diff --git a/docs/current-handoff.md b/docs/current-handoff.md
index 8c2541f..c10adeb 100644
--- a/docs/current-handoff.md
+++ b/docs/current-handoff.md
@@ -78,6 +78,12 @@
 - The 502 live run at `dabd9e2` was PATH NOT OBSERVABLE ON FAILURE: provider path survived provider errors but not successful-provider/later-validation failures.
 - Successful provider results now retain their factual attempted path through later reconstruction-validation failure diagnostics. This corrects only that observability seam; the next unknown remains one real `/api/cases/start` call establishing `/api/chat` versus `/api/generate`.
 
+## Zod 4 migration
+
+- Zod upgraded from 3.25.76 to 4.5.4. Reconstruction and direct Zod-owner suites pass, and native `z.toJSONSchema()` converts the production reconstruction schema with required closed enum constraints.
+- Zod-3-specific error-message assertions are version-independent while retaining semantic validation evidence. Production schema semantics are unchanged and no third-party JSON-schema conversion dependency was added.
+- Next unknown: whether Ollama/Qwen accepts and obeys the generated schema through `/api/chat` format.
+
 ## Current product architecture
 
 Three distinct routes, not a single page:
diff --git a/package-lock.json b/package-lock.json
index f089f7d..88cd085 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -11,7 +11,7 @@
         "next": "^14.2.0",
         "react": "^18.3.0",
         "react-dom": "^18.3.0",
-        "zod": "^3.23.0"
+        "zod": "^4.5.4"
       },
       "devDependencies": {
         "@playwright/test": "^1.62.1",
@@ -7646,9 +7646,9 @@
       }
     },
     "node_modules/zod": {
-      "version": "3.25.76",
-      "resolved": "https://registry.npmjs.org/zod/-/zod-3.25.76.tgz",
-      "integrity": "sha512-gzUt/qt81nXsFGKIFcC3YnfEAx5NkunCfnDlvuBSSFS02bcXu4Lmea0AFIUwbLWxWPx3d9p8S5QoaujKcNQxcQ==",
+      "version": "4.5.4",
+      "resolved": "https://registry.npmjs.org/zod/-/zod-4.5.4.tgz",
+      "integrity": "sha512-sC95tT5iHHH9gtpj6A81kh+NEaRAUFN+qlUPDUbRfOMvNf5QCBqsb3WgvnpVtK5Y+4UfA6KqufotuTvMGiTlsA==",
       "license": "MIT",
       "funding": {
         "url": "https://github.com/sponsors/colinhacks"
diff --git a/package.json b/package.json
index 6dca8bf..30d5861 100644
--- a/package.json
+++ b/package.json
@@ -16,7 +16,7 @@
     "next": "^14.2.0",
     "react": "^18.3.0",
     "react-dom": "^18.3.0",
-    "zod": "^3.23.0"
+    "zod": "^4.5.4"
   },
   "devDependencies": {
     "@playwright/test": "^1.62.1",
diff --git a/tests/reconstruction.test.js b/tests/reconstruction.test.js
index fb90fb4..6754afb 100644
--- a/tests/reconstruction.test.js
+++ b/tests/reconstruction.test.js
@@ -36,7 +36,8 @@ describe("reconstruction schema", () => {
     const result = reconstructionSchema.safeParse(input);
     expect(result.success).toBe(false);
     if (!result.success) {
-      expect(result.error.issues[0].message).toContain("Expected");
+      expect(result.error.issues.length).toBeGreaterThan(0);
+      expect(result.error.issues[0].path).toContain("confidence");
     }
   });
 
diff --git a/tests/reconstruction/compatibility.test.js b/tests/reconstruction/compatibility.test.js
index 23ebbfe..e7454e8 100644
--- a/tests/reconstruction/compatibility.test.js
+++ b/tests/reconstruction/compatibility.test.js
@@ -254,10 +254,10 @@ describe("analyseScenario compatibility", () => {
     expect(result.validationIssues).toContainEqual(expect.objectContaining({
       path: ["reconstruction", "observedStates", 0, "description"],
       code: "invalid_type",
-      message: "Required",
+      expected: "string",
     }));
     expect(result.providerApiPath).toBe("/api/chat");
-    expect(result.errors).toContain("reconstruction: Required");
+    expect(result.errors).toContainEqual(expect.stringMatching(/^reconstruction:/));
   });
 
   it("succeeds when reported_claim is the only evidenceType mismatch", async () => {