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gsxdsm 5b4ec4c08f FN-8042: add merger fallback model lane

Add configurable project fallback models for AI merger retries.

- Resolve complete project merger fallback pairs before the shared global fallback.
- Expose merger fallback model and thinking controls in Project Models with reset-aware persistence.
- Apply the fallback lane across merger, review, PR-response, and recovery sessions with tests and documentation.

Files changed:
 .changeset/fn-8042-merger-fallback-model.md        |  7 +++
 docs/settings-reference.md                         |  4 +-
 .../core/src/__tests__/model-resolution.test.ts    | 24 ++++++++++
 .../core/src/__tests__/settings-parity.test.ts     |  3 ++
 packages/core/src/index.gate.ts                    |  1 +
 packages/core/src/index.ts                         |  1 +
 packages/core/src/model-resolution.ts              | 22 +++++++++
 packages/core/src/settings-schema.ts               |  4 ++
 packages/core/src/types.ts                         | 13 ++++++
 .../app/__tests__/settings-save-split.test.ts      | 41 +++++++++++++++++
 .../app/__tests__/settings-sections.test.tsx       | 36 +++++++++++++++
 .../app/components/settings/save-split.ts          |  7 +--
 .../settings/sections/ProjectModelsSection.tsx     | 53 +++++++++++++++++++++-
 .../settings-default-descriptions.test.tsx         |  3 ++
 .../src/__tests__/agent-session-helpers.test.ts    |  1 +
 .../__tests__/mcp-pr-response-forwarding.test.ts   |  1 +
 packages/engine/src/agent-session-helpers.ts       | 10 +++-
 packages/engine/src/merger-ai.ts                   | 17 +++++--
 packages/engine/src/merger.ts                      | 43 +++++++++++++-----
 packages/engine/src/pr-response-run-ops.ts         | 11 +++--
 20 files changed, 276 insertions(+), 26 deletions(-)

Fusion-Task-Id: FN-8042

Fusion-Task-Lineage: 31762a72-461e-438c-a12d-2816580283fd

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>

2026-07-16 04:29:17 -07:00