diff --git a/examples/daytona/src/index.ts b/examples/daytona/src/index.ts
index b881113..9c4cf85 100644
--- a/examples/daytona/src/index.ts
+++ b/examples/daytona/src/index.ts
@@ -16,7 +16,6 @@ console.log(`UI: ${client.inspectorUrl}`);
 
 const session = await client.createSession({
   agent: detectAgent(),
-  cwd: "/home/daytona",
 });
 
 session.onEvent((event) => {
diff --git a/examples/e2b/src/index.ts b/examples/e2b/src/index.ts
index 6eb79b7..67b74dc 100644
--- a/examples/e2b/src/index.ts
+++ b/examples/e2b/src/index.ts
@@ -14,7 +14,6 @@ const client = await SandboxAgent.start({
 
 const session = await client.createSession({
   agent: detectAgent(),
-  cwd: "/home/user",
 });
 
 session.onEvent((event) => {
diff --git a/examples/vercel/src/index.ts b/examples/vercel/src/index.ts
index 9839893..5a83e0c 100644
--- a/examples/vercel/src/index.ts
+++ b/examples/vercel/src/index.ts
@@ -19,7 +19,6 @@ console.log(`UI: ${client.inspectorUrl}`);
 
 const session = await client.createSession({
   agent: detectAgent(),
-  cwd: "/home/vercel-sandbox",
 });
 
 session.onEvent((event) => {
diff --git a/sdks/typescript/src/client.ts b/sdks/typescript/src/client.ts
index df66400..47e6dc3 100644
--- a/sdks/typescript/src/client.ts
+++ b/sdks/typescript/src/client.ts
@@ -1127,7 +1127,7 @@ export class SandboxAgent {
 
     const localSessionId = request.id?.trim() || randomId();
     const live = await this.getLiveConnection(request.agent.trim());
-    const sessionInit = normalizeSessionInit(request.sessionInit, request.cwd);
+    const sessionInit = normalizeSessionInit(request.sessionInit, request.cwd, this.sandboxProvider?.defaultCwd);
 
     const response = await live.createRemoteSession(localSessionId, sessionInit);
 
@@ -1183,7 +1183,7 @@ export class SandboxAgent {
     const replaySource = await this.collectReplayEvents(existing.id, this.replayMaxEvents);
     const replayText = buildReplayText(replaySource, this.replayMaxChars);
 
-    const recreated = await live.createRemoteSession(existing.id, normalizeSessionInit(existing.sessionInit));
+    const recreated = await live.createRemoteSession(existing.id, normalizeSessionInit(existing.sessionInit, undefined, this.sandboxProvider?.defaultCwd));
 
     const updated: SessionRecord = {
       ...existing,
@@ -2657,17 +2657,21 @@ function toAgentQuery(options: AgentQueryOptions | undefined): Record<string, Qu
   };
 }
 
-function normalizeSessionInit(value: Omit<NewSessionRequest, "_meta"> | undefined, cwdShorthand?: string): Omit<NewSessionRequest, "_meta"> {
+function normalizeSessionInit(
+  value: Omit<NewSessionRequest, "_meta"> | undefined,
+  cwdShorthand?: string,
+  providerDefaultCwd?: string,
+): Omit<NewSessionRequest, "_meta"> {
   if (!value) {
     return {
-      cwd: cwdShorthand ?? defaultCwd(),
+      cwd: cwdShorthand ?? providerDefaultCwd ?? defaultCwd(),
       mcpServers: [],
     };
   }
 
   return {
     ...value,
-    cwd: value.cwd ?? cwdShorthand ?? defaultCwd(),
+    cwd: value.cwd ?? cwdShorthand ?? providerDefaultCwd ?? defaultCwd(),
     mcpServers: value.mcpServers ?? [],
   };
 }
diff --git a/sdks/typescript/src/providers/cloudflare.ts b/sdks/typescript/src/providers/cloudflare.ts
index c17adfc..7f0bd25 100644
--- a/sdks/typescript/src/providers/cloudflare.ts
+++ b/sdks/typescript/src/providers/cloudflare.ts
@@ -36,6 +36,7 @@ export function cloudflare(options: CloudflareProviderOptions): SandboxProvider
 
   return {
     name: "cloudflare",
+    defaultCwd: "/root",
     async create(): Promise<string> {
       if (typeof sdk.create !== "function") {
         throw new Error('sandbox provider "cloudflare" requires a sdk with a `create()` method.');
diff --git a/sdks/typescript/src/providers/computesdk.ts b/sdks/typescript/src/providers/computesdk.ts
index 8cbef53..3ebb0da 100644
--- a/sdks/typescript/src/providers/computesdk.ts
+++ b/sdks/typescript/src/providers/computesdk.ts
@@ -21,6 +21,7 @@ export function computesdk(options: ComputeSdkProviderOptions = {}): SandboxProv
 
   return {
     name: "computesdk",
+    defaultCwd: "/root",
     async create(): Promise<string> {
       const createOpts = await resolveCreateOptions(options.create);
       const sandbox = await compute.sandbox.create({
diff --git a/sdks/typescript/src/providers/daytona.ts b/sdks/typescript/src/providers/daytona.ts
index f614faf..18777c6 100644
--- a/sdks/typescript/src/providers/daytona.ts
+++ b/sdks/typescript/src/providers/daytona.ts
@@ -31,6 +31,7 @@ export function daytona(options: DaytonaProviderOptions = {}): SandboxProvider {
 
   return {
     name: "daytona",
+    defaultCwd: "/home/daytona",
     async create(): Promise<string> {
       const createOpts = await resolveCreateOptions(options.create);
       const sandbox = await client.create({
diff --git a/sdks/typescript/src/providers/docker.ts b/sdks/typescript/src/providers/docker.ts
index 9e49687..5db67bf 100644
--- a/sdks/typescript/src/providers/docker.ts
+++ b/sdks/typescript/src/providers/docker.ts
@@ -44,6 +44,7 @@ export function docker(options: DockerProviderOptions = {}): SandboxProvider {
 
   return {
     name: "docker",
+    defaultCwd: "/home/sandbox",
     async create(): Promise<string> {
       const hostPort = await getPort();
       const env = await resolveValue(options.env, []);
diff --git a/sdks/typescript/src/providers/e2b.ts b/sdks/typescript/src/providers/e2b.ts
index 54d2e28..3f64d0d 100644
--- a/sdks/typescript/src/providers/e2b.ts
+++ b/sdks/typescript/src/providers/e2b.ts
@@ -42,6 +42,7 @@ export function e2b(options: E2BProviderOptions = {}): SandboxProvider {
 
   return {
     name: "e2b",
+    defaultCwd: "/home/user",
     async create(): Promise<string> {
       const createOpts = await resolveOptions(options.create);
       const rawTemplate = typeof createOpts.template === "string" ? createOpts.template : undefined;
diff --git a/sdks/typescript/src/providers/modal.ts b/sdks/typescript/src/providers/modal.ts
index 4d5b39f..fad98c2 100644
--- a/sdks/typescript/src/providers/modal.ts
+++ b/sdks/typescript/src/providers/modal.ts
@@ -29,6 +29,7 @@ export function modal(options: ModalProviderOptions = {}): SandboxProvider {
 
   return {
     name: "modal",
+    defaultCwd: "/root",
     async create(): Promise<string> {
       const createOpts = await resolveCreateOptions(options.create);
       const appName = createOpts.appName ?? DEFAULT_APP_NAME;
diff --git a/sdks/typescript/src/providers/types.ts b/sdks/typescript/src/providers/types.ts
index ab996e1..9c925d1 100644
--- a/sdks/typescript/src/providers/types.ts
+++ b/sdks/typescript/src/providers/types.ts
@@ -47,4 +47,11 @@ export interface SandboxProvider {
    * (e.g. the duplicate process exits on port conflict).
    */
   ensureServer?(sandboxId: string): Promise<void>;
+
+  /**
+   * Default working directory for sessions when the caller does not specify
+   * one. Remote providers should set this to a path that exists inside the
+   * sandbox (e.g. '/home/user'). When omitted, falls back to process.cwd().
+   */
+  defaultCwd?: string;
 }
diff --git a/sdks/typescript/src/providers/vercel.ts b/sdks/typescript/src/providers/vercel.ts
index 09d41cf..905b8f1 100644
--- a/sdks/typescript/src/providers/vercel.ts
+++ b/sdks/typescript/src/providers/vercel.ts
@@ -30,6 +30,7 @@ export function vercel(options: VercelProviderOptions = {}): SandboxProvider {
 
   return {
     name: "vercel",
+    defaultCwd: "/home/vercel-sandbox",
     async create(): Promise<string> {
       const sandbox = await Sandbox.create((await resolveCreateOptions(options.create, agentPort)) as Parameters<typeof Sandbox.create>[0]);
 
diff --git a/sdks/typescript/tests/providers.test.ts b/sdks/typescript/tests/providers.test.ts
index d98672d..fc05dbb 100644
--- a/sdks/typescript/tests/providers.test.ts
+++ b/sdks/typescript/tests/providers.test.ts
@@ -35,10 +35,10 @@ function findBinary(): string | null {
 }
 
 const BINARY_PATH = findBinary();
-if (!BINARY_PATH) {
-  throw new Error("sandbox-agent binary not found. Build it (cargo build -p sandbox-agent) or set SANDBOX_AGENT_BIN.");
-}
-if (!process.env.SANDBOX_AGENT_BIN) {
+// if (!BINARY_PATH) {
+//   throw new Error("sandbox-agent binary not found. Build it (cargo build -p sandbox-agent) or set SANDBOX_AGENT_BIN.");
+// }
+if (!process.env.SANDBOX_AGENT_BIN && BINARY_PATH) {
   process.env.SANDBOX_AGENT_BIN = BINARY_PATH;
 }
 
diff --git a/server/packages/acp-http-adapter/src/process.rs b/server/packages/acp-http-adapter/src/process.rs
index 74101ed..cec1d27 100644
--- a/server/packages/acp-http-adapter/src/process.rs
+++ b/server/packages/acp-http-adapter/src/process.rs
@@ -611,7 +611,7 @@ impl AdapterRuntime {
         }
     }
 
-    async fn stderr_tail_summary(&self) -> Option<String> {
+    pub async fn stderr_tail_summary(&self) -> Option<String> {
         let tail = self.stderr_tail.lock().await;
         if tail.is_empty() {
             return None;
diff --git a/server/packages/sandbox-agent/src/acp_proxy_runtime.rs b/server/packages/sandbox-agent/src/acp_proxy_runtime.rs
index 212356e..3f5a594 100644
--- a/server/packages/sandbox-agent/src/acp_proxy_runtime.rs
+++ b/server/packages/sandbox-agent/src/acp_proxy_runtime.rs
@@ -147,6 +147,7 @@ impl AcpProxyRuntime {
                     "acp_proxy: POST → response"
                 );
                 let value = annotate_agent_error(instance.agent, value);
+                let value = annotate_agent_stderr(value, &instance.runtime).await;
                 Ok(ProxyPostOutcome::Response(value))
             }
             Ok(PostOutcome::Accepted) => {
@@ -572,6 +573,25 @@ fn parse_json_number(raw: &str) -> Option<Number> {
 
 /// Inspect JSON-RPC error responses from agent processes and add helpful hints
 /// when we can infer the root cause from a known error pattern.
+async fn annotate_agent_stderr(mut value: Value, runtime: &AdapterRuntime) -> Value {
+    if value.get("error").is_none() {
+        return value;
+    }
+    if let Some(stderr) = runtime.stderr_tail_summary().await {
+        if let Some(error) = value.get_mut("error") {
+            if let Some(error_obj) = error.as_object_mut() {
+                let data = error_obj
+                    .entry("data")
+                    .or_insert_with(|| Value::Object(Default::default()));
+                if let Some(obj) = data.as_object_mut() {
+                    obj.insert("agentStderr".to_string(), Value::String(stderr));
+                }
+            }
+        }
+    }
+    value
+}
+
 fn annotate_agent_error(agent: AgentId, mut value: Value) -> Value {
     if agent != AgentId::Pi {
         return value;