diff --git a/.changeset/acp-stdio-mcp-regression.md b/.changeset/acp-stdio-mcp-regression.md
new file mode 100644
index 0000000000..90354b7375
--- /dev/null
+++ b/.changeset/acp-stdio-mcp-regression.md
@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Fix ACP session methods rejecting stdio MCP servers, and fix session/load failing when reloading a closed session.
diff --git a/packages/acp-server/src/convert.ts b/packages/acp-server/src/convert.ts
index 308d400a0d..3e405039a5 100644
--- a/packages/acp-server/src/convert.ts
+++ b/packages/acp-server/src/convert.ts
@@ -176,7 +176,13 @@ export function acpMcpServersToConfigRecord(
   const out: Record<string, McpServerConfig> = {};
   for (const server of servers) {
     if (!('type' in server)) {
-      throw new Error(`ACP stdio MCP server ${server.name} does not declare a runtime identity`);
+      out[server.name] = {
+        transport: 'stdio',
+        command: server.command,
+        args: server.args,
+        env: namedPairsToRecord(server.env),
+      };
+      continue;
     }
     if (server.type === 'http' || server.type === 'sse') {
       out[server.name] = {
diff --git a/packages/acp-server/test/convert.test.ts b/packages/acp-server/test/convert.test.ts
index c5a1c490ca..fe34f16627 100644
--- a/packages/acp-server/test/convert.test.ts
+++ b/packages/acp-server/test/convert.test.ts
@@ -19,7 +19,7 @@ describe('acpMcpServersToConfigRecord', () => {
     expect(acpMcpServersToConfigRecord([])).toBeUndefined();
   });
 
-  it('rejects stdio servers that cannot declare a runtime identity', () => {
+  it('maps stdio servers (no `type` discriminator) with env pairs as a record', () => {
     const servers: McpServer[] = [
       {
         name: 'fs',
@@ -31,9 +31,14 @@ describe('acpMcpServersToConfigRecord', () => {
         ],
       },
     ];
-    expect(() => acpMcpServersToConfigRecord(servers)).toThrow(
-      'ACP stdio MCP server fs does not declare a runtime identity',
-    );
+    expect(acpMcpServersToConfigRecord(servers)).toEqual({
+      fs: {
+        transport: 'stdio',
+        command: '/usr/local/bin/mcp-fs',
+        args: ['--root', '/tmp'],
+        env: { API_KEY: 'secret', DEBUG: '1' },
+      },
+    });
   });
 
   it('maps http and sse servers with header pairs as a record', () => {
diff --git a/packages/acp-server/test/lifecycle.test.ts b/packages/acp-server/test/lifecycle.test.ts
index fe124ca8d8..5b505203f3 100644
--- a/packages/acp-server/test/lifecycle.test.ts
+++ b/packages/acp-server/test/lifecycle.test.ts
@@ -291,10 +291,10 @@ describe('acp-server session lifecycle', () => {
   );
 
   it(
-    'session/new rejects stdio MCP servers without runtime identity',
+    'session/new connects ACP mcpServers as ephemeral session servers',
     async () => {
       const c = await boot();
-      await expect(c.send('session/new', {
+      const created = (await c.send('session/new', {
         cwd: homeDir,
         mcpServers: [
           {
@@ -304,16 +304,19 @@ describe('acp-server session lifecycle', () => {
             env: [{ name: 'KIMI_TEST_MCP_START_DELAY_MS', value: '0' }],
           },
         ],
-      })).rejects.toThrow('ACP stdio MCP server mock does not declare a runtime identity');
+      })) as { sessionId: string };
+      expect(created.sessionId).toMatch(/^session_/);
 
       // Engine-side assertion: the session scope's MCP handle is the overlay
       // view and the converted server ended up connected under its ACP name.
+      const entries = await sessionMcpEntries(c, created.sessionId);
+      expect(entries.find((e) => e.name === 'mock')?.status).toBe('connected');
     },
     30_000,
   );
 
   it(
-    'session/load rejects stdio MCP servers without runtime identity',
+    'session/load forwards mcpServers to the re-materialized session',
     async () => {
       const c = await boot();
       const created = (await c.send('session/new', { cwd: homeDir, mcpServers: [] })) as {
@@ -321,13 +324,16 @@ describe('acp-server session lifecycle', () => {
       };
       await c.send('session/close', { sessionId: created.sessionId });
 
-      await expect(c.send('session/load', {
+      await c.send('session/load', {
         sessionId: created.sessionId,
         cwd: homeDir,
         mcpServers: [
           { name: 'mock', command: process.execPath, args: [STDIO_MCP_FIXTURE], env: [] },
         ],
-      })).rejects.toThrow('ACP stdio MCP server mock does not declare a runtime identity');
+      });
+
+      const entries = await sessionMcpEntries(c, created.sessionId);
+      expect(entries.find((e) => e.name === 'mock')?.status).toBe('connected');
     },
     30_000,
   );
diff --git a/packages/agent-core-v2/src/runtime/runtimeUnitHost.ts b/packages/agent-core-v2/src/runtime/runtimeUnitHost.ts
index 9554fbff4c..585b5e0707 100644
--- a/packages/agent-core-v2/src/runtime/runtimeUnitHost.ts
+++ b/packages/agent-core-v2/src/runtime/runtimeUnitHost.ts
@@ -295,7 +295,7 @@ class SharedRuntimeUnitHost implements RuntimeUnitHost {
       },
       registerRuntime: (runtime) => {
         if (!active) throw new Error('runtime unit transaction is disposed');
-        if (runtimes.some((entry) => entry.runtime.identity.runtimeId === runtime.identity.runtimeId)) {
+        if (runtimes.some((entry) => entry.active && entry.runtime.identity.runtimeId === runtime.identity.runtimeId)) {
           throw new Error(`runtime ${runtime.identity.runtimeId} is registered twice in one transaction`);
         }
         const staged: StagedRuntime = { runtime, active: true };
diff --git a/packages/agent-core-v2/src/workspace/workspaceMcp/workspaceMcpService.ts b/packages/agent-core-v2/src/workspace/workspaceMcp/workspaceMcpService.ts
index 951088adbf..e8c21a4ed9 100644
--- a/packages/agent-core-v2/src/workspace/workspaceMcp/workspaceMcpService.ts
+++ b/packages/agent-core-v2/src/workspace/workspaceMcp/workspaceMcpService.ts
@@ -125,7 +125,6 @@ export class WorkspaceMcpService extends Disposable implements IWorkspaceMcpServ
       runtimeResolver: this.runtimeResolver,
       workspaceId: this.workspaceId,
       runtimeId: 'local',
-      requireStdioRuntimeId: true,
       resolveDefaultTimeouts: () => this.mcpConfig.tunables(),
       resolveClientName: this.resolveClientName,
     });
diff --git a/packages/agent-core-v2/test/runtime/runtimeUnitHost.test.ts b/packages/agent-core-v2/test/runtime/runtimeUnitHost.test.ts
index 9b9eec0e4a..f29e3ab117 100644
--- a/packages/agent-core-v2/test/runtime/runtimeUnitHost.test.ts
+++ b/packages/agent-core-v2/test/runtime/runtimeUnitHost.test.ts
@@ -225,6 +225,30 @@ describe('RuntimeUnitHost', () => {
     disposables.dispose();
   });
 
+  it('allows re-registering a runtime id whose earlier registration was removed', async () => {
+    const { disposables, host, registry } = setup();
+    let providerHost!: RuntimeProviderHost;
+    const handle = await host.provide(emptyImports(), async (provider) => {
+      providerHost = provider;
+      return { dispose: () => {} };
+    });
+
+    const first = runtime('one');
+    const registration = providerHost.registerRuntime(first);
+    await registration.remove();
+    expect(registry.current('local')).toBeUndefined();
+
+    const second = runtime('two');
+    providerHost.registerRuntime(second);
+    expect(registry.current('local')).toBe(second);
+
+    await handle.remove();
+    expect(registry.current('local')).toBeUndefined();
+    expect(second.disposed).toBe(true);
+    await host.dispose();
+    disposables.dispose();
+  });
+
   it('waits for in-flight prepare, rejects new transactions, and tears down in reverse order', async () => {
     const { disposables, host } = setup();
     const order: string[] = [];