Omit zero cost_details on generations

src/langfuse.ts

@@ -14,6 +14,19 @@ import { Context as EffectContext, Effect } from "effect";
 
 import { PLUGIN_VERSION } from "./version.js";
 
+function getCostAttributes(cost: number) {
+  // Custom/proxy providers without per-model pricing in opencode.json
+  // can report cost: 0 on completed assistant messages / generation steps.
+  // Omit these costs so Langfuse can calculate them from its model pricing instead.
+  // OpenCode's pricing fallback: https://github.com/anomalyco/opencode/blob/4c9a4309ce7f4f5926416792a30a3b27281d9923/packages/opencode/src/provider/provider.ts#L1502-L1509
+  if (cost > 0) {
+    return {
+      "langfuse.observation.cost_details": JSON.stringify({ total: cost }),
+    };
+  }
+  return {};
+}
+
 export class LangfuseClient {
   readonly baseUrl: string;
   readonly forceFlush: Effect.Effect<void, unknown>;
@@ -758,10 +771,7 @@ export class LangfuseClient {
             input.tokens.input + input.tokens.output + input.tokens.reasoning,
         }),
       );
-      step.span.setAttribute(
-        "langfuse.observation.cost_details",
-        JSON.stringify({ total: input.cost }),
-      );
+      step.span.setAttributes(getCostAttributes(input.cost));
       step.span.setAttribute(
         "langfuse.observation.metadata",
         JSON.stringify({
@@ -821,9 +831,7 @@ export class LangfuseClient {
               input.tokens.total ??
               input.tokens.input + input.tokens.output + input.tokens.reasoning,
           }),
-          "langfuse.observation.cost_details": JSON.stringify({
-            total: input.cost,
-          }),
+          ...getCostAttributes(input.cost),
           "langfuse.observation.metadata": JSON.stringify({
             messageID: input.messageID,
             parentID: input.parentID,

test/integration/v1.test.ts

@@ -394,6 +394,7 @@ const completeGeneration = async (input: {
   started: number;
   completed: number;
   text?: string;
+  cost?: number;
 }) => {
   storeMessage(input.sessionID, {
     info: {
@@ -443,7 +444,7 @@ const completeGeneration = async (input: {
         providerID: "test-provider",
         path: { cwd: "/test", root: "/test" },
         finish: "stop",
-        cost: 0.01,
+        cost: input.cost ?? 0.01,
         tokens: {
           input: 10,
           output: 5,
@@ -3491,6 +3492,56 @@ describe("built plugin", { concurrent: false }, () => {
     });
   });
 
+  test("omits cost_details when OpenCode reports zero cost", async () => {
+    const sessionID = "zero-cost-session";
+    const started = startedAt;
+
+    await sendUserMessage({
+      sessionID,
+      messageID: "zero-cost-user",
+      text: "Use a custom provider without OpenCode cost config",
+      started,
+    });
+    await startGeneration({
+      id: "zero-cost-step",
+      sessionID,
+      started: started + 100,
+    });
+    await completeGeneration({
+      sessionID,
+      userMessageID: "zero-cost-user",
+      assistantMessageID: "zero-cost-assistant-1",
+      started: started + 100,
+      completed: started + 500,
+      text: "Step output",
+      cost: 0,
+    });
+    await completeGeneration({
+      sessionID,
+      userMessageID: "zero-cost-user",
+      assistantMessageID: "zero-cost-assistant-2",
+      started: started + 600,
+      completed: started + 900,
+      text: "Fallback output",
+      cost: 0,
+    });
+
+    const { spans } = await flushSession(sessionID);
+    const generations = spans.filter(
+      (span) => span.name === "opencode.generation",
+    );
+
+    expect(generations).toHaveLength(2);
+    for (const generation of generations) {
+      expect(getAttributes(generation)).toHaveProperty(
+        "langfuse.observation.usage_details",
+      );
+      expect(getAttributes(generation)).not.toHaveProperty(
+        "langfuse.observation.cost_details",
+      );
+    }
+  });
+
   test("can be disposed repeatedly", async () => {
     expect(hooks.dispose).toBeDefined();
     await hooks.dispose?.();