diff --git a/codex-rs/app-server/README.md b/codex-rs/app-server/README.md
index 8558cabb7f..35e1b08b61 100644
--- a/codex-rs/app-server/README.md
+++ b/codex-rs/app-server/README.md
@@ -1842,6 +1842,8 @@ Order of messages:
 
 When stdin approvals are enabled, a `write_stdin` approval sets `kind: "writeStdin"`, references the original terminal command's `itemId`, and has its own `approvalId`. The request belongs to the current turn, which may differ from the turn that opened the terminal. With `approvalsReviewer: "auto_review"`, the `item/autoApprovalReview/*` notifications likewise target the original command item and carry an action of type `writeStdin` with `approvalId`, `processId`, `stdin`, and `cwd`. For stdin approvals, `cwd` is the terminal’s launch directory, not its current working directory. Approving or denying a stdin write does not start, complete, or change the status of the parent command-execution item.
 
+For reviewed stdin, the complete formatted action and approval reason must fit within 8,000 bytes. Oversized or truncated actions are rejected before any bytes reach the terminal, rather than reviewing a shortened input and executing the full input.
+
 ### File change approvals
 
 Order of messages:
diff --git a/codex-rs/core/src/guardian/mod.rs b/codex-rs/core/src/guardian/mod.rs
index 6c63588a0b..9f09b4c0e2 100644
--- a/codex-rs/core/src/guardian/mod.rs
+++ b/codex-rs/core/src/guardian/mod.rs
@@ -244,8 +244,7 @@ impl GuardianRejectionCircuitBreaker {
     }
 }
 
-#[cfg(test)]
-use approval_request::format_guardian_action_pretty;
+pub(crate) use approval_request::format_guardian_action_pretty;
 #[cfg(test)]
 use approval_request::guardian_assessment_action;
 #[cfg(test)]
diff --git a/codex-rs/core/src/tools/approvals.rs b/codex-rs/core/src/tools/approvals.rs
index 81a7cfced3..90b16c78d7 100644
--- a/codex-rs/core/src/tools/approvals.rs
+++ b/codex-rs/core/src/tools/approvals.rs
@@ -266,7 +266,9 @@ impl ApprovalAction {
         }
     }
 
-    fn into_guardian_request(self) -> std::io::Result<crate::guardian::GuardianApprovalRequest> {
+    pub(crate) fn into_guardian_request(
+        self,
+    ) -> std::io::Result<crate::guardian::GuardianApprovalRequest> {
         Ok(match self {
             Self::ExecCommand {
                 id,
diff --git a/codex-rs/core/src/unified_exec/process_manager.rs b/codex-rs/core/src/unified_exec/process_manager.rs
index 2558833b01..1424239d50 100644
--- a/codex-rs/core/src/unified_exec/process_manager.rs
+++ b/codex-rs/core/src/unified_exec/process_manager.rs
@@ -798,6 +798,22 @@ impl UnifiedExecProcessManager {
         };
         let _interaction_guard = locked_process.interaction_lock().lock_owned().await;
         if let Some(approval) = approval {
+            let approval_reason = "Send input to an existing escalated terminal. The cwd is its launch directory; the terminal's current directory and state may have changed.".to_string();
+            let reviewed = crate::guardian::format_guardian_action_pretty(
+                &approval.clone().into_guardian_request().map_err(|err| {
+                    UnifiedExecError::StdinApproval(ToolError::Rejected(err.to_string()))
+                })?,
+            )
+            .map_err(|err| UnifiedExecError::StdinApproval(ToolError::Rejected(err.to_string())))?;
+            // Bound the entire serialized action plus its reason, including JSON
+            // escaping. Reject, never execute an unreviewed tail.
+            if reviewed.truncated
+                || reviewed.text.len().saturating_add(approval_reason.len()) > 8_000
+            {
+                return Err(UnifiedExecError::StdinApproval(ToolError::Rejected(
+                    "terminal input and permission details are too large to review safely; use a smaller input or start a new terminal with fewer grants".to_string(),
+                )));
+            }
             let strict_auto_review = context
                 .session
                 .active_turn_context_and_strict_auto_review()
@@ -809,7 +825,7 @@ impl UnifiedExecProcessManager {
                 call_id: context.call_id.clone(),
                 tool_name: ToolName::plain("write_stdin"),
                 strict_auto_review,
-                approval_reason: Some("Send input to an existing escalated terminal. The cwd is its launch directory; the terminal's current directory and state may have changed.".to_string()),
+                approval_reason: Some(approval_reason),
                 retry_reason: None,
                 network_approval_context: None,
             };
diff --git a/codex-rs/core/tests/suite/mod.rs b/codex-rs/core/tests/suite/mod.rs
index a34c08a902..5aba823765 100644
--- a/codex-rs/core/tests/suite/mod.rs
+++ b/codex-rs/core/tests/suite/mod.rs
@@ -170,6 +170,7 @@ mod turn_input_submission;
 mod turn_state;
 mod unified_exec;
 mod unified_exec_process_events;
+mod unified_exec_stdin_review_size;
 #[cfg(unix)]
 mod unified_exec_zsh_fork_approvals;
 mod unstable_features_warning;
diff --git a/codex-rs/core/tests/suite/unified_exec_stdin_review_size.rs b/codex-rs/core/tests/suite/unified_exec_stdin_review_size.rs
new file mode 100644
index 0000000000..d4c8bb78ab
--- /dev/null
+++ b/codex-rs/core/tests/suite/unified_exec_stdin_review_size.rs
@@ -0,0 +1,123 @@
+//! Stdin approval must cover the complete input before any bytes reach a terminal.
+
+use anyhow::Result;
+use codex_core::TurnInputRequest;
+use codex_features::Feature;
+use codex_protocol::approvals::ExecApprovalKind;
+use codex_protocol::protocol::AskForApproval;
+use codex_protocol::protocol::EventMsg;
+use codex_protocol::protocol::Op;
+use codex_protocol::protocol::ReviewDecision;
+use codex_protocol::protocol::SandboxPolicy;
+use codex_protocol::protocol::ThreadSettingsOverrides;
+use codex_protocol::user_input::UserInput;
+use core_test_support::responses::ev_completed;
+use core_test_support::responses::ev_function_call;
+use core_test_support::responses::mount_sse_sequence;
+use core_test_support::responses::sse;
+use core_test_support::responses::start_mock_server;
+use core_test_support::skip_if_no_network;
+use core_test_support::skip_if_sandbox;
+use core_test_support::skip_if_target_windows;
+use core_test_support::test_codex::test_codex;
+use core_test_support::wait_for_event;
+use pretty_assertions::assert_eq;
+use serde_json::json;
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn oversized_stdin_is_rejected_before_approval_or_execution() -> Result<()> {
+    skip_if_target_windows!(Ok(()), "uses a POSIX interactive shell");
+    skip_if_no_network!(Ok(()));
+    skip_if_sandbox!(Ok(()));
+    let server = start_mock_server().await;
+    let mut builder = test_codex().with_config(|config| {
+        config
+            .features
+            .enable(Feature::WriteStdinApproval)
+            .expect("enable stdin approvals");
+    });
+    let test = builder.build_with_auto_env(&server).await?;
+    // The raw input fits in 8KB, but its JSON escapes do not. None of the
+    // rejected input, including the trailing assignment, may reach the shell.
+    let oversized = format!("#{}\nREJECTED=1\n", "\"".repeat(4_100));
+    let mut sequence = Vec::new();
+    for (id, tool, args) in [
+        (
+            "open",
+            "exec_command",
+            json!({"cmd":"/bin/bash --noprofile --norc", "tty":true, "yield_time_ms":200, "sandbox_permissions":"require_escalated"}),
+        ),
+        (
+            "oversized",
+            "write_stdin",
+            json!({"session_id":1000, "chars":oversized, "yield_time_ms":1000}),
+        ),
+        (
+            "allowed",
+            "write_stdin",
+            json!({"session_id":1000, "chars":"test -z \"${REJECTED-}\" && printf 'STDIN_%s_OK\\n' REVIEW; exit\n", "yield_time_ms":1000}),
+        ),
+    ] {
+        sequence.push(sse(vec![
+            ev_function_call(id, tool, &args.to_string()),
+            ev_completed(id),
+        ]));
+    }
+    sequence.push(sse(vec![ev_completed("done")]));
+    let responses = mount_sse_sequence(&server, sequence).await;
+    test.codex
+        .start_or_steer_turn(
+            TurnInputRequest::user_input(vec![UserInput::Text {
+                text: "send terminal input".to_string(),
+                text_elements: Vec::new(),
+            }])
+            .with_thread_settings(ThreadSettingsOverrides {
+                approval_policy: Some(AskForApproval::OnRequest),
+                sandbox_policy: Some(SandboxPolicy::ReadOnly {
+                    network_access: false,
+                }),
+                ..Default::default()
+            }),
+        )
+        .await?;
+    let mut approvals = Vec::new();
+    loop {
+        match wait_for_event(&test.codex, |_| true).await {
+            EventMsg::ExecApprovalRequest(request) => {
+                let id = request.effective_approval_id();
+                approvals.push((request.kind, id.clone()));
+                test.codex
+                    .submit(Op::ExecApproval {
+                        id,
+                        turn_id: Some(request.turn_id),
+                        decision: ReviewDecision::Approved,
+                    })
+                    .await?;
+            }
+            EventMsg::TurnComplete(_) => break,
+            _ => {}
+        }
+    }
+    assert_eq!(
+        approvals,
+        vec![
+            (ExecApprovalKind::Command, "open".to_string()),
+            (ExecApprovalKind::WriteStdin, "allowed".to_string()),
+        ]
+    );
+    let requests = responses.requests();
+    let last = requests.last().expect("final model request");
+    let rejected = last.function_call_output("oversized");
+    assert!(
+        rejected.to_string().contains("too large to review safely"),
+        "{rejected}"
+    );
+    let allowed = last.function_call_output("allowed");
+    assert!(allowed.to_string().contains("STDIN_REVIEW_OK"), "{allowed}");
+    test.codex.submit(Op::Shutdown).await?;
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::ShutdownComplete)
+    })
+    .await;
+    Ok(())
+}