diff --git a/dev-tools/tests/triples/load_test_triples.py b/dev-tools/tests/triples/load_test_triples.py
new file mode 100755
index 00000000..a147d041
--- /dev/null
+++ b/dev-tools/tests/triples/load_test_triples.py
@@ -0,0 +1,227 @@
+#!/usr/bin/env python3
+"""
+Load test triples into the triple store for testing tg-query-graph.
+
+Tests all graph features:
+- SPO with IRI objects
+- SPO with literal objects
+- Literals with XML datatypes
+- Literals with language tags
+- Quoted triples (RDF-star)
+- Named graphs
+"""
+
+import asyncio
+import json
+import os
+import websockets
+
+# Configuration
+API_URL = os.getenv("TRUSTGRAPH_URL", "http://localhost:8088/")
+TOKEN = os.getenv("TRUSTGRAPH_TOKEN", None)
+FLOW = "default"
+USER = "trustgraph"
+COLLECTION = "default"
+DOCUMENT_ID = "test-triples-001"
+
+# Namespaces
+EX = "http://example.org/"
+RDF = "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+RDFS = "http://www.w3.org/2000/01/rdf-schema#"
+XSD = "http://www.w3.org/2001/XMLSchema#"
+TG = "https://trustgraph.ai/ns/"
+
+
+def iri(value):
+    """Build IRI term."""
+    return {"t": "i", "i": value}
+
+
+def literal(value, datatype=None, language=None):
+    """Build literal term with optional datatype or language."""
+    term = {"t": "l", "v": value}
+    if datatype:
+        term["dt"] = datatype
+    if language:
+        term["ln"] = language
+    return term
+
+
+def quoted_triple(s, p, o):
+    """Build quoted triple term (RDF-star)."""
+    return {
+        "t": "t",
+        "tr": {"s": s, "p": p, "o": o}
+    }
+
+
+def triple(s, p, o, g=None):
+    """Build a complete triple dict."""
+    t = {"s": s, "p": p, "o": o}
+    if g:
+        t["g"] = g
+    return t
+
+
+# Test triples covering all features
+TEST_TRIPLES = [
+    # 1. Basic SPO with IRI object
+    triple(
+        iri(f"{EX}marie-curie"),
+        iri(f"{RDF}type"),
+        iri(f"{EX}Scientist")
+    ),
+
+    # 2. SPO with IRI object (relationship)
+    triple(
+        iri(f"{EX}marie-curie"),
+        iri(f"{EX}discovered"),
+        iri(f"{EX}radium")
+    ),
+
+    # 3. Simple literal (no datatype/language)
+    triple(
+        iri(f"{EX}marie-curie"),
+        iri(f"{RDFS}label"),
+        literal("Marie Curie")
+    ),
+
+    # 4. Literal with language tag (English)
+    triple(
+        iri(f"{EX}marie-curie"),
+        iri(f"{RDFS}label"),
+        literal("Marie Curie", language="en")
+    ),
+
+    # 5. Literal with language tag (French)
+    triple(
+        iri(f"{EX}marie-curie"),
+        iri(f"{RDFS}label"),
+        literal("Marie Curie", language="fr")
+    ),
+
+    # 6. Literal with language tag (Polish)
+    triple(
+        iri(f"{EX}marie-curie"),
+        iri(f"{RDFS}label"),
+        literal("Maria Sk\u0142odowska-Curie", language="pl")
+    ),
+
+    # 7. Literal with xsd:integer datatype
+    triple(
+        iri(f"{EX}marie-curie"),
+        iri(f"{EX}birthYear"),
+        literal("1867", datatype=f"{XSD}integer")
+    ),
+
+    # 8. Literal with xsd:date datatype
+    triple(
+        iri(f"{EX}marie-curie"),
+        iri(f"{EX}birthDate"),
+        literal("1867-11-07", datatype=f"{XSD}date")
+    ),
+
+    # 9. Literal with xsd:boolean datatype
+    triple(
+        iri(f"{EX}marie-curie"),
+        iri(f"{EX}nobelLaureate"),
+        literal("true", datatype=f"{XSD}boolean")
+    ),
+
+    # 10. Quoted triple in object position (RDF 1.2 style)
+    # "Wikipedia asserts that Marie Curie discovered radium"
+    triple(
+        iri(f"{EX}wikipedia"),
+        iri(f"{TG}asserts"),
+        quoted_triple(
+            iri(f"{EX}marie-curie"),
+            iri(f"{EX}discovered"),
+            iri(f"{EX}radium")
+        )
+    ),
+
+    # 11. Quoted triple with literal inside (object position)
+    # "NLP-v1.0 extracted that Marie Curie has label Marie Curie"
+    triple(
+        iri(f"{EX}nlp-v1"),
+        iri(f"{TG}extracted"),
+        quoted_triple(
+            iri(f"{EX}marie-curie"),
+            iri(f"{RDFS}label"),
+            literal("Marie Curie")
+        )
+    ),
+
+    # 12. Triple in a named graph (g is plain string, not Term)
+    triple(
+        iri(f"{EX}radium"),
+        iri(f"{RDF}type"),
+        iri(f"{EX}Element"),
+        g=f"{EX}chemistry-graph"
+    ),
+
+    # 13. Another triple in the same named graph
+    triple(
+        iri(f"{EX}radium"),
+        iri(f"{EX}atomicNumber"),
+        literal("88", datatype=f"{XSD}integer"),
+        g=f"{EX}chemistry-graph"
+    ),
+
+    # 14. Triple in a different named graph
+    triple(
+        iri(f"{EX}pierre-curie"),
+        iri(f"{EX}spouseOf"),
+        iri(f"{EX}marie-curie"),
+        g=f"{EX}biography-graph"
+    ),
+]
+
+
+async def load_triples():
+    """Load test triples via WebSocket bulk import."""
+    # Convert HTTP URL to WebSocket URL
+    ws_url = API_URL.replace("http://", "ws://").replace("https://", "wss://")
+    ws_url = f"{ws_url.rstrip('/')}/api/v1/flow/{FLOW}/import/triples"
+    if TOKEN:
+        ws_url = f"{ws_url}?token={TOKEN}"
+
+    metadata = {
+        "id": DOCUMENT_ID,
+        "metadata": [],
+        "user": USER,
+        "collection": COLLECTION
+    }
+
+    print(f"Connecting to {ws_url}...")
+    async with websockets.connect(ws_url, ping_interval=20, ping_timeout=60) as websocket:
+        message = {
+            "metadata": metadata,
+            "triples": TEST_TRIPLES
+        }
+        print(f"Sending {len(TEST_TRIPLES)} test triples...")
+        await websocket.send(json.dumps(message))
+        print("Triples sent successfully!")
+
+    print("\nTest triples loaded:")
+    print("  - 2 basic IRI triples (type, relationship)")
+    print("  - 4 literal triples (plain + 3 languages: en, fr, pl)")
+    print("  - 3 typed literal triples (xsd:integer, xsd:date, xsd:boolean)")
+    print("  - 2 quoted triples (RDF-star provenance)")
+    print("  - 3 triples in named graphs (chemistry-graph, biography-graph)")
+    print(f"\nTotal: {len(TEST_TRIPLES)} triples")
+    print(f"User: {USER}, Collection: {COLLECTION}")
+
+
+def main():
+    print("Loading test triples for tg-query-graph testing\n")
+    asyncio.run(load_triples())
+    print("\nDone! Now test with:")
+    print("  tg-query-graph -s http://example.org/marie-curie")
+    print("  tg-query-graph -p http://www.w3.org/2000/01/rdf-schema#label")
+    print("  tg-query-graph -o 'Marie Curie' --object-language en")
+    print("  tg-query-graph --format json | jq .")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tests/unit/test_agent/test_callback_message_id.py b/tests/unit/test_agent/test_callback_message_id.py
new file mode 100644
index 00000000..7cb0ee54
--- /dev/null
+++ b/tests/unit/test_agent/test_callback_message_id.py
@@ -0,0 +1,122 @@
+"""
+Tests that streaming callbacks set message_id on AgentResponse.
+"""
+
+import pytest
+from unittest.mock import AsyncMock, MagicMock
+
+from trustgraph.agent.orchestrator.pattern_base import PatternBase
+from trustgraph.schema import AgentResponse
+
+
+@pytest.fixture
+def pattern():
+    processor = MagicMock()
+    return PatternBase(processor)
+
+
+class TestThinkCallbackMessageId:
+
+    @pytest.mark.asyncio
+    async def test_streaming_think_has_message_id(self, pattern):
+        responses = []
+        async def capture(r):
+            responses.append(r)
+
+        msg_id = "urn:trustgraph:agent:sess/i1/thought"
+        think = pattern.make_think_callback(capture, streaming=True, message_id=msg_id)
+        await think("hello", is_final=False)
+
+        assert len(responses) == 1
+        assert responses[0].message_id == msg_id
+        assert responses[0].chunk_type == "thought"
+
+    @pytest.mark.asyncio
+    async def test_non_streaming_think_has_message_id(self, pattern):
+        responses = []
+        async def capture(r):
+            responses.append(r)
+
+        msg_id = "urn:trustgraph:agent:sess/i1/thought"
+        think = pattern.make_think_callback(capture, streaming=False, message_id=msg_id)
+        await think("hello")
+
+        assert responses[0].message_id == msg_id
+        assert responses[0].end_of_message is True
+
+
+class TestObserveCallbackMessageId:
+
+    @pytest.mark.asyncio
+    async def test_streaming_observe_has_message_id(self, pattern):
+        responses = []
+        async def capture(r):
+            responses.append(r)
+
+        msg_id = "urn:trustgraph:agent:sess/i1/observation"
+        observe = pattern.make_observe_callback(capture, streaming=True, message_id=msg_id)
+        await observe("result", is_final=True)
+
+        assert responses[0].message_id == msg_id
+        assert responses[0].chunk_type == "observation"
+
+
+class TestAnswerCallbackMessageId:
+
+    @pytest.mark.asyncio
+    async def test_streaming_answer_has_message_id(self, pattern):
+        responses = []
+        async def capture(r):
+            responses.append(r)
+
+        msg_id = "urn:trustgraph:agent:sess/final"
+        answer = pattern.make_answer_callback(capture, streaming=True, message_id=msg_id)
+        await answer("the answer")
+
+        assert responses[0].message_id == msg_id
+        assert responses[0].chunk_type == "answer"
+
+    @pytest.mark.asyncio
+    async def test_no_message_id_default(self, pattern):
+        responses = []
+        async def capture(r):
+            responses.append(r)
+
+        answer = pattern.make_answer_callback(capture, streaming=True)
+        await answer("the answer")
+
+        assert responses[0].message_id == ""
+
+
+class TestSendFinalResponseMessageId:
+
+    @pytest.mark.asyncio
+    async def test_streaming_final_has_message_id(self, pattern):
+        responses = []
+        async def capture(r):
+            responses.append(r)
+
+        msg_id = "urn:trustgraph:agent:sess/final"
+        await pattern.send_final_response(
+            capture, streaming=True, answer_text="answer",
+            message_id=msg_id,
+        )
+
+        # Should get content chunk + end-of-dialog marker
+        assert all(r.message_id == msg_id for r in responses)
+
+    @pytest.mark.asyncio
+    async def test_non_streaming_final_has_message_id(self, pattern):
+        responses = []
+        async def capture(r):
+            responses.append(r)
+
+        msg_id = "urn:trustgraph:agent:sess/final"
+        await pattern.send_final_response(
+            capture, streaming=False, answer_text="answer",
+            message_id=msg_id,
+        )
+
+        assert len(responses) == 1
+        assert responses[0].message_id == msg_id
+        assert responses[0].end_of_dialog is True
diff --git a/tests/unit/test_agent/test_explainability_parsing.py b/tests/unit/test_agent/test_explainability_parsing.py
index 7035318d..d75ea604 100644
--- a/tests/unit/test_agent/test_explainability_parsing.py
+++ b/tests/unit/test_agent/test_explainability_parsing.py
@@ -22,6 +22,7 @@ from trustgraph.api.explainability import (
     TG_SYNTHESIS,
     TG_ANSWER_TYPE,
     TG_OBSERVATION_TYPE,
+    TG_TOOL_USE,
     TG_ANALYSIS,
     TG_CONCLUSION,
     TG_DOCUMENT,
@@ -76,6 +77,13 @@ class TestFromTriplesDispatch:
         entity = ExplainEntity.from_triples("urn:a", triples)
         assert isinstance(entity, Analysis)
 
+    def test_dispatches_analysis_with_tooluse(self):
+        """Analysis+ToolUse mixin still dispatches to Analysis."""
+        triples = _make_triples("urn:a",
+                                [PROV_ENTITY, TG_ANALYSIS, TG_TOOL_USE])
+        entity = ExplainEntity.from_triples("urn:a", triples)
+        assert isinstance(entity, Analysis)
+
     def test_dispatches_observation(self):
         triples = _make_triples("urn:o", [PROV_ENTITY, TG_OBSERVATION_TYPE])
         entity = ExplainEntity.from_triples("urn:o", triples)
diff --git a/tests/unit/test_agent/test_on_action_callback.py b/tests/unit/test_agent/test_on_action_callback.py
new file mode 100644
index 00000000..4a1c0c3b
--- /dev/null
+++ b/tests/unit/test_agent/test_on_action_callback.py
@@ -0,0 +1,132 @@
+"""
+Tests for the on_action callback in react() — verifies that it fires
+after action selection but before tool execution.
+"""
+
+import pytest
+from unittest.mock import AsyncMock, MagicMock
+
+from trustgraph.agent.react.agent_manager import AgentManager
+from trustgraph.agent.react.types import Action, Final, Tool, Argument
+
+
+class TestOnActionCallback:
+
+    @pytest.mark.asyncio
+    async def test_on_action_called_for_tool_use(self):
+        """on_action fires when react() selects a tool (not Final)."""
+        call_log = []
+
+        async def fake_on_action(act):
+            call_log.append(("on_action", act.name))
+
+        # Tool that records when it's invoked
+        async def tool_invoke(**kwargs):
+            call_log.append(("tool_invoke",))
+            return "tool result"
+
+        tool_impl = MagicMock()
+        tool_impl.return_value.invoke = AsyncMock(side_effect=tool_invoke)
+
+        tools = {
+            "search": Tool(
+                name="search",
+                description="Search",
+                implementation=tool_impl,
+                arguments=[Argument(name="query", type="string", description="q")],
+                config={},
+            ),
+        }
+
+        agent = AgentManager(tools=tools)
+
+        # Mock reason() to return an Action
+        action = Action(thought="thinking", name="search", arguments={"query": "test"}, observation="")
+        agent.reason = AsyncMock(return_value=action)
+
+        think = AsyncMock()
+        observe = AsyncMock()
+        context = MagicMock()
+
+        await agent.react(
+            question="test",
+            history=[],
+            think=think,
+            observe=observe,
+            context=context,
+            on_action=fake_on_action,
+        )
+
+        # on_action should fire before tool_invoke
+        assert len(call_log) == 2
+        assert call_log[0] == ("on_action", "search")
+        assert call_log[1] == ("tool_invoke",)
+
+    @pytest.mark.asyncio
+    async def test_on_action_not_called_for_final(self):
+        """on_action does not fire when react() returns Final."""
+        called = []
+
+        async def fake_on_action(act):
+            called.append(act)
+
+        agent = AgentManager(tools={})
+        agent.reason = AsyncMock(
+            return_value=Final(thought="done", final="answer")
+        )
+
+        think = AsyncMock()
+        observe = AsyncMock()
+        context = MagicMock()
+
+        result = await agent.react(
+            question="test",
+            history=[],
+            think=think,
+            observe=observe,
+            context=context,
+            on_action=fake_on_action,
+        )
+
+        assert isinstance(result, Final)
+        assert len(called) == 0
+
+    @pytest.mark.asyncio
+    async def test_on_action_none_accepted(self):
+        """react() works fine when on_action is None (default)."""
+        async def tool_invoke(**kwargs):
+            return "result"
+
+        tool_impl = MagicMock()
+        tool_impl.return_value.invoke = AsyncMock(side_effect=tool_invoke)
+
+        tools = {
+            "search": Tool(
+                name="search",
+                description="Search",
+                implementation=tool_impl,
+                arguments=[],
+                config={},
+            ),
+        }
+
+        agent = AgentManager(tools=tools)
+        agent.reason = AsyncMock(
+            return_value=Action(thought="t", name="search", arguments={}, observation="")
+        )
+
+        think = AsyncMock()
+        observe = AsyncMock()
+        context = MagicMock()
+
+        result = await agent.react(
+            question="test",
+            history=[],
+            think=think,
+            observe=observe,
+            context=context,
+            # on_action not passed — defaults to None
+        )
+
+        assert isinstance(result, Action)
+        assert result.observation == "result"
diff --git a/tests/unit/test_agent/test_parse_chunk_message_id.py b/tests/unit/test_agent/test_parse_chunk_message_id.py
new file mode 100644
index 00000000..38942f1e
--- /dev/null
+++ b/tests/unit/test_agent/test_parse_chunk_message_id.py
@@ -0,0 +1,74 @@
+"""
+Tests that _parse_chunk propagates message_id from wire format
+to AgentThought, AgentObservation, and AgentAnswer.
+"""
+
+import pytest
+
+from trustgraph.api.socket_client import SocketClient
+from trustgraph.api.types import AgentThought, AgentObservation, AgentAnswer
+
+
+@pytest.fixture
+def client():
+    # We only need _parse_chunk — don't connect
+    c = object.__new__(SocketClient)
+    return c
+
+
+class TestParseChunkMessageId:
+
+    def test_thought_message_id(self, client):
+        resp = {
+            "chunk_type": "thought",
+            "content": "thinking...",
+            "end_of_message": False,
+            "message_id": "urn:trustgraph:agent:sess/i1/thought",
+        }
+        chunk = client._parse_chunk(resp)
+        assert isinstance(chunk, AgentThought)
+        assert chunk.message_id == "urn:trustgraph:agent:sess/i1/thought"
+
+    def test_observation_message_id(self, client):
+        resp = {
+            "chunk_type": "observation",
+            "content": "result",
+            "end_of_message": True,
+            "message_id": "urn:trustgraph:agent:sess/i1/observation",
+        }
+        chunk = client._parse_chunk(resp)
+        assert isinstance(chunk, AgentObservation)
+        assert chunk.message_id == "urn:trustgraph:agent:sess/i1/observation"
+
+    def test_answer_message_id(self, client):
+        resp = {
+            "chunk_type": "answer",
+            "content": "the answer",
+            "end_of_message": False,
+            "end_of_dialog": False,
+            "message_id": "urn:trustgraph:agent:sess/final",
+        }
+        chunk = client._parse_chunk(resp)
+        assert isinstance(chunk, AgentAnswer)
+        assert chunk.message_id == "urn:trustgraph:agent:sess/final"
+
+    def test_thought_missing_message_id(self, client):
+        resp = {
+            "chunk_type": "thought",
+            "content": "thinking...",
+            "end_of_message": False,
+        }
+        chunk = client._parse_chunk(resp)
+        assert isinstance(chunk, AgentThought)
+        assert chunk.message_id == ""
+
+    def test_answer_missing_message_id(self, client):
+        resp = {
+            "chunk_type": "answer",
+            "content": "answer",
+            "end_of_message": True,
+            "end_of_dialog": True,
+        }
+        chunk = client._parse_chunk(resp)
+        assert isinstance(chunk, AgentAnswer)
+        assert chunk.message_id == ""
diff --git a/tests/unit/test_provenance/test_agent_provenance.py b/tests/unit/test_provenance/test_agent_provenance.py
index d3f0ef8c..c548ef9d 100644
--- a/tests/unit/test_provenance/test_agent_provenance.py
+++ b/tests/unit/test_provenance/test_agent_provenance.py
@@ -12,6 +12,7 @@ from trustgraph.provenance.agent import (
     agent_iteration_triples,
     agent_observation_triples,
     agent_final_triples,
+    agent_synthesis_triples,
 )
 
 from trustgraph.provenance.namespaces import (
@@ -21,7 +22,7 @@ from trustgraph.provenance.namespaces import (
     TG_QUERY, TG_THOUGHT, TG_ACTION, TG_ARGUMENTS,
     TG_QUESTION, TG_ANALYSIS, TG_CONCLUSION, TG_DOCUMENT,
     TG_ANSWER_TYPE, TG_REFLECTION_TYPE, TG_THOUGHT_TYPE, TG_OBSERVATION_TYPE,
-    TG_TOOL_USE,
+    TG_TOOL_USE, TG_SYNTHESIS,
     TG_AGENT_QUESTION,
 )
 
@@ -105,6 +106,25 @@ class TestAgentSessionTriples:
         )
         assert len(triples) == 6
 
+    def test_session_parent_uri(self):
+        """Subagent sessions derive from a parent entity (e.g. Decomposition)."""
+        parent = "urn:trustgraph:agent:parent/decompose"
+        triples = agent_session_triples(
+            self.SESSION_URI, "Q", "2024-01-01T00:00:00Z",
+            parent_uri=parent,
+        )
+        derived = find_triple(triples, PROV_WAS_DERIVED_FROM, self.SESSION_URI)
+        assert derived is not None
+        assert derived.o.iri == parent
+
+    def test_session_no_parent_uri(self):
+        """Top-level sessions have no wasDerivedFrom."""
+        triples = agent_session_triples(
+            self.SESSION_URI, "Q", "2024-01-01T00:00:00Z"
+        )
+        derived = find_triple(triples, PROV_WAS_DERIVED_FROM, self.SESSION_URI)
+        assert derived is None
+
 
 # ---------------------------------------------------------------------------
 # agent_iteration_triples
@@ -358,3 +378,59 @@ class TestAgentFinalTriples:
         )
         doc = find_triple(triples, TG_DOCUMENT, self.FINAL_URI)
         assert doc is None
+
+
+# ---------------------------------------------------------------------------
+# agent_synthesis_triples
+# ---------------------------------------------------------------------------
+
+class TestAgentSynthesisTriples:
+
+    SYNTH_URI = "urn:trustgraph:agent:test-session/synthesis"
+    FINDING_0 = "urn:trustgraph:agent:test-session/finding/0"
+    FINDING_1 = "urn:trustgraph:agent:test-session/finding/1"
+    FINDING_2 = "urn:trustgraph:agent:test-session/finding/2"
+
+    def test_synthesis_types(self):
+        triples = agent_synthesis_triples(self.SYNTH_URI, self.FINDING_0)
+        assert has_type(triples, self.SYNTH_URI, PROV_ENTITY)
+        assert has_type(triples, self.SYNTH_URI, TG_SYNTHESIS)
+        assert has_type(triples, self.SYNTH_URI, TG_ANSWER_TYPE)
+
+    def test_synthesis_single_parent_string(self):
+        """Single parent passed as string."""
+        triples = agent_synthesis_triples(self.SYNTH_URI, self.FINDING_0)
+        derived = find_triples(triples, PROV_WAS_DERIVED_FROM, self.SYNTH_URI)
+        assert len(derived) == 1
+        assert derived[0].o.iri == self.FINDING_0
+
+    def test_synthesis_multiple_parents(self):
+        """Multiple parents for supervisor fan-in."""
+        parents = [self.FINDING_0, self.FINDING_1, self.FINDING_2]
+        triples = agent_synthesis_triples(self.SYNTH_URI, parents)
+        derived = find_triples(triples, PROV_WAS_DERIVED_FROM, self.SYNTH_URI)
+        assert len(derived) == 3
+        derived_uris = {t.o.iri for t in derived}
+        assert derived_uris == set(parents)
+
+    def test_synthesis_single_parent_as_list(self):
+        """Single parent passed as list."""
+        triples = agent_synthesis_triples(self.SYNTH_URI, [self.FINDING_0])
+        derived = find_triples(triples, PROV_WAS_DERIVED_FROM, self.SYNTH_URI)
+        assert len(derived) == 1
+        assert derived[0].o.iri == self.FINDING_0
+
+    def test_synthesis_document(self):
+        triples = agent_synthesis_triples(
+            self.SYNTH_URI, self.FINDING_0,
+            document_id="urn:doc:synth",
+        )
+        doc = find_triple(triples, TG_DOCUMENT, self.SYNTH_URI)
+        assert doc is not None
+        assert doc.o.iri == "urn:doc:synth"
+
+    def test_synthesis_label(self):
+        triples = agent_synthesis_triples(self.SYNTH_URI, self.FINDING_0)
+        label = find_triple(triples, RDFS_LABEL, self.SYNTH_URI)
+        assert label is not None
+        assert label.o.value == "Synthesis"
diff --git a/tests/unit/test_provenance/test_explainability.py b/tests/unit/test_provenance/test_explainability.py
index e2c7fcd1..a6d655a7 100644
--- a/tests/unit/test_provenance/test_explainability.py
+++ b/tests/unit/test_provenance/test_explainability.py
@@ -558,3 +558,96 @@ class TestExplainabilityClientDetectSessionType:
         mock_flow = MagicMock()
         client = ExplainabilityClient(mock_flow, retry_delay=0.0)
         assert client.detect_session_type("urn:trustgraph:docrag:abc") == "docrag"
+
+
+class TestChainWalkerFollowsSubTraceTerminal:
+    """Test that _follow_provenance_chain continues from a sub-trace's
+    Synthesis to find downstream entities like Observation."""
+
+    def test_observation_found_via_subtrace_synthesis(self):
+        """
+        DAG: Question -> Analysis -> GraphRAG Question -> Synthesis -> Observation
+        The walker should find Analysis, the sub-trace, then follow from
+        Synthesis to discover Observation.
+        """
+        # Entity triples (s, p, o)
+        entity_data = {
+            "urn:agent:q": [
+                ("urn:agent:q", RDF_TYPE, TG_AGENT_QUESTION),
+                ("urn:agent:q", TG_QUERY, "test"),
+            ],
+            "urn:agent:analysis": [
+                ("urn:agent:analysis", RDF_TYPE, TG_ANALYSIS),
+                ("urn:agent:analysis", PROV_WAS_DERIVED_FROM, "urn:agent:q"),
+            ],
+            "urn:graphrag:q": [
+                ("urn:graphrag:q", RDF_TYPE, TG_QUESTION),
+                ("urn:graphrag:q", RDF_TYPE, TG_GRAPH_RAG_QUESTION),
+                ("urn:graphrag:q", TG_QUERY, "test"),
+                ("urn:graphrag:q", PROV_WAS_DERIVED_FROM, "urn:agent:analysis"),
+            ],
+            "urn:graphrag:synth": [
+                ("urn:graphrag:synth", RDF_TYPE, TG_SYNTHESIS),
+                ("urn:graphrag:synth", PROV_WAS_DERIVED_FROM, "urn:graphrag:q"),
+            ],
+            "urn:agent:obs": [
+                ("urn:agent:obs", RDF_TYPE, TG_OBSERVATION_TYPE),
+                ("urn:agent:obs", PROV_WAS_DERIVED_FROM, "urn:graphrag:synth"),
+            ],
+            "urn:agent:conclusion": [
+                ("urn:agent:conclusion", RDF_TYPE, TG_CONCLUSION),
+                ("urn:agent:conclusion", PROV_WAS_DERIVED_FROM, "urn:agent:obs"),
+            ],
+        }
+
+        # Build a mock flow that answers triples queries
+        # Query by s= returns that entity's triples
+        # Query by p=wasDerivedFrom, o=X returns entities derived from X
+        def mock_triples_query(s=None, p=None, o=None, **kwargs):
+            if s and not p:
+                # Fetch entity triples
+                tuples = entity_data.get(s, [])
+                return _make_wire_triples(tuples)
+            elif p == PROV_WAS_DERIVED_FROM and o:
+                # Find entities derived from o
+                results = []
+                for uri, tuples in entity_data.items():
+                    for _, pred, obj in tuples:
+                        if pred == PROV_WAS_DERIVED_FROM and obj == o:
+                            results.append((uri, pred, obj))
+                return _make_wire_triples(results)
+            return []
+
+        mock_flow = MagicMock()
+        mock_flow.triples_query.side_effect = mock_triples_query
+
+        client = ExplainabilityClient(mock_flow, retry_delay=0.0, max_retries=2)
+
+        # Mock fetch_graphrag_trace to return a trace with a synthesis
+        synth_entity = Synthesis(uri="urn:graphrag:synth", entity_type="synthesis")
+        client.fetch_graphrag_trace = MagicMock(return_value={
+            "question": Question(uri="urn:graphrag:q", entity_type="question",
+                                 question_type="graph-rag"),
+            "synthesis": synth_entity,
+        })
+
+        trace = client.fetch_agent_trace(
+            "urn:agent:q",
+            graph="urn:graph:retrieval",
+        )
+
+        # Should have found all steps
+        step_types = [
+            type(s).__name__ if not isinstance(s, dict) else s.get("type")
+            for s in trace["steps"]
+        ]
+
+        assert "Analysis" in step_types, f"Missing Analysis in {step_types}"
+        assert "sub-trace" in step_types, f"Missing sub-trace in {step_types}"
+        assert "Observation" in step_types, f"Missing Observation in {step_types}"
+        assert "Conclusion" in step_types, f"Missing Conclusion in {step_types}"
+
+        # Observation should come after the sub-trace
+        subtrace_idx = step_types.index("sub-trace")
+        obs_idx = step_types.index("Observation")
+        assert obs_idx > subtrace_idx, "Observation should appear after sub-trace"