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feat(F08): Fallback Chains - X-Nexus-Fallback-Model Header#99

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leocamello merged 5 commits intomainfrom
feature/f08-fallback-chains
Feb 8, 2026
Merged

feat(F08): Fallback Chains - X-Nexus-Fallback-Model Header#99
leocamello merged 5 commits intomainfrom
feature/f08-fallback-chains

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@leocamello leocamello commented Feb 8, 2026

Summary

Completes F08 by implementing the X-Nexus-Fallback-Model HTTP response header, providing visibility into when fallback models are used.

Changes

T07: RoutingResult Struct

  • Created RoutingResult struct containing:
    • backend: Arc<Backend> - The selected backend
    • actual_model: String - The actual model used (may differ if fallback)
    • fallback_used: bool - True if a fallback model was used
  • Updated select_backend() to return RoutingResult instead of Arc<Backend>
  • Added NoHealthyBackend error variant for proper 503 responses

T08: X-Nexus-Fallback-Model Header

  • Added FALLBACK_HEADER constant: "x-nexus-fallback-model" (lowercase for HTTP/2)
  • Implemented header in both streaming and non-streaming responses
  • Header only present when fallback_used is true

T09-T10: Tests

  • Unit tests for RoutingResult with alias+fallback edge cases
  • Integration tests using wiremock for full HTTP verification
  • Streaming response header test

Acceptance Criteria

  • AC-01: Fallback chains configurable (F06)
  • AC-02: Tries each fallback in order (F06)
  • AC-03: Logs fallback usage at WARN level (F06)
  • AC-04: Returns 503 if all fallbacks exhausted (F06)
  • AC-05: Response model field shows requested model (F06)
  • AC-06: X-Nexus-Fallback-Model header indicates actual model used (F08) ✅

Test Results

cargo test: All tests passing
cargo clippy: 0 warnings
cargo fmt: Formatted

Verification

  • requirements-validation.md: 51 passed, 14 N/A, 0 unchecked
  • verification.md: 95 passed, 14 N/A, 0 unchecked

Closes #95
Closes #96
Closes #97
Closes #98

@leocamello
docs(F08): add X-Nexus-Fallback-Model header tasks
0450e66 
Updated spec.md:
- Added US-04 for fallback transparency header
- Updated AC-06 to require header implementation

Updated plan.md:
- Added implementation approach for RoutingResult struct
- Documented header integration strategy

Updated tasks.md:
- Added T07-T10 for header implementation with TDD tests
- Documented existing T01-T06 completed in F06
@leocamello
docs(F08): complete requirements validation checklist
1d42c0c 
51 passed, 14 N/A, 0 unchecked
Ready for implementation
@leocamello
docs(F08): add TDD enforcement protocol to tasks
5177941 
- Add RED phase checkpoint requirements
- Link T08 to AC-06 explicitly
- Add executable test commands to acceptance criteria
- Restructure T07-T10 with explicit Steps 1-4
@leocamello
feat(F08): implement X-Nexus-Fallback-Model header
bf86846 
T07: RoutingResult struct
- Create RoutingResult with backend, actual_model, fallback_used
- Update select_backend() to return RoutingResult
- Add NoHealthyBackend error variant for 503 responses

T08: X-Nexus-Fallback-Model header
- Add FALLBACK_HEADER constant (lowercase for HTTP/2)
- Implement header in both streaming and non-streaming responses
- Header only present when fallback_used is true

T09: Header unit tests
- routing_result_contains_fallback_info
- routing_result_no_fallback_when_primary_used
- routing_result_with_alias_and_fallback
- routing_result_no_fallback_info_when_no_fallback_configured

T10: Header integration tests
- api_response_includes_fallback_header
- response_no_fallback_header_when_primary_used
- streaming_response_includes_fallback_header

Closes #95
Closes #96
Closes #97
Closes #98
@leocamello
docs(F08): complete verification checklist and update spec status
5bbb6e9 
- Mark spec.md as ✅ Implemented
- Mark all AC as complete including AC-06
- Complete verification.md: 95 passed, 14 N/A, 0 unchecked
@leocamello leocamello added enhancement New feature or request P1 Phase 1 Priority fallback-chains F08: Fallback Chains feature labels Feb 8, 2026
@leocamello leocamello merged commit dab2ea0 into main Feb 8, 2026
8 checks passed
@leocamello leocamello deleted the feature/f08-fallback-chains branch February 8, 2026 21:35
leocamello added a commit that referenced this pull request Feb 17, 2026
@leocamello
feat(F08): Fallback Chains - X-Nexus-Fallback-Model Header (#99)
2580a50 
* docs(F08): add X-Nexus-Fallback-Model header tasks

Updated spec.md:
- Added US-04 for fallback transparency header
- Updated AC-06 to require header implementation

Updated plan.md:
- Added implementation approach for RoutingResult struct
- Documented header integration strategy

Updated tasks.md:
- Added T07-T10 for header implementation with TDD tests
- Documented existing T01-T06 completed in F06

* docs(F08): complete requirements validation checklist

51 passed, 14 N/A, 0 unchecked
Ready for implementation

* docs(F08): add TDD enforcement protocol to tasks

- Add RED phase checkpoint requirements
- Link T08 to AC-06 explicitly
- Add executable test commands to acceptance criteria
- Restructure T07-T10 with explicit Steps 1-4

* feat(F08): implement X-Nexus-Fallback-Model header

T07: RoutingResult struct
- Create RoutingResult with backend, actual_model, fallback_used
- Update select_backend() to return RoutingResult
- Add NoHealthyBackend error variant for 503 responses

T08: X-Nexus-Fallback-Model header
- Add FALLBACK_HEADER constant (lowercase for HTTP/2)
- Implement header in both streaming and non-streaming responses
- Header only present when fallback_used is true

T09: Header unit tests
- routing_result_contains_fallback_info
- routing_result_no_fallback_when_primary_used
- routing_result_with_alias_and_fallback
- routing_result_no_fallback_info_when_no_fallback_configured

T10: Header integration tests
- api_response_includes_fallback_header
- response_no_fallback_header_when_primary_used
- streaming_response_includes_fallback_header

Closes #95
Closes #96
Closes #97
Closes #98

* docs(F08): complete verification checklist and update spec status

- Mark spec.md as ✅ Implemented
- Mark all AC as complete including AC-06
- Complete verification.md: 95 passed, 14 N/A, 0 unchecked
leocamello added a commit that referenced this pull request Feb 17, 2026
@leocamello
feat(F08): Fallback Chains - X-Nexus-Fallback-Model Header (#99)
b6fea0b 
* docs(F08): add X-Nexus-Fallback-Model header tasks

Updated spec.md:
- Added US-04 for fallback transparency header
- Updated AC-06 to require header implementation

Updated plan.md:
- Added implementation approach for RoutingResult struct
- Documented header integration strategy

Updated tasks.md:
- Added T07-T10 for header implementation with TDD tests
- Documented existing T01-T06 completed in F06

* docs(F08): complete requirements validation checklist

51 passed, 14 N/A, 0 unchecked
Ready for implementation

* docs(F08): add TDD enforcement protocol to tasks

- Add RED phase checkpoint requirements
- Link T08 to AC-06 explicitly
- Add executable test commands to acceptance criteria
- Restructure T07-T10 with explicit Steps 1-4

* feat(F08): implement X-Nexus-Fallback-Model header

T07: RoutingResult struct
- Create RoutingResult with backend, actual_model, fallback_used
- Update select_backend() to return RoutingResult
- Add NoHealthyBackend error variant for 503 responses

T08: X-Nexus-Fallback-Model header
- Add FALLBACK_HEADER constant (lowercase for HTTP/2)
- Implement header in both streaming and non-streaming responses
- Header only present when fallback_used is true

T09: Header unit tests
- routing_result_contains_fallback_info
- routing_result_no_fallback_when_primary_used
- routing_result_with_alias_and_fallback
- routing_result_no_fallback_info_when_no_fallback_configured

T10: Header integration tests
- api_response_includes_fallback_header
- response_no_fallback_header_when_primary_used
- streaming_response_includes_fallback_header

Closes #95
Closes #96
Closes #97
Closes #98

* docs(F08): complete verification checklist and update spec status

- Mark spec.md as ✅ Implemented
- Mark all AC as complete including AC-06
- Complete verification.md: 95 passed, 14 N/A, 0 unchecked
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enhancement New feature or request fallback-chains F08: Fallback Chains feature P1 Phase 1 Priority

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F08-T10: Header Integration Tests F08-T09: Header Unit Tests F08-T08: X-Nexus-Fallback-Model Header F08-T07: RoutingResult Struct

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