diff --git a/examples/cli/README.md b/examples/cli/README.md
index 6be5a1b..44b8857 100644
--- a/examples/cli/README.md
+++ b/examples/cli/README.md
@@ -297,7 +297,7 @@ Expand from one or more seed entries:
 .venv/bin/python -m citegeist --db library.sqlite3 expand langton1989artificial1
 ```
 
-Choose the source:
+Choose the graph expansion source:
 
 ```bash
 .venv/bin/python -m citegeist --db library.sqlite3 expand langton1989artificial1 --source openalex
@@ -333,12 +333,14 @@ Override the topic phrase:
 .venv/bin/python -m citegeist --db library.sqlite3 expand-topic artificial-life --topic-phrase "artificial life alife artificial organisms"
 ```
 
-Choose source and relation:
+Choose graph expansion source and relation:
 
 ```bash
 .venv/bin/python -m citegeist --db library.sqlite3 expand-topic artificial-life --source openalex --relation cited_by
 ```
 
+Note: graph expansion currently supports `openalex` and `crossref`. Metadata resolution, verification, and bootstrap topic seeding also use `datacite` and `pubmed`.
+
 Control seed and discovery limits:
 
 ```bash
diff --git a/examples/literature-explorer/README.md b/examples/literature-explorer/README.md
index 039d7de..7ecb44a 100644
--- a/examples/literature-explorer/README.md
+++ b/examples/literature-explorer/README.md
@@ -45,6 +45,11 @@ The adapter exposes JSON-serializable methods suitable for a local web bridge:
 - `verify_bibtex(bibtex_text, context="", limit=5)`
 - `graph(seed_keys, relation_types=None, depth=1, review_status=None, missing_only=False)`
 
+The `capabilities()` payload also distinguishes between broader metadata/search sources and narrower graph-expansion sources:
+
+- metadata and topic seeding currently use `crossref`, `datacite`, `openalex`, and `pubmed`
+- topic graph expansion currently uses `crossref` and `openalex`
+
 ## Browser Contract
 
 A demo app can expose a browser object like:
diff --git a/examples/literature-explorer/index.html b/examples/literature-explorer/index.html
index d555117..0f60b0a 100644
--- a/examples/literature-explorer/index.html
+++ b/examples/literature-explorer/index.html
@@ -561,7 +561,7 @@
           </label>
           <div class="row-3">
             <label>
-              Source
+              Expansion Source
               <select id="expand-source">
                 <option value="openalex">openalex</option>
                 <option value="crossref">crossref</option>
@@ -611,6 +611,7 @@
           <div id="expand-summary" class="summary-box">
             <strong>Expansion Policy</strong>
             <p>Use <code>cites</code> to bias toward newer work, or <code>both</code> for broader graph growth. Recursive rounds stop once the recent-entry target is met.</p>
+            <p id="expand-source-note">Topic graph expansion currently supports openalex and crossref. Metadata search and bootstrap seeding also use datacite and pubmed.</p>
           </div>
         </section>
 
@@ -827,6 +828,7 @@
       expandPreviewButton: document.getElementById("expand-preview-button"),
       expandCommitButton: document.getElementById("expand-commit-button"),
       expandSummary: document.getElementById("expand-summary"),
+      expandSourceNote: document.getElementById("expand-source-note"),
       searchQuery: document.getElementById("search-query"),
       searchTopic: document.getElementById("search-topic"),
       searchButton: document.getElementById("search-button"),
@@ -1081,6 +1083,43 @@
       `;
     }
 
+    function populateSelectOptions(select, values, preferredValue) {
+      if (!select || !Array.isArray(values) || !values.length) return;
+      const previousValue = select.value;
+      select.innerHTML = "";
+      values.forEach((value) => {
+        const option = document.createElement("option");
+        option.value = value;
+        option.textContent = value;
+        select.appendChild(option);
+      });
+      if (values.includes(previousValue)) {
+        select.value = previousValue;
+      } else if (values.includes(preferredValue)) {
+        select.value = preferredValue;
+      } else {
+        select.value = values[0];
+      }
+    }
+
+    function applyCapabilities(capabilities) {
+      const topicExpansionSources = capabilities?.topic_expansion_sources || capabilities?.graph_expansion_sources || [];
+      const relationTypes = capabilities?.graph_relation_types || [];
+      const metadataSources = capabilities?.metadata_sources || [];
+      if (topicExpansionSources.length) {
+        populateSelectOptions(els.expandSource, topicExpansionSources, "openalex");
+      }
+      if (relationTypes.length) {
+        populateSelectOptions(els.expandRelation, relationTypes, "cites");
+      }
+      if (els.expandSourceNote) {
+        const expansionText = topicExpansionSources.length ? topicExpansionSources.join(", ") : "openalex, crossref";
+        const metadataText = metadataSources.length ? metadataSources.join(", ") : "crossref, datacite, openalex, pubmed";
+        els.expandSourceNote.textContent =
+          `Topic graph expansion currently supports ${expansionText}. Metadata search and bootstrap seeding also use ${metadataText}.`;
+      }
+    }
+
     async function connect() {
       setBusy(els.connectButton, true);
       try {
@@ -1092,6 +1131,7 @@
         const client = createLiteratureExplorerClient(bridge);
         const capabilities = await client.capabilities();
         state.client = client;
+        applyCapabilities(capabilities);
         setStatus(`Connected to ${state.bridgeUrl}`, "ok");
         setLastOp("connect");
         logActivity("capabilities", capabilities);
diff --git a/src/citegeist/app_api.py b/src/citegeist/app_api.py
index a3ae29b..163f805 100644
--- a/src/citegeist/app_api.py
+++ b/src/citegeist/app_api.py
@@ -9,6 +9,10 @@ from .extract import extract_references
 from .storage import BibliographyStore
 from .verify import BibliographyVerifier
 
+METADATA_SOURCES = ["crossref", "datacite", "openalex", "pubmed"]
+GRAPH_EXPANSION_SOURCES = ["crossref", "openalex"]
+GRAPH_RELATION_TYPES = ["cites", "cited_by", "both"]
+
 
 class LiteratureExplorerApi:
     """JSON-serializable adapter layer for browser or local UI bridges."""
@@ -41,6 +45,11 @@ class LiteratureExplorerApi:
                 "graph",
             ],
             "preview_operations": ["bootstrap", "expand_topic"],
+            "metadata_sources": list(METADATA_SOURCES),
+            "topic_seed_sources": list(METADATA_SOURCES),
+            "graph_expansion_sources": list(GRAPH_EXPANSION_SOURCES),
+            "topic_expansion_sources": list(GRAPH_EXPANSION_SOURCES),
+            "graph_relation_types": list(GRAPH_RELATION_TYPES),
         }
 
     def search(self, query: str, *, limit: int = 20, topic_slug: str | None = None) -> dict[str, object]:
diff --git a/src/citegeist/cli.py b/src/citegeist/cli.py
index 201d4df..c4f440e 100644
--- a/src/citegeist/cli.py
+++ b/src/citegeist/cli.py
@@ -198,7 +198,7 @@ def build_parser() -> argparse.ArgumentParser:
         "--source",
         choices=["crossref", "openalex"],
         default="crossref",
-        help="External source used for graph expansion",
+        help="Graph expansion source",
     )
     expand_parser.add_argument(
         "--relation",
@@ -221,7 +221,7 @@ def build_parser() -> argparse.ArgumentParser:
         "--source",
         choices=["crossref", "openalex"],
         default="openalex",
-        help="External source used for topic expansion",
+        help="Topic graph expansion source",
     )
     expand_topic_parser.add_argument(
         "--relation",
diff --git a/tests/test_app_api.py b/tests/test_app_api.py
index 6b9dd45..4f9c6ad 100644
--- a/tests/test_app_api.py
+++ b/tests/test_app_api.py
@@ -107,6 +107,22 @@ def test_literature_explorer_api_search_and_show_entry():
         store.close()
 
 
+def test_literature_explorer_api_capabilities_distinguish_metadata_and_expansion_sources():
+    store = BibliographyStore()
+    try:
+        api = LiteratureExplorerApi(store)
+
+        payload = api.capabilities()
+
+        assert payload["metadata_sources"] == ["crossref", "datacite", "openalex", "pubmed"]
+        assert payload["topic_seed_sources"] == ["crossref", "datacite", "openalex", "pubmed"]
+        assert payload["graph_expansion_sources"] == ["crossref", "openalex"]
+        assert payload["topic_expansion_sources"] == ["crossref", "openalex"]
+        assert payload["graph_relation_types"] == ["cites", "cited_by", "both"]
+    finally:
+        store.close()
+
+
 def test_literature_explorer_api_bootstrap_returns_topic_payload():
     store = BibliographyStore()
     try: