diff --git a/examples/acp-agent/tests/acp.snapshot.ts b/examples/acp-agent/tests/acp.snapshot.ts
index 582df37b82..9f99ce9913 100644
--- a/examples/acp-agent/tests/acp.snapshot.ts
+++ b/examples/acp-agent/tests/acp.snapshot.ts
@@ -37,6 +37,7 @@ interface Scenario {
 
 const SCENARIOS: Scenario[] = [
   { name: 'handshake', hasModelTurn: false, recorded: false },
+  { name: 'reject-extra-dirs', hasModelTurn: false, recorded: false },
   { name: 'text-turn', hasModelTurn: true, recorded: true },
   { name: 'tool-call-turn', hasModelTurn: true, recorded: true },
   { name: 'workspace-edit', hasModelTurn: true, recorded: true },
diff --git a/examples/acp-agent/tests/snapshot-harness.ts b/examples/acp-agent/tests/snapshot-harness.ts
index de05c8ca98..d66ee45d7f 100644
--- a/examples/acp-agent/tests/snapshot-harness.ts
+++ b/examples/acp-agent/tests/snapshot-harness.ts
@@ -55,6 +55,7 @@ const repoTsconfig = fileURLToPath(new URL('../../../tsconfig.json', import.meta
 type InputStep =
   | { op: 'initialize'; terminalOutput?: boolean }
   | { op: 'newSession' }
+  | { op: 'newSessionExpectError'; additionalDirectories?: string[] }
   | { op: 'prompt'; text: string }
   | { op: 'promptExpectError'; text: string }
   | { op: 'promptAndCancel'; text: string }
@@ -228,6 +229,21 @@ async function runStep(
       setSessionId(sessionId)
       return
     }
+    case 'newSessionExpectError': {
+      // The bridge rejects a session/new that widens the workspace scope
+      // (non-empty additionalDirectories / mcpServers — unimplemented). The SDK
+      // surfaces that as a rejected RPC; swallow it so the run completes and the
+      // error frame is captured in the transcript.
+      await client.newSession({
+        cwd,
+        mcpServers: [],
+        ...step.additionalDirectories !== undefined ? { additionalDirectories: step.additionalDirectories } : {},
+      }).then(
+        () => { throw new Error('snapshot-harness: expected session/new to be rejected but it succeeded') },
+        () => { /* expected: the bridge rejected the unsupported workspace scope */ },
+      )
+      return
+    }
     case 'prompt': {
       const sessionId = getSessionId()
       if (sessionId === undefined) throw new Error('snapshot-harness: prompt before newSession')
diff --git a/examples/acp-agent/tests/snapshots/reject-extra-dirs/input.json b/examples/acp-agent/tests/snapshots/reject-extra-dirs/input.json
new file mode 100644
index 0000000000..6a10f9a40c
--- /dev/null
+++ b/examples/acp-agent/tests/snapshots/reject-extra-dirs/input.json
@@ -0,0 +1,6 @@
+{
+  "steps": [
+    { "op": "initialize" },
+    { "op": "newSessionExpectError", "additionalDirectories": ["/extra-dir"] }
+  ]
+}
diff --git a/examples/acp-agent/tests/snapshots/reject-extra-dirs/session.jsonl b/examples/acp-agent/tests/snapshots/reject-extra-dirs/session.jsonl
new file mode 100644
index 0000000000..ab44090be6
--- /dev/null
+++ b/examples/acp-agent/tests/snapshots/reject-extra-dirs/session.jsonl
@@ -0,0 +1 @@
+{"type":"session","version":1,"id":"00000000-0000-0000-0000-000000000000","createdAt":0}
diff --git a/examples/acp-agent/tests/snapshots/reject-extra-dirs/stdout.golden.jsonl b/examples/acp-agent/tests/snapshots/reject-extra-dirs/stdout.golden.jsonl
new file mode 100644
index 0000000000..4b864fe7f3
--- /dev/null
+++ b/examples/acp-agent/tests/snapshots/reject-extra-dirs/stdout.golden.jsonl
@@ -0,0 +1,2 @@
+{"jsonrpc":"2.0","id":1,"result":{"protocolVersion":1,"agentInfo":{"name":"deepseek-harness-acp","version":"0.0.1"},"agentCapabilities":{"loadSession":true,"promptCapabilities":{"image":false,"audio":false,"embeddedContext":false}},"authMethods":[]}}
+{"jsonrpc":"2.0","id":2,"error":{"code":-32602,"message":"Invalid params: additionalDirectories is not supported in this MVP"}}