diff --git a/src/lib/chatStream.test.ts b/src/lib/chatStream.test.ts
index a2e5103..6cf3f16 100644
--- a/src/lib/chatStream.test.ts
+++ b/src/lib/chatStream.test.ts
@@ -97,6 +97,35 @@ describe("streamAgentChat", () => {
     });
   });
 
+  it("parses legacy tool_call arguments when params is empty", async () => {
+    apiFetch.mockResolvedValue({
+      ok: true,
+      body: makeStream([
+        'event: tool_call\ndata: {"conversationId":"agent-1e75dd01-29e","tool":"locate_features","params":{},"arguments":"{\\"ids\\":[\\"142902\\"],\\"feature_type\\":\\"junction\\"}"}\n\n',
+        'event: done\ndata: {"session_id":"agent-1e75dd01-29e"}\n\n',
+      ]),
+    });
+
+    const events: Array<{
+      type: string;
+      sessionId?: string;
+      tool?: string;
+      params?: Record<string, unknown>;
+    }> = [];
+
+    await streamAgentChat({
+      message: "hi",
+      onEvent: (event) => events.push(event),
+    });
+
+    expect(events[0]).toEqual({
+      type: "tool_call",
+      sessionId: "agent-1e75dd01-29e",
+      tool: "locate_features",
+      params: { ids: ["142902"], feature_type: "junction" },
+    });
+  });
+
   it("emits error when response is not ok", async () => {
     apiFetch.mockResolvedValue({
       ok: false,
diff --git a/src/lib/chatStream.ts b/src/lib/chatStream.ts
index 4d2746a..187cd59 100644
--- a/src/lib/chatStream.ts
+++ b/src/lib/chatStream.ts
@@ -52,6 +52,30 @@ const parseEventBlock = (block: string): { event?: string; data?: string } => {
   };
 };
 
+const isObjectRecord = (value: unknown): value is Record<string, unknown> =>
+  typeof value === "object" && value !== null && !Array.isArray(value);
+
+const resolveToolParams = (
+  params: unknown,
+  argumentsPayload: unknown,
+): Record<string, unknown> => {
+  if (isObjectRecord(params) && Object.keys(params).length > 0) {
+    return params;
+  }
+  if (isObjectRecord(argumentsPayload)) {
+    return argumentsPayload;
+  }
+  if (typeof argumentsPayload === "string") {
+    try {
+      const parsed = JSON.parse(argumentsPayload) as unknown;
+      return isObjectRecord(parsed) ? parsed : {};
+    } catch {
+      return {};
+    }
+  }
+  return isObjectRecord(params) ? params : {};
+};
+
 export const streamAgentChat = async ({
   message,
   sessionId,
@@ -125,11 +149,13 @@ export const streamAgentChat = async ({
       try {
         const parsed = JSON.parse(data) as {
           session_id?: string;
+          conversationId?: string;
           content?: string;
           message?: string;
           detail?: string;
           tool?: string;
           params?: Record<string, unknown>;
+          arguments?: unknown;
           id?: string;
           phase?: string;
           status?: "running" | "completed" | "error";
@@ -166,9 +192,9 @@ export const streamAgentChat = async ({
         } else if (event === "tool_call") {
           onEvent({
             type: "tool_call",
-            sessionId: parsed.session_id ?? "",
+            sessionId: parsed.session_id ?? parsed.conversationId ?? "",
             tool: parsed.tool ?? "",
-            params: parsed.params ?? {},
+            params: resolveToolParams(parsed.params, parsed.arguments),
           });
         }
       } catch {