diff --git a/rust/crates/runtime/src/lib.rs b/rust/crates/runtime/src/lib.rs
index 5678af8..7d23f35 100644
--- a/rust/crates/runtime/src/lib.rs
+++ b/rust/crates/runtime/src/lib.rs
@@ -38,6 +38,7 @@ mod session;
 #[cfg(test)]
 mod session_control;
 mod sse;
+pub mod stale_base;
 pub mod stale_branch;
 pub mod summary_compression;
 pub mod task_packet;
@@ -150,6 +151,10 @@ pub use session::{
     SessionFork, SessionPromptEntry,
 };
 pub use sse::{IncrementalSseParser, SseEvent};
+pub use stale_base::{
+    check_base_commit, format_stale_base_warning, read_claw_base_file, resolve_expected_base,
+    BaseCommitSource, BaseCommitState,
+};
 pub use stale_branch::{
     apply_policy, check_freshness, BranchFreshness, StaleBranchAction, StaleBranchEvent,
     StaleBranchPolicy,
diff --git a/rust/crates/runtime/src/stale_base.rs b/rust/crates/runtime/src/stale_base.rs
new file mode 100644
index 0000000..b432d30
--- /dev/null
+++ b/rust/crates/runtime/src/stale_base.rs
@@ -0,0 +1,429 @@
+#![allow(clippy::must_use_candidate)]
+use std::path::Path;
+use std::process::Command;
+
+/// Outcome of comparing the worktree HEAD against the expected base commit.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum BaseCommitState {
+    /// HEAD matches the expected base commit.
+    Matches,
+    /// HEAD has diverged from the expected base.
+    Diverged { expected: String, actual: String },
+    /// No expected base was supplied (neither flag nor file).
+    NoExpectedBase,
+    /// The working directory is not inside a git repository.
+    NotAGitRepo,
+}
+
+/// Where the expected base commit originated from.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum BaseCommitSource {
+    Flag(String),
+    File(String),
+}
+
+/// Read the `.claw-base` file from the given directory and return the trimmed
+/// commit hash, or `None` when the file is absent or empty.
+pub fn read_claw_base_file(cwd: &Path) -> Option<String> {
+    let path = cwd.join(".claw-base");
+    let content = std::fs::read_to_string(path).ok()?;
+    let trimmed = content.trim();
+    if trimmed.is_empty() {
+        None
+    } else {
+        Some(trimmed.to_string())
+    }
+}
+
+/// Resolve the expected base commit: prefer the `--base-commit` flag value,
+/// fall back to reading `.claw-base` from `cwd`.
+pub fn resolve_expected_base(flag_value: Option<&str>, cwd: &Path) -> Option<BaseCommitSource> {
+    if let Some(value) = flag_value {
+        let trimmed = value.trim();
+        if !trimmed.is_empty() {
+            return Some(BaseCommitSource::Flag(trimmed.to_string()));
+        }
+    }
+    read_claw_base_file(cwd).map(BaseCommitSource::File)
+}
+
+/// Verify that the worktree HEAD matches `expected_base`.
+///
+/// Returns [`BaseCommitState::NoExpectedBase`] when no expected commit is
+/// provided (the check is effectively a no-op in that case).
+pub fn check_base_commit(cwd: &Path, expected_base: Option<&BaseCommitSource>) -> BaseCommitState {
+    let Some(source) = expected_base else {
+        return BaseCommitState::NoExpectedBase;
+    };
+    let expected_raw = match source {
+        BaseCommitSource::Flag(value) | BaseCommitSource::File(value) => value.as_str(),
+    };
+
+    let Some(head_sha) = resolve_head_sha(cwd) else {
+        return BaseCommitState::NotAGitRepo;
+    };
+
+    let Some(expected_sha) = resolve_rev(cwd, expected_raw) else {
+        // If the expected ref cannot be resolved, compare raw strings as a
+        // best-effort fallback (e.g. partial SHA provided by the caller).
+        return if head_sha.starts_with(expected_raw) || expected_raw.starts_with(&head_sha) {
+            BaseCommitState::Matches
+        } else {
+            BaseCommitState::Diverged {
+                expected: expected_raw.to_string(),
+                actual: head_sha,
+            }
+        };
+    };
+
+    if head_sha == expected_sha {
+        BaseCommitState::Matches
+    } else {
+        BaseCommitState::Diverged {
+            expected: expected_sha,
+            actual: head_sha,
+        }
+    }
+}
+
+/// Format a human-readable warning when the base commit has diverged.
+///
+/// Returns `None` for non-warning states (`Matches`, `NoExpectedBase`).
+pub fn format_stale_base_warning(state: &BaseCommitState) -> Option<String> {
+    match state {
+        BaseCommitState::Diverged { expected, actual } => Some(format!(
+            "warning: worktree HEAD ({actual}) does not match expected base commit ({expected}). \
+             Session may run against a stale codebase."
+        )),
+        BaseCommitState::NotAGitRepo => {
+            Some("warning: stale-base check skipped — not inside a git repository.".to_string())
+        }
+        BaseCommitState::Matches | BaseCommitState::NoExpectedBase => None,
+    }
+}
+
+fn resolve_head_sha(cwd: &Path) -> Option<String> {
+    resolve_rev(cwd, "HEAD")
+}
+
+fn resolve_rev(cwd: &Path, rev: &str) -> Option<String> {
+    let output = Command::new("git")
+        .args(["rev-parse", rev])
+        .current_dir(cwd)
+        .output()
+        .ok()?;
+    if !output.status.success() {
+        return None;
+    }
+    let sha = String::from_utf8(output.stdout).ok()?;
+    let trimmed = sha.trim();
+    if trimmed.is_empty() {
+        None
+    } else {
+        Some(trimmed.to_string())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs;
+    use std::process::Command;
+    use std::time::{SystemTime, UNIX_EPOCH};
+
+    fn temp_dir() -> std::path::PathBuf {
+        let nanos = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .expect("time should be after epoch")
+            .as_nanos();
+        std::env::temp_dir().join(format!("runtime-stale-base-{nanos}"))
+    }
+
+    fn init_repo(path: &std::path::Path) {
+        fs::create_dir_all(path).expect("create repo dir");
+        run(path, &["init", "--quiet", "-b", "main"]);
+        run(path, &["config", "user.email", "tests@example.com"]);
+        run(path, &["config", "user.name", "Stale Base Tests"]);
+        fs::write(path.join("init.txt"), "initial\n").expect("write init file");
+        run(path, &["add", "."]);
+        run(path, &["commit", "-m", "initial commit", "--quiet"]);
+    }
+
+    fn run(cwd: &std::path::Path, args: &[&str]) {
+        let status = Command::new("git")
+            .args(args)
+            .current_dir(cwd)
+            .status()
+            .unwrap_or_else(|e| panic!("git {} failed to execute: {e}", args.join(" ")));
+        assert!(
+            status.success(),
+            "git {} exited with {status}",
+            args.join(" ")
+        );
+    }
+
+    fn commit_file(repo: &std::path::Path, name: &str, msg: &str) {
+        fs::write(repo.join(name), format!("{msg}\n")).expect("write file");
+        run(repo, &["add", name]);
+        run(repo, &["commit", "-m", msg, "--quiet"]);
+    }
+
+    fn head_sha(repo: &std::path::Path) -> String {
+        let output = Command::new("git")
+            .args(["rev-parse", "HEAD"])
+            .current_dir(repo)
+            .output()
+            .expect("git rev-parse HEAD");
+        String::from_utf8(output.stdout)
+            .expect("valid utf8")
+            .trim()
+            .to_string()
+    }
+
+    #[test]
+    fn matches_when_head_equals_expected_base() {
+        // given
+        let root = temp_dir();
+        init_repo(&root);
+        let sha = head_sha(&root);
+        let source = BaseCommitSource::Flag(sha);
+
+        // when
+        let state = check_base_commit(&root, Some(&source));
+
+        // then
+        assert_eq!(state, BaseCommitState::Matches);
+        fs::remove_dir_all(&root).expect("cleanup");
+    }
+
+    #[test]
+    fn diverged_when_head_moved_past_expected_base() {
+        // given
+        let root = temp_dir();
+        init_repo(&root);
+        let old_sha = head_sha(&root);
+        commit_file(&root, "extra.txt", "move head forward");
+        let new_sha = head_sha(&root);
+        let source = BaseCommitSource::Flag(old_sha.clone());
+
+        // when
+        let state = check_base_commit(&root, Some(&source));
+
+        // then
+        assert_eq!(
+            state,
+            BaseCommitState::Diverged {
+                expected: old_sha,
+                actual: new_sha,
+            }
+        );
+        fs::remove_dir_all(&root).expect("cleanup");
+    }
+
+    #[test]
+    fn no_expected_base_when_source_is_none() {
+        // given
+        let root = temp_dir();
+        init_repo(&root);
+
+        // when
+        let state = check_base_commit(&root, None);
+
+        // then
+        assert_eq!(state, BaseCommitState::NoExpectedBase);
+        fs::remove_dir_all(&root).expect("cleanup");
+    }
+
+    #[test]
+    fn not_a_git_repo_when_outside_repo() {
+        // given
+        let root = temp_dir();
+        fs::create_dir_all(&root).expect("create dir");
+        let source = BaseCommitSource::Flag("abc1234".to_string());
+
+        // when
+        let state = check_base_commit(&root, Some(&source));
+
+        // then
+        assert_eq!(state, BaseCommitState::NotAGitRepo);
+        fs::remove_dir_all(&root).expect("cleanup");
+    }
+
+    #[test]
+    fn reads_claw_base_file() {
+        // given
+        let root = temp_dir();
+        fs::create_dir_all(&root).expect("create dir");
+        fs::write(root.join(".claw-base"), "abc1234def5678\n").expect("write .claw-base");
+
+        // when
+        let value = read_claw_base_file(&root);
+
+        // then
+        assert_eq!(value, Some("abc1234def5678".to_string()));
+        fs::remove_dir_all(&root).expect("cleanup");
+    }
+
+    #[test]
+    fn returns_none_for_missing_claw_base_file() {
+        // given
+        let root = temp_dir();
+        fs::create_dir_all(&root).expect("create dir");
+
+        // when
+        let value = read_claw_base_file(&root);
+
+        // then
+        assert!(value.is_none());
+        fs::remove_dir_all(&root).expect("cleanup");
+    }
+
+    #[test]
+    fn returns_none_for_empty_claw_base_file() {
+        // given
+        let root = temp_dir();
+        fs::create_dir_all(&root).expect("create dir");
+        fs::write(root.join(".claw-base"), "  \n").expect("write empty .claw-base");
+
+        // when
+        let value = read_claw_base_file(&root);
+
+        // then
+        assert!(value.is_none());
+        fs::remove_dir_all(&root).expect("cleanup");
+    }
+
+    #[test]
+    fn resolve_expected_base_prefers_flag_over_file() {
+        // given
+        let root = temp_dir();
+        fs::create_dir_all(&root).expect("create dir");
+        fs::write(root.join(".claw-base"), "from_file\n").expect("write .claw-base");
+
+        // when
+        let source = resolve_expected_base(Some("from_flag"), &root);
+
+        // then
+        assert_eq!(
+            source,
+            Some(BaseCommitSource::Flag("from_flag".to_string()))
+        );
+        fs::remove_dir_all(&root).expect("cleanup");
+    }
+
+    #[test]
+    fn resolve_expected_base_falls_back_to_file() {
+        // given
+        let root = temp_dir();
+        fs::create_dir_all(&root).expect("create dir");
+        fs::write(root.join(".claw-base"), "from_file\n").expect("write .claw-base");
+
+        // when
+        let source = resolve_expected_base(None, &root);
+
+        // then
+        assert_eq!(
+            source,
+            Some(BaseCommitSource::File("from_file".to_string()))
+        );
+        fs::remove_dir_all(&root).expect("cleanup");
+    }
+
+    #[test]
+    fn resolve_expected_base_returns_none_when_nothing_available() {
+        // given
+        let root = temp_dir();
+        fs::create_dir_all(&root).expect("create dir");
+
+        // when
+        let source = resolve_expected_base(None, &root);
+
+        // then
+        assert!(source.is_none());
+        fs::remove_dir_all(&root).expect("cleanup");
+    }
+
+    #[test]
+    fn format_warning_returns_message_for_diverged() {
+        // given
+        let state = BaseCommitState::Diverged {
+            expected: "abc1234".to_string(),
+            actual: "def5678".to_string(),
+        };
+
+        // when
+        let warning = format_stale_base_warning(&state);
+
+        // then
+        let message = warning.expect("should produce warning");
+        assert!(message.contains("abc1234"));
+        assert!(message.contains("def5678"));
+        assert!(message.contains("stale codebase"));
+    }
+
+    #[test]
+    fn format_warning_returns_none_for_matches() {
+        // given
+        let state = BaseCommitState::Matches;
+
+        // when
+        let warning = format_stale_base_warning(&state);
+
+        // then
+        assert!(warning.is_none());
+    }
+
+    #[test]
+    fn format_warning_returns_none_for_no_expected_base() {
+        // given
+        let state = BaseCommitState::NoExpectedBase;
+
+        // when
+        let warning = format_stale_base_warning(&state);
+
+        // then
+        assert!(warning.is_none());
+    }
+
+    #[test]
+    fn matches_with_claw_base_file_in_real_repo() {
+        // given
+        let root = temp_dir();
+        init_repo(&root);
+        let sha = head_sha(&root);
+        fs::write(root.join(".claw-base"), format!("{sha}\n")).expect("write .claw-base");
+        let source = resolve_expected_base(None, &root);
+
+        // when
+        let state = check_base_commit(&root, source.as_ref());
+
+        // then
+        assert_eq!(state, BaseCommitState::Matches);
+        fs::remove_dir_all(&root).expect("cleanup");
+    }
+
+    #[test]
+    fn diverged_with_claw_base_file_after_new_commit() {
+        // given
+        let root = temp_dir();
+        init_repo(&root);
+        let old_sha = head_sha(&root);
+        fs::write(root.join(".claw-base"), format!("{old_sha}\n")).expect("write .claw-base");
+        commit_file(&root, "new.txt", "advance head");
+        let new_sha = head_sha(&root);
+        let source = resolve_expected_base(None, &root);
+
+        // when
+        let state = check_base_commit(&root, source.as_ref());
+
+        // then
+        assert_eq!(
+            state,
+            BaseCommitState::Diverged {
+                expected: old_sha,
+                actual: new_sha,
+            }
+        );
+        fs::remove_dir_all(&root).expect("cleanup");
+    }
+}
diff --git a/rust/crates/rusty-claude-cli/src/main.rs b/rust/crates/rusty-claude-cli/src/main.rs
index c1b77cd..ada8995 100644
--- a/rust/crates/rusty-claude-cli/src/main.rs
+++ b/rust/crates/rusty-claude-cli/src/main.rs
@@ -42,12 +42,13 @@ use init::initialize_repo;
 use plugins::{PluginHooks, PluginManager, PluginManagerConfig, PluginRegistry};
 use render::{MarkdownStreamState, Spinner, TerminalRenderer};
 use runtime::{
-    clear_oauth_credentials, format_usd, generate_pkce_pair, generate_state,
-    load_oauth_credentials, load_system_prompt, parse_oauth_callback_request_target,
-    pricing_for_model, resolve_sandbox_status, save_oauth_credentials, ApiClient, ApiRequest,
-    AssistantEvent, CompactionConfig, ConfigLoader, ConfigSource, ContentBlock,
-    ConversationMessage, ConversationRuntime, McpServer, McpServerManager, McpServerSpec, McpTool,
-    MessageRole, ModelPricing, OAuthAuthorizationRequest, OAuthConfig, OAuthTokenExchangeRequest,
+    check_base_commit, clear_oauth_credentials, format_stale_base_warning, format_usd,
+    generate_pkce_pair, generate_state, load_oauth_credentials, load_system_prompt,
+    parse_oauth_callback_request_target, pricing_for_model, resolve_expected_base,
+    resolve_sandbox_status, save_oauth_credentials, ApiClient, ApiRequest, AssistantEvent,
+    CompactionConfig, ConfigLoader, ConfigSource, ContentBlock, ConversationMessage,
+    ConversationRuntime, McpServer, McpServerManager, McpServerSpec, McpTool, MessageRole,
+    ModelPricing, OAuthAuthorizationRequest, OAuthConfig, OAuthTokenExchangeRequest,
     PermissionMode, PermissionPolicy, ProjectContext, PromptCacheEvent, ResolvedPermissionMode,
     RuntimeError, Session, TokenUsage, ToolError, ToolExecutor, UsageTracker,
 };
@@ -88,6 +89,7 @@ const CLI_OPTION_SUGGESTIONS: &[&str] = &[
     "--resume",
     "--print",
     "--compact",
+    "--base-commit",
     "-p",
 ];
 
@@ -198,7 +200,9 @@ fn run() -> Result<(), Box<dyn std::error::Error>> {
             allowed_tools,
             permission_mode,
             compact: _,
+            base_commit,
         } => {
+            run_stale_base_preflight(base_commit.as_deref());
             let stdin_context = read_piped_stdin();
             let effective_prompt = merge_prompt_with_stdin(&prompt, stdin_context.as_deref());
             LiveCli::new(model, true, allowed_tools, permission_mode)?
@@ -217,7 +221,8 @@ fn run() -> Result<(), Box<dyn std::error::Error>> {
             model,
             allowed_tools,
             permission_mode,
-        } => run_repl(model, allowed_tools, permission_mode)?,
+            base_commit,
+        } => run_repl(model, allowed_tools, permission_mode, base_commit)?,
         CliAction::HelpTopic(topic) => print_help_topic(topic),
         CliAction::Help { output_format } => print_help(output_format)?,
     }
@@ -277,6 +282,7 @@ enum CliAction {
         allowed_tools: Option<AllowedToolSet>,
         permission_mode: PermissionMode,
         compact: bool,
+        base_commit: Option<String>,
     },
     Login {
         output_format: CliOutputFormat,
@@ -299,6 +305,7 @@ enum CliAction {
         model: String,
         allowed_tools: Option<AllowedToolSet>,
         permission_mode: PermissionMode,
+        base_commit: Option<String>,
     },
     HelpTopic(LocalHelpTopic),
     // prompt-mode formatting is only supported for non-interactive runs
@@ -341,6 +348,7 @@ fn parse_args(args: &[String]) -> Result<CliAction, String> {
     let mut wants_version = false;
     let mut allowed_tool_values = Vec::new();
     let mut compact = false;
+    let mut base_commit: Option<String> = None;
     let mut rest = Vec::new();
     let mut index = 0;
 
@@ -395,6 +403,17 @@ fn parse_args(args: &[String]) -> Result<CliAction, String> {
                 compact = true;
                 index += 1;
             }
+            "--base-commit" => {
+                let value = args
+                    .get(index + 1)
+                    .ok_or_else(|| "missing value for --base-commit".to_string())?;
+                base_commit = Some(value.clone());
+                index += 2;
+            }
+            flag if flag.starts_with("--base-commit=") => {
+                base_commit = Some(flag[14..].to_string());
+                index += 1;
+            }
             "-p" => {
                 // Claw Code compat: -p "prompt" = one-shot prompt
                 let prompt = args[index + 1..].join(" ");
@@ -409,6 +428,7 @@ fn parse_args(args: &[String]) -> Result<CliAction, String> {
                     permission_mode: permission_mode_override
                         .unwrap_or_else(default_permission_mode),
                     compact,
+                    base_commit: base_commit.clone(),
                 });
             }
             "--print" => {
@@ -466,6 +486,7 @@ fn parse_args(args: &[String]) -> Result<CliAction, String> {
             model,
             allowed_tools,
             permission_mode,
+            base_commit,
         });
     }
     if rest.first().map(String::as_str) == Some("--resume") {
@@ -503,6 +524,7 @@ fn parse_args(args: &[String]) -> Result<CliAction, String> {
                     allowed_tools,
                     permission_mode,
                     compact,
+                    base_commit,
                 }),
                 SkillSlashDispatch::Local => Ok(CliAction::Skills {
                     args,
@@ -527,6 +549,7 @@ fn parse_args(args: &[String]) -> Result<CliAction, String> {
                 allowed_tools,
                 permission_mode,
                 compact,
+                base_commit: base_commit.clone(),
             })
         }
         other if other.starts_with('/') => parse_direct_slash_cli_action(
@@ -536,6 +559,7 @@ fn parse_args(args: &[String]) -> Result<CliAction, String> {
             allowed_tools,
             permission_mode,
             compact,
+            base_commit,
         ),
         _other => Ok(CliAction::Prompt {
             prompt: rest.join(" "),
@@ -544,6 +568,7 @@ fn parse_args(args: &[String]) -> Result<CliAction, String> {
             allowed_tools,
             permission_mode,
             compact,
+            base_commit,
         }),
     }
 }
@@ -635,6 +660,7 @@ fn parse_direct_slash_cli_action(
     allowed_tools: Option<AllowedToolSet>,
     permission_mode: PermissionMode,
     compact: bool,
+    base_commit: Option<String>,
 ) -> Result<CliAction, String> {
     let raw = rest.join(" ");
     match SlashCommand::parse(&raw) {
@@ -661,6 +687,7 @@ fn parse_direct_slash_cli_action(
                     allowed_tools,
                     permission_mode,
                     compact,
+                    base_commit,
                 }),
                 SkillSlashDispatch::Local => Ok(CliAction::Skills {
                     args,
@@ -2665,11 +2692,28 @@ fn run_resume_command(
     }
 }
 
+/// Stale-base preflight: verify the worktree HEAD matches the expected base
+/// commit (from `--base-commit` flag or `.claw-base` file). Emits a warning to
+/// stderr when the HEAD has diverged.
+fn run_stale_base_preflight(flag_value: Option<&str>) {
+    let cwd = match env::current_dir() {
+        Ok(cwd) => cwd,
+        Err(_) => return,
+    };
+    let source = resolve_expected_base(flag_value, &cwd);
+    let state = check_base_commit(&cwd, source.as_ref());
+    if let Some(warning) = format_stale_base_warning(&state) {
+        eprintln!("{warning}");
+    }
+}
+
 fn run_repl(
     model: String,
     allowed_tools: Option<AllowedToolSet>,
     permission_mode: PermissionMode,
+    base_commit: Option<String>,
 ) -> Result<(), Box<dyn std::error::Error>> {
+    run_stale_base_preflight(base_commit.as_deref());
     let resolved_model = resolve_repl_model(model);
     let mut cli = LiveCli::new(resolved_model, true, allowed_tools, permission_mode)?;
     let mut editor =
@@ -7980,6 +8024,7 @@ mod tests {
                 model: DEFAULT_MODEL.to_string(),
                 allowed_tools: None,
                 permission_mode: PermissionMode::DangerFullAccess,
+                base_commit: None,
             }
         );
     }
@@ -8142,6 +8187,7 @@ mod tests {
                 allowed_tools: None,
                 permission_mode: PermissionMode::DangerFullAccess,
                 compact: false,
+                base_commit: None,
             }
         );
     }