fix(client): preserve agent_uri trailing slash; widen MCP URL fallbacks

src/adcp/protocols/mcp.py

@@ -350,13 +350,17 @@ async def _get_session(self) -> ClientSession:
                 else:
                     headers[self.agent_config.auth_header] = self.agent_config.auth_token
 
-            # Try the user's exact URL first
-            urls_to_try = [self.agent_config.agent_uri]
-
-            # If URL doesn't end with /mcp, also try with /mcp suffix
-            if not self.agent_config.agent_uri.rstrip("/").endswith("/mcp"):
-                base_uri = self.agent_config.agent_uri.rstrip("/")
-                urls_to_try.append(f"{base_uri}/mcp")
+            # Try the user's exact URL first, then the alternate slash form, then
+            # /mcp discovery paths. MCP servers disagree on whether their endpoint
+            # is at /mcp or /mcp/ — try both rather than silently normalizing.
+            uri = self.agent_config.agent_uri
+            base = uri.rstrip("/")
+            urls_to_try = [uri]
+            if base.endswith("/mcp"):
+                # User pointed at the MCP endpoint; also try the other slash form.
+                urls_to_try.append(f"{base}/" if not uri.endswith("/") else base)
+            else:
+                urls_to_try.extend([f"{base}/mcp", f"{base}/mcp/"])
 
             # RFC 9421 auto-signing: if ADCPClient installed a signing request
             # hook, wire it into streamable_http via a custom httpx client

src/adcp/signing/capability_cache.py

@@ -151,7 +151,9 @@ def build_capability_cache_key(
     Format matches the JS SDK exactly:
     ``agent_uri[::sha256(auth_token)[:16]][::sig=signer_fingerprint]``
     """
-    parts = [agent_uri]
+    # Normalize trailing slash so http://host/mcp and http://host/mcp/ resolve
+    # to the same cache entry — the validator no longer strips it.
+    parts = [agent_uri.rstrip("/")]
     if auth_token:
         token_digest = hashlib.sha256(auth_token.encode("utf-8")).hexdigest()[:16]
         parts.append(f"::{token_digest}")

src/adcp/types/core.py

@@ -44,8 +44,7 @@ def validate_agent_uri(cls, v: str) -> str:
                 "Example: https://agent.example.com"
             )
 
-        # Remove trailing slash for consistency
-        return v.rstrip("/")
+        return v
 
     @field_validator("timeout")
     @classmethod

tests/test_client.py

@@ -20,6 +20,20 @@ def test_agent_config_creation():
     assert config.protocol == Protocol.A2A
 
 
+@pytest.mark.parametrize(
+    "input_uri,expected_uri",
+    [
+        ("https://example.com/mcp/", "https://example.com/mcp/"),
+        ("https://example.com/mcp", "https://example.com/mcp"),
+        ("https://example.com", "https://example.com"),
+        ("https://example.com/", "https://example.com/"),
+    ],
+)
+def test_agent_uri_preserves_user_supplied_form(input_uri: str, expected_uri: str) -> None:
+    cfg = AgentConfig(id="x", agent_uri=input_uri, protocol=Protocol.MCP)
+    assert cfg.agent_uri == expected_uri
+
+
 def test_client_creation():
     """Test creating ADCP client."""
     config = AgentConfig(

tests/test_helpers.py

@@ -62,8 +62,7 @@ def test_mcp_config_structure():
     """Test TEST_AGENT_MCP_CONFIG has correct structure."""
     assert TEST_AGENT_MCP_CONFIG.id == "test-agent-mcp"
     assert TEST_AGENT_MCP_CONFIG.protocol == Protocol.MCP
-    # AgentConfig validator strips trailing slashes for consistency
-    assert TEST_AGENT_MCP_CONFIG.agent_uri == "https://test-agent.adcontextprotocol.org/mcp"
+    assert TEST_AGENT_MCP_CONFIG.agent_uri == "https://test-agent.adcontextprotocol.org/mcp/"
     assert TEST_AGENT_MCP_CONFIG.auth_token is not None
 
 
@@ -195,7 +194,7 @@ def test_mcp_no_auth_config_structure():
     """Test TEST_AGENT_MCP_NO_AUTH_CONFIG has correct structure."""
     assert TEST_AGENT_MCP_NO_AUTH_CONFIG.id == "test-agent-mcp-no-auth"
     assert TEST_AGENT_MCP_NO_AUTH_CONFIG.protocol == Protocol.MCP
-    assert TEST_AGENT_MCP_NO_AUTH_CONFIG.agent_uri == "https://test-agent.adcontextprotocol.org/mcp"
+    assert TEST_AGENT_MCP_NO_AUTH_CONFIG.agent_uri == "https://test-agent.adcontextprotocol.org/mcp/"
     assert TEST_AGENT_MCP_NO_AUTH_CONFIG.auth_token is None
 
 

tests/test_protocols.py

@@ -1832,6 +1832,66 @@ async def test_cleanup_handles_exception_group_with_cancelled_error(self, mcp_co
         assert adapter._session is None
 
 
+class TestMCPUrlFallback:
+    """Tests for the MCP URL fallback list built in _get_session."""
+
+    @pytest.mark.parametrize(
+        "agent_uri,expected_urls",
+        [
+            # Slash-terminated /mcp/ — also try no-slash form
+            (
+                "https://host/mcp/",
+                ["https://host/mcp/", "https://host/mcp"],
+            ),
+            # No-slash /mcp — also try slash form
+            (
+                "https://host/mcp",
+                ["https://host/mcp", "https://host/mcp/"],
+            ),
+            # Bare host — discovery: try both /mcp and /mcp/
+            (
+                "https://host",
+                ["https://host", "https://host/mcp", "https://host/mcp/"],
+            ),
+            # Host with trailing slash — discovery: try both /mcp and /mcp/
+            (
+                "https://host/",
+                ["https://host/", "https://host/mcp", "https://host/mcp/"],
+            ),
+        ],
+    )
+    @pytest.mark.asyncio
+    async def test_urls_to_try(self, agent_uri: str, expected_urls: list[str]) -> None:
+        from unittest.mock import patch
+
+        from adcp.protocols.mcp import MCPAdapter
+        from adcp.types.core import AgentConfig, Protocol
+
+        cfg = AgentConfig(id="t", agent_uri=agent_uri, protocol=Protocol.MCP)
+        adapter = MCPAdapter(cfg)
+
+        real_urls: list[str] = []
+
+        class _FakeCM:
+            async def __aenter__(self) -> None:
+                raise ConnectionError("abort")
+
+            async def __aexit__(self, *_: object) -> None:
+                pass
+
+        def capture_url(url: str, **_kw: object) -> _FakeCM:
+            real_urls.append(url)
+            return _FakeCM()
+
+        with patch("adcp.protocols.mcp.streamablehttp_client", side_effect=capture_url):
+            try:
+                await adapter._get_session()
+            except Exception:
+                pass
+
+        assert real_urls == expected_urls
+
+
 class TestFromMcpClientFactory:
     """Tests for ADCPClient.from_mcp_client() factory method."""