diff --git a/core/src/main/java/com/google/adk/tools/mcp/McpToolset.java b/core/src/main/java/com/google/adk/tools/mcp/McpToolset.java
index 4cafb9681..dae35f77e 100644
--- a/core/src/main/java/com/google/adk/tools/mcp/McpToolset.java
+++ b/core/src/main/java/com/google/adk/tools/mcp/McpToolset.java
@@ -25,6 +25,7 @@
 import com.google.adk.tools.BaseTool;
 import com.google.adk.tools.BaseToolset;
 import com.google.adk.tools.ToolPredicate;
+import com.google.common.annotations.VisibleForTesting;
 import com.google.common.collect.ImmutableList;
 import com.google.common.primitives.Booleans;
 import io.modelcontextprotocol.client.McpSyncClient;
@@ -416,10 +417,7 @@ public static McpToolset fromConfig(BaseTool.ToolConfig config, String configAbs
       }
 
       List<String> toolNames = mcpToolsetConfig.toolFilter();
-      Object connectionParameters =
-          Optional.<Object>ofNullable(mcpToolsetConfig.stdioConnectionParams())
-              .or(() -> Optional.ofNullable(mcpToolsetConfig.sseServerParams()))
-              .orElse(mcpToolsetConfig.stdioConnectionParams());
+      Object connectionParameters = resolveConnectionParameters(mcpToolsetConfig);
 
       // Create McpToolset with McpSessionManager having appropriate connection parameters
       if (toolNames != null) {
@@ -431,4 +429,20 @@ public static McpToolset fromConfig(BaseTool.ToolConfig config, String configAbs
       throw new ConfigurationException("Failed to parse McpToolsetConfig from ToolArgsConfig", e);
     }
   }
+
+  /**
+   * Resolves the single connection-parameters object from an already-validated config. {@code
+   * stdioServerParams} is converted to the MCP SDK {@link ServerParameters}, the type {@link
+   * DefaultMcpTransportBuilder} accepts; the other variants pass through unchanged.
+   */
+  @VisibleForTesting
+  static Object resolveConnectionParameters(McpToolsetConfig mcpToolsetConfig) {
+    return Optional.<Object>ofNullable(mcpToolsetConfig.stdioConnectionParams())
+        .or(() -> Optional.ofNullable(mcpToolsetConfig.sseServerParams()))
+        .or(
+            () ->
+                Optional.ofNullable(mcpToolsetConfig.stdioServerParams())
+                    .map(StdioServerParameters::toServerParameters))
+        .orElseThrow(() -> new IllegalStateException("Validated MCP connection params missing."));
+  }
 }
diff --git a/core/src/test/java/com/google/adk/tools/mcp/McpToolsetTest.java b/core/src/test/java/com/google/adk/tools/mcp/McpToolsetTest.java
index f00091d2d..164105aa6 100644
--- a/core/src/test/java/com/google/adk/tools/mcp/McpToolsetTest.java
+++ b/core/src/test/java/com/google/adk/tools/mcp/McpToolsetTest.java
@@ -31,6 +31,7 @@
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import io.modelcontextprotocol.client.McpSyncClient;
+import io.modelcontextprotocol.client.transport.ServerParameters;
 import io.modelcontextprotocol.json.McpJsonDefaults;
 import io.modelcontextprotocol.json.McpJsonMapper;
 import io.modelcontextprotocol.spec.McpSchema;
@@ -369,4 +370,45 @@ public void getTools_succeedsOnLastRetryAttempt() {
     verify(mockMcpSessionManager, times(3)).createSession();
     verify(mockMcpSyncClient, times(3)).listTools();
   }
+
+  @Test
+  public void resolveConnectionParameters_stdioServerParams_convertsToSdkServerParameters() {
+    McpToolsetConfig config = new McpToolsetConfig();
+    config.setStdioServerParams(StdioServerParameters.builder().command("my-command").build());
+
+    Object resolved = McpToolset.resolveConnectionParameters(config);
+
+    // Regression, Finding 1: this branch used to resolve to null (stdioServerParams was never
+    // consulted), deferring the failure to an NPE in DefaultMcpTransportBuilder.build(null).
+    assertThat(resolved).isInstanceOf(ServerParameters.class);
+  }
+
+  @Test
+  public void resolveConnectionParameters_sseServerParams_passesThrough() {
+    McpToolsetConfig config = new McpToolsetConfig();
+    SseServerParameters sseParams =
+        SseServerParameters.builder().url("http://localhost:8080").build();
+    config.setSseServerParams(sseParams);
+
+    assertThat(McpToolset.resolveConnectionParameters(config)).isSameInstanceAs(sseParams);
+  }
+
+  @Test
+  public void resolveConnectionParameters_stdioConnectionParams_passesThrough() {
+    McpToolsetConfig config = new McpToolsetConfig();
+    StdioConnectionParameters connectionParams =
+        StdioConnectionParameters.builder()
+            .serverParams(StdioServerParameters.builder().command("my-command").build())
+            .build();
+    config.setStdioConnectionParams(connectionParams);
+
+    assertThat(McpToolset.resolveConnectionParameters(config)).isSameInstanceAs(connectionParams);
+  }
+
+  @Test
+  public void resolveConnectionParameters_nothingSet_throwsIllegalStateException() {
+    assertThrows(
+        IllegalStateException.class,
+        () -> McpToolset.resolveConnectionParameters(new McpToolsetConfig()));
+  }
 }