diff --git a/server/src/agents/merge/squash.rs b/server/src/agents/merge/squash.rs
index 526808d6..8b70882d 100644
--- a/server/src/agents/merge/squash.rs
+++ b/server/src/agents/merge/squash.rs
@@ -28,6 +28,34 @@ pub(crate) fn run_squash_merge(
         .lock()
         .map_err(|e| format!("Merge lock poisoned: {e}"))?;
 
+    // ── Pre-flight: verify the branch has commits ahead of base ──────────────
+    // A zero-commit branch produces an empty squash and a silent "nothing to
+    // commit" failure.  Catch it early with a grep-able error before any merge
+    // work starts.
+    let base_branch = crate::config::ProjectConfig::load(project_root)
+        .ok()
+        .and_then(|c| c.base_branch.clone())
+        .unwrap_or_else(|| "master".to_string());
+
+    let ahead_out = Command::new("git")
+        .args(["rev-list", "--count", &format!("{base_branch}..{branch}")])
+        .current_dir(project_root)
+        .output()
+        .map_err(|e| format!("Failed to count commits ahead: {e}"))?;
+
+    if ahead_out.status.success() {
+        let ahead: u64 = String::from_utf8_lossy(&ahead_out.stdout)
+            .trim()
+            .parse()
+            .unwrap_or(1); // parse failure → don't false-positive; let merge proceed
+        if ahead == 0 {
+            return Err(format!(
+                "{story_id}: no commits to merge — feature branch '{branch}' \
+                 has 0 commits ahead of '{base_branch}'"
+            ));
+        }
+    }
+
     let mut all_output = String::new();
     let merge_branch = format!("merge-queue/{story_id}");
     let merge_wt_path = project_root.join(".huskies").join("merge_workspace");
@@ -827,16 +855,25 @@ mod tests {
             .output()
             .unwrap();
 
-        let result = run_squash_merge(repo, "feature/story-empty_test", "empty_test").unwrap();
+        let result = run_squash_merge(repo, "feature/story-empty_test", "empty_test");
 
-        // Bug 226: empty diff must NOT be treated as success.
-        assert!(
-            !result.success,
-            "empty diff merge must fail, not silently succeed: {}",
-            result.output
-        );
+        // Bug 226 / 675: a zero-commit branch must not be treated as success.
+        // The pre-flight check (bug 675) returns Err for zero commits ahead;
+        // the older code path returned Ok(SquashMergeResult { success: false }).
+        // Either form is a failure — just not success.
+        match result {
+            Ok(r) => assert!(
+                !r.success,
+                "empty diff merge must fail, not silently succeed: {}",
+                r.output
+            ),
+            Err(e) => assert!(
+                e.contains("no commits to merge") || e.contains("nothing to commit"),
+                "unexpected error: {e}"
+            ),
+        }
 
-        // Cleanup should still happen.
+        // Cleanup should still happen (no workspace was created for the Err path).
         assert!(
             !repo.join(".huskies/merge_workspace").exists(),
             "merge workspace should be cleaned up"
diff --git a/server/src/agents/pool/pipeline/merge.rs b/server/src/agents/pool/pipeline/merge.rs
index d3900dd2..27f32c39 100644
--- a/server/src/agents/pool/pipeline/merge.rs
+++ b/server/src/agents/pool/pipeline/merge.rs
@@ -614,4 +614,165 @@ mod tests {
             MergeJobStatus::Running => panic!("should not still be running"),
         }
     }
+
+    // ── bug 675: zero commits ahead must fail with "no commits to merge" ─────
+
+    /// Regression test for bug 675: when the feature branch has zero commits
+    /// ahead of master the pipeline must fail with a clear "no commits to merge"
+    /// error and the story must remain in `4_merge` (not advance to `5_done`).
+    #[tokio::test]
+    async fn merge_agent_work_zero_commits_ahead_stays_in_merge_stage() {
+        use std::fs;
+        use tempfile::tempdir;
+
+        let tmp = tempdir().unwrap();
+        let repo = tmp.path();
+        init_git_repo(repo);
+
+        // Feature branch is created at the same commit as master — zero commits ahead.
+        Command::new("git")
+            .args(["checkout", "-b", "feature/story-675_zero_commits"])
+            .current_dir(repo)
+            .output()
+            .unwrap();
+        Command::new("git")
+            .args(["checkout", "master"])
+            .current_dir(repo)
+            .output()
+            .unwrap();
+
+        // Place the story file in 4_merge so we can verify it stays there.
+        let merge_dir = repo.join(".huskies/work/4_merge");
+        fs::create_dir_all(&merge_dir).unwrap();
+        fs::write(
+            merge_dir.join("675_zero_commits.md"),
+            "---\nname: Zero commits test\n---\n",
+        )
+        .unwrap();
+        Command::new("git")
+            .args(["add", "."])
+            .current_dir(repo)
+            .output()
+            .unwrap();
+        Command::new("git")
+            .args(["commit", "-m", "place story in 4_merge"])
+            .current_dir(repo)
+            .output()
+            .unwrap();
+
+        let pool = Arc::new(AgentPool::new_test(3001));
+        let job = run_merge_to_completion(&pool, repo, "675_zero_commits").await;
+
+        // The job must have failed with a "no commits to merge" error.
+        match &job.status {
+            MergeJobStatus::Failed(e) => {
+                assert!(
+                    e.contains("no commits to merge"),
+                    "error must contain 'no commits to merge', got: {e}"
+                );
+                assert!(
+                    e.contains("675_zero_commits"),
+                    "error must name the story_id, got: {e}"
+                );
+            }
+            MergeJobStatus::Completed(report) => {
+                panic!(
+                    "expected Failed status, got Completed with success={}: {}",
+                    report.success, report.gate_output
+                );
+            }
+            MergeJobStatus::Running => panic!("should not still be running"),
+        }
+
+        // Story file must still be in 4_merge — NOT advanced to 5_done.
+        assert!(
+            merge_dir.join("675_zero_commits.md").exists(),
+            "story file must remain in 4_merge when merge fails"
+        );
+        assert!(
+            !repo
+                .join(".huskies/work/5_done/675_zero_commits.md")
+                .exists(),
+            "story must NOT advance to 5_done when merge fails with no commits"
+        );
+    }
+
+    /// Non-regression test for bug 675: a feature branch with exactly one commit
+    /// ahead of master must continue to merge successfully (happy path).
+    #[tokio::test]
+    async fn merge_agent_work_one_commit_ahead_merges_successfully() {
+        use std::fs;
+        use tempfile::tempdir;
+
+        let tmp = tempdir().unwrap();
+        let repo = tmp.path();
+        init_git_repo(repo);
+
+        // Feature branch: one commit ahead of master.
+        Command::new("git")
+            .args(["checkout", "-b", "feature/story-675_one_commit"])
+            .current_dir(repo)
+            .output()
+            .unwrap();
+        fs::write(repo.join("feature_675.txt"), "feature content\n").unwrap();
+        Command::new("git")
+            .args(["add", "."])
+            .current_dir(repo)
+            .output()
+            .unwrap();
+        Command::new("git")
+            .args(["commit", "-m", "add feature file"])
+            .current_dir(repo)
+            .output()
+            .unwrap();
+        Command::new("git")
+            .args(["checkout", "master"])
+            .current_dir(repo)
+            .output()
+            .unwrap();
+
+        // Place the story file in 4_merge.
+        let merge_dir = repo.join(".huskies/work/4_merge");
+        fs::create_dir_all(&merge_dir).unwrap();
+        fs::write(
+            merge_dir.join("675_one_commit.md"),
+            "---\nname: One commit test\n---\n",
+        )
+        .unwrap();
+        Command::new("git")
+            .args(["add", "."])
+            .current_dir(repo)
+            .output()
+            .unwrap();
+        Command::new("git")
+            .args(["commit", "-m", "place story in 4_merge"])
+            .current_dir(repo)
+            .output()
+            .unwrap();
+
+        let pool = Arc::new(AgentPool::new_test(3001));
+        let job = run_merge_to_completion(&pool, repo, "675_one_commit").await;
+
+        // The merge must not fail with "no commits to merge".
+        match &job.status {
+            MergeJobStatus::Failed(e) => {
+                assert!(
+                    !e.contains("no commits to merge"),
+                    "one-commit-ahead branch must NOT fail with 'no commits to merge': {e}"
+                );
+                // Gate failures (no script/test) are acceptable in test env.
+            }
+            MergeJobStatus::Completed(report) => {
+                // Success or gate failure — both acceptable; the key invariant is
+                // that we didn't fail with the zero-commits early-exit.
+                assert!(
+                    report.success || !report.gates_passed,
+                    "unexpected state: success={} gates_passed={}",
+                    report.success,
+                    report.gates_passed
+                );
+            }
+            MergeJobStatus::Running => panic!("should not still be running"),
+        }
+    }
 }