From f83dcb0b34011d0fd438fb7a22604d7ac77afaae Mon Sep 17 00:00:00 2001
From: Timothee Guerin <tiguerin@microsoft.com>
Date: Thu, 5 Mar 2026 12:24:57 -0500
Subject: [PATCH 1/2] Fix running multiple versioning mutators

---
 packages/versioning/src/versioning.ts         | 21 +++++++-
 .../apply-snapshot-versioning.test.ts         | 52 +++++++++++++++++++
 2 files changed, 71 insertions(+), 2 deletions(-)

diff --git a/packages/versioning/src/versioning.ts b/packages/versioning/src/versioning.ts
index 5089fb2f0d7..0999228273f 100644
--- a/packages/versioning/src/versioning.ts
+++ b/packages/versioning/src/versioning.ts
@@ -391,13 +391,30 @@ export function getAvailabilityMapInTimeline(
   )
     return undefined;
 
-  added = resolveWhenFirstAdded(added, removed, parentAdded);
+  // Only keep versioning info related to this timeline
+  const timelineAdded = added.filter((x) => timeline.getIndex(x) !== -1);
+  const timelineRemoved = removed.filter((x) => timeline.getIndex(x) !== -1);
+  const hasTypeChangedInTimeline =
+    typeChanged !== undefined && [...typeChanged.keys()].some((v) => timeline.getIndex(v) !== -1);
+  const hasReturnTypeChangedInTimeline =
+    returnTypeChanged !== undefined &&
+    [...returnTypeChanged.keys()].some((v) => timeline.getIndex(v) !== -1);
+
+  if (
+    !timelineAdded.length &&
+    !timelineRemoved.length &&
+    !hasTypeChangedInTimeline &&
+    !hasReturnTypeChangedInTimeline
+  )
+    return undefined;
+
+  added = resolveWhenFirstAdded(timelineAdded, timelineRemoved, parentAdded);
 
   // something isn't available by default
   let isAvail = false;
   for (const [index, moment] of timeline.entries()) {
     const add = added.find((x) => timeline.getIndex(x) === index);
-    const rem = removed.find((x) => timeline.getIndex(x) === index);
+    const rem = timelineRemoved.find((x) => timeline.getIndex(x) === index);
     if (rem) {
       isAvail = false;
       avail.set(moment, Availability.Removed);
diff --git a/packages/versioning/test/mutations/apply-snapshot-versioning.test.ts b/packages/versioning/test/mutations/apply-snapshot-versioning.test.ts
index 421f43ebc65..9c997ca69ea 100644
--- a/packages/versioning/test/mutations/apply-snapshot-versioning.test.ts
+++ b/packages/versioning/test/mutations/apply-snapshot-versioning.test.ts
@@ -1,5 +1,6 @@
 import type { Namespace, Scalar, Type } from "@typespec/compiler";
 import { unsafe_mutateSubgraphWithNamespace } from "@typespec/compiler/experimental";
+import { t } from "@typespec/compiler/testing";
 import { strictEqual } from "assert";
 import { describe, expect, it } from "vitest";
 import { getVersioningMutators } from "../../src/mutator.js";
@@ -211,3 +212,54 @@ describe("operations in interface", () => {
     (decorators) => `interface Test { ${decorators} A(): void; }`,
   );
 });
+
+describe("apply multiple versioning mutators", () => {
+  // https://github.com/microsoft/typespec/issues/9927
+  it("properties with @added from different services are preserved", async () => {
+    const { ServiceA, ServiceB, program } = await Tester.compile(t.code`
+      @versioned(VersionsA)
+      namespace ${t.namespace("ServiceA")} {
+        enum VersionsA { av1, av2 }
+        model Foo {
+          name: string;
+          @added(VersionsA.av2)
+          description?: string;
+        }
+      }
+
+      @versioned(VersionsB)
+      namespace ${t.namespace("ServiceB")} {
+        enum VersionsB { bv1, bv2 }
+        model Bar {
+          id: int32;
+          @added(VersionsB.bv2)
+          value?: string;
+        }
+      }
+    `);
+
+    const serviceAMutators = getVersioningMutators(program, ServiceA);
+    const serviceBMutators = getVersioningMutators(program, ServiceB);
+
+    strictEqual(serviceAMutators?.kind, "versioned");
+    strictEqual(serviceBMutators?.kind, "versioned");
+
+    const serviceAV2 = serviceAMutators.snapshots[1].mutator;
+    const serviceBV2 = serviceBMutators.snapshots[1].mutator;
+
+    const globalNs = program.getGlobalNamespaceType();
+    const result = unsafe_mutateSubgraphWithNamespace(program, [serviceAV2, serviceBV2], globalNs);
+
+    const mutatedGlobal = result.type as Namespace;
+    const serviceA = mutatedGlobal.namespaces.get("ServiceA")!;
+    const serviceB = mutatedGlobal.namespaces.get("ServiceB")!;
+
+    const foo = serviceA.models.get("Foo")!;
+    expect(foo.properties.has("name")).toBe(true);
+    expect(foo.properties.has("description")).toBe(true);
+
+    const bar = serviceB.models.get("Bar")!;
+    expect(bar.properties.has("id")).toBe(true);
+    expect(bar.properties.has("value")).toBe(true);
+  });
+});

From 454c7096ab4091f34bf6f22f9333017288b73e59 Mon Sep 17 00:00:00 2001
From: Timothee Guerin <tiguerin@microsoft.com>
Date: Thu, 5 Mar 2026 12:25:56 -0500
Subject: [PATCH 2/2] m

---
 .../fix-multi-mutators-versioning-2026-2-5-12-25-51.md     | 7 +++++++
 1 file changed, 7 insertions(+)
 create mode 100644 .chronus/changes/fix-multi-mutators-versioning-2026-2-5-12-25-51.md

diff --git a/.chronus/changes/fix-multi-mutators-versioning-2026-2-5-12-25-51.md b/.chronus/changes/fix-multi-mutators-versioning-2026-2-5-12-25-51.md
new file mode 100644
index 00000000000..e0b309a0c03
--- /dev/null
+++ b/.chronus/changes/fix-multi-mutators-versioning-2026-2-5-12-25-51.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/versioning"
+---
+
+[API] Fix running multiple versioning mutators together
\ No newline at end of file