fix: string-level edge splicing in reweave merge — no yaml.dump reformatting

Two fixes from Ganymede review:
1. CRITICAL: blank line before closing --- compounded on repeat reweaves.
   Body starts with \n---, so \n{body} created \n\n---. Fixed by checking
   body prefix.
2. Replaced yaml.dump round-trip with _serialize_edge_fields() that splices
   only edge arrays into raw frontmatter text. Non-edge fields (title,
   confidence, type, quotes, flow styles) stay byte-identical to main HEAD.

_parse_yaml_frontmatter now returns 3-tuple: (dict, raw_fm_text, body).
_serialize_frontmatter takes (raw_fm_text, merged_edges_dict, body).

26 tests pass including idempotency (5x serialize), formatting preservation,
and no-blank-line regression test.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/merge.py

@@ -393,25 +393,27 @@ async def _cherry_pick_onto_main(branch: str) -> tuple[bool, str]:
 REWEAVE_EDGE_FIELDS = ("supports", "challenges", "depends_on", "related", "reweave_edges")
 
 
-def _parse_yaml_frontmatter(text: str) -> tuple[dict | None, str]:
+def _parse_yaml_frontmatter(text: str) -> tuple[dict | None, str, str]:
     """Parse YAML frontmatter from markdown text.
 
-    Returns (frontmatter_dict, body_text_including_closing_delimiter).
-    Returns (None, text) if no valid frontmatter found.
+    Returns (frontmatter_dict, raw_fm_text, body_text_including_closing_delimiter).
+    Returns (None, "", text) if no valid frontmatter found.
+    raw_fm_text is the text between the --- delimiters (no delimiters, no leading newline).
     """
     import yaml
 
     if not text.startswith("---"):
-        return None, text
+        return None, "", text
     end = text.find("\n---", 3)
     if end == -1:
-        return None, text
+        return None, "", text
     try:
-        fm = yaml.safe_load(text[3:end])
-        body = text[end:]  # includes closing --- and body
-        return fm if isinstance(fm, dict) else None, body
+        raw_fm_text = text[4:end]  # skip "---\n", stop before "\n---"
+        fm = yaml.safe_load(raw_fm_text)
+        body = text[end:]  # includes closing \n--- and body
+        return (fm if isinstance(fm, dict) else None), raw_fm_text, body
     except Exception:
-        return None, text
+        return None, "", text
 
 
 def _union_edge_lists(main_edges: list, branch_edges: list) -> list:
@@ -435,12 +437,74 @@ def _union_edge_lists(main_edges: list, branch_edges: list) -> list:
     return result
 
 
-def _serialize_frontmatter(fm: dict, body: str) -> str:
-    """Serialize frontmatter dict + body back to markdown text."""
+def _serialize_edge_fields(raw_fm_text: str, merged_edges: dict[str, list]) -> str:
+    """Splice merged edge fields into raw frontmatter text, preserving all other fields byte-identical.
+
+    Only modifies REWEAVE_EDGE_FIELDS lines. All other frontmatter (title, confidence, type, etc.)
+    stays exactly as it was in the source text — no yaml.dump reformatting.
+
+    Args:
+        raw_fm_text: The raw YAML text between the --- delimiters (no delimiters included).
+        merged_edges: {field_name: [edge_values]} for each edge field that should be present.
+    """
+    import re
     import yaml
 
-    fm_str = yaml.dump(fm, default_flow_style=False, allow_unicode=True, sort_keys=False).rstrip("\n")
-    return f"---\n{fm_str}\n{body}"
+    lines = raw_fm_text.split("\n")
+    result_lines = []
+    i = 0
+    fields_written = set()
+
+    while i < len(lines):
+        line = lines[i]
+        # Check if this line starts an edge field
+        matched_field = None
+        for field in REWEAVE_EDGE_FIELDS:
+            if line.startswith(f"{field}:"):
+                matched_field = field
+                break
+
+        if matched_field:
+            fields_written.add(matched_field)
+            # Skip the old field and its list items
+            i += 1
+            while i < len(lines) and lines[i].startswith("- "):
+                i += 1
+            # Write the merged version
+            edges = merged_edges.get(matched_field, [])
+            if edges:
+                result_lines.append(f"{matched_field}:")
+                for edge in edges:
+                    result_lines.append(f"- {edge}")
+            # Don't increment i — it's already past the old field
+            continue
+        else:
+            result_lines.append(line)
+            i += 1
+
+    # Append any new edge fields that didn't exist in the original
+    for field in REWEAVE_EDGE_FIELDS:
+        if field not in fields_written:
+            edges = merged_edges.get(field, [])
+            if edges:
+                result_lines.append(f"{field}:")
+                for edge in edges:
+                    result_lines.append(f"- {edge}")
+
+    return "\n".join(result_lines)
+
+
+def _serialize_frontmatter(raw_fm_text: str, merged_edges: dict[str, list], body: str) -> str:
+    """Rebuild markdown file: splice merged edges into raw frontmatter, append body.
+
+    Uses string-level surgery — only edge fields are modified. All other frontmatter
+    stays byte-identical to the source. No yaml.dump reformatting.
+    """
+    spliced = _serialize_edge_fields(raw_fm_text, merged_edges)
+    # body starts with \n--- (closing delimiter + body text)
+    if body.startswith("\n"):
+        return f"---\n{spliced}{body}"
+    return f"---\n{spliced}\n{body}"
 
 
 async def _merge_reweave_pr(branch: str) -> tuple[bool, str]:
@@ -510,8 +574,8 @@ async def _merge_reweave_pr(branch: str) -> tuple[bool, str]:
                 continue
 
             # Parse frontmatter from both versions
-            main_fm, main_body = _parse_yaml_frontmatter(main_content)
-            branch_fm, branch_body = _parse_yaml_frontmatter(branch_content)
+            main_fm, main_raw_fm, main_body = _parse_yaml_frontmatter(main_content)
+            branch_fm, _branch_raw_fm, branch_body = _parse_yaml_frontmatter(branch_content)
 
             if main_fm is None or branch_fm is None:
                 # Parse failure = something unexpected. Fail the merge, don't fallback
@@ -540,8 +604,8 @@ async def _merge_reweave_pr(branch: str) -> tuple[bool, str]:
                     )
                     skipped_non_superset.append(f"{fpath}:{field}")
 
-            # Union edge lists: main's edges first (order-preserved), branch-new appended
-            merged_fm = dict(main_fm)  # Start with main's full frontmatter
+            # Collect merged edge fields for string-level splicing
+            merged_edges = {}
             for field in REWEAVE_EDGE_FIELDS:
                 main_list = main_fm.get(field, [])
                 branch_list = branch_fm.get(field, [])
@@ -549,15 +613,14 @@ async def _merge_reweave_pr(branch: str) -> tuple[bool, str]:
                     main_list = [main_list] if main_list else []
                 if not isinstance(branch_list, list):
                     branch_list = [branch_list] if branch_list else []
-
                 if main_list or branch_list:
-                    merged_fm[field] = _union_edge_lists(main_list, branch_list)
+                    merged_edges[field] = _union_edge_lists(main_list, branch_list)
 
-            # Write merged file — use main's body (reweave doesn't touch body text)
+            # Write merged file — splice edges into main's raw frontmatter, use main's body
             full_path = os.path.join(worktree_path, fpath)
             os.makedirs(os.path.dirname(full_path), exist_ok=True)
             with open(full_path, "w") as f:
-                f.write(_serialize_frontmatter(merged_fm, main_body))
+                f.write(_serialize_frontmatter(main_raw_fm, merged_edges, main_body))
             await _git("add", fpath, cwd=worktree_path)
             merged_count += 1
 

tests/test_reweave_merge.py

@@ -1,4 +1,4 @@
-"""Tests for _merge_reweave_pr helpers — frontmatter union, order-preserving dedup, superset assertion.
+"""Tests for _merge_reweave_pr helpers — frontmatter union, order-preserving dedup, string-level splicing.
 
 These test the pure functions used by _merge_reweave_pr in lib/merge.py.
 Copied here because lib/merge.py's relative imports make direct import impractical in tests.
@@ -13,18 +13,19 @@ import yaml
 REWEAVE_EDGE_FIELDS = ("supports", "challenges", "depends_on", "related", "reweave_edges")
 
 
-def _parse_yaml_frontmatter(text: str) -> tuple[dict | None, str]:
+def _parse_yaml_frontmatter(text: str) -> tuple[dict | None, str, str]:
     if not text.startswith("---"):
-        return None, text
+        return None, "", text
     end = text.find("\n---", 3)
     if end == -1:
-        return None, text
+        return None, "", text
     try:
-        fm = yaml.safe_load(text[3:end])
+        raw_fm_text = text[4:end]
+        fm = yaml.safe_load(raw_fm_text)
         body = text[end:]
-        return fm if isinstance(fm, dict) else None, body
+        return (fm if isinstance(fm, dict) else None), raw_fm_text, body
     except Exception:
-        return None, text
+        return None, "", text
 
 
 def _union_edge_lists(main_edges: list, branch_edges: list) -> list:
@@ -43,31 +44,75 @@ def _union_edge_lists(main_edges: list, branch_edges: list) -> list:
     return result
 
 
-def _serialize_frontmatter(fm: dict, body: str) -> str:
-    fm_str = yaml.dump(fm, default_flow_style=False, allow_unicode=True, sort_keys=False).rstrip("\n")
-    return f"---\n{fm_str}\n{body}"
+def _serialize_edge_fields(raw_fm_text: str, merged_edges: dict[str, list]) -> str:
+    lines = raw_fm_text.split("\n")
+    result_lines = []
+    i = 0
+    fields_written = set()
+
+    while i < len(lines):
+        line = lines[i]
+        matched_field = None
+        for field in REWEAVE_EDGE_FIELDS:
+            if line.startswith(f"{field}:"):
+                matched_field = field
+                break
+
+        if matched_field:
+            fields_written.add(matched_field)
+            i += 1
+            while i < len(lines) and lines[i].startswith("- "):
+                i += 1
+            edges = merged_edges.get(matched_field, [])
+            if edges:
+                result_lines.append(f"{matched_field}:")
+                for edge in edges:
+                    result_lines.append(f"- {edge}")
+            continue
+        else:
+            result_lines.append(line)
+            i += 1
+
+    for field in REWEAVE_EDGE_FIELDS:
+        if field not in fields_written:
+            edges = merged_edges.get(field, [])
+            if edges:
+                result_lines.append(f"{field}:")
+                for edge in edges:
+                    result_lines.append(f"- {edge}")
+
+    return "\n".join(result_lines)
+
+
+def _serialize_frontmatter(raw_fm_text: str, merged_edges: dict[str, list], body: str) -> str:
+    spliced = _serialize_edge_fields(raw_fm_text, merged_edges)
+    if body.startswith("\n"):
+        return f"---\n{spliced}{body}"
+    return f"---\n{spliced}\n{body}"
 
 # --- End copied functions ---
 
 
 class TestParseYamlFrontmatter:
     def test_basic(self):
-        text = "---\ntitle: Test Claim\nsupports:\n  - claim-a\n---\nBody text here."
-        fm, body = _parse_yaml_frontmatter(text)
+        text = "---\ntitle: Test Claim\nsupports:\n- claim-a\n---\nBody text here."
+        fm, raw, body = _parse_yaml_frontmatter(text)
         assert fm is not None
         assert fm["title"] == "Test Claim"
         assert fm["supports"] == ["claim-a"]
         assert body.startswith("\n---")
+        assert "title: Test Claim" in raw
 
     def test_no_frontmatter(self):
         text = "Just plain text"
-        fm, body = _parse_yaml_frontmatter(text)
+        fm, raw, body = _parse_yaml_frontmatter(text)
         assert fm is None
+        assert raw == ""
         assert body == text
 
     def test_malformed_yaml(self):
         text = "---\n: invalid: yaml: {{{\n---\nBody"
-        fm, body = _parse_yaml_frontmatter(text)
+        fm, raw, body = _parse_yaml_frontmatter(text)
         assert fm is None
 
 
@@ -110,18 +155,90 @@ class TestUnionEdgeLists:
         assert result == ["claim-a", "claim-b"]
 
 
-class TestSerializeFrontmatter:
-    def test_roundtrip(self):
-        fm = {"title": "Test", "supports": ["claim-a", "claim-b"]}
-        body = "\n---\nBody text here."
-        text = _serialize_frontmatter(fm, body)
-        assert text.startswith("---\n")
-        assert "title: Test" in text
-        assert "Body text here." in text
+class TestSerializeEdgeFields:
+    def test_replaces_existing_field(self):
+        raw = "title: Test\nsupports:\n- old-claim"
+        merged = {"supports": ["old-claim", "new-claim"]}
+        result = _serialize_edge_fields(raw, merged)
+        assert "- old-claim" in result
+        assert "- new-claim" in result
+        assert "title: Test" in result
 
-        fm2, body2 = _parse_yaml_frontmatter(text)
-        assert fm2["title"] == "Test"
-        assert fm2["supports"] == ["claim-a", "claim-b"]
+    def test_preserves_non_edge_fields_exactly(self):
+        raw = "title: 'Quoted Title'\nconfidence: 0.85\ntype: claim"
+        merged = {"related": ["new-claim"]}
+        result = _serialize_edge_fields(raw, merged)
+        assert "title: 'Quoted Title'" in result
+        assert "confidence: 0.85" in result
+        assert "type: claim" in result
+        assert "related:" in result
+        assert "- new-claim" in result
+
+    def test_appends_new_field(self):
+        raw = "title: Test\ntype: claim"
+        merged = {"supports": ["claim-a"]}
+        result = _serialize_edge_fields(raw, merged)
+        assert "title: Test" in result
+        assert "supports:" in result
+        assert "- claim-a" in result
+
+    def test_empty_edges_removes_field(self):
+        raw = "title: Test\nsupports:\n- old-claim\ntype: claim"
+        merged = {}  # no edges to write
+        result = _serialize_edge_fields(raw, merged)
+        assert "supports:" not in result
+        assert "title: Test" in result
+        assert "type: claim" in result
+
+    def test_multiple_edge_fields(self):
+        raw = "title: Test\nsupports:\n- a\nchallenges:\n- b"
+        merged = {"supports": ["a", "c"], "challenges": ["b", "d"]}
+        result = _serialize_edge_fields(raw, merged)
+        lines = result.split("\n")
+        # supports and challenges both present with merged values
+        assert "- a" in result
+        assert "- c" in result
+        assert "- b" in result
+        assert "- d" in result
+
+
+class TestSerializeFrontmatter:
+    def test_roundtrip_preserves_formatting(self):
+        original = "---\ntitle: 'Quoted Title'\nconfidence: 0.85\nsupports:\n- claim-a\n---\nBody text here."
+        fm, raw, body = _parse_yaml_frontmatter(original)
+        merged_edges = {"supports": ["claim-a", "claim-b"]}
+        result = _serialize_frontmatter(raw, merged_edges, body)
+
+        # Non-edge fields preserved exactly
+        assert "title: 'Quoted Title'" in result
+        assert "confidence: 0.85" in result
+        # Edge fields updated
+        assert "- claim-a" in result
+        assert "- claim-b" in result
+        # Structure preserved
+        assert result.startswith("---\n")
+        assert "\n---\n" in result
+        assert result.endswith("Body text here.")
+
+    def test_no_blank_line_before_closing_delimiter(self):
+        """Ganymede critical: no extra blank line compounds on repeat reweaves."""
+        original = "---\ntitle: Test\nsupports:\n- a\n---\nBody."
+        fm, raw, body = _parse_yaml_frontmatter(original)
+        merged_edges = {"supports": ["a", "b"]}
+        result = _serialize_frontmatter(raw, merged_edges, body)
+        # Should NOT have \n\n--- (double newline before closing)
+        assert "\n\n---" not in result
+
+    def test_repeated_serialize_no_drift(self):
+        """Repeated serialization should be idempotent — no accumulating blank lines."""
+        text = "---\ntitle: Test\nsupports:\n- a\n---\nBody."
+        merged_edges = {"supports": ["a", "b"]}
+
+        for _ in range(5):
+            fm, raw, body = _parse_yaml_frontmatter(text)
+            text = _serialize_frontmatter(raw, merged_edges, body)
+
+        assert text.count("\n\n") == 0  # no double newlines anywhere
 
 
 class TestSupersetDetection:
@@ -155,22 +272,22 @@ class TestFullUnionWorkflow:
         """Main got new edges after branch was created. Union includes both."""
         main_text = (
             "---\ntitle: Test Claim\nconfidence: 0.8\n"
-            "supports:\n  - claim-a\n  - claim-b\n"
-            "related:\n  - claim-x\n"
+            "supports:\n- claim-a\n- claim-b\n"
+            "related:\n- claim-x\n"
             "---\nBody text."
         )
         branch_text = (
             "---\ntitle: Test Claim\nconfidence: 0.8\n"
-            "supports:\n  - claim-a\n"
-            "related:\n  - claim-x\n  - claim-y\n"
-            "reweave_edges:\n  - \"claim-y|related|2026-04-04\"\n"
+            "supports:\n- claim-a\n"
+            "related:\n- claim-x\n- claim-y\n"
+            "reweave_edges:\n- \"claim-y|related|2026-04-04\"\n"
             "---\nBody text."
         )
 
-        main_fm, main_body = _parse_yaml_frontmatter(main_text)
-        branch_fm, _ = _parse_yaml_frontmatter(branch_text)
+        main_fm, main_raw, main_body = _parse_yaml_frontmatter(main_text)
+        branch_fm, _, _ = _parse_yaml_frontmatter(branch_text)
 
-        merged_fm = dict(main_fm)
+        merged_edges = {}
         for field in REWEAVE_EDGE_FIELDS:
             main_list = main_fm.get(field, [])
             branch_list = branch_fm.get(field, [])
@@ -179,23 +296,27 @@ class TestFullUnionWorkflow:
             if not isinstance(branch_list, list):
                 branch_list = [branch_list] if branch_list else []
             if main_list or branch_list:
-                merged_fm[field] = _union_edge_lists(main_list, branch_list)
+                merged_edges[field] = _union_edge_lists(main_list, branch_list)
 
-        assert merged_fm["supports"] == ["claim-a", "claim-b"]
-        assert "claim-x" in merged_fm["related"]
-        assert "claim-y" in merged_fm["related"]
-        assert len(merged_fm.get("reweave_edges", [])) == 1
-        assert merged_fm["confidence"] == 0.8
+        assert merged_edges["supports"] == ["claim-a", "claim-b"]
+        assert "claim-x" in merged_edges["related"]
+        assert "claim-y" in merged_edges["related"]
+        assert len(merged_edges.get("reweave_edges", [])) == 1
+
+        # Verify non-edge fields preserved in serialization
+        result = _serialize_frontmatter(main_raw, merged_edges, main_body)
+        assert "confidence: 0.8" in result
+        assert "title: Test Claim" in result
 
     def test_no_edge_fields_untouched(self):
         """Non-edge fields (title, confidence, type) come from main unchanged."""
         main_text = "---\ntitle: Original\nconfidence: 0.9\ntype: claim\n---\nBody."
-        branch_text = "---\ntitle: Original\nconfidence: 0.9\ntype: claim\nrelated:\n  - new-claim\n---\nBody."
+        branch_text = "---\ntitle: Original\nconfidence: 0.9\ntype: claim\nrelated:\n- new-claim\n---\nBody."
 
-        main_fm, main_body = _parse_yaml_frontmatter(main_text)
-        branch_fm, _ = _parse_yaml_frontmatter(branch_text)
+        main_fm, main_raw, main_body = _parse_yaml_frontmatter(main_text)
+        branch_fm, _, _ = _parse_yaml_frontmatter(branch_text)
 
-        merged_fm = dict(main_fm)
+        merged_edges = {}
         for field in REWEAVE_EDGE_FIELDS:
             main_list = main_fm.get(field, [])
             branch_list = branch_fm.get(field, [])
@@ -204,12 +325,13 @@ class TestFullUnionWorkflow:
             if not isinstance(branch_list, list):
                 branch_list = [branch_list] if branch_list else []
             if main_list or branch_list:
-                merged_fm[field] = _union_edge_lists(main_list, branch_list)
+                merged_edges[field] = _union_edge_lists(main_list, branch_list)
 
-        assert merged_fm["title"] == "Original"
-        assert merged_fm["confidence"] == 0.9
-        assert merged_fm["type"] == "claim"
-        assert merged_fm["related"] == ["new-claim"]
+        result = _serialize_frontmatter(main_raw, merged_edges, main_body)
+        assert "title: Original" in result
+        assert "confidence: 0.9" in result
+        assert "type: claim" in result
+        assert "- new-claim" in result
 
     def test_scalar_edge_field_converted_to_list(self):
         """Edge fields stored as scalars (not lists) are handled gracefully."""
@@ -225,3 +347,16 @@ class TestFullUnionWorkflow:
 
         result = _union_edge_lists(main_list, branch_list)
         assert result == ["single-claim", "new-claim"]
+
+    def test_yaml_formatting_preserved_across_reweave(self):
+        """The key test: non-edge YAML formatting stays byte-identical."""
+        # Use unusual but valid YAML formatting
+        main_text = "---\ntitle: 'A \"Quoted\" Title'\nconfidence: 0.85\nsome_custom_field: [1, 2, 3]\nsupports:\n- old-claim\n---\nBody."
+        fm, raw, body = _parse_yaml_frontmatter(main_text)
+        merged_edges = {"supports": ["old-claim", "new-claim"]}
+        result = _serialize_frontmatter(raw, merged_edges, body)
+
+        # These non-edge fields must be byte-identical to source
+        assert "title: 'A \"Quoted\" Title'" in result
+        assert "confidence: 0.85" in result
+        assert "some_custom_field: [1, 2, 3]" in result