diff --git a/workspace/tests/test_shared_runtime_helpers.py b/workspace/tests/test_shared_runtime_helpers.py
new file mode 100644
index 00000000..8d5f3c04
--- /dev/null
+++ b/workspace/tests/test_shared_runtime_helpers.py
@@ -0,0 +1,300 @@
+"""Test coverage for shared_runtime helpers (issue #366).
+
+Six helper functions previously had zero test coverage:
+  _extract_part_text, extract_message_text, format_conversation_history,
+  build_task_text, append_peer_guidance, brief_task
+"""
+from __future__ import annotations
+
+
+from shared_runtime import (
+    _extract_part_text,
+    append_peer_guidance,
+    brief_task,
+    build_task_text,
+    extract_message_text,
+    format_conversation_history,
+)
+
+
+# =============================================================================
+# _extract_part_text
+# =============================================================================
+
+class TestExtractPartText:
+    """Coverage for shared_runtime._extract_part_text()."""
+
+    def test_dict_with_text_field(self):
+        assert _extract_part_text({"text": "hello"}) == "hello"
+
+    def test_dict_without_text_field(self):
+        assert _extract_part_text({"type": "image"}) == ""
+
+    def test_dict_with_empty_text_field(self):
+        assert _extract_part_text({"text": ""}) == ""
+
+    def test_dict_with_root_nesting(self):
+        """Text buried in part['root']['text'] is extracted."""
+        assert _extract_part_text({"root": {"text": "nested"}}) == "nested"
+
+    def test_dict_with_root_non_dict(self):
+        """part['root'] that is not a dict is safely skipped."""
+        assert _extract_part_text({"root": "string", "text": "top"}) == "top"
+
+    def test_object_with_text_attribute(self):
+        class FakePart:
+            text = "attr-text"
+
+        assert _extract_part_text(FakePart()) == "attr-text"
+
+    def test_object_with_root_object_with_text(self):
+        """Object with root.attr.text is extracted (A2A v1 object style)."""
+
+        class FakeRoot:
+            text = "root-attr-text"
+
+        class FakePart:
+            root = FakeRoot()
+
+        assert _extract_part_text(FakePart()) == "root-attr-text"
+
+    def test_object_with_empty_text_attribute(self):
+        class FakePart:
+            text = ""
+
+        assert _extract_part_text(FakePart()) == ""
+
+    def test_none_input(self):
+        assert _extract_part_text(None) == ""
+
+    def test_unexpected_type(self):
+        """Plain int/float/bool falls through to empty string."""
+        assert _extract_part_text(42) == ""
+
+
+# =============================================================================
+# extract_message_text
+# =============================================================================
+
+class TestExtractMessageText:
+    """Coverage for shared_runtime.extract_message_text()."""
+
+    def test_list_of_dict_parts(self):
+        parts = [{"text": "hello"}, {"text": "world"}]
+        assert extract_message_text(parts) == "hello world"
+
+    def test_single_part(self):
+        assert extract_message_text([{"text": "single"}]) == "single"
+
+    def test_context_object_with_message_parts(self):
+        """RequestContext-like: .message.parts is the parts list."""
+
+        class FakeContext:
+            class _Msg:
+                parts = [{"text": "from context"}]
+
+            message = _Msg()
+
+        assert extract_message_text(FakeContext()) == "from context"
+
+    def test_context_object_without_message(self):
+        """No .message attr → falls back to treating input as a parts list."""
+
+        class FakeContext:
+            pass  # no .message
+
+        # Pass a list directly as the context-like object
+        assert extract_message_text([{"text": "fallback"}]) == "fallback"
+
+    def test_whitespace_normalized(self):
+        """Leading/trailing whitespace is stripped; internal newlines are preserved."""
+        parts = [{"text": "  hello  "}, {"text": "\nworld\n"}]
+        result = extract_message_text(parts)
+        # Leading/trailing stripped, but internal \n stays (join uses single space)
+        assert result == "hello   \nworld"
+        assert not result.startswith(" ")
+        assert not result.endswith(" ")
+
+    def test_empty_parts_list(self):
+        assert extract_message_text([]) == ""
+
+
+# =============================================================================
+# format_conversation_history
+# =============================================================================
+
+class TestFormatConversationHistory:
+    """Coverage for shared_runtime.format_conversation_history()."""
+
+    def test_single_user_message(self):
+        hist = [("human", "hello")]
+        out = format_conversation_history(hist)
+        assert out == "User: hello"
+
+    def test_single_agent_message(self):
+        hist = [("ai", "response")]
+        out = format_conversation_history(hist)
+        assert out == "Agent: response"
+
+    def test_interleaved_history(self):
+        hist = [
+            ("human", "hello"),
+            ("ai", "hi there"),
+            ("human", "what is 2+2?"),
+            ("ai", "four"),
+        ]
+        out = format_conversation_history(hist)
+        lines = out.split("\n")
+        assert lines[0] == "User: hello"
+        assert lines[1] == "Agent: hi there"
+        assert lines[2] == "User: what is 2+2?"
+        assert lines[3] == "Agent: four"
+
+    def test_empty_history(self):
+        assert format_conversation_history([]) == ""
+
+
+# =============================================================================
+# build_task_text
+# =============================================================================
+
+class TestBuildTaskText:
+    """Coverage for shared_runtime.build_task_text()."""
+
+    def test_no_history_returns_user_message_unchanged(self):
+        assert build_task_text("do the thing", []) == "do the thing"
+
+    def test_history_prepends_transcript(self):
+        hist = [("human", "hello"), ("ai", "hi")]
+        result = build_task_text("follow-up", hist)
+        assert "Conversation so far:" in result
+        assert "User: hello" in result
+        assert "Agent: hi" in result
+        assert "follow-up" in result
+
+    def test_user_message_after_conversation_header(self):
+        hist = [("human", "hello")]
+        result = build_task_text("do it", hist)
+        assert result.startswith("Conversation so far:")
+        assert result.endswith("Current request: do it")
+
+    def test_empty_user_message_with_history(self):
+        """Empty user_message is still rendered with history."""
+        hist = [("human", "hello")]
+        result = build_task_text("", hist)
+        assert "Conversation so far:" in result
+        assert "Current request:" in result
+
+
+# =============================================================================
+# append_peer_guidance
+# =============================================================================
+
+class TestAppendPeerGuidance:
+    """Coverage for shared_runtime.append_peer_guidance()."""
+
+    def test_base_text_appended(self):
+        result = append_peer_guidance(
+            "base text",
+            peers_info="alpha: ws-1",
+            default_text="default",
+            tool_name="delegate_task",
+        )
+        assert result.startswith("base text")
+        assert "## Peers" in result
+        assert "alpha: ws-1" in result
+        assert "Use delegate_task" in result
+
+    def test_null_base_text_uses_default(self):
+        result = append_peer_guidance(
+            None,
+            peers_info="peer info",
+            default_text="DEFAULT_TEXT",
+            tool_name="tool",
+        )
+        assert result.startswith("DEFAULT_TEXT")
+
+    def test_whitespace_base_text_strips_to_empty_peers_still_added(self):
+        """Whitespace-only base_text is stripped but default_text is NOT used
+        (only None triggers the fallback). The peers section is still appended."""
+        result = append_peer_guidance(
+            "  ",
+            peers_info="peer",
+            default_text="DEF",
+            tool_name="t",
+        )
+        # "  ".strip() == ""; default_text is NOT substituted for whitespace
+        assert "## Peers" in result
+        assert "peer" in result
+        assert "DEF" not in result  # default_text only on None, not whitespace
+
+    def test_none_base_text_uses_default(self):
+        """None base_text triggers fallback to default_text."""
+        result = append_peer_guidance(
+            None,
+            peers_info="peer",
+            default_text="DEFAULT",
+            tool_name="tool",
+        )
+        assert result.startswith("DEFAULT")
+        assert "## Peers" in result
+
+    def test_empty_peers_info_skips_section(self):
+        result = append_peer_guidance(
+            "base",
+            peers_info="",
+            default_text="def",
+            tool_name="tool",
+        )
+        # No "## Peers" section when peers_info is empty
+        assert result == "base"
+
+    def test_whitespace_in_base_and_peers_normalized(self):
+        result = append_peer_guidance(
+            "  base  \n",
+            peers_info="  peer-1  \n",
+            default_text="def",
+            tool_name="tool",
+        )
+        # Base should be stripped of leading/trailing whitespace
+        assert result.startswith("base")
+        # Peer info should be appended
+        assert "peer-1" in result
+
+
+# =============================================================================
+# brief_task
+# =============================================================================
+
+class TestBriefTask:
+    """Coverage for shared_runtime.brief_task()."""
+
+    def test_short_text_returned_unchanged(self):
+        assert brief_task("hello", limit=60) == "hello"
+
+    def test_exact_limit_no_ellipsis(self):
+        text = "A" * 60
+        assert brief_task(text, limit=60) == text
+        assert "..." not in text
+
+    def test_truncated_with_ellipsis(self):
+        text = "A" * 80
+        result = brief_task(text, limit=60)
+        assert len(result) == 63  # 60 chars + "..."
+        assert result.endswith("...")
+
+    def test_limit_10_shortens(self):
+        result = brief_task("hello world", limit=10)
+        assert len(result) == 13  # 10 chars + "..."
+        assert result.endswith("...")
+
+    def test_limit_0_returns_ellipsis(self):
+        """limit=0 → 0-char slice + "..." since len("hello") > 0."""
+        result = brief_task("hello", limit=0)
+        assert result == "..."
+
+    def test_limit_1_single_char_plus_ellipsis(self):
+        result = brief_task("hello", limit=1)
+        assert len(result) == 4  # 1 char + "..."
+        assert result.startswith("h")
+        assert result.endswith("...")