fix(server): emit valid MCP list_agents payloads

Explicit `undefined` keys on optional snapshot fields were converted
to `null` by `ensureValidJson`, failing `AgentSnapshotPayloadSchema`
validation on the wire. Stop writing those keys so they remain absent,
which is what `.optional()` expects.

Also adds a test that parses `list_agents` output through the schema
to catch this class of bug.

packages/server/src/server/agent/agent-projections.ts

@@ -94,7 +94,7 @@ export function toAgentPayload(
     model: agent.config.model ?? null,
     thinkingOptionId,
     effectiveThinkingOptionId,
-    runtimeInfo,
+    ...(runtimeInfo ? { runtimeInfo } : {}),
     createdAt: agent.createdAt.toISOString(),
     updatedAt: agent.updatedAt.toISOString(),
     lastUserMessageAt: agent.lastUserMessageAt ? agent.lastUserMessageAt.toISOString() : null,
@@ -192,8 +192,6 @@ export function buildStoredAgentPayload(
     availableModes: [],
     pendingPermissions: [],
     persistence: toAgentPersistenceHandle(logger, providerRegistry, record.persistence),
-    lastUsage: undefined,
-    lastError: undefined,
     title: record.title ?? record.config?.title ?? null,
     requiresAttention: record.requiresAttention ?? false,
     attentionReason: record.attentionReason ?? null,

packages/server/src/server/agent/mcp-server.test.ts

@@ -2,12 +2,14 @@ import { describe, expect, it, vi } from "vitest";
 import { mkdtemp, mkdir, rm } from "node:fs/promises";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
+import { z } from "zod";
 
 import { createTestLogger } from "../../test-utils/test-logger.js";
 import { createAgentMcpServer } from "./mcp-server.js";
 import type { AgentManager, ManagedAgent } from "./agent-manager.js";
 import type { AgentStorage, StoredAgentRecord } from "./agent-storage.js";
 import type { ProviderDefinition } from "./provider-registry.js";
+import { AgentSnapshotPayloadSchema } from "../../shared/messages.js";
 
 type TestDeps = {
   agentManager: AgentManager;
@@ -702,6 +704,59 @@ describe("agent snapshot MCP serialization", () => {
     ]);
   });
 
+  it("emits list_agents payloads that satisfy the declared output schema", async () => {
+    const { agentManager, agentStorage, spies } = createTestDeps();
+    const liveAgent = {
+      id: "live-agent",
+      provider: "claude",
+      cwd: "/tmp/live-project",
+      config: {},
+      runtimeInfo: undefined,
+      createdAt: new Date("2026-04-11T00:00:00.000Z"),
+      updatedAt: new Date("2026-04-11T00:00:00.000Z"),
+      lastUserMessageAt: null,
+      lifecycle: "idle",
+      capabilities: {
+        supportsStreaming: false,
+        supportsSessionPersistence: false,
+        supportsDynamicModes: false,
+        supportsMcpServers: true,
+        supportsReasoningStream: false,
+        supportsToolInvocations: true,
+      },
+      currentModeId: null,
+      availableModes: [],
+      features: [],
+      pendingPermissions: new Map(),
+      persistence: null,
+      labels: {},
+      attention: { requiresAttention: false },
+    } as unknown as ManagedAgent;
+    spies.agentManager.listAgents.mockReturnValue([liveAgent]);
+    spies.agentStorage.list.mockResolvedValue([
+      createStoredRecord({ id: "stored-non-archived", archivedAt: null }),
+      createStoredRecord({ id: "stored-archived", archivedAt: "2026-04-12T00:00:00.000Z" }),
+    ]);
+
+    const server = await createAgentMcpServer({
+      agentManager,
+      agentStorage,
+      logger,
+      providerRegistry: {
+        claude: createProviderDefinition({}),
+      } as any,
+    });
+    const tool = (server as any)._registeredTools["list_agents"];
+    const response = await tool.callback({ includeArchived: true });
+
+    const parsed = z.array(AgentSnapshotPayloadSchema).safeParse(response.structuredContent.agents);
+    if (!parsed.success) {
+      throw new Error(
+        `list_agents response failed AgentSnapshotPayloadSchema: ${JSON.stringify(parsed.error.issues, null, 2)}`,
+      );
+    }
+  });
+
   it("loads archived agents before reading get_agent_activity", async () => {
     const { agentManager, agentStorage, spies } = createTestDeps();
     const record = createStoredRecord({ id: "archived-activity-agent" });

packages/server/src/server/persistence-hooks.ts

@@ -167,7 +167,7 @@ export function toAgentPersistenceHandle(
   return {
     provider,
     sessionId: handle.sessionId,
-    nativeHandle: handle.nativeHandle,
-    metadata: handle.metadata,
+    ...(handle.nativeHandle !== undefined ? { nativeHandle: handle.nativeHandle } : {}),
+    ...(handle.metadata !== undefined ? { metadata: handle.metadata } : {}),
   } satisfies AgentPersistenceHandle;
 }