diff --git a/tests/unit/handlers/spare.parts.handlers.test.js b/tests/unit/handlers/spare.parts.handlers.test.js
index 66da474..15399d3 100644
--- a/tests/unit/handlers/spare.parts.handlers.test.js
+++ b/tests/unit/handlers/spare.parts.handlers.test.js
@@ -12,6 +12,12 @@ const PART = {
   rack: PART_RACK,
   info: { location: 'Lab', status: 'active' }
 }
+const REPLACED_PART = {
+  id: 'p-out',
+  code: 'CB-AM-CB_1111-02',
+  rack: PART_RACK,
+  info: { location: 'miner.room', status: 'faulty' }
+}
 const OPEN_WO = {
   id: 'wo-1',
   code: 'IVI-2-0001',
@@ -32,12 +38,13 @@ const userMeta = (email = 'op@test') => ({
   _info: { authToken: 'tok', user: { metadata: { email } } }
 })
 
-function buildCtx ({ wo = OPEN_WO, part = PART, pushResults = {} } = {}) {
+function buildCtx ({ wo = OPEN_WO, part = PART, replaced = REPLACED_PART, pushResults = {} } = {}) {
   const pushed = []
   const handler = async (_key, method, params) => {
     if (method === 'listThings') {
       if (params.query?.type === 'inventory-work_order') return wo ? [wo] : []
       if (params.query?.id === PART.id) return part ? [part] : []
+      if (params.query?.code) return replaced && params.query.code === replaced.code ? [replaced] : []
       return []
     }
     if (method === 'pushAction') {
@@ -153,6 +160,83 @@ test('handlers: updateSparePart skips WO checks when only non-move fields change
   t.absent(pushed[0].params[0].info.workOrderId, 'no WO injected for non-move')
 })
 
+test('handlers: updateSparePart records top-level remarks on the move entry and keeps it off the part', async (t) => {
+  const { ctx, pushed } = buildCtx()
+  await handlers.updateSparePart(ctx, {
+    ...userMeta(),
+    params: { id: PART.id },
+    body: { rackId: PART_RACK, workOrderId: 'wo-1', remarks: 'Not reachable', info: { location: 'miner.room' } }
+  })
+  const woAction = pushed.find(p => p.params[0].rackId === WO_RACK)
+  const move = woAction.params[0].info.partsMoves[0]
+  t.is(move.remarks, 'Not reachable')
+  t.is(move.role, 'original')
+  const partAction = pushed.find(p => p.params[0].rackId === PART_RACK)
+  t.absent(partAction.params[0].info.remarks, 'remarks are move metadata, not written to the part record')
+})
+
+test('handlers: updateSparePart falls back to info.reason for remarks', async (t) => {
+  const { ctx, pushed } = buildCtx()
+  await handlers.updateSparePart(ctx, {
+    ...userMeta(),
+    params: { id: PART.id },
+    body: { rackId: PART_RACK, workOrderId: 'wo-1', info: { location: 'miner.room', reason: 'hardware_error' } }
+  })
+  const woAction = pushed.find(p => p.params[0].rackId === WO_RACK)
+  t.is(woAction.params[0].info.partsMoves[0].remarks, 'hardware_error')
+})
+
+test('handlers: updateSparePart marks a replacement and links the replaced part', async (t) => {
+  const { ctx, pushed } = buildCtx()
+  const out = await handlers.updateSparePart(ctx, {
+    ...userMeta(),
+    params: { id: PART.id },
+    body: { rackId: PART_RACK, workOrderId: 'wo-1', replacesPartCode: REPLACED_PART.code, info: { location: 'site.warehouse' } }
+  })
+  const woAction = pushed.find(p => p.params[0].rackId === WO_RACK)
+  const move = woAction.params[0].info.partsMoves[0]
+  t.is(move.role, 'replacement')
+  t.is(move.replacesPartCode, REPLACED_PART.code)
+  t.is(move.replacesPartId, REPLACED_PART.id, 'server resolves the replaced part id from its code')
+  t.is(out.move.role, 'replacement')
+})
+
+test('handlers: updateSparePart 400s when replacesPartCode does not resolve to a part', async (t) => {
+  const { ctx } = buildCtx({ replaced: null })
+  await t.exception(
+    () => handlers.updateSparePart(ctx, {
+      ...userMeta(),
+      params: { id: PART.id },
+      body: { rackId: PART_RACK, workOrderId: 'wo-1', replacesPartCode: 'NOPE-0001', info: { location: 'site.warehouse' } }
+    }),
+    /ERR_REPLACES_PART_NOT_FOUND/
+  )
+})
+
+test('handlers: updateSparePart rejects a part replacing itself', async (t) => {
+  const { ctx } = buildCtx()
+  await t.exception(
+    () => handlers.updateSparePart(ctx, {
+      ...userMeta(),
+      params: { id: PART.id },
+      body: { rackId: PART_RACK, workOrderId: 'wo-1', replacesPartCode: PART.code, info: { location: 'site.warehouse' } }
+    }),
+    /ERR_REPLACES_PART_SELF/
+  )
+})
+
+test('handlers: updateSparePart treats a replacement as a move and requires a workOrderId', async (t) => {
+  const { ctx } = buildCtx()
+  await t.exception(
+    () => handlers.updateSparePart(ctx, {
+      ...userMeta(),
+      params: { id: PART.id },
+      body: { rackId: PART_RACK, replacesPartCode: REPLACED_PART.code, info: {} }
+    }),
+    /ERR_PART_MOVE_REQUIRES_WO/
+  )
+})
+
 function buildRegisterCtx ({ pushResult } = {}) {
   const pushed = []
   const handler = async (_key, method, params) => {
diff --git a/workers/lib/server/handlers/spare.parts.handlers.js b/workers/lib/server/handlers/spare.parts.handlers.js
index 2817f56..f94d7e0 100644
--- a/workers/lib/server/handlers/spare.parts.handlers.js
+++ b/workers/lib/server/handlers/spare.parts.handlers.js
@@ -29,11 +29,19 @@ async function _loadSparePart (ctx, partId) {
   return flattenRpcResults(results).find(t => t?.type !== WORK_ORDER_THING_TYPE) || null
 }
 
+async function _loadSparePartByCode (ctx, code) {
+  const results = await ctx.dataProxy.requestData('listThings', {
+    query: { code }
+  })
+  return flattenRpcResults(results).find(t => t?.type !== WORK_ORDER_THING_TYPE) || null
+}
+
 async function updateSparePart (ctx, req) {
   const { id } = req.params
-  const { rackId, workOrderId, info } = req.body
+  const { rackId, workOrderId, info, remarks, replacesPartCode } = req.body
 
-  const movesPart = info.location !== undefined || info.status !== undefined
+  const isReplacement = typeof replacesPartCode === 'string' && replacesPartCode.length > 0
+  const movesPart = info.location !== undefined || info.status !== undefined || isReplacement
 
   if (movesPart && !workOrderId) {
     const err = new Error('ERR_PART_MOVE_REQUIRES_WO')
@@ -43,10 +51,12 @@ async function updateSparePart (ctx, req) {
 
   let workOrderCode
   let part
+  let replacedPart
   if (movesPart) {
-    const [wo, p] = await Promise.all([
+    const [wo, p, rp] = await Promise.all([
       _loadWorkOrder(ctx, workOrderId),
-      _loadSparePart(ctx, id)
+      _loadSparePart(ctx, id),
+      isReplacement ? _loadSparePartByCode(ctx, replacesPartCode) : Promise.resolve(null)
     ])
     if (!wo) {
       const err = new Error('ERR_WORK_ORDER_NOT_FOUND')
@@ -63,8 +73,21 @@ async function updateSparePart (ctx, req) {
       err.statusCode = 404
       throw err
     }
+    if (isReplacement) {
+      if (replacesPartCode === p.code) {
+        const err = new Error('ERR_REPLACES_PART_SELF')
+        err.statusCode = 400
+        throw err
+      }
+      if (!rp) {
+        const err = new Error('ERR_REPLACES_PART_NOT_FOUND')
+        err.statusCode = 400
+        throw err
+      }
+    }
     workOrderCode = wo.code
     part = p
+    replacedPart = rp
   }
 
   const { permissions } = await ctx.authLib.getTokenPerms(req._info.authToken)
@@ -97,6 +120,7 @@ async function updateSparePart (ctx, req) {
     throw err
   }
 
+  const moveRemarks = remarks ?? info.reason
   const moveEntry = {
     partId: id,
     partCode: part.code,
@@ -106,10 +130,15 @@ async function updateSparePart (ctx, req) {
     toLocation: info.location ?? part.info?.location ?? null,
     fromStatus: part.info?.status ?? null,
     toStatus: info.status ?? part.info?.status ?? null,
-    role: 'original',
+    role: isReplacement ? 'replacement' : 'original',
     ts: Date.now(),
     user: voter
   }
+  if (moveRemarks !== undefined && moveRemarks !== null) moveEntry.remarks = moveRemarks
+  if (isReplacement) {
+    moveEntry.replacesPartCode = replacesPartCode
+    moveEntry.replacesPartId = replacedPart.id
+  }
 
   const woRackId = await getWorkOrderRackId(ctx)
   const wo = await _loadWorkOrder(ctx, workOrderId)
diff --git a/workers/lib/server/schemas/spare.parts.schemas.js b/workers/lib/server/schemas/spare.parts.schemas.js
index b5c59f1..3e51204 100644
--- a/workers/lib/server/schemas/spare.parts.schemas.js
+++ b/workers/lib/server/schemas/spare.parts.schemas.js
@@ -54,6 +54,8 @@ const update = {
     properties: {
       rackId: { type: 'string', minLength: 1 },
       workOrderId: { type: 'string', minLength: 1 },
+      remarks: { type: 'string', maxLength: 4000 },
+      replacesPartCode: { type: 'string', minLength: 1, maxLength: 200 },
       info: {
         type: 'object',
         additionalProperties: true,