Improve MCP text response decoding

src/index.ts

@@ -145,9 +145,21 @@ function removeUndefinedValues(input: Record<string, unknown>): Record<string, u
 
 function decodeToolResult<Result>(result: unknown): Result {
   if (isTextContentResult(result)) {
-    const text = result.content.find((item) => item.type === "text")?.text;
-    if (typeof text === "string") {
+    const text = result.content
+      .filter((item) => item.type === "text" && typeof item.text === "string")
+      .map((item) => item.text)
+      .join("\n")
+      .trim();
+    if ((result as { isError?: unknown }).isError === true) {
+      throw new Error(text || "MCP tool call failed.");
+    }
+    if (!text) {
+      throw new Error("MCP tool response did not include text content.");
+    }
+    try {
       return JSON.parse(text) as Result;
+    } catch (error) {
+      throw new Error(`MCP tool response was not valid JSON: ${error instanceof Error ? error.message : String(error)}`);
     }
   }
   return result as Result;

test/client.test.ts

@@ -167,4 +167,21 @@ describe("AgentDispatchMcpClient", () => {
 
     await expect(client.listProviders()).resolves.toEqual(["aws", "gcp"]);
   });
+
+  it("throws clear errors for MCP error and malformed text responses", async () => {
+    const errorClient = new AgentDispatchMcpClient({
+      callTool: async () => ({ content: [{ type: "text", text: "bad request" }], isError: true })
+    });
+    await expect(errorClient.listProviders()).rejects.toThrow("bad request");
+
+    const malformedClient = new AgentDispatchMcpClient({
+      callTool: async () => ({ content: [{ type: "text", text: "not json" }] })
+    });
+    await expect(malformedClient.listProviders()).rejects.toThrow("not valid JSON");
+
+    const emptyClient = new AgentDispatchMcpClient({
+      callTool: async () => ({ content: [{ type: "image", data: "..." }] })
+    });
+    await expect(emptyClient.listProviders()).rejects.toThrow("did not include text content");
+  });
 });