feat: add ecc2 merge queue reporting

ecc2/src/main.rs

@@ -244,6 +244,12 @@ enum Commands {
         #[arg(long)]
         keep_worktree: bool,
     },
+    /// Show the merge queue for inactive worktrees and any branch-to-branch blockers
+    MergeQueue {
+        /// Emit machine-readable JSON instead of the human summary
+        #[arg(long)]
+        json: bool,
+    },
     /// Prune worktrees for inactive sessions and report any active sessions still holding one
     PruneWorktrees {
         /// Emit machine-readable JSON instead of the human summary
@@ -837,6 +843,14 @@ async fn main() -> Result<()> {
                 }
             }
         }
+        Some(Commands::MergeQueue { json }) => {
+            let report = session::manager::build_merge_queue(&db)?;
+            if json {
+                println!("{}", serde_json::to_string_pretty(&report)?);
+            } else {
+                println!("{}", format_merge_queue_human(&report));
+            }
+        }
         Some(Commands::PruneWorktrees { json }) => {
             let outcome = session::manager::prune_inactive_worktrees(&db, &cfg).await?;
             if json {
@@ -1588,6 +1602,59 @@ fn format_prune_worktrees_human(outcome: &session::manager::WorktreePruneOutcome
     lines.join("\n")
 }
 
+fn format_merge_queue_human(report: &session::manager::MergeQueueReport) -> String {
+    let mut lines = Vec::new();
+    lines.push(format!(
+        "Merge queue: {} ready / {} blocked",
+        report.ready_entries.len(),
+        report.blocked_entries.len()
+    ));
+
+    if report.ready_entries.is_empty() {
+        lines.push("No merge-ready worktrees queued".to_string());
+    } else {
+        lines.push("Ready".to_string());
+        for entry in &report.ready_entries {
+            lines.push(format!(
+                "- #{} {} [{}] | {} / {} | {}",
+                entry.queue_position.unwrap_or(0),
+                entry.session_id,
+                entry.branch,
+                entry.project,
+                entry.task_group,
+                entry.task
+            ));
+        }
+    }
+
+    if !report.blocked_entries.is_empty() {
+        lines.push(String::new());
+        lines.push("Blocked".to_string());
+        for entry in &report.blocked_entries {
+            lines.push(format!(
+                "- {} [{}] | {} / {} | {}",
+                entry.session_id, entry.branch, entry.project, entry.task_group, entry.suggested_action
+            ));
+            for blocker in entry.blocked_by.iter().take(2) {
+                lines.push(format!(
+                    "  blocker {} [{}] | {}",
+                    blocker.session_id, blocker.branch, blocker.summary
+                ));
+                for conflict in blocker.conflicts.iter().take(3) {
+                    lines.push(format!("    conflict {conflict}"));
+                }
+                if let Some(preview) = blocker.conflicting_patch_preview.as_ref() {
+                    for line in preview.lines().take(6) {
+                        lines.push(format!("    {}", line));
+                    }
+                }
+            }
+        }
+    }
+
+    lines.join("\n")
+}
+
 fn build_otel_export(
     db: &session::store::StateStore,
     session_id: Option<&str>,
@@ -2535,6 +2602,17 @@ mod tests {
         }
     }
 
+    #[test]
+    fn cli_parses_merge_queue_json_flag() {
+        let cli = Cli::try_parse_from(["ecc", "merge-queue", "--json"])
+            .expect("merge-queue --json should parse");
+
+        match cli.command {
+            Some(Commands::MergeQueue { json }) => assert!(json),
+            _ => panic!("expected merge-queue subcommand"),
+        }
+    }
+
     #[test]
     fn format_worktree_status_human_includes_readiness_and_conflicts() {
         let report = WorktreeStatusReport {
@@ -2666,6 +2744,58 @@ mod tests {
         assert!(text.contains("Cleanup removed worktree and branch"));
     }
 
+    #[test]
+    fn format_merge_queue_human_reports_ready_and_blocked_entries() {
+        let text = format_merge_queue_human(&session::manager::MergeQueueReport {
+            ready_entries: vec![session::manager::MergeQueueEntry {
+                session_id: "alpha1234".to_string(),
+                task: "merge alpha".to_string(),
+                project: "ecc".to_string(),
+                task_group: "checkout".to_string(),
+                branch: "ecc/alpha1234".to_string(),
+                base_branch: "main".to_string(),
+                state: session::SessionState::Stopped,
+                worktree_health: worktree::WorktreeHealth::InProgress,
+                dirty: false,
+                queue_position: Some(1),
+                ready_to_merge: true,
+                blocked_by: Vec::new(),
+                suggested_action: "merge in queue order #1".to_string(),
+            }],
+            blocked_entries: vec![session::manager::MergeQueueEntry {
+                session_id: "beta5678".to_string(),
+                task: "merge beta".to_string(),
+                project: "ecc".to_string(),
+                task_group: "checkout".to_string(),
+                branch: "ecc/beta5678".to_string(),
+                base_branch: "main".to_string(),
+                state: session::SessionState::Stopped,
+                worktree_health: worktree::WorktreeHealth::InProgress,
+                dirty: false,
+                queue_position: None,
+                ready_to_merge: false,
+                blocked_by: vec![session::manager::MergeQueueBlocker {
+                    session_id: "alpha1234".to_string(),
+                    branch: "ecc/alpha1234".to_string(),
+                    state: session::SessionState::Stopped,
+                    conflicts: vec!["README.md".to_string()],
+                    summary: "merge after alpha1234 to avoid branch conflicts".to_string(),
+                    conflicting_patch_preview: Some("--- Branch diff vs main ---\nREADME.md".to_string()),
+                    blocker_patch_preview: None,
+                }],
+                suggested_action: "merge after alpha1234".to_string(),
+            }],
+        });
+
+        assert!(text.contains("Merge queue: 1 ready / 1 blocked"));
+        assert!(text.contains("Ready"));
+        assert!(text.contains("#1 alpha1234"));
+        assert!(text.contains("Blocked"));
+        assert!(text.contains("beta5678"));
+        assert!(text.contains("blocker alpha1234"));
+        assert!(text.contains("conflict README.md"));
+    }
+
     #[test]
     fn format_bulk_worktree_merge_human_reports_summary_and_skips() {
         let text = format_bulk_worktree_merge_human(&session::manager::WorktreeBulkMergeOutcome {

ecc2/src/session/manager.rs

@@ -964,6 +964,191 @@ pub async fn prune_inactive_worktrees(
     })
 }
 
+#[derive(Debug, Clone, Serialize)]
+pub struct MergeQueueBlocker {
+    pub session_id: String,
+    pub branch: String,
+    pub state: SessionState,
+    pub conflicts: Vec<String>,
+    pub summary: String,
+    pub conflicting_patch_preview: Option<String>,
+    pub blocker_patch_preview: Option<String>,
+}
+
+#[derive(Debug, Clone, Serialize)]
+pub struct MergeQueueEntry {
+    pub session_id: String,
+    pub task: String,
+    pub project: String,
+    pub task_group: String,
+    pub branch: String,
+    pub base_branch: String,
+    pub state: SessionState,
+    pub worktree_health: worktree::WorktreeHealth,
+    pub dirty: bool,
+    pub queue_position: Option<usize>,
+    pub ready_to_merge: bool,
+    pub blocked_by: Vec<MergeQueueBlocker>,
+    pub suggested_action: String,
+}
+
+#[derive(Debug, Clone, Serialize)]
+pub struct MergeQueueReport {
+    pub ready_entries: Vec<MergeQueueEntry>,
+    pub blocked_entries: Vec<MergeQueueEntry>,
+}
+
+pub fn build_merge_queue(db: &StateStore) -> Result<MergeQueueReport> {
+    let mut sessions = db
+        .list_sessions()?
+        .into_iter()
+        .filter(|session| session.worktree.is_some())
+        .collect::<Vec<_>>();
+    sessions.sort_by(|left, right| {
+        merge_queue_priority(left)
+            .cmp(&merge_queue_priority(right))
+            .then_with(|| left.project.cmp(&right.project))
+            .then_with(|| left.task_group.cmp(&right.task_group))
+            .then_with(|| left.updated_at.cmp(&right.updated_at))
+            .then_with(|| left.id.cmp(&right.id))
+    });
+
+    let mut entries = Vec::new();
+    let mut mergeable_sessions = Vec::<Session>::new();
+    let mut next_position = 1usize;
+
+    for session in sessions {
+        let Some(worktree) = session.worktree.clone() else {
+            continue;
+        };
+
+        let worktree_health = worktree::health(&worktree)?;
+        let dirty = worktree::has_uncommitted_changes(&worktree)?;
+        let mut blocked_by = Vec::new();
+
+        if matches!(
+            session.state,
+            SessionState::Pending | SessionState::Running | SessionState::Idle | SessionState::Stale
+        ) {
+            blocked_by.push(MergeQueueBlocker {
+                session_id: session.id.clone(),
+                branch: worktree.branch.clone(),
+                state: session.state.clone(),
+                conflicts: Vec::new(),
+                summary: format!("session is still {}", session_state_label(&session.state)),
+                conflicting_patch_preview: None,
+                blocker_patch_preview: None,
+            });
+        } else if worktree_health == worktree::WorktreeHealth::Conflicted {
+            let readiness = worktree::merge_readiness(&worktree)?;
+            blocked_by.push(MergeQueueBlocker {
+                session_id: session.id.clone(),
+                branch: worktree.branch.clone(),
+                state: session.state.clone(),
+                conflicts: readiness.conflicts,
+                summary: readiness.summary,
+                conflicting_patch_preview: worktree::diff_patch_preview(&worktree, 18)?,
+                blocker_patch_preview: None,
+            });
+        } else if dirty {
+            blocked_by.push(MergeQueueBlocker {
+                session_id: session.id.clone(),
+                branch: worktree.branch.clone(),
+                state: session.state.clone(),
+                conflicts: Vec::new(),
+                summary: "worktree has uncommitted changes".to_string(),
+                conflicting_patch_preview: worktree::diff_patch_preview(&worktree, 18)?,
+                blocker_patch_preview: None,
+            });
+        } else {
+            for blocker in &mergeable_sessions {
+                let Some(blocker_worktree) = blocker.worktree.as_ref() else {
+                    continue;
+                };
+                let Some(conflict) =
+                    worktree::branch_conflict_preview(&worktree, blocker_worktree, 12)?
+                else {
+                    continue;
+                };
+
+                blocked_by.push(MergeQueueBlocker {
+                    session_id: blocker.id.clone(),
+                    branch: blocker_worktree.branch.clone(),
+                    state: blocker.state.clone(),
+                    conflicts: conflict.conflicts,
+                    summary: format!(
+                        "merge after {} to avoid branch conflicts",
+                        blocker.id
+                    ),
+                    conflicting_patch_preview: conflict.right_patch_preview,
+                    blocker_patch_preview: conflict.left_patch_preview,
+                });
+            }
+        }
+
+        let ready_to_merge = blocked_by.is_empty();
+        let queue_position = if ready_to_merge {
+            let position = next_position;
+            next_position += 1;
+            mergeable_sessions.push(session.clone());
+            Some(position)
+        } else {
+            None
+        };
+
+        let suggested_action = if let Some(position) = queue_position {
+            format!("merge in queue order #{position}")
+        } else if blocked_by.iter().any(|blocker| blocker.session_id == session.id) {
+            blocked_by
+                .first()
+                .map(|blocker| blocker.summary.clone())
+                .unwrap_or_else(|| "resolve merge blockers".to_string())
+        } else {
+            format!(
+                "merge after {}",
+                blocked_by
+                    .iter()
+                    .map(|blocker| blocker.session_id.as_str())
+                    .collect::<Vec<_>>()
+                    .join(", ")
+            )
+        };
+
+        entries.push(MergeQueueEntry {
+            session_id: session.id,
+            task: session.task,
+            project: session.project,
+            task_group: session.task_group,
+            branch: worktree.branch,
+            base_branch: worktree.base_branch,
+            state: session.state,
+            worktree_health,
+            dirty,
+            queue_position,
+            ready_to_merge,
+            blocked_by,
+            suggested_action,
+        });
+    }
+
+    let mut ready_entries = entries
+        .iter()
+        .filter(|entry| entry.ready_to_merge)
+        .cloned()
+        .collect::<Vec<_>>();
+    ready_entries.sort_by_key(|entry| entry.queue_position.unwrap_or(usize::MAX));
+
+    let blocked_entries = entries
+        .into_iter()
+        .filter(|entry| !entry.ready_to_merge)
+        .collect::<Vec<_>>();
+
+    Ok(MergeQueueReport {
+        ready_entries,
+        blocked_entries,
+    })
+}
+
 pub async fn delete_session(db: &StateStore, id: &str) -> Result<()> {
     let session = resolve_session(db, id)?;
 
@@ -1326,6 +1511,14 @@ fn attached_worktree_count(db: &StateStore) -> Result<usize> {
         .count())
 }
 
+fn merge_queue_priority(session: &Session) -> (u8, chrono::DateTime<chrono::Utc>) {
+    let active_rank = match session.state {
+        SessionState::Completed | SessionState::Failed | SessionState::Stopped => 0,
+        SessionState::Pending | SessionState::Running | SessionState::Idle | SessionState::Stale => 1,
+    };
+    (active_rank, session.updated_at)
+}
+
 async fn spawn_session_runner(
     task: &str,
     session_id: &str,
@@ -3020,6 +3213,97 @@ mod tests {
         Ok(())
     }
 
+    #[tokio::test(flavor = "current_thread")]
+    async fn build_merge_queue_orders_ready_sessions_and_blocks_conflicts() -> Result<()> {
+        let tempdir = TestDir::new("manager-merge-queue")?;
+        let repo_root = tempdir.path().join("repo");
+        init_git_repo(&repo_root)?;
+
+        let cfg = build_config(tempdir.path());
+        let db = StateStore::open(&cfg.db_path)?;
+        let now = Utc::now();
+
+        let alpha_worktree = worktree::create_for_session_in_repo("alpha", &cfg, &repo_root)?;
+        fs::write(alpha_worktree.path.join("README.md"), "alpha\n")?;
+        run_git(&alpha_worktree.path, ["add", "README.md"])?;
+        run_git(&alpha_worktree.path, ["commit", "-m", "alpha change"])?;
+
+        let beta_worktree = worktree::create_for_session_in_repo("beta", &cfg, &repo_root)?;
+        fs::write(beta_worktree.path.join("README.md"), "beta\n")?;
+        run_git(&beta_worktree.path, ["add", "README.md"])?;
+        run_git(&beta_worktree.path, ["commit", "-m", "beta change"])?;
+
+        let gamma_worktree = worktree::create_for_session_in_repo("gamma", &cfg, &repo_root)?;
+        fs::write(gamma_worktree.path.join("src.txt"), "gamma\n")?;
+        run_git(&gamma_worktree.path, ["add", "src.txt"])?;
+        run_git(&gamma_worktree.path, ["commit", "-m", "gamma change"])?;
+
+        db.insert_session(&Session {
+            id: "alpha".to_string(),
+            task: "alpha merge".to_string(),
+            project: "ecc".to_string(),
+            task_group: "merge".to_string(),
+            agent_type: "claude".to_string(),
+            working_dir: alpha_worktree.path.clone(),
+            state: SessionState::Stopped,
+            pid: None,
+            worktree: Some(alpha_worktree),
+            created_at: now - Duration::minutes(3),
+            updated_at: now - Duration::minutes(3),
+            last_heartbeat_at: now - Duration::minutes(3),
+            metrics: SessionMetrics::default(),
+        })?;
+        db.insert_session(&Session {
+            id: "beta".to_string(),
+            task: "beta merge".to_string(),
+            project: "ecc".to_string(),
+            task_group: "merge".to_string(),
+            agent_type: "claude".to_string(),
+            working_dir: beta_worktree.path.clone(),
+            state: SessionState::Stopped,
+            pid: None,
+            worktree: Some(beta_worktree),
+            created_at: now - Duration::minutes(2),
+            updated_at: now - Duration::minutes(2),
+            last_heartbeat_at: now - Duration::minutes(2),
+            metrics: SessionMetrics::default(),
+        })?;
+        db.insert_session(&Session {
+            id: "gamma".to_string(),
+            task: "gamma merge".to_string(),
+            project: "ecc".to_string(),
+            task_group: "merge".to_string(),
+            agent_type: "claude".to_string(),
+            working_dir: gamma_worktree.path.clone(),
+            state: SessionState::Stopped,
+            pid: None,
+            worktree: Some(gamma_worktree),
+            created_at: now - Duration::minutes(1),
+            updated_at: now - Duration::minutes(1),
+            last_heartbeat_at: now - Duration::minutes(1),
+            metrics: SessionMetrics::default(),
+        })?;
+
+        let queue = build_merge_queue(&db)?;
+        assert_eq!(queue.ready_entries.len(), 2);
+        assert_eq!(queue.ready_entries[0].session_id, "alpha");
+        assert_eq!(queue.ready_entries[0].queue_position, Some(1));
+        assert_eq!(queue.ready_entries[1].session_id, "gamma");
+        assert_eq!(queue.ready_entries[1].queue_position, Some(2));
+
+        assert_eq!(queue.blocked_entries.len(), 1);
+        let blocked = &queue.blocked_entries[0];
+        assert_eq!(blocked.session_id, "beta");
+        assert_eq!(blocked.blocked_by.len(), 1);
+        assert_eq!(blocked.blocked_by[0].session_id, "alpha");
+        assert!(blocked.blocked_by[0]
+            .conflicts
+            .contains(&"README.md".to_string()));
+        assert!(blocked.suggested_action.contains("merge after alpha"));
+
+        Ok(())
+    }
+
     #[tokio::test(flavor = "current_thread")]
     async fn delete_session_removes_inactive_session_and_worktree() -> Result<()> {
         let tempdir = TestDir::new("manager-delete-session")?;

ecc2/src/tui/dashboard.rs

@@ -4371,6 +4371,35 @@ impl Dashboard {
                         lines.push(format!("- conflict {conflict}"));
                     }
                 }
+                if let Ok(merge_queue) = manager::build_merge_queue(&self.db) {
+                    let entry = merge_queue
+                        .ready_entries
+                        .iter()
+                        .chain(merge_queue.blocked_entries.iter())
+                        .find(|entry| entry.session_id == session.id);
+                    if let Some(entry) = entry {
+                        lines.push("Merge queue".to_string());
+                        if let Some(position) = entry.queue_position {
+                            lines.push(format!(
+                                "- ready #{} | {}",
+                                position, entry.suggested_action
+                            ));
+                        } else {
+                            lines.push(format!("- blocked | {}", entry.suggested_action));
+                        }
+                        for blocker in entry.blocked_by.iter().take(2) {
+                            lines.push(format!(
+                                "  blocker {} [{}] | {}",
+                                format_session_id(&blocker.session_id),
+                                blocker.branch,
+                                blocker.summary
+                            ));
+                            for conflict in blocker.conflicts.iter().take(3) {
+                                lines.push(format!("    conflict {conflict}"));
+                            }
+                        }
+                    }
+                }
             }
 
             lines.push(format!(

ecc2/src/worktree/mod.rs

@@ -19,7 +19,7 @@ pub struct MergeReadiness {
     pub conflicts: Vec<String>,
 }
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, Serialize, PartialEq, Eq)]
 pub enum WorktreeHealth {
     Clear,
     InProgress,
@@ -33,6 +33,15 @@ pub struct MergeOutcome {
     pub already_up_to_date: bool,
 }
 
+#[derive(Debug, Clone, Serialize, PartialEq, Eq)]
+pub struct BranchConflictPreview {
+    pub left_branch: String,
+    pub right_branch: String,
+    pub conflicts: Vec<String>,
+    pub left_patch_preview: Option<String>,
+    pub right_patch_preview: Option<String>,
+}
+
 /// Create a new git worktree for an agent session.
 pub fn create_for_session(session_id: &str, cfg: &Config) -> Result<WorktreeInfo> {
     let repo_root = std::env::current_dir().context("Failed to resolve repository root")?;
@@ -255,15 +264,45 @@ pub fn diff_patch_preview(worktree: &WorktreeInfo, max_lines: usize) -> Result<O
 }
 
 pub fn merge_readiness(worktree: &WorktreeInfo) -> Result<MergeReadiness> {
+    let mut readiness = merge_readiness_for_branches(
+        &base_checkout_path(worktree)?,
+        &worktree.base_branch,
+        &worktree.branch,
+    )?;
+    readiness.summary = match readiness.status {
+        MergeReadinessStatus::Ready => format!("Merge ready into {}", worktree.base_branch),
+        MergeReadinessStatus::Conflicted => {
+            let conflict_summary = readiness
+                .conflicts
+                .iter()
+                .take(3)
+                .cloned()
+                .collect::<Vec<_>>()
+                .join(", ");
+            let overflow = readiness.conflicts.len().saturating_sub(3);
+            let detail = if overflow > 0 {
+                format!("{conflict_summary}, +{overflow} more")
+            } else {
+                conflict_summary
+            };
+            format!(
+                "Merge blocked by {} conflict(s): {detail}",
+                readiness.conflicts.len()
+            )
+        }
+    };
+    Ok(readiness)
+}
+
+pub fn merge_readiness_for_branches(
+    repo_root: &Path,
+    left_branch: &str,
+    right_branch: &str,
+) -> Result<MergeReadiness> {
     let output = Command::new("git")
         .arg("-C")
-        .arg(&worktree.path)
+        .arg(repo_root)
-        .args([
+        .args(["merge-tree", "--write-tree", left_branch, right_branch])
-            "merge-tree",
-            "--write-tree",
-            &worktree.base_branch,
-            &worktree.branch,
-        ])
         .output()
         .context("Failed to generate merge readiness preview")?;
 
@@ -280,7 +319,7 @@ pub fn merge_readiness(worktree: &WorktreeInfo) -> Result<MergeReadiness> {
     if output.status.success() {
         return Ok(MergeReadiness {
             status: MergeReadinessStatus::Ready,
-            summary: format!("Merge ready into {}", worktree.base_branch),
+            summary: format!("Merge ready: {right_branch} into {left_branch}"),
             conflicts: Vec::new(),
         });
     }
@@ -301,7 +340,10 @@ pub fn merge_readiness(worktree: &WorktreeInfo) -> Result<MergeReadiness> {
 
         return Ok(MergeReadiness {
             status: MergeReadinessStatus::Conflicted,
-            summary: format!("Merge blocked by {} conflict(s): {detail}", conflicts.len()),
+            summary: format!(
+                "Merge blocked between {left_branch} and {right_branch} by {} conflict(s): {detail}",
+                conflicts.len()
+            ),
             conflicts,
         });
     }
@@ -310,6 +352,30 @@ pub fn merge_readiness(worktree: &WorktreeInfo) -> Result<MergeReadiness> {
     anyhow::bail!("git merge-tree failed: {stderr}");
 }
 
+pub fn branch_conflict_preview(
+    left: &WorktreeInfo,
+    right: &WorktreeInfo,
+    max_lines: usize,
+) -> Result<Option<BranchConflictPreview>> {
+    if left.base_branch != right.base_branch {
+        return Ok(None);
+    }
+
+    let repo_root = base_checkout_path(left)?;
+    let readiness = merge_readiness_for_branches(&repo_root, &left.branch, &right.branch)?;
+    if readiness.status != MergeReadinessStatus::Conflicted {
+        return Ok(None);
+    }
+
+    Ok(Some(BranchConflictPreview {
+        left_branch: left.branch.clone(),
+        right_branch: right.branch.clone(),
+        conflicts: readiness.conflicts.clone(),
+        left_patch_preview: diff_patch_preview_for_paths(left, &readiness.conflicts, max_lines)?,
+        right_patch_preview: diff_patch_preview_for_paths(right, &readiness.conflicts, max_lines)?,
+    }))
+}
+
 pub fn health(worktree: &WorktreeInfo) -> Result<WorktreeHealth> {
     let merge_readiness = merge_readiness(worktree)?;
     if merge_readiness.status == MergeReadinessStatus::Conflicted {
@@ -462,6 +528,79 @@ fn git_diff_patch_lines(worktree_path: &Path, extra_args: &[&str]) -> Result<Vec
     Ok(parse_nonempty_lines(&output.stdout))
 }
 
+fn git_diff_patch_lines_for_paths(
+    worktree_path: &Path,
+    extra_args: &[&str],
+    paths: &[String],
+) -> Result<Vec<String>> {
+    if paths.is_empty() {
+        return Ok(Vec::new());
+    }
+
+    let mut command = Command::new("git");
+    command
+        .arg("-C")
+        .arg(worktree_path)
+        .arg("diff")
+        .args(["--stat", "--patch", "--find-renames"]);
+    command.args(extra_args);
+    command.arg("--");
+    for path in paths {
+        command.arg(path);
+    }
+
+    let output = command
+        .output()
+        .context("Failed to generate filtered worktree patch preview")?;
+
+    if !output.status.success() {
+        let stderr = String::from_utf8_lossy(&output.stderr);
+        tracing::warn!(
+            "Filtered worktree patch preview warning for {}: {stderr}",
+            worktree_path.display()
+        );
+        return Ok(Vec::new());
+    }
+
+    Ok(parse_nonempty_lines(&output.stdout))
+}
+
+pub fn diff_patch_preview_for_paths(
+    worktree: &WorktreeInfo,
+    paths: &[String],
+    max_lines: usize,
+) -> Result<Option<String>> {
+    if paths.is_empty() {
+        return Ok(None);
+    }
+
+    let mut remaining = max_lines.max(1);
+    let mut sections = Vec::new();
+    let base_ref = format!("{}...HEAD", worktree.base_branch);
+
+    let committed = git_diff_patch_lines_for_paths(&worktree.path, &[&base_ref], paths)?;
+    if !committed.is_empty() && remaining > 0 {
+        let taken = take_preview_lines(&committed, &mut remaining);
+        sections.push(format!(
+            "--- Branch diff vs {} ---\n{}",
+            worktree.base_branch,
+            taken.join("\n")
+        ));
+    }
+
+    let working = git_diff_patch_lines_for_paths(&worktree.path, &[], paths)?;
+    if !working.is_empty() && remaining > 0 {
+        let taken = take_preview_lines(&working, &mut remaining);
+        sections.push(format!("--- Working tree diff ---\n{}", taken.join("\n")));
+    }
+
+    if sections.is_empty() {
+        Ok(None)
+    } else {
+        Ok(Some(sections.join("\n\n")))
+    }
+}
+
 fn git_status_short(worktree_path: &Path) -> Result<Vec<String>> {
     let output = Command::new("git")
         .arg("-C")
@@ -901,4 +1040,81 @@ mod tests {
         let _ = fs::remove_dir_all(root);
         Ok(())
     }
+
+    #[test]
+    fn branch_conflict_preview_reports_conflicting_branches() -> Result<()> {
+        let root = std::env::temp_dir()
+            .join(format!("ecc2-worktree-branch-conflict-preview-{}", Uuid::new_v4()));
+        let repo = init_repo(&root)?;
+
+        let left_dir = root.join("wt-left");
+        run_git(
+            &repo,
+            &[
+                "worktree",
+                "add",
+                "-b",
+                "ecc/left",
+                left_dir.to_str().expect("utf8 path"),
+                "HEAD",
+            ],
+        )?;
+        fs::write(left_dir.join("README.md"), "left\n")?;
+        run_git(&left_dir, &["add", "README.md"])?;
+        run_git(&left_dir, &["commit", "-m", "left change"])?;
+
+        let right_dir = root.join("wt-right");
+        run_git(
+            &repo,
+            &[
+                "worktree",
+                "add",
+                "-b",
+                "ecc/right",
+                right_dir.to_str().expect("utf8 path"),
+                "HEAD",
+            ],
+        )?;
+        fs::write(right_dir.join("README.md"), "right\n")?;
+        run_git(&right_dir, &["add", "README.md"])?;
+        run_git(&right_dir, &["commit", "-m", "right change"])?;
+
+        let left = WorktreeInfo {
+            path: left_dir.clone(),
+            branch: "ecc/left".to_string(),
+            base_branch: "main".to_string(),
+        };
+        let right = WorktreeInfo {
+            path: right_dir.clone(),
+            branch: "ecc/right".to_string(),
+            base_branch: "main".to_string(),
+        };
+
+        let preview = branch_conflict_preview(&left, &right, 12)?
+            .expect("expected branch conflict preview");
+        assert_eq!(preview.conflicts, vec!["README.md".to_string()]);
+        assert!(preview
+            .left_patch_preview
+            .as_ref()
+            .is_some_and(|preview| preview.contains("README.md")));
+        assert!(preview
+            .right_patch_preview
+            .as_ref()
+            .is_some_and(|preview| preview.contains("README.md")));
+
+        let _ = Command::new("git")
+            .arg("-C")
+            .arg(&repo)
+            .args(["worktree", "remove", "--force"])
+            .arg(&left_dir)
+            .output();
+        let _ = Command::new("git")
+            .arg("-C")
+            .arg(&repo)
+            .args(["worktree", "remove", "--force"])
+            .arg(&right_dir)
+            .output();
+        let _ = fs::remove_dir_all(root);
+        Ok(())
+    }
 }