diff --git a/.changeset/fix-branch-group-name-collision-triage.md b/.changeset/fix-branch-group-name-collision-triage.md
new file mode 100644
index 0000000000..2c503d15bc
--- /dev/null
+++ b/.changeset/fix-branch-group-name-collision-triage.md
@@ -0,0 +1,9 @@
+---
+"@runfusion/fusion": patch
+---
+
+Fix mission triage silently stranding features when two missions share a base branch.
+
+`branch_groups.branchName` is globally unique, but `ensureBranchGroupForSource` only checked for an existing group by `(sourceType, sourceId)`. When a second mission's shared-branch triage resolved to a base branch (e.g. `main`) that another mission already owned a branch group for, `createBranchGroup` threw `UNIQUE constraint failed: branch_groups.branchName`. That error escaped `triageFeature` and was swallowed by both of its callers (the validation-failure auto-triage and the startup/maintenance reconcile sweep), leaving the mission's `defined` features — including auto-generated fix features — permanently un-triaged and the mission unable to progress.
+
+`ensureBranchGroupForSource` now reuses an existing open group for the same branch name (matching the established `getBranchGroupByBranchName(...) ?? ensureBranchGroupForSource(...)` idiom) instead of colliding on the unique constraint.
diff --git a/docs/solutions/logic-errors/branch-group-name-collision-strands-mission-triage.md b/docs/solutions/logic-errors/branch-group-name-collision-strands-mission-triage.md
new file mode 100644
index 0000000000..d970439557
--- /dev/null
+++ b/docs/solutions/logic-errors/branch-group-name-collision-strands-mission-triage.md
@@ -0,0 +1,83 @@
+---
+title: "Branch-group name collision silently strands mission triage"
+date: 2026-06-03
+category: docs/solutions/logic-errors
+module: "core/store (branch_groups) + engine mission triage"
+problem_type: logic_error
+component: database
+symptoms:
+  - "Mission's defined features (incl. auto-generated fix features) are never triaged into tasks and the mission stops progressing"
+  - "No Fix: tasks exist and no triage audit event is emitted, despite repeated startups"
+  - "Engine log shows 'UNIQUE constraint failed: branch_groups.branchName' (only in stdout — never persisted)"
+  - "Triage works for one mission but fails for another that shares the same base branch"
+root_cause: logic_error
+resolution_type: code_fix
+severity: high
+related_components:
+  - "packages/core/src/store.ts (ensureBranchGroupForSource, createBranchGroup, getBranchGroupByBranchName)"
+  - "packages/core/src/mission-store.ts (triageFeature)"
+  - "packages/engine/src/mission-execution-loop.ts (handleValidationFail auto-triage)"
+  - "packages/engine/src/scheduler.ts (reconcileAllMissionFeatures)"
+tags:
+  - mission-system
+  - branch-groups
+  - triage
+  - unique-constraint
+  - swallowed-error
+  - idempotency
+---
+
+# Branch-group name collision silently strands mission triage
+
+## Problem
+
+`MissionStore.triageFeature` throws `UNIQUE constraint failed: branch_groups.branchName` for a mission whose shared-branch base collides with a branch group another mission already owns. The throw is swallowed by both triage callers, so the mission's `defined` features — including auto-generated **Fix** features from failed validations — are never turned into tasks and the mission silently stops progressing.
+
+## Symptoms
+
+- A mission stops advancing; `defined`/Fix features accumulate in active slices and never become tasks.
+- No `Fix:` tasks exist and no triage audit event (`mission:stranded-feature-triaged`) is emitted, even across many engine restarts.
+- The only trace is in engine **stdout** (never persisted): `Error triaging fix feature …: UNIQUE constraint failed: branch_groups.branchName` and `Failed to triage stranded feature … during reconciliation: …`.
+- Triage succeeds for one mission but consistently fails for another — the one whose shared base resolves to a branch name (e.g. `main`) already claimed by the first mission's branch group.
+
+## What Didn't Work
+
+- **Reasoning from code alone** suggested `triageFeature` looked robust (the branch-assignment helpers don't obviously throw), which nearly led to dismissing the triage-throw hypothesis. The error sites are also silent (logged, not persisted), so the audit/activity tables showed nothing.
+- The breakthrough was **reproducing against a `VACUUM INTO` snapshot of the live mission DB**: instantiating a real `TaskStore`, pulling its `MissionStore`, and calling `triageFeature` on a stuck fix feature surfaced the exact exception and stack immediately.
+
+## Solution
+
+`ensureBranchGroupForSource` was only idempotent by `(sourceType, sourceId)`, but `branch_groups.branchName` is globally **UNIQUE**. When the source had no group yet and another source already owned a group with that branch name, `createBranchGroup` violated the unique constraint and threw.
+
+Reuse an existing open group for the same branch name before creating one (the idiom already used in `register-task-workflow-routes.ts`):
+
+```ts
+// packages/core/src/store.ts — ensureBranchGroupForSource
+const existing = this.getBranchGroupBySource(sourceType, sourceId);
+if (existing) return existing;
+
+// branch_groups.branchName is globally UNIQUE — one open group per branch.
+// Reuse it instead of colliding on the constraint.
+const existingByBranch = this.getBranchGroupByBranchName(init.branchName);
+if (existingByBranch) return existingByBranch;
+
+return this.createBranchGroup({ sourceType, sourceId, ...init });
+```
+
+The low-level `createBranchGroup` still enforces uniqueness (unchanged).
+
+## Why This Works
+
+The mission had an empty `branchStrategy`, so `missionBranchStrategyDefaults(undefined)` returned `assignmentMode: "shared"`, and the shared base fell through to `settings.defaultBranch = "main"`. Triaging any `defined` feature then called `ensureBranchGroupForSource("mission", missionId, { branchName: "main" })`; a different mission already owned the `"main"` group, so the insert threw. The error escaped `triageFeature` into its two callers — the validation-failure auto-triage (`mission-execution-loop.ts`) and the reconcile sweep (`scheduler.ts`) — both of which catch-and-log without persisting, so features stayed `defined` forever. Reusing the existing open group removes the only failing operation; verified against the live snapshot (`triageFeature` threw before, returned `status: triaged` with a new task after).
+
+## Prevention
+
+- **An "ensure"-named helper keyed on one identity can still violate a UNIQUE constraint on a *different* column.** Make idempotency cover every uniqueness dimension the table enforces — here, both `(sourceType, sourceId)` and the unique `branchName`.
+- **Swallowed errors in triage/reconcile paths cause silent stalls.** When a catch-and-continue site guards a step that work depends on (triage, validation, advancement), emit a persisted signal (audit event / mission event), not just a stdout log — otherwise the failure is invisible in the DB and impossible to diagnose post-hoc.
+- **When a state machine stalls with no error, snapshot the live DB read-only (`VACUUM INTO` / `?mode=ro`) and drive the real code path against it.** Code-reading alone misled this investigation; the exact exception came from reproduction.
+- Known limitation / follow-up: this reuses an *open* same-name group; a *closed/finalized* group on the same branch would still hit the UNIQUE constraint (branch-name retirement is a separate, arguably by-design concern).
+
+## Related Issues
+
+- `docs/solutions/logic-errors/mission-autopilot-stalled-by-stranded-done-feature.md` — sibling mission-stall learning (PR #1345). Same family: a mission silently wedges and an error/edge in a triage/recovery path is the cause. Both reinforce "swallowed triage-path errors → silent mission stalls."
+- PR #1348 — the fix for this bug.
diff --git a/packages/core/src/__tests__/branch-group-store.test.ts b/packages/core/src/__tests__/branch-group-store.test.ts
index ae26efd00d..17d641af37 100644
--- a/packages/core/src/__tests__/branch-group-store.test.ts
+++ b/packages/core/src/__tests__/branch-group-store.test.ts
@@ -67,6 +67,34 @@ describe("TaskStore branch groups", () => {
     expect(second.autoMerge).toBe(true);
   });
 
+  it("reuses an existing open group with the same branchName across sources instead of throwing", () => {
+    // Regression: branch_groups.branchName is globally UNIQUE. When one mission
+    // already owns an open group for a shared base branch, a second source whose
+    // triage resolves to the same branch must reuse that group rather than crash
+    // on the UNIQUE constraint. (Mission triage discards the result and only needs
+    // it not to throw; a thrown error there silently strands "defined" features.)
+    const owner = store.createBranchGroup({ sourceType: "mission", sourceId: "M-OWNER", branchName: "main" });
+
+    let reusedByMission!: ReturnType<typeof store.ensureBranchGroupForSource>;
+    expect(() => {
+      reusedByMission = store.ensureBranchGroupForSource("mission", "M-OTHER", {
+        branchName: "main",
+        autoMerge: true,
+      });
+    }).not.toThrow();
+    expect(reusedByMission.id).toBe(owner.id);
+
+    // Invariant holds across the other source types that share this helper.
+    const reusedByNewTask = store.ensureBranchGroupForSource("new-task", "shared/main", { branchName: "main" });
+    expect(reusedByNewTask.id).toBe(owner.id);
+
+    const reusedByPlanning = store.ensureBranchGroupForSource("planning", "PS-main", { branchName: "main" });
+    expect(reusedByPlanning.id).toBe(owner.id);
+
+    // No duplicate rows were created for the shared branch.
+    expect(store.listBranchGroups().filter((g) => g.branchName === "main")).toHaveLength(1);
+  });
+
   it("supports new-task branch group sources and round-trips through lookups", () => {
     const group = store.ensureBranchGroupForSource("new-task", "shared/onboarding", {
       branchName: "shared/onboarding",
diff --git a/packages/core/src/store.ts b/packages/core/src/store.ts
index 9c3b1f6434..463852e1d2 100644
--- a/packages/core/src/store.ts
+++ b/packages/core/src/store.ts
@@ -4416,6 +4416,17 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
       return existing;
     }
 
+    // `branch_groups.branchName` is globally UNIQUE — a branch is represented by
+    // exactly one open group. If another source already owns an open group for
+    // this branch, reuse it rather than calling createBranchGroup and violating
+    // the UNIQUE constraint. Without this, two missions whose shared base resolves
+    // to the same branch (e.g. "main") collide: the throw escapes triageFeature
+    // and is swallowed by its callers, silently stranding "defined" features.
+    const existingByBranch = this.getBranchGroupByBranchName(init.branchName);
+    if (existingByBranch) {
+      return existingByBranch;
+    }
+
     return this.createBranchGroup({
       sourceType,
       sourceId,