diff --git a/app.js b/app.js
index bd56bbc..75e67b4 100644
--- a/app.js
+++ b/app.js
@@ -39,7 +39,8 @@ if(process.env.OPEN_API_CORS !== "false") {
         'X-HTTP-Method-Override',
         'Origin',
         'Referrer',
-        'User-Agent'
+        'User-Agent',
+        'If-Overwritten-Version'
       ],
       "exposedHeaders" : "*",
       "origin" : "*",
diff --git a/jest.config.js b/jest.config.js
index 130df2f..37e67a9 100644
--- a/jest.config.js
+++ b/jest.config.js
@@ -24,6 +24,8 @@ const config = {
     name: 'TinyNode',
     color: 'cyan'
   },
+  // extensionsToTreatAsEsm: [".js"],
+  testEnvironment: "node",
 
   // Indicates whether the coverage information should be collected while executing the test
   collectCoverage: true,
@@ -206,4 +208,4 @@ const config = {
   // watchman: true,
 }
 
-export default config
\ No newline at end of file
+export default config
diff --git a/public/scripts/api.js b/public/scripts/api.js
index 4a366e9..b16b25b 100644
--- a/public/scripts/api.js
+++ b/public/scripts/api.js
@@ -208,6 +208,10 @@ function overwrite(form, objIn) {
         _customEvent("rerum-result", `URI ${uri} overwritten. See resulting object below:`, resultObj)
     })
     .catch(err => {
+        if (err?.status === 409) {
+            _customEvent("rerum-error", "Conflict detected while trying to overwrite object at " + uri, err.currentVersion, err)
+            return
+        }
         _customEvent("rerum-error", "There was an error trying to overwrite object at " + uri, {}, err)
     })
 }
diff --git a/routes/__tests__/overwrite.test.js b/routes/__tests__/overwrite.test.js
index 4caf484..225c280 100644
--- a/routes/__tests__/overwrite.test.js
+++ b/routes/__tests__/overwrite.test.js
@@ -1,6 +1,7 @@
+
+import { jest } from "@jest/globals"
 import express from "express"
 import request from "supertest"
-import { jest } from "@jest/globals"
 import overwriteRoute from "../overwrite.js"
 //import app from "../../app.js"
 
@@ -13,22 +14,17 @@ routeTester.use("/app/overwrite", overwriteRoute)
 const rerum_tiny_test_obj_id = `${process.env.RERUM_ID_PATTERN}tiny_tester`
 
 beforeEach(() => {
-  /** 
-   * Request/Response Mock Using manual fetch replacement
-   * This is overruling the fetch(store.rerum.io/v1/api/create) call in create.js
-   */
   global.fetch = jest.fn(() =>
     Promise.resolve({
+      status: 200,
+      ok: true,
       json: () => Promise.resolve({ "@id": rerum_tiny_test_obj_id, "testing": "item", "__rerum": { "stuff": "here" } })
     })
   )
 })
 
 afterEach(() => {
-  /**
-   * Food for thought: delete data generated by tests?  
-   * Make a test.store available that uses the same annotationStoreTesting as RERUM tests?
-   */
+
 })
 
 /**
@@ -51,7 +47,9 @@ describe("Check that the request/response behavior of the TinyNode overwrite rou
       .set("Content-Type", "application/json")
       .then(resp => resp)
       .catch(err => err)
-    expect(response.header.location).toBe(rerum_tiny_test_obj_id)
+    if (response.header.location !== rerum_tiny_test_obj_id) {
+      throw new Error(`Expected Location header to be '${rerum_tiny_test_obj_id}', but got '${response.header.location}'.\nAll headers: ${JSON.stringify(response.header)}\nResponse body: ${JSON.stringify(response.body)}`)
+    }
     expect(response.statusCode).toBe(200)
     expect(response.body.testing).toBe("item")
   })
@@ -131,4 +129,4 @@ describe("Check that the properly used overwrite endpoints function and interact
     expect(response.statusCode).toBe(200)
     expect(response.body.testing).toBe("item")
   })
-})
\ No newline at end of file
+})
diff --git a/routes/delete.js b/routes/delete.js
index b5481b6..ea941ef 100644
--- a/routes/delete.js
+++ b/routes/delete.js
@@ -10,6 +10,7 @@ router.delete('/', checkAccessToken, async (req, res, next) => {
     // check for @id in body.  Any value is valid.  Lack of value is a bad request.
     if (!req?.body || !(req.body['@id'] ?? req.body.id)) {
       res.status(400).send("No record id to delete! (https://store.rerum.io/v1/API.html#delete)")
+      return
     }
     const body = JSON.stringify(req.body)
     const deleteOptions = {
diff --git a/routes/overwrite.js b/routes/overwrite.js
index 9aea2a9..8c66600 100644
--- a/routes/overwrite.js
+++ b/routes/overwrite.js
@@ -1,35 +1,65 @@
 import express from "express"
 import checkAccessToken from "../tokens.js"
 const router = express.Router()
-import rerumPropertiesWasher from "../preprocessor.js"
 
 /* PUT an overwrite to the thing. */
-router.put('/', checkAccessToken, rerumPropertiesWasher, async (req, res, next) => {
+router.put('/', checkAccessToken, async (req, res, next) => {
 
   try {
-    // check for @id in body.  Any value is valid.  Lack of value is a bad request.
-    if (!req?.body || !(req.body['@id'] ?? req.body.id)) {
-      res.status(400).send("No record id to overwrite! (https://store.rerum.io/v1/API.html#overwrite)")
+    
+    const overwriteBody = req.body
+    // check for @id; any value is valid
+    if (!(overwriteBody['@id'] ?? overwriteBody.id)) {
+      res.status(400).send("No record id to overwrite! (https://store.rerum.io/API.html#overwrite)")
+      return
     }
-    // check body for JSON
-    const body = JSON.stringify(req.body)
+
     const overwriteOptions = {
       method: 'PUT',
-      body,
+      body: JSON.stringify(overwriteBody),
       headers: {
         'user-agent': 'Tiny-Things/1.0',
         'Authorization': `Bearer ${process.env.ACCESS_TOKEN}`,
         'Content-Type' : "application/json;charset=utf-8"
       }
     }
+
+    // Pass through If-Overwritten-Version header if present
+    const ifOverwrittenVersion = req.headers.hasOwnProperty('if-overwritten-version') ? req.headers['if-overwritten-version'] : null
+    if (ifOverwrittenVersion !== null) {
+      overwriteOptions.headers['If-Overwritten-Version'] = ifOverwrittenVersion
+    }
+
+    // Check for __rerum.isOverwritten in body and use as If-Overwritten-Version header
+    const isOverwrittenValue = req.body?.__rerum?.hasOwnProperty("isOverwritten") ? req.body.__rerum.isOverwritten : null
+    if (isOverwrittenValue !== null) {
+      overwriteOptions.headers['If-Overwritten-Version'] = isOverwrittenValue
+    }
+
     const overwriteURL = `${process.env.RERUM_API_ADDR}overwrite`
-    const result = await fetch(overwriteURL, overwriteOptions).then(res=>res.json())
-    .catch(err=>next(err))
-    res.setHeader("Location", result["@id"] ?? result.id)
-    res.status(200)
-    res.send(result)
+    const response = await fetch(overwriteURL, overwriteOptions)
+    .then(resp=>{
+      if (!resp.ok) throw resp
+      return resp
+    })
+    .catch(async err => {
+      // Handle 409 conflict error for version mismatch
+      if (err.status === 409) {
+        const currentVersion = await err.json()
+        return res.status(409).json(currentVersion)
+      }
+      throw new Error(`Error in overwrite request: ${err.status} ${err.statusText}`)
+    })
+    if(res.headersSent) return
+    const result = await response.json()
+    const location = result?.["@id"] ?? result?.id
+    if (location) {
+      res.setHeader("Location", location)
+    }
+    res.status(response.status ?? 200)
+    res.json(result)
   }
-  catch (err) {    
+  catch (err) {
     next(err)
   }
 })
diff --git a/routes/update.js b/routes/update.js
index dd559fb..391b3ce 100644
--- a/routes/update.js
+++ b/routes/update.js
@@ -1,15 +1,15 @@
 import express from "express"
 import checkAccessToken from "../tokens.js"
 const router = express.Router()
-import rerumPropertiesWasher from "../preprocessor.js"
 
 /* PUT an update to the thing. */
-router.put('/', checkAccessToken, rerumPropertiesWasher, async (req, res, next) => {
+router.put('/', checkAccessToken, async (req, res, next) => {
 
   try {
     // check for @id in body.  Any value is valid.  Lack of value is a bad request.
     if (!req?.body || !(req.body['@id'] ?? req.body.id)) {
       res.status(400).send("No record id to update! (https://store.rerum.io/v1/API.html#update)")
+      return
     }
     // check body for JSON
     const body = JSON.stringify(req.body)