diff --git a/scripts/build/deps/webkit.ts b/scripts/build/deps/webkit.ts
index 61b5aee41661..4bf1f48acd11 100644
--- a/scripts/build/deps/webkit.ts
+++ b/scripts/build/deps/webkit.ts
@@ -3,7 +3,7 @@
  * for local mode. Override via `--webkit-version=<hash>` to test a branch.
  * From https://github.com/oven-sh/WebKit releases.
  */
-export const WEBKIT_VERSION = "7d0c8a7d095864cbf89b43c73ff2d5e5287b1931";
+export const WEBKIT_VERSION = "autobuild-preview-pr-850-3f27c8af";
 
 /**
  * WebKit (JavaScriptCore) — the JS engine.
diff --git a/test/js/bun/jsc/delete-property-transition.test.ts b/test/js/bun/jsc/delete-property-transition.test.ts
new file mode 100644
index 000000000000..96c8081607eb
--- /dev/null
+++ b/test/js/bun/jsc/delete-property-transition.test.ts
@@ -0,0 +1,339 @@
+import { describe as describeCell } from "bun:jsc";
+import { describe, expect, test } from "bun:test";
+import { bunEnv, bunExe } from "harness";
+
+// JavaScriptCore keeps, in the Structure that an object has, the Structure that the delete of a property moves it
+// to. The next object that loses the same property gets that Structure again. The lookup never found what was
+// stored for a property that is non-enumerable, read-only or an accessor. So each such delete made a Structure of
+// its own, each property added after it made one more, and no inline cache ever took such a delete.
+
+function structureOf(value: object): string {
+  const description = describeCell(value);
+  const id = /StructureID: (\d+)/.exec(description)?.[1];
+  if (id === undefined) throw new Error("no StructureID in: " + description);
+  return id;
+}
+
+/** All the objects stay alive while they are counted: a Structure that nothing uses is collected and its id is used again. */
+function structuresOf(make: (i: number) => object, count = 100): number {
+  const made = Array.from({ length: count }, (_, i) => make(i));
+  return new Set(made.map(structureOf)).size;
+}
+
+const define = (object: object, key: PropertyKey, descriptor: PropertyDescriptor): any =>
+  Object.defineProperty(object, key, { configurable: true, ...descriptor });
+
+const symbol = Symbol("a");
+
+// This file is a module, so the function is strict and `length` is an own, non-enumerable property from the start.
+function argumentsOf(_a?: unknown, _b?: unknown): any {
+  return arguments;
+}
+
+class Reported extends Error {
+  // The statements of the constructor of SQL.PostgresError.
+  constructor(message: string, options: Record<string, string>) {
+    super(message);
+    delete (this as any).line;
+    delete (this as any).column;
+    this.name = "Reported";
+    for (const key of ["code", "detail", "line", "column"]) {
+      if (options[key] !== undefined) (this as any)[key] = options[key];
+    }
+  }
+}
+
+// Each one deletes a property whose attributes are not 0. The number is how many Structures 100 objects end on.
+const shapes: [name: string, structures: number, make: (i: number) => object][] = [
+  [
+    "a non-enumerable property, then an add",
+    1,
+    () => {
+      const o = define({}, "a", { value: 1, writable: true });
+      delete o.a;
+      o.b = 1;
+      return o;
+    },
+  ],
+  [
+    "a read-only property",
+    1,
+    () => {
+      const o = define({}, "a", { value: 1, enumerable: true });
+      delete o.a;
+      return o;
+    },
+  ],
+  [
+    "a getter",
+    1,
+    () => {
+      const o: any = {
+        get a() {
+          return 1;
+        },
+      };
+      delete o.a;
+      return o;
+    },
+  ],
+  [
+    "a symbol",
+    1,
+    () => {
+      const o = define({}, symbol, { value: 1 });
+      delete o[symbol];
+      return o;
+    },
+  ],
+  [
+    "Reflect.deleteProperty",
+    1,
+    () => {
+      const o = define({}, "a", { value: 1 });
+      Reflect.deleteProperty(o, "a");
+      return o;
+    },
+  ],
+  [
+    "a Proxy without a trap",
+    1,
+    () => {
+      const target = define({}, "a", { value: 1 });
+      delete new Proxy(target, {}).a;
+      return target;
+    },
+  ],
+  [
+    "a property that became non-enumerable later",
+    1,
+    () => {
+      const o: any = { a: 1 };
+      define(o, "a", { enumerable: false });
+      delete o.a;
+      return o;
+    },
+  ],
+  [
+    "a property that is added again",
+    1,
+    () => {
+      const o = define({}, "a", { value: 1 });
+      delete o.a;
+      o.a = 1;
+      return o;
+    },
+  ],
+  [
+    "two properties",
+    1,
+    () => {
+      const o = define(define({}, "a", { value: 1 }), "b", { value: 2 });
+      delete o.a;
+      delete o.b;
+      return o;
+    },
+  ],
+  [
+    "one of two properties",
+    2,
+    i => {
+      const o = define(define({}, "a", { value: 1 }), "b", { value: 2 });
+      if (i & 1) delete o.a;
+      else delete o.b;
+      return o;
+    },
+  ],
+  [
+    "the stack of an Error",
+    1,
+    () => {
+      const error = new Error("message");
+      delete error.stack;
+      return error;
+    },
+  ],
+  [
+    "the name of a function",
+    1,
+    () => {
+      const f: any = function () {};
+      delete f.name;
+      return f;
+    },
+  ],
+  [
+    "the length of arguments",
+    1,
+    () => {
+      const args = argumentsOf(1, 2);
+      delete args.length;
+      return args;
+    },
+  ],
+  [
+    "a second Error.captureStackTrace",
+    1,
+    () => {
+      const error = new Error("message");
+      Error.captureStackTrace(error);
+      Error.captureStackTrace(error);
+      return error;
+    },
+  ],
+  ["a class that deletes the line and column of its Error", 1, () => new Reported("message", { code: "C" })],
+  [
+    "that class with the fields of two kinds of error",
+    2,
+    i => new Reported("message", i & 1 ? { code: "C", line: "12", column: "c" } : { code: "C", detail: "d" }),
+  ],
+];
+
+describe("objects that lose the same property get the same Structure", () => {
+  test.each(shapes)("%s", (_, structures, make) => {
+    expect(structuresOf(make)).toBe(structures);
+  });
+});
+
+// The cases below need a delete site that the compilers have. One child runs them all. The compilers run on its main
+// thread, so the same number of calls gets a site to the same compiler each time.
+const hotSites = `
+  const vm = require("node:vm");
+  const { noInline, setTimeZone } = require("bun:jsc");
+  const results = {};
+
+  {
+    const descriptors = [
+      { value: 1, writable: true, configurable: true },
+      { value: 1, enumerable: true, configurable: true },
+      { get() { return 1; }, set(value) {}, enumerable: true, configurable: true },
+      { value: 1, configurable: true },
+    ];
+    function deleteA(o) { return delete o.a; }
+    function deleteKey(o, key) { return delete o[key]; }
+    noInline(deleteA);
+    noInline(deleteKey);
+
+    const wrong = new Set();
+    for (let i = 0; i < 3000; i++) {
+      const kind = i & 3;
+      const o = Object.defineProperty({ x: 1 }, "a", descriptors[kind]);
+      o.y = 2;
+      const result = i & 4 ? deleteA(o) : deleteKey(o, "a");
+      if (result !== true || "a" in o || o.x !== 1 || o.y !== 2) wrong.add("kind " + kind);
+      o.a = i;
+      if (o.a !== i) wrong.add("add after kind " + kind);
+    }
+
+    // A property that is not configurable is still refused at these sites: false, or a TypeError in strict code.
+    const fixed = Object.defineProperty({ x: 1 }, "a", { value: 1 });
+    const refused = [o => deleteA(o), o => deleteKey(o, "a")].map(site => {
+      try {
+        return site(fixed) === false;
+      } catch (error) {
+        return error instanceof TypeError;
+      }
+    });
+    results.attributes = { wrong: [...wrong], refused, value: fixed.a };
+  }
+
+  {
+    const { strictArguments, sloppyArguments, deleteCallee, deleteLength, deleteCalleeAfterLength } = new Function(
+      "function strict() { 'use strict'; return arguments; }" +
+        "function target(a, b) { return inner(); }" +
+        "function inner() { return target.arguments; }" +
+        "return {" +
+        "  strictArguments: () => strict(1, 2)," +
+        "  sloppyArguments: () => target(1, 2)," +
+        "  deleteCallee(o) { return delete o.callee; }," +
+        "  deleteLength(o) { return delete o.length; }," +
+        "  deleteCalleeAfterLength(o) { return delete o.callee; }," +
+        "};",
+    )();
+    noInline(deleteCallee);
+    noInline(deleteLength);
+    noInline(deleteCalleeAfterLength);
+
+    const wrong = { strict: 0, strictAfterLength: 0, sloppy: 0, sloppyAfterLength: 0 };
+    for (let i = 0; i < 1500; i++) {
+      if (deleteCallee(strictArguments()) !== false) wrong.strict++;
+      const args = strictArguments();
+      deleteLength(args);
+      if (deleteCalleeAfterLength(args) !== false) wrong.strictAfterLength++;
+    }
+    for (let i = 0; i < 100; i++) {
+      let args = sloppyArguments();
+      if (deleteCallee(args) !== true || "callee" in args) wrong.sloppy++;
+      args = sloppyArguments();
+      deleteLength(args);
+      if (deleteCalleeAfterLength(args) !== true || "callee" in args || args[0] !== 1) wrong.sloppyAfterLength++;
+    }
+    results.functionArguments = wrong;
+  }
+
+  {
+    const deleteGlobal = new Function("return delete declaredLater;");
+    noInline(deleteGlobal);
+    let missing = 0;
+    for (let i = 0; i < 1500; i++) missing += deleteGlobal() === true;
+    vm.runInThisContext('var declaredLater = {"a": 1};');
+    results.globalVar = { missing, result: deleteGlobal(), value: declaredLater.a };
+  }
+
+  {
+    function deleteTZ() { return delete process.env.TZ; }
+    noInline(deleteTZ);
+    deleteTZ();
+    const zone = () => Intl.DateTimeFormat().resolvedOptions().timeZone;
+    const zoneOfTheHost = zone();
+    for (let i = 0; i < 300; i++) deleteTZ();
+    setTimeZone("Asia/Tokyo");
+    const hourInTokyo = new Date(0).getHours();
+    const result = deleteTZ();
+    results.timeZone = { result, hourInTokyo, resetToTheZoneOfTheHost: zone() === zoneOfTheHost };
+  }
+
+  console.log(JSON.stringify(results));
+`;
+
+let child: Promise<Record<string, unknown>> | undefined;
+async function runHotSites() {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", hotSites],
+    env: { ...bunEnv, BUN_JSC_useConcurrentJIT: "0" },
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+  expect({ stderr, exitCode }).toEqual({ stderr: "", exitCode: 0 });
+  return JSON.parse(stdout);
+}
+
+describe("a delete site that is hot", () => {
+  test("does for such a property what the first delete did", async () => {
+    const { attributes } = await (child ??= runHotSites());
+    expect(attributes).toEqual({ wrong: [], refused: [true, true], value: 1 });
+  });
+
+  test("does not answer for function.arguments after it saw the arguments of strict functions", async () => {
+    // `callee` is made when it is asked for: an accessor that is not configurable for the arguments of a strict
+    // function, an ordinary property for function.arguments of a sloppy one. Both kinds have one Structure until then,
+    // also after a delete of `length`. The sites are sloppy: a strict delete of the accessor throws and is never cached.
+    const { functionArguments } = await (child ??= runHotSites());
+    expect(functionArguments).toEqual({ strict: 0, strictAfterLength: 0, sloppy: 0, sloppyAfterLength: 0 });
+  });
+
+  test("refuses a global var that a script declared after the site was hot", async () => {
+    // A script that is only `var name = <JSON>` declares the variable and leaves the Structure of the global object
+    // as it is. The global object answers for its variables from a table of its own.
+    const { globalVar } = await (child ??= runHotSites());
+    expect(globalVar).toEqual({ missing: 1500, result: false, value: 1 });
+  });
+
+  test("still resets the time zone for process.env.TZ", async () => {
+    // The delete of TZ also drops the time zone that is in use. The object that process.env is has that in its own
+    // delete, which a cached delete does not call.
+    const { timeZone } = await (child ??= runHotSites());
+    expect(timeZone).toEqual({ result: true, hourInTokyo: 9, resetToTheZoneOfTheHost: true });
+  });
+});
diff --git a/test/js/sql/sql-errors.test.ts b/test/js/sql/sql-errors.test.ts
index de4ffacd223f..5372e19f4a5d 100644
--- a/test/js/sql/sql-errors.test.ts
+++ b/test/js/sql/sql-errors.test.ts
@@ -1,4 +1,5 @@
 import { SQL } from "bun";
+import { describe as describeCell } from "bun:jsc";
 import { expect, test } from "bun:test";
 import { bunEnv, bunExe, describeWithContainer } from "harness";
 
@@ -51,6 +52,11 @@ function layout(error: object, ignored: string[]) {
 // An Error that JavaScript makes has these. They describe the place where it was made.
 const stackKeys = ["stack", "sourceURL", "originalLine", "originalColumn"];
 
+/** How many Structures the errors have. They are all alive here, so no id is one that was used again. */
+function structuresOf(errors: object[]): number {
+  return new Set(errors.map(error => /StructureID: (\d+)/.exec(describeCell(error))![1])).size;
+}
+
 class UserError extends Error {}
 const thrownValues: [kind: string, value: unknown][] = [
   ["an Error subclass", new UserError("thrown by user code")],
@@ -186,6 +192,21 @@ describeWithContainer("mysql", { image: "mysql_plain", concurrent: true }, conta
     );
     await expectUsable(sql);
   });
+
+  test("errors with the same fields have one Structure", async () => {
+    await container.ready;
+    await using sql = connect();
+
+    const fromServer: object[] = [];
+    for (let i = 0; i < 20; i++) fromServer.push(await rejectionOf(sql`SELECT * FROM no_such_table_for_sql_errors`));
+    const madeHere = fromServer.map(({ message, code, errno, sqlState }: any) => {
+      return new SQL.MySQLError(message, { code, errno, sqlState });
+    });
+    expect({ fromServer: structuresOf(fromServer), madeHere: structuresOf(madeHere) }).toEqual({
+      fromServer: 1,
+      madeHere: 1,
+    });
+  });
 });
 
 describeWithContainer("postgres", { image: "postgres_plain", concurrent: true }, container => {
@@ -298,6 +319,20 @@ describeWithContainer("postgres", { image: "postgres_plain", concurrent: true },
     );
     await expectUsable(sql);
   });
+
+  test("errors with the same fields have one Structure", async () => {
+    await container.ready;
+    await using sql = connect();
+
+    // Each one loses the `line` and `column` that JavaScriptCore gave it, and gets the fields of the server after that.
+    const fromServer: object[] = [];
+    for (let i = 0; i < 20; i++) fromServer.push(await rejectionOf(sql`SELECT * FROM no_such_table_for_sql_errors`));
+    const madeHere = fromServer.map((error: any) => new SQL.PostgresError(error.message, { ...error }));
+    expect({ fromServer: structuresOf(fromServer), madeHere: structuresOf(madeHere) }).toEqual({
+      fromServer: 1,
+      madeHere: 1,
+    });
+  });
 });
 
 for (const [adapter, image, url, failing, message] of [