diff --git a/graphify/build.py b/graphify/build.py
index dd21f6f2..a0dbcae0 100644
--- a/graphify/build.py
+++ b/graphify/build.py
@@ -30,6 +30,26 @@ import networkx as nx
 from .validate import validate_extraction
 
 
+# Synonym mapper for known invalid file_type values that LLM subagents commonly
+# emit. Keeps semantic intent close (markdown→document, tool→code) and falls
+# back to "concept" for any other invalid value (see #840).
+_FILE_TYPE_SYNONYMS = {
+    "markdown": "document",
+    "text": "document",
+    "tool": "code",
+    "library": "code",
+    "pattern": "concept",
+    "principle": "concept",
+    "constraint": "concept",
+    "tech": "concept",
+    "technology": "concept",
+    "data-source": "concept",
+    "data_source": "concept",
+    "gotcha": "concept",
+    "framework": "concept",
+}
+
+
 def _normalize_id(s: str) -> str:
     r"""Normalize an ID string the same way extract._make_id does.
 
@@ -105,6 +125,9 @@ def build_from_json(extraction: dict, *, directed: bool = False) -> nx.Graph:
         # trigger spurious "invalid file_type 'None'" validator warnings (#660).
         if node.get("file_type") in (None, ""):
             node["file_type"] = "concept"
+        ft = node.get("file_type", "")
+        if ft and ft not in {"code", "document", "paper", "image", "rationale", "concept"}:
+            node["file_type"] = _FILE_TYPE_SYNONYMS.get(ft, "concept")
 
     errors = validate_extraction(extraction)
     # Dangling edges (stdlib/external imports) are expected - only warn about real schema errors.
diff --git a/graphify/llm.py b/graphify/llm.py
index 26787ed3..c1932697 100644
--- a/graphify/llm.py
+++ b/graphify/llm.py
@@ -122,7 +122,7 @@ Node ID format: lowercase, only [a-z0-9_], no dots or slashes.
 Format: {stem}_{entity} where stem = filename without extension, entity = symbol name (both normalised).
 
 Output exactly this schema:
-{"nodes":[{"id":"stem_entity","label":"Human Readable Name","file_type":"code|document|paper|image|concept","source_file":"relative/path","source_location":null,"source_url":null,"captured_at":null,"author":null,"contributor":null}],"edges":[{"source":"node_id","target":"node_id","relation":"calls|implements|references|cites|conceptually_related_to|shares_data_with|semantically_similar_to","confidence":"EXTRACTED|INFERRED|AMBIGUOUS","confidence_score":1.0,"source_file":"relative/path","source_location":null,"weight":1.0}],"hyperedges":[],"input_tokens":0,"output_tokens":0}
+{"nodes":[{"id":"stem_entity","label":"Human Readable Name","file_type":"code|document|paper|image|rationale|concept","source_file":"relative/path","source_location":null,"source_url":null,"captured_at":null,"author":null,"contributor":null}],"edges":[{"source":"node_id","target":"node_id","relation":"calls|implements|references|cites|conceptually_related_to|shares_data_with|semantically_similar_to","confidence":"EXTRACTED|INFERRED|AMBIGUOUS","confidence_score":1.0,"source_file":"relative/path","source_location":null,"weight":1.0}],"hyperedges":[],"input_tokens":0,"output_tokens":0}
 """
 
 
diff --git a/graphify/skill.md b/graphify/skill.md
index edee6975..9e7f5d26 100644
--- a/graphify/skill.md
+++ b/graphify/skill.md
@@ -315,7 +315,7 @@ Rules:
 
 Code files: focus on semantic edges AST cannot find (call relationships, shared data, arch patterns).
   Do not re-extract imports - AST already has those.
-Doc/paper files: extract named concepts, entities, citations. For rationale (WHY decisions were made, trade-offs, design intent): store as a `rationale` attribute on the relevant concept node — do NOT create a separate rationale node or fragment node. Only create a node for something that is itself a named entity or concept. Use `file_type:"rationale"` for concept-like nodes (ideas, principles, mechanisms, design patterns). Do NOT invent file_types like `concept` — valid values are only `code|document|paper|image|rationale`.
+Doc/paper files: extract named concepts, entities, citations. For rationale (WHY decisions were made, trade-offs, design intent): store as a `rationale` attribute on the relevant concept node — do NOT create a separate rationale node or fragment node. Only create a node for something that is itself a named entity or concept. Use `file_type:"rationale"` for concept-like nodes (ideas, principles, mechanisms, design patterns). `file_type` MUST be one of exactly these six values: `code`, `document`, `paper`, `image`, `rationale`, `concept`. Any other value is invalid and will be rejected.
 Code files: when adding `calls` edges, source MUST be the caller (the function/class doing the calling), target MUST be the callee. Never reverse this direction.
 Image files: use vision to understand what the image IS - do not just OCR.
   UI screenshot: layout patterns, design decisions, key elements, purpose.
@@ -360,7 +360,7 @@ confidence_score is REQUIRED on every edge - never omit it, never use 0.5 as a d
 Node ID format: lowercase, only `[a-z0-9_]`, no dots or slashes. Format: `{stem}_{entity}` where stem is the filename without extension and entity is the symbol name, both normalized (lowercase, non-alphanumeric chars replaced with `_`). Example: `src/auth/session.py` + `ValidateToken` → `session_validatetoken`. This must match the ID the AST extractor generates so cross-references between code and semantic nodes connect correctly. CRITICAL: never append chunk numbers, sequence numbers, or any suffix to an ID (no `_c1`, `_c2`, `_chunk2`, etc.). IDs must be deterministic from the label alone — the same entity must always produce the same ID regardless of which chunk processes it.
 
 Generate the extraction JSON matching this schema exactly:
-{"nodes":[{"id":"session_validatetoken","label":"Human Readable Name","file_type":"code|document|paper|image|rationale","source_file":"relative/path","source_location":null,"source_url":null,"captured_at":null,"author":null,"contributor":null}],"edges":[{"source":"node_id","target":"node_id","relation":"calls|implements|references|cites|conceptually_related_to|shares_data_with|semantically_similar_to|rationale_for","confidence":"EXTRACTED|INFERRED|AMBIGUOUS","confidence_score":1.0,"source_file":"relative/path","source_location":null,"weight":1.0}],"hyperedges":[{"id":"snake_case_id","label":"Human Readable Label","nodes":["node_id1","node_id2","node_id3"],"relation":"participate_in|implement|form","confidence":"EXTRACTED|INFERRED","confidence_score":0.75,"source_file":"relative/path"}],"input_tokens":0,"output_tokens":0}
+{"nodes":[{"id":"session_validatetoken","label":"Human Readable Name","file_type":"code|document|paper|image|rationale|concept","source_file":"relative/path","source_location":null,"source_url":null,"captured_at":null,"author":null,"contributor":null}],"edges":[{"source":"node_id","target":"node_id","relation":"calls|implements|references|cites|conceptually_related_to|shares_data_with|semantically_similar_to|rationale_for","confidence":"EXTRACTED|INFERRED|AMBIGUOUS","confidence_score":1.0,"source_file":"relative/path","source_location":null,"weight":1.0}],"hyperedges":[{"id":"snake_case_id","label":"Human Readable Label","nodes":["node_id1","node_id2","node_id3"],"relation":"participate_in|implement|form","confidence":"EXTRACTED|INFERRED","confidence_score":0.75,"source_file":"relative/path"}],"input_tokens":0,"output_tokens":0}
 
 Then write the JSON to disk using the Write tool at this exact absolute path (no relative paths — Write resolves relative paths against an undefined cwd and the file will be silently lost):
 CHUNK_PATH
@@ -672,7 +672,9 @@ from graphify.detect import save_manifest
 
 # Save manifest for --update
 detect = json.loads(Path('graphify-out/.graphify_detect.json').read_text(encoding=\"utf-8\"))
-save_manifest(detect['files'])
+# In --update mode, 'all_files' carries the full corpus; 'files' is the changed
+# subset. Full-rebuild mode populates only 'files', so the fallback handles that.
+save_manifest(detect.get('all_files') or detect['files'])
 
 # Update cumulative cost tracker
 extract = json.loads(Path('graphify-out/.graphify_extract.json').read_text(encoding=\"utf-8\"))
@@ -772,6 +774,24 @@ print(f'{new_total} new/changed file(s) to re-extract.')
 "
 ```
 
+Then populate `.graphify_detect.json` so Steps 3A–6 (which read it unconditionally) see the right state for an incremental run. `files` carries the changed subset (drives Step 3A AST + Step 3B0 cache check on only what changed); `all_files` carries the full corpus for any step that needs corpus-wide context:
+
+```bash
+$(cat graphify-out/.graphify_python) -c "
+import json
+from pathlib import Path
+r = json.loads(Path('graphify-out/.graphify_incremental.json').read_text(encoding=\"utf-8\"))
+Path('graphify-out/.graphify_detect.json').write_text(json.dumps({
+    'files': r.get('new_files', {}),
+    'all_files': r.get('files', {}),
+    'total_files': r.get('new_total', 0),
+    'total_words': r.get('total_words', 0),
+    'skipped_sensitive': r.get('skipped_sensitive', []),
+    'needs_graph': True,
+}, ensure_ascii=False), encoding=\"utf-8\")
+"
+```
+
 If new files exist, first check whether all changed files are code files:
 
 ```bash
diff --git a/tests/test_build.py b/tests/test_build.py
index 95acc1d4..54497b46 100644
--- a/tests/test_build.py
+++ b/tests/test_build.py
@@ -116,8 +116,9 @@ def test_missing_file_type_defaults_to_concept(capsys):
     assert G.nodes["n1"]["file_type"] == "concept"
 
 
-def test_real_invalid_file_type_still_warns(capsys):
-    """Truly invalid file_type values (not None, not empty) must still warn."""
+def test_real_invalid_file_type_coerced_to_concept():
+    """Unknown file_type values are coerced through the synonym mapper, falling
+    back to 'concept' for anything that isn't a known LLM synonym (#840)."""
     ext = {
         "nodes": [
             {"id": "n1", "label": "Bad", "file_type": "weird_type", "source_file": "a.py"},
@@ -126,10 +127,26 @@ def test_real_invalid_file_type_still_warns(capsys):
         "input_tokens": 0,
         "output_tokens": 0,
     }
-    build_from_json(ext)
-    err = capsys.readouterr().err
-    assert "invalid file_type" in err
-    assert "weird_type" in err
+    G = build_from_json(ext)
+    assert G.nodes["n1"]["file_type"] == "concept"
+
+
+def test_file_type_synonym_mapping():
+    """Known invalid file_type values map to their canonical equivalents."""
+    ext = {
+        "nodes": [
+            {"id": "n1", "label": "MD", "file_type": "markdown", "source_file": "a.md"},
+            {"id": "n2", "label": "Tool", "file_type": "tool", "source_file": "b.py"},
+            {"id": "n3", "label": "Pat", "file_type": "pattern", "source_file": "c.md"},
+        ],
+        "edges": [],
+        "input_tokens": 0,
+        "output_tokens": 0,
+    }
+    G = build_from_json(ext)
+    assert G.nodes["n1"]["file_type"] == "document"
+    assert G.nodes["n2"]["file_type"] == "code"
+    assert G.nodes["n3"]["file_type"] == "concept"
 
 
 def test_build_merge_preserves_call_edge_direction(tmp_path):