diff --git a/apps/server/src/vcs/GitVcsDriverCore.test.ts b/apps/server/src/vcs/GitVcsDriverCore.test.ts
index af5ac9ed72570..25401ec98544c 100644
--- a/apps/server/src/vcs/GitVcsDriverCore.test.ts
+++ b/apps/server/src/vcs/GitVcsDriverCore.test.ts
@@ -6507,7 +6507,17 @@ it.layer(layerTest)("GitVcsDriver core integration", (it) => {
           assert.equal(yield* git(cwd, ["rev-parse", "HEAD"]), mainHead);
           assert.equal(yield* git(cwd, ["ls-files", "--unmerged"]), unmerged);
 
-          if (conflict) yield* writeTextFile(cwd, "README.md", "resolved version\n");
+          if (conflict) {
+            const unresolved = yield* driver
+              .commit(commitCwd, "Unresolved merge", "", { stage: {} })
+              .pipe(Effect.result);
+            assert.isTrue(Result.isFailure(unresolved), "committed an unresolved conflict");
+            assert.equal(yield* git(cwd, ["rev-parse", "HEAD"]), mainHead);
+            assert.equal(yield* git(cwd, ["ls-files", "--unmerged"]), unmerged);
+            // A lone "=======" is a heading underline, not a conflict boundary.
+            yield* writeTextFile(cwd, "README.md", "Heading\n=======\n\nresolved\n");
+            assert.isNotNull(yield* driver.prepareCommitContext(commitCwd));
+          }
           yield* driver.commit(commitCwd, "Finish merge", "", { stage: {} });
           assert.equal(
             yield* git(cwd, ["show", "-s", "--format=%P", "HEAD"]),
@@ -6517,7 +6527,10 @@ it.layer(layerTest)("GitVcsDriver core integration", (it) => {
           assert.equal(yield* git(cwd, ["show", "HEAD:feature.txt"]), "feature");
           assert.equal(yield* git(cwd, ["show", "HEAD:unrelated.txt"]), "unrelated");
           if (conflict)
-            assert.equal(yield* git(cwd, ["show", "HEAD:README.md"]), "resolved version");
+            assert.equal(
+              yield* git(cwd, ["show", "HEAD:README.md"]),
+              "Heading\n=======\n\nresolved",
+            );
         }),
     );
 
diff --git a/apps/server/src/vcs/GitVcsDriverCore.ts b/apps/server/src/vcs/GitVcsDriverCore.ts
index f9043e504cde7..a8e4fb9265f3d 100644
--- a/apps/server/src/vcs/GitVcsDriverCore.ts
+++ b/apps/server/src/vcs/GitVcsDriverCore.ts
@@ -2543,6 +2543,78 @@ export const makeGitVcsDriverCore = Effect.fn("makeGitVcsDriverCore")(function*
         },
       );
     } else {
+      // add -A marks unmerged paths resolved, so refuse while one still has conflict markers.
+      const unmergedPaths = (yield* runGitStdoutWithOptions(
+        "GitVcsDriver.commit.unmergedPaths",
+        cwd,
+        ["-c", "diff.relative=false", "diff", "--name-only", "--diff-filter=U", "-z"],
+        env ? { env } : {},
+      ))
+        .split("\0")
+        .filter((file) => file.length > 0);
+      if (unmergedPaths.length > 0) {
+        const check = yield* executeGit(
+          "GitVcsDriver.commit.conflictMarkers",
+          cwd,
+          [
+            "-c",
+            "diff.relative=false",
+            "diff",
+            "--check",
+            "--no-color",
+            "--",
+            ...unmergedPaths.map((file) => `:(top,literal)${file}`),
+          ],
+          {
+            ...(env ? { env } : {}),
+            allowNonZeroExit: true,
+            maxOutputBytes: REVIEW_METADATA_MAX_OUTPUT_BYTES,
+            appendTruncationMarker: true,
+          },
+        );
+        const reports = check.stdout.split("\n");
+        const conflicted: string[] = [];
+        let topLevel: string | undefined;
+        for (const file of unmergedPaths) {
+          const markerLines = new Set(
+            reports.flatMap((line) => {
+              const match = line.startsWith(`${file}:`)
+                ? /^(\d+): leftover conflict marker$/.exec(line.slice(file.length + 1))
+                : null;
+              return match ? [Number(match[1]) - 1] : [];
+            }),
+          );
+          if (markerLines.size === 0) continue;
+          topLevel ??= (yield* runGitStdout("GitVcsDriver.commit.topLevel", cwd, [
+            "rev-parse",
+            "--show-toplevel",
+          ])).trim();
+          const lines = (yield* fileSystem.readFileString(path.join(topLevel, file)).pipe(
+            Effect.mapError(
+              (cause) =>
+                new GitCommandError({
+                  operation: "GitVcsDriver.commit.conflictMarkers",
+                  command: "git commit",
+                  cwd,
+                  detail: `Could not check ${file} for conflict markers.`,
+                  cause,
+                }),
+            ),
+          )).split("\n");
+          // Git also reports a lone "=======", which is valid content such as a heading underline.
+          const reported = (marker: string) =>
+            lines.some((line, index) => markerLines.has(index) && line.startsWith(marker));
+          if (reported("<") && reported(">")) conflicted.push(file);
+        }
+        if (conflicted.length > 0) {
+          return yield* new GitCommandError({
+            operation: "GitVcsDriver.commit",
+            command: "git commit",
+            cwd,
+            detail: `Cannot commit with unresolved merge conflicts in ${conflicted.join(", ")}. Resolve them, then commit.`,
+          });
+        }
+      }
       yield* runGit(
         "GitVcsDriver.commit.addAll",
         cwd,