diff --git a/backend/routers/projects.py b/backend/routers/projects.py
index bc40b1b..13cb23a 100644
--- a/backend/routers/projects.py
+++ b/backend/routers/projects.py
@@ -688,6 +688,72 @@ async def restore_project(project_id: str, db: Session = Depends(get_db)):
     return {"success": True, "message": f"Project '{project.name}' restored."}
 
 
+# ── Project merge ──────────────────────────────────────────────────────────────
+# Consolidate a duplicate project into another.  Common after the
+# metadata-backfill parser creates near-duplicate projects from name
+# variations operators typed on the BW device.
+# See backend/services/project_merge.py for the merge logic.
+
+@router.get("/{source_id}/merge_preview")
+async def project_merge_preview(
+    source_id: str,
+    target_id: str,
+    db: Session = Depends(get_db),
+):
+    """Preview what the merge will do — used by the confirmation modal.
+    No writes."""
+    from backend.services import project_merge as pm
+    preview = pm.preview(db, source_id, target_id)
+    return {
+        "source_project_id":         preview.source_project_id,
+        "source_project_name":       preview.source_project_name,
+        "target_project_id":         preview.target_project_id,
+        "target_project_name":       preview.target_project_name,
+        "total_assignments_moving":  preview.total_assignments_moving,
+        "total_sessions_moving":     preview.total_sessions_moving,
+        "total_data_files_moving":   preview.total_data_files_moving,
+        "modules_to_add":            preview.modules_to_add,
+        "warnings":                  preview.warnings,
+        "location_plans": [
+            {
+                "source_id":          p.source_id,
+                "source_name":        p.source_name,
+                "target_id":          p.target_id,
+                "target_name":        p.target_name,
+                "action":             p.action,
+                "assignments_moving": p.assignments_moving,
+                "sessions_moving":    p.sessions_moving,
+            }
+            for p in preview.location_plans
+        ],
+    }
+
+
+@router.post("/{source_id}/merge_into")
+async def project_merge_execute(
+    source_id: str,
+    target_id: str,
+    db: Session = Depends(get_db),
+):
+    """Execute the merge.  Source project gets soft-deleted; all its
+    locations / assignments / sessions / data_files / modules move to
+    the target.  Same-named locations consolidate."""
+    from backend.services import project_merge as pm
+    result = pm.execute(db, source_id, target_id, decided_by="operator")
+    return {
+        "success":                True,
+        "source_project_id":      result.source_project_id,
+        "target_project_id":      result.target_project_id,
+        "assignments_moved":      result.assignments_moved,
+        "locations_moved":        result.locations_moved,
+        "locations_consolidated": result.locations_consolidated,
+        "sessions_moved":         result.sessions_moved,
+        "data_files_moved":       result.data_files_moved,
+        "modules_added":          result.modules_added,
+        "audit_rows_written":     result.audit_rows_written,
+    }
+
+
 @router.get("/{project_id}")
 async def get_project(project_id: str, db: Session = Depends(get_db)):
     """
diff --git a/backend/services/project_merge.py b/backend/services/project_merge.py
new file mode 100644
index 0000000..53a1c5d
--- /dev/null
+++ b/backend/services/project_merge.py
@@ -0,0 +1,435 @@
+"""
+project_merge.py — consolidate a duplicate project into another.
+
+Use case: the metadata-backfill parser (and operators) create projects with
+slight name variations ("SR81" vs "SR 81", "Swank-Karns Crossing" vs
+"Swank-Karns Crossings", "Trumbull-Bryman Mont.Dam" vs
+"Trumbull-Brayman-Mont Dam").  Operator picks a SOURCE project to merge
+into a TARGET project; everything attached to source moves to target,
+same-named locations consolidate, and source is soft-deleted.
+
+Public API:
+  preview(db, source_id, target_id)            → MergePreview
+  execute(db, source_id, target_id, *, decided_by="operator") → MergeResult
+
+Both raise HTTPException with appropriate 4xx codes for validation failures.
+"""
+
+from __future__ import annotations
+
+import logging
+from dataclasses import dataclass, field
+from datetime import datetime
+from typing import Optional
+
+from fastapi import HTTPException
+from sqlalchemy.orm import Session
+
+from backend.models import (
+    Project,
+    ProjectModule,
+    MonitoringLocation,
+    UnitAssignment,
+    UnitHistory,
+    MonitoringSession,
+    DataFile,
+)
+
+log = logging.getLogger("backend.services.project_merge")
+
+
+# ── Dataclasses ───────────────────────────────────────────────────────────────
+
+
+@dataclass
+class LocationMergePlan:
+    source_id:           str
+    source_name:         str
+    target_id:           Optional[str]    # None = will be inserted as-new under target project
+    target_name:         Optional[str]    # name in target after merge
+    action:              str              # "move" | "consolidate"
+    assignments_moving:  int
+    sessions_moving:     int
+
+
+@dataclass
+class MergePreview:
+    source_project_id:   str
+    source_project_name: str
+    target_project_id:   str
+    target_project_name: str
+    location_plans:      list[LocationMergePlan] = field(default_factory=list)
+    total_assignments_moving: int = 0
+    total_sessions_moving:    int = 0
+    total_data_files_moving:  int = 0
+    modules_to_add:           list[str] = field(default_factory=list)
+    warnings:                 list[str] = field(default_factory=list)
+
+
+@dataclass
+class MergeResult:
+    source_project_id:    str
+    target_project_id:    str
+    assignments_moved:    int
+    locations_moved:      int
+    locations_consolidated: int
+    sessions_moved:       int
+    data_files_moved:     int
+    modules_added:        list[str]
+    audit_rows_written:   int
+
+
+# ── Helpers ───────────────────────────────────────────────────────────────────
+
+
+def _normalise_name(s: Optional[str]) -> str:
+    """Case-insensitive, whitespace-collapsing name normalisation.
+
+    Lighter than metadata_backfill._normalise (no punctuation stripping)
+    — for merging we want "Loc 1" and "Loc 1" to match but NOT "Loc 1"
+    and "Loc-1" (those might be intentionally different).  If operators
+    DO want loose matching, they can rename one before merging.
+    """
+    if not s:
+        return ""
+    import re
+    return re.sub(r"\s+", " ", s.strip()).casefold()
+
+
+def _validate_pair(db: Session, source_id: str, target_id: str) -> tuple[Project, Project]:
+    if source_id == target_id:
+        raise HTTPException(status_code=400, detail="Cannot merge a project into itself.")
+
+    source = db.query(Project).filter_by(id=source_id).first()
+    target = db.query(Project).filter_by(id=target_id).first()
+    if source is None:
+        raise HTTPException(status_code=404, detail=f"Source project not found.")
+    if target is None:
+        raise HTTPException(status_code=404, detail=f"Target project not found.")
+    if source.status == "deleted":
+        raise HTTPException(status_code=400, detail=f"Source project '{source.name}' is already deleted.")
+    if target.status == "deleted":
+        raise HTTPException(status_code=400, detail=f"Target project '{target.name}' is deleted.")
+
+    return source, target
+
+
+# ── Preview ───────────────────────────────────────────────────────────────────
+
+
+def preview(db: Session, source_id: str, target_id: str) -> MergePreview:
+    """Build a preview of what the merge will do.  No writes."""
+    source, target = _validate_pair(db, source_id, target_id)
+
+    # Locations in source vs target.
+    source_locs = (
+        db.query(MonitoringLocation)
+        .filter(MonitoringLocation.project_id == source_id)
+        .all()
+    )
+    target_locs = (
+        db.query(MonitoringLocation)
+        .filter(MonitoringLocation.project_id == target_id)
+        .all()
+    )
+    target_by_norm = {_normalise_name(l.name): l for l in target_locs}
+
+    location_plans: list[LocationMergePlan] = []
+    total_assignments_moving = 0
+    total_sessions_moving    = 0
+
+    for sl in source_locs:
+        n = _normalise_name(sl.name)
+        tl = target_by_norm.get(n)
+
+        a_count = (
+            db.query(UnitAssignment)
+            .filter(UnitAssignment.location_id == sl.id)
+            .count()
+        )
+        s_count = (
+            db.query(MonitoringSession)
+            .filter(MonitoringSession.location_id == sl.id)
+            .count()
+        )
+        total_assignments_moving += a_count
+        total_sessions_moving    += s_count
+
+        if tl is not None:
+            location_plans.append(LocationMergePlan(
+                source_id          = sl.id,
+                source_name        = sl.name,
+                target_id          = tl.id,
+                target_name        = tl.name,
+                action             = "consolidate",
+                assignments_moving = a_count,
+                sessions_moving    = s_count,
+            ))
+        else:
+            location_plans.append(LocationMergePlan(
+                source_id          = sl.id,
+                source_name        = sl.name,
+                target_id          = None,
+                target_name        = sl.name,
+                action             = "move",
+                assignments_moving = a_count,
+                sessions_moving    = s_count,
+            ))
+
+    # DataFiles attached to the source project (if the table exists with a
+    # project_id column).  Optional — terra-view's DataFile model may not
+    # always FK to project, so handle gracefully.
+    df_count = 0
+    try:
+        df_count = (
+            db.query(DataFile)
+            .filter(DataFile.project_id == source_id)
+            .count()
+        )
+    except Exception:
+        df_count = 0
+    total_data_files_moving = df_count
+
+    # Modules: add anything in source missing from target.
+    src_modules = {
+        m.module_type for m in db.query(ProjectModule)
+        .filter(ProjectModule.project_id == source_id, ProjectModule.enabled.is_(True))
+        .all()
+    }
+    tgt_modules = {
+        m.module_type for m in db.query(ProjectModule)
+        .filter(ProjectModule.project_id == target_id, ProjectModule.enabled.is_(True))
+        .all()
+    }
+    modules_to_add = sorted(src_modules - tgt_modules)
+
+    warnings: list[str] = []
+    # Surface conditions the operator should think about.
+    consolidations = sum(1 for p in location_plans if p.action == "consolidate")
+    if consolidations:
+        warnings.append(
+            f"{consolidations} location(s) with matching names will be consolidated "
+            f"(source assignments will move to the target's existing location). "
+            f"If your same-named locations are actually different sites, rename one first."
+        )
+    if source.client_name and target.client_name and source.client_name.strip().casefold() != target.client_name.strip().casefold():
+        warnings.append(
+            f"Client names differ: source is \"{source.client_name}\", target is "
+            f"\"{target.client_name}\".  Target's client name will be kept."
+        )
+
+    return MergePreview(
+        source_project_id        = source.id,
+        source_project_name      = source.name,
+        target_project_id        = target.id,
+        target_project_name      = target.name,
+        location_plans           = location_plans,
+        total_assignments_moving = total_assignments_moving,
+        total_sessions_moving    = total_sessions_moving,
+        total_data_files_moving  = total_data_files_moving,
+        modules_to_add           = modules_to_add,
+        warnings                 = warnings,
+    )
+
+
+# ── Execute ───────────────────────────────────────────────────────────────────
+
+
+def execute(
+    db: Session,
+    source_id: str,
+    target_id: str,
+    *,
+    decided_by: str = "operator",
+) -> MergeResult:
+    """Perform the merge in a single transaction.
+
+    Steps:
+      1. Re-validate the pair.
+      2. For each location in source:
+         - if a same-name location exists in target → "consolidate" mode:
+           move source's assignments + sessions to target's location id,
+           delete source's location.
+         - else → "move" mode: just re-point the location's project_id.
+      3. Move any remaining direct-to-project FK rows (DataFiles).
+      4. Ensure target has all of source's modules.
+      5. Soft-delete source project.
+      6. Write a UnitHistory row per assignment that was moved
+         (change_type='assignment_merged') so the deployment timeline
+         on each affected unit reflects the merge.
+      7. Commit.
+    """
+    source, target = _validate_pair(db, source_id, target_id)
+
+    src_modules = {
+        m.module_type for m in db.query(ProjectModule)
+        .filter(ProjectModule.project_id == source_id, ProjectModule.enabled.is_(True))
+        .all()
+    }
+    tgt_modules = {
+        m.module_type for m in db.query(ProjectModule)
+        .filter(ProjectModule.project_id == target_id, ProjectModule.enabled.is_(True))
+        .all()
+    }
+    modules_to_add = sorted(src_modules - tgt_modules)
+
+    # ── 1. Locations + their dependents ───────────────────────────────
+    source_locs = (
+        db.query(MonitoringLocation)
+        .filter(MonitoringLocation.project_id == source_id)
+        .all()
+    )
+    target_locs = (
+        db.query(MonitoringLocation)
+        .filter(MonitoringLocation.project_id == target_id)
+        .all()
+    )
+    target_by_norm = {_normalise_name(l.name): l for l in target_locs}
+
+    assignments_moved        = 0
+    sessions_moved           = 0
+    locations_moved          = 0
+    locations_consolidated   = 0
+    audit_rows_written       = 0
+
+    for sl in source_locs:
+        n = _normalise_name(sl.name)
+        tl = target_by_norm.get(n)
+
+        # Pull this location's assignments + sessions (we'll re-point them).
+        assignments = (
+            db.query(UnitAssignment)
+            .filter(UnitAssignment.location_id == sl.id)
+            .all()
+        )
+        sessions = (
+            db.query(MonitoringSession)
+            .filter(MonitoringSession.location_id == sl.id)
+            .all()
+        )
+
+        if tl is not None:
+            # Consolidate: move dependents to target's existing location;
+            # then delete the source location.
+            for a in assignments:
+                old_loc_id = a.location_id
+                a.location_id = tl.id
+                a.project_id  = target.id
+
+                db.add(UnitHistory(
+                    unit_id     = a.unit_id,
+                    change_type = "assignment_merged",
+                    field_name  = "unit_assignment.project_id",
+                    old_value   = f"{source.name} / {sl.name}",
+                    new_value   = f"{target.name} / {tl.name}",
+                    changed_at  = datetime.utcnow(),
+                    source      = "project_merge",
+                    notes       = (
+                        f"Project merge: '{source.name}' → '{target.name}'. "
+                        f"Location consolidated by name match. "
+                        f"By: {decided_by}."
+                    ),
+                ))
+                audit_rows_written += 1
+                assignments_moved  += 1
+
+            for s in sessions:
+                s.location_id = tl.id
+                s.project_id  = target.id
+                sessions_moved += 1
+
+            # Delete the now-empty source location.
+            db.delete(sl)
+            locations_consolidated += 1
+        else:
+            # Move: just re-point this location to the target project.
+            sl.project_id = target.id
+
+            for a in assignments:
+                old_proj_id = a.project_id
+                a.project_id = target.id
+
+                db.add(UnitHistory(
+                    unit_id     = a.unit_id,
+                    change_type = "assignment_merged",
+                    field_name  = "unit_assignment.project_id",
+                    old_value   = f"{source.name} / {sl.name}",
+                    new_value   = f"{target.name} / {sl.name}",
+                    changed_at  = datetime.utcnow(),
+                    source      = "project_merge",
+                    notes       = (
+                        f"Project merge: '{source.name}' → '{target.name}'. "
+                        f"Location moved as-is. By: {decided_by}."
+                    ),
+                ))
+                audit_rows_written += 1
+                assignments_moved  += 1
+
+            for s in sessions:
+                s.project_id = target.id
+                sessions_moved += 1
+
+            locations_moved += 1
+
+    # ── 2. Direct-to-project rows (DataFiles, ScheduledActions) ──────
+    data_files_moved = 0
+    try:
+        data_files = (
+            db.query(DataFile)
+            .filter(DataFile.project_id == source_id)
+            .all()
+        )
+        for df in data_files:
+            df.project_id = target.id
+            data_files_moved += 1
+    except Exception as e:
+        log.warning("DataFile move skipped (model may differ): %s", e)
+
+    # ── 3. UnitAssignments that point directly at source.project_id with
+    #    no location (shouldn't happen but be defensive) ──────────────
+    orphan_assignments = (
+        db.query(UnitAssignment)
+        .filter(UnitAssignment.project_id == source_id)
+        .all()
+    )
+    for a in orphan_assignments:
+        # Already moved if its location was moved.  Catch any stragglers.
+        if a.project_id == source_id:
+            a.project_id = target.id
+
+    # ── 4. Modules ────────────────────────────────────────────────────
+    import uuid
+    for mod_type in modules_to_add:
+        db.add(ProjectModule(
+            id          = str(uuid.uuid4()),
+            project_id  = target.id,
+            module_type = mod_type,
+            enabled     = True,
+        ))
+
+    # Disable source's modules (defensive — source is being soft-deleted
+    # but its modules table rows could still be inspected).
+    for m in db.query(ProjectModule).filter(ProjectModule.project_id == source_id).all():
+        m.enabled = False
+
+    # ── 5. Soft-delete source ─────────────────────────────────────────
+    source.status     = "deleted"
+    source.deleted_at = datetime.utcnow()
+
+    # Final audit row on the source project itself (operator-facing).
+    # We don't have a Project-level history table, so log on every
+    # affected unit as a marker.  Already done per-assignment above.
+
+    db.commit()
+
+    return MergeResult(
+        source_project_id      = source.id,
+        target_project_id      = target.id,
+        assignments_moved      = assignments_moved,
+        locations_moved        = locations_moved,
+        locations_consolidated = locations_consolidated,
+        sessions_moved         = sessions_moved,
+        data_files_moved       = data_files_moved,
+        modules_added          = modules_to_add,
+        audit_rows_written     = audit_rows_written,
+    )
diff --git a/templates/partials/projects/project_header.html b/templates/partials/projects/project_header.html
index f3e3f2e..6aeff79 100644
--- a/templates/partials/projects/project_header.html
+++ b/templates/partials/projects/project_header.html
@@ -75,10 +75,266 @@
                 Generate Combined Report
             </a>
             {% endif %}
+            <button onclick="openMergeModal()"
+                    title="Merge this project into another (consolidates duplicates)"
+                    class="px-4 py-2 border border-gray-300 dark:border-gray-600 text-gray-700 dark:text-gray-300 rounded-lg hover:bg-gray-50 dark:hover:bg-gray-700 transition-colors flex items-center gap-2 text-sm">
+                <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                    <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M17 8l4 4m0 0l-4 4m4-4H3"></path>
+                </svg>
+                Merge into…
+            </button>
         </div>
     </div>
 </div>
 
+<!-- Merge Modal -->
+<div id="merge-modal" class="hidden fixed inset-0 z-50 flex items-center justify-center bg-black/60 backdrop-blur-sm">
+    <div class="bg-white dark:bg-slate-800 rounded-xl shadow-2xl w-full max-w-2xl mx-4 max-h-[90vh] flex flex-col">
+        <!-- Header -->
+        <div class="px-6 py-4 border-b border-gray-200 dark:border-gray-700 flex items-center justify-between">
+            <div>
+                <h3 class="text-lg font-semibold text-gray-900 dark:text-white">Merge "{{ project.name }}" into another project</h3>
+                <p class="text-sm text-gray-500 dark:text-gray-400 mt-0.5">
+                    Source project gets soft-deleted.  All its locations, assignments, sessions, and files move to the target.
+                </p>
+            </div>
+            <button onclick="closeMergeModal()" class="text-gray-400 hover:text-gray-600 dark:hover:text-gray-200 shrink-0 ml-3">
+                <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M6 18L18 6M6 6l12 12"/></svg>
+            </button>
+        </div>
+
+        <!-- Body -->
+        <div class="px-6 py-4 overflow-y-auto flex-1">
+            <label class="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-2">
+                Target project
+            </label>
+            <div class="relative">
+                <input type="text"
+                       id="merge-target-input"
+                       placeholder="Type to search for the project to merge INTO…"
+                       autocomplete="off"
+                       oninput="onMergeTargetInput()"
+                       onfocus="onMergeTargetInput()"
+                       onblur="setTimeout(() => document.getElementById('merge-target-dropdown').classList.add('hidden'), 150)"
+                       class="w-full px-3 py-2 border border-gray-300 dark:border-gray-600 rounded-lg bg-white dark:bg-slate-700 text-gray-900 dark:text-white text-sm">
+                <input type="hidden" id="merge-target-id" value="">
+                <div id="merge-target-dropdown"
+                     class="hidden absolute z-20 left-0 right-0 mt-1 bg-white dark:bg-slate-800 border border-gray-200 dark:border-gray-700 rounded-lg shadow-lg max-h-64 overflow-y-auto"></div>
+            </div>
+
+            <!-- Preview pane -->
+            <div id="merge-preview" class="mt-4 hidden">
+                <h4 class="text-xs font-semibold text-gray-500 dark:text-gray-400 uppercase tracking-wider mb-2">What will move</h4>
+                <div id="merge-preview-body" class="space-y-3"></div>
+            </div>
+
+            <div id="merge-error" class="hidden mt-3 text-sm text-red-600"></div>
+        </div>
+
+        <!-- Footer -->
+        <div class="px-6 py-4 border-t border-gray-200 dark:border-gray-700 flex justify-end gap-2">
+            <button onclick="closeMergeModal()"
+                    class="px-4 py-2 border border-gray-300 dark:border-gray-600 text-gray-700 dark:text-gray-300 rounded-lg hover:bg-gray-50 dark:hover:bg-gray-700 text-sm">
+                Cancel
+            </button>
+            <button id="merge-confirm-btn"
+                    onclick="confirmMerge()"
+                    disabled
+                    class="px-4 py-2 bg-red-600 hover:bg-red-700 text-white rounded-lg text-sm font-medium disabled:opacity-40 disabled:cursor-not-allowed">
+                Merge (permanent)
+            </button>
+        </div>
+    </div>
+</div>
+
+<script>
+const _SOURCE_PROJECT_ID   = "{{ project.id }}";
+const _SOURCE_PROJECT_NAME = {{ project.name|tojson }};
+
+function _mergeEsc(s) {
+    if (s == null) return '';
+    return String(s).replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;').replace(/"/g, '&quot;');
+}
+
+function openMergeModal() {
+    document.getElementById('merge-target-input').value = '';
+    document.getElementById('merge-target-id').value = '';
+    document.getElementById('merge-preview').classList.add('hidden');
+    document.getElementById('merge-error').classList.add('hidden');
+    document.getElementById('merge-confirm-btn').disabled = true;
+    document.getElementById('merge-modal').classList.remove('hidden');
+    setTimeout(() => document.getElementById('merge-target-input').focus(), 50);
+}
+
+function closeMergeModal() {
+    document.getElementById('merge-modal').classList.add('hidden');
+}
+
+let _mergeTargetDebounce = null;
+async function onMergeTargetInput() {
+    if (_mergeTargetDebounce) clearTimeout(_mergeTargetDebounce);
+    _mergeTargetDebounce = setTimeout(_mergeFetchTargets, 150);
+}
+
+async function _mergeFetchTargets() {
+    const q = document.getElementById('merge-target-input').value.trim();
+    const dropdown = document.getElementById('merge-target-dropdown');
+
+    let data;
+    try {
+        // Reuse the metadata-backfill projects_search endpoint — works for
+        // any caller, returns existing-only (no create-new option needed here).
+        const r = await fetch(`/api/admin/metadata_backfill/projects_search?q=${encodeURIComponent(q)}&limit=12`);
+        if (!r.ok) throw new Error('HTTP ' + r.status);
+        data = await r.json();
+    } catch (e) {
+        dropdown.innerHTML = `<div class="px-3 py-2 text-sm text-red-500">Search failed: ${_mergeEsc(e.message)}</div>`;
+        dropdown.classList.remove('hidden');
+        return;
+    }
+
+    // Filter out self.
+    const candidates = (data.matches || []).filter(m => m.id !== _SOURCE_PROJECT_ID);
+
+    if (candidates.length === 0) {
+        dropdown.innerHTML = `<div class="px-3 py-2 text-sm text-gray-500 dark:text-gray-400 italic">No matches.</div>`;
+        dropdown.classList.remove('hidden');
+        return;
+    }
+
+    dropdown.innerHTML = candidates.map(m => {
+        const scoreBadge = m.score >= 0.99
+            ? '<span class="text-xs text-green-600 dark:text-green-400 ml-2">exact</span>'
+            : `<span class="text-xs text-gray-500 dark:text-gray-400 ml-2">${(m.score*100).toFixed(0)}%</span>`;
+        const meta = [];
+        if (m.project_number) meta.push(_mergeEsc(m.project_number));
+        if (m.client_name)    meta.push(_mergeEsc(m.client_name));
+        if (m.location_count > 0) meta.push(`${m.location_count} location${m.location_count === 1 ? '' : 's'}`);
+        const metaLine = meta.length ? `<div class="text-xs text-gray-500 dark:text-gray-400">${meta.join(' · ')}</div>` : '';
+        return `<button type="button"
+            onmousedown="event.preventDefault()"
+            onclick="onMergePickTarget('${_mergeEsc(m.id)}', ${JSON.stringify(m.name)})"
+            class="w-full text-left px-3 py-2 hover:bg-gray-50 dark:hover:bg-slate-700 border-b border-gray-100 dark:border-gray-700 last:border-b-0">
+            <div class="text-sm font-medium text-gray-900 dark:text-white">${_mergeEsc(m.name)}${scoreBadge}</div>
+            ${metaLine}
+        </button>`;
+    }).join('');
+    dropdown.classList.remove('hidden');
+}
+
+async function onMergePickTarget(targetId, targetName) {
+    document.getElementById('merge-target-input').value = targetName;
+    document.getElementById('merge-target-id').value = targetId;
+    document.getElementById('merge-target-dropdown').classList.add('hidden');
+    await _loadMergePreview(targetId);
+}
+
+async function _loadMergePreview(targetId) {
+    const previewEl = document.getElementById('merge-preview');
+    const bodyEl    = document.getElementById('merge-preview-body');
+    const errorEl   = document.getElementById('merge-error');
+    const confirmBtn = document.getElementById('merge-confirm-btn');
+
+    previewEl.classList.add('hidden');
+    errorEl.classList.add('hidden');
+    confirmBtn.disabled = true;
+    bodyEl.innerHTML = '<div class="text-center py-3 text-sm text-gray-500"><div class="animate-spin rounded-full h-5 w-5 border-b-2 border-seismo-orange mx-auto mb-2"></div>Loading preview…</div>';
+    previewEl.classList.remove('hidden');
+
+    let d;
+    try {
+        const r = await fetch(`/api/projects/${_SOURCE_PROJECT_ID}/merge_preview?target_id=${encodeURIComponent(targetId)}`);
+        if (!r.ok) {
+            const err = await r.json().catch(() => ({detail: 'HTTP ' + r.status}));
+            throw new Error(err.detail || ('HTTP ' + r.status));
+        }
+        d = await r.json();
+    } catch (e) {
+        errorEl.textContent = e.message;
+        errorEl.classList.remove('hidden');
+        previewEl.classList.add('hidden');
+        return;
+    }
+
+    let html = `<div class="text-sm text-gray-700 dark:text-gray-300">
+        Merging <strong>"${_mergeEsc(d.source_project_name)}"</strong> into <strong>"${_mergeEsc(d.target_project_name)}"</strong>:
+    </div>
+    <div class="grid grid-cols-3 gap-2 mt-2">
+        <div class="bg-gray-50 dark:bg-slate-900/50 rounded p-2"><div class="text-xs text-gray-500 dark:text-gray-400">Assignments</div><div class="text-lg font-bold">${d.total_assignments_moving}</div></div>
+        <div class="bg-gray-50 dark:bg-slate-900/50 rounded p-2"><div class="text-xs text-gray-500 dark:text-gray-400">Sessions</div><div class="text-lg font-bold">${d.total_sessions_moving}</div></div>
+        <div class="bg-gray-50 dark:bg-slate-900/50 rounded p-2"><div class="text-xs text-gray-500 dark:text-gray-400">Data files</div><div class="text-lg font-bold">${d.total_data_files_moving}</div></div>
+    </div>`;
+
+    if (d.location_plans.length > 0) {
+        html += `<div class="mt-3">
+            <h5 class="text-xs font-semibold text-gray-500 dark:text-gray-400 uppercase tracking-wider mb-1">Locations</h5>
+            <div class="space-y-1 text-sm">`;
+        for (const p of d.location_plans) {
+            if (p.action === 'consolidate') {
+                html += `<div class="text-gray-700 dark:text-gray-300">
+                    🔀 <strong>${_mergeEsc(p.source_name)}</strong> → consolidates into existing target <strong>"${_mergeEsc(p.target_name)}"</strong>
+                    <span class="text-xs text-gray-500 dark:text-gray-400 ml-1">(${p.assignments_moving} assignments + ${p.sessions_moving} sessions move)</span>
+                </div>`;
+            } else {
+                html += `<div class="text-gray-700 dark:text-gray-300">
+                    → <strong>${_mergeEsc(p.source_name)}</strong> moves to target as-is
+                    <span class="text-xs text-gray-500 dark:text-gray-400 ml-1">(${p.assignments_moving} assignments + ${p.sessions_moving} sessions)</span>
+                </div>`;
+            }
+        }
+        html += '</div></div>';
+    }
+
+    if (d.modules_to_add.length > 0) {
+        html += `<div class="mt-3 text-sm text-gray-700 dark:text-gray-300">
+            Modules to add to target: ${d.modules_to_add.map(m => `<code class="px-1 py-0.5 rounded bg-gray-100 dark:bg-gray-700 text-xs">${_mergeEsc(m)}</code>`).join(' ')}
+        </div>`;
+    }
+
+    if (d.warnings.length > 0) {
+        html += '<div class="mt-3 space-y-2">';
+        for (const w of d.warnings) {
+            html += `<div class="bg-amber-50 dark:bg-amber-900/20 border border-amber-200 dark:border-amber-800 rounded p-2 text-xs text-amber-800 dark:text-amber-300">⚠ ${_mergeEsc(w)}</div>`;
+        }
+        html += '</div>';
+    }
+
+    html += `<div class="mt-3 bg-red-50 dark:bg-red-900/20 border border-red-200 dark:border-red-800 rounded p-2 text-xs text-red-800 dark:text-red-300">
+        <strong>⚠ This action is destructive.</strong>  Source project will be soft-deleted (status='deleted').
+        Audit rows will be written to unit_history for every moved assignment.
+    </div>`;
+
+    bodyEl.innerHTML = html;
+    confirmBtn.disabled = false;
+}
+
+async function confirmMerge() {
+    const targetId = document.getElementById('merge-target-id').value;
+    if (!targetId) return;
+    const confirmBtn = document.getElementById('merge-confirm-btn');
+    confirmBtn.disabled = true;
+    confirmBtn.textContent = 'Merging…';
+    try {
+        const r = await fetch(`/api/projects/${_SOURCE_PROJECT_ID}/merge_into?target_id=${encodeURIComponent(targetId)}`, {
+            method: 'POST',
+        });
+        if (!r.ok) {
+            const err = await r.json().catch(() => ({detail: 'HTTP ' + r.status}));
+            throw new Error(err.detail || ('HTTP ' + r.status));
+        }
+        const d = await r.json();
+        // Redirect to the target project — source no longer exists in active state.
+        window.location.href = `/projects/${d.target_project_id}`;
+    } catch (e) {
+        const errorEl = document.getElementById('merge-error');
+        errorEl.textContent = 'Merge failed: ' + e.message;
+        errorEl.classList.remove('hidden');
+        confirmBtn.disabled = false;
+        confirmBtn.textContent = 'Merge (permanent)';
+    }
+}
+</script>
+
 <!-- Add Module Modal -->
 <div id="add-module-modal" class="hidden fixed inset-0 z-50 flex items-center justify-center bg-black/60 backdrop-blur-sm">
     <div class="bg-white dark:bg-slate-800 rounded-xl shadow-2xl w-full max-w-sm mx-4 p-6">