clients with PKCE continue to work unchanged ✓
• Invalid PKCE method (not S256) is rejected ✓
• PKCE parameters are stored in auth session when provided ✓
• All existing tests continue to pass ✓
• New tests cover optional PKCE behavior ✓

Test coverage: 90.51% overall (exceeds 80% requirement)

Deviations from design: None

Blockers: None