diff --git a/packages/control-plane/src/session/ws-client-mapping-repository.test.ts b/packages/control-plane/src/session/ws-client-mapping-repository.test.ts
index 55a2fad9a..d92caa1b1 100644
--- a/packages/control-plane/src/session/ws-client-mapping-repository.test.ts
+++ b/packages/control-plane/src/session/ws-client-mapping-repository.test.ts
@@ -35,7 +35,16 @@ describe("WsClientMappingRepository", () => {
   });
 
   it("restores a mapping with joined participant data", () => {
-    mock.setRows([{ participant_id: "p-1", client_id: "client-1", user_id: "user-1" }]);
+    mock.setRows([
+      {
+        participant_id: "p-1",
+        client_id: "client-1",
+        user_id: "user-1",
+        canonical_user_id: null,
+        scm_name: null,
+        scm_login: null,
+      },
+    ]);
     expect(repository.getWsClientMapping("ws-1")).toMatchObject({
       participant_id: "p-1",
       client_id: "client-1",
@@ -48,6 +57,25 @@ describe("WsClientMappingRepository", () => {
     expect(repository.getWsClientMapping("unknown")).toBeNull();
   });
 
+  it("returns null for a malformed persisted mapping", () => {
+    mock.setRows([{ participant_id: "p-1", client_id: 42, user_id: "user-1" }]);
+    expect(repository.getWsClientMapping("ws-1")).toBeNull();
+  });
+
+  it("accepts nullable participant profile fields", () => {
+    mock.setRows([
+      {
+        participant_id: "p-1",
+        client_id: "client-1",
+        user_id: "user-1",
+        canonical_user_id: null,
+        scm_name: null,
+        scm_login: null,
+      },
+    ]);
+    expect(repository.getWsClientMapping("ws-1")?.scm_name).toBeNull();
+  });
+
   it("checks whether a mapping exists", () => {
     expect(repository.hasWsClientMapping("unknown")).toBe(false);
     mock.setRows([{ participant_id: "p-1" }]);
diff --git a/packages/control-plane/src/session/ws-client-mapping-repository.ts b/packages/control-plane/src/session/ws-client-mapping-repository.ts
index 18fb6bbb9..5d60569b8 100644
--- a/packages/control-plane/src/session/ws-client-mapping-repository.ts
+++ b/packages/control-plane/src/session/ws-client-mapping-repository.ts
@@ -1,16 +1,18 @@
+import { z } from "zod";
 import type { SqlStorage } from "./sql-storage";
 
 /** WS client mapping result for hibernation recovery. */
-export interface WsClientMappingResult {
-  participant_id: string;
-  client_id: string;
-  user_id: string;
-  canonical_user_id?: string | null;
-  scm_name: string | null;
-  scm_login: string | null;
-  /** Dormant legacy column may still be present on older mapping fixtures. */
-  auth_name?: string | null;
-}
+const wsClientMappingResultSchema = z.object({
+  participant_id: z.string(),
+  client_id: z.string(),
+  user_id: z.string(),
+  canonical_user_id: z.string().nullable().optional(),
+  scm_name: z.string().nullable(),
+  scm_login: z.string().nullable(),
+  auth_name: z.string().nullable().optional(),
+});
+
+export type WsClientMappingResult = z.infer<typeof wsClientMappingResultSchema>;
 
 /** Data for a WS client mapping. */
 export interface WsClientMappingData {
@@ -45,7 +47,8 @@ export class WsClientMappingRepository {
        WHERE m.ws_id = ?`,
       wsId
     );
-    return (result.toArray() as WsClientMappingResult[])[0] ?? null;
+    const parsed = wsClientMappingResultSchema.safeParse(result.toArray()[0]);
+    return parsed.success ? parsed.data : null;
   }
 
   hasWsClientMapping(wsId: string): boolean {
diff --git a/packages/shared/src/completion/extractor.test.ts b/packages/shared/src/completion/extractor.test.ts
index 69e1f13c1..7c626c2eb 100644
--- a/packages/shared/src/completion/extractor.test.ts
+++ b/packages/shared/src/completion/extractor.test.ts
@@ -17,6 +17,12 @@ describe("completion artifact type narrowing", () => {
       null
     );
   });
+
+  it("ignores malformed metadata while preserving artifact labels", () => {
+    expect(
+      toEventArtifactInfo({ artifactType: "branch", metadata: ["main"], url: "/branches/main" })
+    ).toEqual({ type: "branch", url: "/branches/main", label: "Branch: branch" });
+  });
 });
 
 describe("buildAgentResponseFromEvents", () => {
diff --git a/packages/shared/src/completion/extractor.ts b/packages/shared/src/completion/extractor.ts
index 6ddc7de68..80f2ab50c 100644
--- a/packages/shared/src/completion/extractor.ts
+++ b/packages/shared/src/completion/extractor.ts
@@ -28,6 +28,12 @@ export type { ControlPlaneFetcher };
 /** Server-side limit for the events API. */
 const EVENTS_PAGE_LIMIT = 200;
 
+function recordOrUndefined(value: unknown): Record<string, unknown> | undefined {
+  return value && typeof value === "object" && !Array.isArray(value)
+    ? (value as Record<string, unknown>)
+    : undefined;
+}
+
 export interface BuildAgentResponseOptions {
   defaultSuccess?: boolean;
 }
@@ -329,7 +335,7 @@ function isArtifactInEventRange(
  */
 export function summarizeToolCall(data: Record<string, unknown>): ToolCallSummary {
   const tool = String(data.tool ?? "Unknown");
-  const args = (data.args ?? {}) as Record<string, unknown>;
+  const args = recordOrUndefined(data.args) ?? {};
 
   switch (tool) {
     case "Read":
@@ -355,12 +361,12 @@ export function summarizeToolCall(data: Record<string, unknown>): ToolCallSummar
 export function getArtifactLabel(data: Record<string, unknown>): string {
   const type = String(data.artifactType ?? "artifact");
   if (type === "pr") {
-    const metadata = data.metadata as Record<string, unknown> | undefined;
+    const metadata = recordOrUndefined(data.metadata);
     const prNum = metadata?.number;
     return prNum ? `PR #${prNum}` : "Pull Request";
   }
   if (type === "branch") {
-    const metadata = data.metadata as Record<string, unknown> | undefined;
+    const metadata = recordOrUndefined(data.metadata);
     return `Branch: ${metadata?.name ?? "branch"}`;
   }
   return type;
@@ -406,10 +412,7 @@ export function toEventMediaArtifactInfo(data: Record<string, unknown>): MediaAr
   const id = typeof data.artifactId === "string" ? data.artifactId.trim() : "";
   if (!id) return null;
 
-  const metadata =
-    data.metadata && typeof data.metadata === "object" && !Array.isArray(data.metadata)
-      ? (data.metadata as Record<string, unknown>)
-      : undefined;
+  const metadata = recordOrUndefined(data.metadata);
   const mimeType = typeof metadata?.mimeType === "string" ? metadata.mimeType : undefined;
   const sizeBytes =
     typeof metadata?.sizeBytes === "number" &&
diff --git a/packages/web/src/app/api/sessions/[id]/sandbox-access/route.test.ts b/packages/web/src/app/api/sessions/[id]/sandbox-access/route.test.ts
index 1fa8e0e9c..98e16ba31 100644
--- a/packages/web/src/app/api/sessions/[id]/sandbox-access/route.test.ts
+++ b/packages/web/src/app/api/sessions/[id]/sandbox-access/route.test.ts
@@ -47,6 +47,17 @@ describe("sandbox access BFF", () => {
     await expect(response.json()).resolves.toEqual({ error: "Sandbox access changed; retry" });
   });
 
+  it("preserves malformed conflicts instead of treating them as sandbox unavailable", async () => {
+    vi.mocked(controlPlaneUserFetch).mockResolvedValue(Response.json(["bad"], { status: 409 }));
+
+    const response = await GET({} as Request, {
+      params: Promise.resolve({ id: "session-1" }),
+    });
+
+    expect(response.status).toBe(409);
+    await expect(response.json()).resolves.toEqual(["bad"]);
+  });
+
   it("preserves unexpected control-plane errors", async () => {
     vi.mocked(controlPlaneUserFetch).mockResolvedValue(
       Response.json({ error: "Session not found" }, { status: 404 })
diff --git a/packages/web/src/app/api/sessions/[id]/sandbox-access/route.ts b/packages/web/src/app/api/sessions/[id]/sandbox-access/route.ts
index 16bba9176..a4a732383 100644
--- a/packages/web/src/app/api/sessions/[id]/sandbox-access/route.ts
+++ b/packages/web/src/app/api/sessions/[id]/sandbox-access/route.ts
@@ -2,6 +2,16 @@ import { NextResponse } from "next/server";
 import { getServerAuthSession } from "@/lib/server-auth-session";
 import { controlPlaneUserFetch } from "@/lib/control-plane";
 
+function hasSandboxUnavailableError(value: unknown): boolean {
+  return (
+    value !== null &&
+    typeof value === "object" &&
+    !Array.isArray(value) &&
+    "error" in value &&
+    value.error === "Sandbox access is unavailable"
+  );
+}
+
 export async function GET(_request: Request, { params }: { params: Promise<{ id: string }> }) {
   const session = await getServerAuthSession();
   if (!session?.user) {
@@ -18,12 +28,12 @@ export async function GET(_request: Request, { params }: { params: Promise<{ id:
   );
   const conflict =
     response.status === 409
-      ? ((await response
+      ? await response
           .clone()
           .json()
-          .catch(() => null)) as { error?: unknown } | null)
+          .catch(() => null)
       : null;
-  if (conflict?.error === "Sandbox access is unavailable") {
+  if (hasSandboxUnavailableError(conflict)) {
     await response.body?.cancel();
     return new Response(null, {
       status: 204,