diff --git a/server/src/agents/gates.rs b/server/src/agents/gates.rs
index d587a3dc..4c124f1d 100644
--- a/server/src/agents/gates.rs
+++ b/server/src/agents/gates.rs
@@ -43,22 +43,47 @@ pub(crate) fn worktree_has_committed_work(wt_path: &Path) -> bool {
 
 /// Run `cargo check` in the given worktree directory to verify committed code compiles.
 ///
-/// Resets any dirty (uncommitted) files first via `git checkout .` so that only
-/// the committed state is evaluated.  Used as part of the "work survived" check
-/// when an agent crashes mid-output (bug 645).
+/// Stashes any dirty (uncommitted) files first so that only the committed state
+/// is evaluated, then restores them afterward.  Uncommitted work in worktrees is
+/// never junk — it may be the next agent session's starting point (bug 651).
+///
+/// Used as part of the "work survived" check when an agent crashes mid-output
+/// (bug 645).
 pub(crate) fn cargo_check_in_worktree(wt_path: &Path) -> bool {
-    // Reset uncommitted changes so cargo check evaluates only committed code.
-    let _ = Command::new("git")
-        .args(["checkout", "."])
+    // Stash uncommitted changes (including untracked files) so cargo check
+    // evaluates only committed code.  We restore them afterward.
+    let stashed = Command::new("git")
+        .args([
+            "stash",
+            "push",
+            "--include-untracked",
+            "-m",
+            "cargo-check-temp",
+        ])
         .current_dir(wt_path)
-        .output();
+        .output()
+        .map(|o| {
+            o.status.success()
+                && !String::from_utf8_lossy(&o.stdout).contains("No local changes to save")
+        })
+        .unwrap_or(false);
 
-    Command::new("cargo")
+    let result = Command::new("cargo")
         .args(["check"])
         .current_dir(wt_path)
         .output()
         .map(|o| o.status.success())
-        .unwrap_or(false)
+        .unwrap_or(false);
+
+    // Restore stashed uncommitted changes.
+    if stashed {
+        let _ = Command::new("git")
+            .args(["stash", "pop"])
+            .current_dir(wt_path)
+            .output();
+    }
+
+    result
 }
 
 /// Check whether the given directory has any uncommitted git changes.
@@ -457,9 +482,10 @@ mod tests {
 
     // ── cargo_check_in_worktree tests ────────────────────────────────────────
 
-    /// Bug 645: cargo_check_in_worktree resets dirty files before checking.
+    /// Bug 645 + 651: cargo_check_in_worktree stashes dirty files before
+    /// checking committed code and restores them afterward.
     #[test]
-    fn cargo_check_in_worktree_resets_dirty_files() {
+    fn cargo_check_in_worktree_stashes_and_restores_dirty_files() {
         use std::fs;
         let tmp = tempfile::tempdir().unwrap();
         let project_root = tmp.path().join("project");
@@ -530,11 +556,24 @@ mod tests {
 
         // Now simulate a crash leaving dirty files (broken syntax).
         fs::write(wt_path.join("src/lib.rs"), "THIS IS BROKEN SYNTAX!!!\n").unwrap();
+        // Also add an untracked file.
+        fs::write(wt_path.join("crash_residue.txt"), "untracked junk").unwrap();
 
-        // cargo_check_in_worktree should reset dirty files and check committed code.
+        // cargo_check_in_worktree should stash dirty files, check committed code, and restore.
         assert!(
             cargo_check_in_worktree(&wt_path),
-            "cargo check should pass on committed code after resetting dirty files"
+            "cargo check should pass on committed code after stashing dirty files"
+        );
+
+        // Bug 651: dirty files must be restored after cargo check.
+        assert_eq!(
+            fs::read_to_string(wt_path.join("src/lib.rs")).unwrap(),
+            "THIS IS BROKEN SYNTAX!!!\n",
+            "modified tracked file should be restored after cargo check"
+        );
+        assert!(
+            wt_path.join("crash_residue.txt").exists(),
+            "untracked file should be restored after cargo check"
         );
     }
 }
diff --git a/server/src/agents/pool/pipeline/completion.rs b/server/src/agents/pool/pipeline/completion.rs
index 9d53a7f9..4fbd6fc8 100644
--- a/server/src/agents/pool/pipeline/completion.rs
+++ b/server/src/agents/pool/pipeline/completion.rs
@@ -235,31 +235,51 @@ pub(in crate::agents::pool) async fn run_server_owned_completion(
             // If the worktree is dirty, check whether committed work survived.
             // An agent crash (e.g. Claude Code CLI's `output.write(&bytes).is_ok()`
             // assertion — bug 645) can leave uncommitted files behind even though
-            // the agent already committed valid work.  In that case, clean up the
+            // the agent already committed valid work.  In that case, stash the
             // dirty files and proceed with gates on the committed code.
-            if let Err(dirty_msg) = crate::agents::gates::check_uncommitted_changes(&path) {
-                if crate::agents::gates::worktree_has_committed_work(&path) {
-                    crate::slog!(
-                        "[agents] Worktree dirty but committed work exists — \
-                         resetting uncommitted changes and proceeding with gates. \
+            // Uncommitted work is never junk — it may be the next agent session's
+            // starting point (bug 651).
+            let stashed =
+                if let Err(dirty_msg) = crate::agents::gates::check_uncommitted_changes(&path) {
+                    if crate::agents::gates::worktree_has_committed_work(&path) {
+                        crate::slog!(
+                            "[agents] Worktree dirty but committed work exists — \
+                         stashing uncommitted changes and proceeding with gates. \
                          Dirty state: {dirty_msg}"
-                    );
-                    // Reset dirty files so gates run against committed code only.
-                    let _ = std::process::Command::new("git")
-                        .args(["checkout", "."])
-                        .current_dir(&path)
-                        .output();
-                    let _ = std::process::Command::new("git")
-                        .args(["clean", "-fd"])
-                        .current_dir(&path)
-                        .output();
+                        );
+                        // Stash dirty files so gates run against committed code only.
+                        // They will be restored after gates complete.
+                        std::process::Command::new("git")
+                            .args([
+                                "stash",
+                                "push",
+                                "--include-untracked",
+                                "-m",
+                                "server-completion-temp",
+                            ])
+                            .current_dir(&path)
+                            .output()
+                            .map(|o| {
+                                o.status.success()
+                                    && !String::from_utf8_lossy(&o.stdout)
+                                        .contains("No local changes to save")
+                            })
+                            .unwrap_or(false)
+                    } else {
+                        return Ok((false, dirty_msg));
+                    }
                 } else {
-                    return Ok((false, dirty_msg));
-                }
-            }
+                    false
+                };
             // AC5: Fail early if the coder finished with no commits on the feature branch.
             // This prevents empty-diff stories from advancing through QA to merge.
             if !crate::agents::gates::worktree_has_committed_work(&path) {
+                if stashed {
+                    let _ = std::process::Command::new("git")
+                        .args(["stash", "pop"])
+                        .current_dir(&path)
+                        .output();
+                }
                 return Ok((
                     false,
                     "Agent exited with no commits on the feature branch. \
@@ -267,7 +287,15 @@ pub(in crate::agents::pool) async fn run_server_owned_completion(
                         .to_string(),
                 ));
             }
-            crate::agents::gates::run_acceptance_gates(&path)
+            let result = crate::agents::gates::run_acceptance_gates(&path);
+            // Restore stashed uncommitted changes.
+            if stashed {
+                let _ = std::process::Command::new("git")
+                    .args(["stash", "pop"])
+                    .current_dir(&path)
+                    .output();
+            }
+            result
         })
         .await
         {
@@ -667,11 +695,11 @@ mod tests {
         );
     }
 
-    /// Bug 645: when an agent crashes leaving dirty files but committed work,
-    /// server-owned completion should clean up the dirty files and run gates
-    /// on the committed code instead of failing immediately.
+    /// Bug 645 + 651: when an agent crashes leaving dirty files but committed
+    /// work, server-owned completion should stash the dirty files during gates
+    /// and restore them afterward.  Uncommitted work is never junk.
     #[tokio::test]
-    async fn server_owned_completion_cleans_dirty_worktree_with_committed_work() {
+    async fn server_owned_completion_preserves_dirty_worktree_with_committed_work() {
         use std::fs;
         use tempfile::tempdir;
 
@@ -730,10 +758,15 @@ mod tests {
         )
         .await;
 
-        // The dirty file should have been cleaned up.
+        // Bug 651: The dirty file must be PRESERVED — uncommitted work is never junk.
         assert!(
-            !wt_path.join("dirty.txt").exists(),
-            "dirty file should be cleaned up after server-owned completion"
+            wt_path.join("dirty.txt").exists(),
+            "dirty file should be preserved after server-owned completion (bug 651)"
+        );
+        assert_eq!(
+            fs::read_to_string(wt_path.join("dirty.txt")).unwrap(),
+            "crash residue",
+            "dirty file contents should be unchanged"
         );
 
         // A Done event should have been emitted (completion ran, didn't fail
@@ -744,4 +777,195 @@ mod tests {
             "expected Done event, got: {event:?}"
         );
     }
+
+    /// AC3 (bug 651): simulate an agent killed by the watchdog with a
+    /// substantive uncommitted diff.  After the orchestrator's full
+    /// post-termination handling (gates, completion, advance check),
+    /// `git status --short` must still show the same modified files.
+    #[tokio::test]
+    async fn watchdog_kill_preserves_uncommitted_diff() {
+        use std::fs;
+        use tempfile::tempdir;
+
+        let tmp = tempdir().unwrap();
+        let project_root = tmp.path().join("project");
+        fs::create_dir_all(&project_root).unwrap();
+        init_git_repo(&project_root);
+
+        // Create a worktree on a feature branch with committed code.
+        let wt_path = tmp.path().join("wt");
+        Command::new("git")
+            .args([
+                "worktree",
+                "add",
+                &wt_path.to_string_lossy(),
+                "-b",
+                "feature/story-651_watchdog",
+            ])
+            .current_dir(&project_root)
+            .output()
+            .unwrap();
+
+        // Commit some work.
+        fs::write(wt_path.join("committed.txt"), "committed work").unwrap();
+        Command::new("git")
+            .args(["add", "."])
+            .current_dir(&wt_path)
+            .output()
+            .unwrap();
+        Command::new("git")
+            .args(["commit", "-m", "coder: add committed work"])
+            .current_dir(&wt_path)
+            .output()
+            .unwrap();
+
+        // Simulate substantive uncommitted diff left by watchdog kill.
+        fs::write(wt_path.join("in_progress.rs"), "fn wip() {}").unwrap();
+        fs::write(wt_path.join("committed.txt"), "modified after commit").unwrap();
+
+        // Snapshot git status before completion.
+        let status_before = Command::new("git")
+            .args(["status", "--short"])
+            .current_dir(&wt_path)
+            .output()
+            .unwrap();
+        let status_before = String::from_utf8_lossy(&status_before.stdout)
+            .trim()
+            .to_string();
+        assert!(
+            !status_before.is_empty(),
+            "pre-condition: worktree should be dirty"
+        );
+
+        let pool = AgentPool::new_test(3001);
+        pool.inject_test_agent_with_path(
+            "651_watchdog",
+            "coder-1",
+            AgentStatus::Running,
+            wt_path.clone(),
+        );
+
+        run_server_owned_completion(
+            &pool.agents,
+            pool.port,
+            "651_watchdog",
+            "coder-1",
+            Some("sess-651".to_string()),
+            pool.watcher_tx.clone(),
+        )
+        .await;
+
+        // After full post-termination handling, git status must be unchanged.
+        let status_after = Command::new("git")
+            .args(["status", "--short"])
+            .current_dir(&wt_path)
+            .output()
+            .unwrap();
+        let status_after = String::from_utf8_lossy(&status_after.stdout)
+            .trim()
+            .to_string();
+
+        assert_eq!(
+            status_before, status_after,
+            "Bug 651: uncommitted diff must survive post-termination handling.\n\
+             Before: {status_before}\nAfter: {status_after}"
+        );
+
+        // Verify file contents are intact.
+        assert_eq!(
+            fs::read_to_string(wt_path.join("in_progress.rs")).unwrap(),
+            "fn wip() {}",
+        );
+        assert_eq!(
+            fs::read_to_string(wt_path.join("committed.txt")).unwrap(),
+            "modified after commit",
+        );
+    }
+
+    /// AC4 (bug 651 regression for 645): when an agent crashes with committed
+    /// work AND uncommitted noise, the auto-advance still picks up the
+    /// committed work.  The committed-state check is authoritative; the
+    /// uncommitted state is just preserved on disk for the next agent.
+    #[tokio::test]
+    async fn committed_work_advances_despite_uncommitted_noise() {
+        use std::fs;
+        use tempfile::tempdir;
+
+        let tmp = tempdir().unwrap();
+        let project_root = tmp.path().join("project");
+        fs::create_dir_all(&project_root).unwrap();
+        init_git_repo(&project_root);
+
+        // Create a minimal Cargo project so cargo check works.
+        fs::write(
+            project_root.join("Cargo.toml"),
+            "[package]\nname = \"test_proj\"\nversion = \"0.1.0\"\nedition = \"2021\"\n",
+        )
+        .unwrap();
+        fs::create_dir_all(project_root.join("src")).unwrap();
+        fs::write(project_root.join("src/lib.rs"), "// empty\n").unwrap();
+        Command::new("git")
+            .args(["add", "."])
+            .current_dir(&project_root)
+            .output()
+            .unwrap();
+        Command::new("git")
+            .args(["commit", "-m", "add cargo project"])
+            .current_dir(&project_root)
+            .output()
+            .unwrap();
+
+        // Create a worktree on a feature branch.
+        let wt_path = tmp.path().join("wt");
+        Command::new("git")
+            .args([
+                "worktree",
+                "add",
+                &wt_path.to_string_lossy(),
+                "-b",
+                "feature/story-651_regression",
+            ])
+            .current_dir(&project_root)
+            .output()
+            .unwrap();
+
+        // Commit valid code on the feature branch.
+        fs::write(wt_path.join("src/lib.rs"), "pub fn good() {}\n").unwrap();
+        Command::new("git")
+            .args(["add", "."])
+            .current_dir(&wt_path)
+            .output()
+            .unwrap();
+        Command::new("git")
+            .args(["commit", "-m", "add good fn"])
+            .current_dir(&wt_path)
+            .output()
+            .unwrap();
+
+        // Simulate crash leaving uncommitted noise (broken syntax in tracked file).
+        fs::write(wt_path.join("src/lib.rs"), "THIS IS BROKEN SYNTAX!!!\n").unwrap();
+        fs::write(wt_path.join("crash_junk.tmp"), "untracked noise").unwrap();
+
+        // The "work survived" check should detect committed work and pass cargo check
+        // despite the dirty worktree, WITHOUT destroying the dirty files.
+        assert!(
+            crate::agents::gates::worktree_has_committed_work(&wt_path),
+            "committed work should be detected"
+        );
+        assert!(
+            crate::agents::gates::cargo_check_in_worktree(&wt_path),
+            "cargo check should pass on committed code (stash/pop, not reset)"
+        );
+
+        // Dirty files must still exist after the check.
+        assert_eq!(
+            fs::read_to_string(wt_path.join("src/lib.rs")).unwrap(),
+            "THIS IS BROKEN SYNTAX!!!\n",
+            "uncommitted noise in tracked file must be preserved"
+        );
+        assert!(
+            wt_path.join("crash_junk.tmp").exists(),
+            "untracked noise file must be preserved"
+        );
+    }
 }