diff --git a/milestones/README.md b/milestones/README.md
index 38d03f3..821ffc9 100644
--- a/milestones/README.md
+++ b/milestones/README.md
@@ -5,10 +5,13 @@ repositories that install `docgen` and maintain their own demo bundle. The
 library no longer ships an in-repo dogfood; consumers are the integration test
 of record.
 
-**Active:** **[pages-config-strings.md](pages-config-strings.md)** —
-`pages.docs_dir` / title / extra_links must be typed at config load.
+**Active:** **[visual-map-sources.md](visual-map-sources.md)** —
+`visual_map` mixed `sources` must be a YAML list of strings at config
+load.
 
 **Shipped:**
+- **[pages-config-strings.md](pages-config-strings.md)** —
+  `pages.docs_dir` / title / extra_links must be typed (#111).
 - **[generation-segment-strings.md](generation-segment-strings.md)** —
   per-segment narration / scene-generation prompts must be YAML strings
   (#110).
diff --git a/milestones/pages-config-strings.md b/milestones/pages-config-strings.md
index 3cf3f6f..783fcdd 100644
--- a/milestones/pages-config-strings.md
+++ b/milestones/pages-config-strings.md
@@ -1,7 +1,7 @@
 # Milestone: pages docs_dir / title / extra_links must be typed
 
-**Status:** Active  
-**PR:** [#111](https://github.com/jmjava/documentation-generator/pull/111)  
+**Status:** Shipped  
+**PR:** #111  
 **Depends on:** `milestones/generation-segment-strings.md` (PR #110),
 `milestones/path-config-strings.md` (PR #109),
 `milestones/concat-segment-lists.md` (PR #84)
diff --git a/milestones/visual-map-sources.md b/milestones/visual-map-sources.md
new file mode 100644
index 0000000..94c1321
--- /dev/null
+++ b/milestones/visual-map-sources.md
@@ -0,0 +1,35 @@
+# Milestone: visual_map mixed sources must be a list of strings
+
+**Status:** Active  
+**PR:** [#112](https://github.com/jmjava/documentation-generator/pull/112)  
+**Depends on:** `milestones/visual-map-field-strings.md` (PR #108),
+`milestones/pages-config-strings.md` (PR #111)
+
+## Problem
+
+`visual_map.<id>.source` is already a YAML string. Mixed rows use
+**`sources:`** instead:
+
+1. A YAML **string** (`sources: clip.mp4`) was iterated as characters
+   in `Composer.compose_segments`, looking up one-letter paths.
+2. A nested list item was passed to `_resolve_source` and Path-joined.
+
+Missing `sources` remains allowed (empty mixed row).
+
+## Goal
+
+Fail closed at `Config.from_yaml`. Present `visual_map.<id>.sources`
+must be a YAML list of non-empty strings.
+
+## Done when
+
+- [x] Present `visual_map.<id>.sources` must be a YAML list of strings.
+- [x] Tests for a string value and a nested-list item.
+- [x] `ruff check src/ tests/`
+- [x] `pytest tests/`
+- [x] `docgen benchmark` (no clock change)
+
+## Out of scope
+
+- `wizard.default_guidance` type gating is separate.
+- Empty `sources: []` remains allowed.
diff --git a/src/docgen/config.py b/src/docgen/config.py
index efb3c89..8b32b60 100644
--- a/src/docgen/config.py
+++ b/src/docgen/config.py
@@ -338,6 +338,13 @@ def __post_init__(self) -> None:
                         f"{src}: visual_map.{sid_s}.{fname} must be a YAML string, "
                         f"not {type(val).__name__}"
                     )
+            if spec.get("sources") is not None:
+                string_list_block(
+                    spec,
+                    "sources",
+                    label=f"visual_map.{sid_s}.sources",
+                    source=src,
+                )
         wiz = self._block("wizard")
         if wiz.get("exclude_patterns") is not None:
             string_list_block(
diff --git a/tests/test_config.py b/tests/test_config.py
index f1459e6..6426176 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -741,3 +741,25 @@ def test_from_yaml_list_pages_segment_title_raises(tmp_path: Path) -> None:
     )
     with pytest.raises(ConfigError, match="pages.segments.01.title must be a YAML string"):
         Config.from_yaml(p)
+
+
+def test_from_yaml_string_visual_map_sources_raises(tmp_path: Path) -> None:
+    p = tmp_path / "docgen.yaml"
+    p.write_text(
+        'visual_map:\n  "01":\n    type: mixed\n    sources: clip.mp4\n',
+        encoding="utf-8",
+    )
+    with pytest.raises(ConfigError, match="visual_map.01.sources must be a YAML list"):
+        Config.from_yaml(p)
+
+
+def test_from_yaml_list_visual_map_sources_item_raises(tmp_path: Path) -> None:
+    p = tmp_path / "docgen.yaml"
+    p.write_text(
+        'visual_map:\n  "01":\n    type: mixed\n    sources:\n      - - clip.mp4\n',
+        encoding="utf-8",
+    )
+    with pytest.raises(
+        ConfigError, match=r"visual_map.01.sources\[0\] must be a YAML string"
+    ):
+        Config.from_yaml(p)