fix(opencode): route Union Alpha through Messages API

Route union-alpha to /zen/v1/messages with targetFormat claude, add anthropic-version header, and register model capabilities (vision, 262K context, 131K max output).

open-sse/executors/opencode.js

@@ -5,13 +5,14 @@ import { getThinkingLevels } from "../providers/thinkingLevels.js";
 import { injectReasoningContent } from "../utils/reasoningContentInjector.js";
 import { resolveSessionId } from "../utils/sessionManager.js";
 import { isMuseSparkModel } from "../providers/models/helpers.js";
+import { ANTHROPIC_API_VERSION } from "../providers/shared.js";
 
 const OPENCODE_UA = "opencode";
-// Models served by /zen/v1/responses; every other model stays on /chat/completions.
 const RESPONSES_MODELS = new Set([
   "muse-spark-1.2-contributor-free",
   "muse-spark-1.3-contributor-free",
 ]);
+const MESSAGES_MODELS = new Set(["union-alpha"]);
 
 function generateRequestId() {
   return `msg_${crypto.randomUUID().replace(/-/g, "")}`;
@@ -31,6 +32,10 @@ function isResponsesModel(model) {
   return RESPONSES_MODELS.has(base) || isMuseSparkModel(base);
 }
 
+function isMessagesModel(model) {
+  return MESSAGES_MODELS.has(baseModelId(model));
+}
+
 function resolveOpencodeSession(body, credentials) {
   const headers = credentials?.rawHeaders || {};
   return resolveSessionId({
@@ -89,12 +94,12 @@ export class OpenCodeExecutor extends BaseExecutor {
 
   buildUrl(model) {
     const base = this.config.baseUrl;
-    return isResponsesModel(model)
-      ? `${base}/zen/v1/responses`
-      : `${base}/zen/v1/chat/completions`;
+    if (isResponsesModel(model)) return `${base}/zen/v1/responses`;
+    if (isMessagesModel(model)) return `${base}/zen/v1/messages`;
+    return `${base}/zen/v1/chat/completions`;
   }
 
-  buildHeaders(credentials, stream = true) {
+  buildHeaders(credentials, stream = true, url = "") {
     const raw = credentials?.rawHeaders || {};
     const lower = {};
     for (const [k, v] of Object.entries(raw)) lower[k.toLowerCase()] = v;
@@ -102,7 +107,7 @@ export class OpenCodeExecutor extends BaseExecutor {
     const downstreamUa = lower["user-agent"] || "";
     const isOpencodeDownstream = downstreamUa.toLowerCase().includes("opencode");
 
-    return {
+    const headers = {
       "Content-Type": "application/json",
       "Authorization": "Bearer public",
       "User-Agent": isOpencodeDownstream ? downstreamUa : OPENCODE_UA,
@@ -112,5 +117,7 @@ export class OpenCodeExecutor extends BaseExecutor {
       "x-opencode-project": lower["x-opencode-project"] || "global",
       "Accept": stream ? "text/event-stream" : "*/*",
     };
+    if (url.endsWith("/messages")) headers["anthropic-version"] = ANTHROPIC_API_VERSION;
+    return headers;
   }
 }

open-sse/providers/capabilities.js

@@ -141,6 +141,8 @@ export const MODEL_CAPABILITIES = {
   // via OpenAI Responses input_image; reasoning supports up to xhigh.
   "muse-spark-1.2-contributor-free": { vision: true, reasoning: true, thinkingFormat: "openai", contextWindow: 1048576, maxOutput: 131072 },
   "muse-spark-1.3-contributor-free": { vision: true, reasoning: true, thinkingFormat: "openai", contextWindow: 1048576, maxOutput: 131072 },
+  // OpenCode Free Union Alpha — multimodal (text+vision), 262K context, 131K max output
+  "union-alpha": { vision: true, contextWindow: 262144, maxOutput: 131072 },
 };
 
 const KIRO_GPT_5_6_CAPABILITIES = { vision: true, reasoning: true, search: true, thinkingFormat: "openai", contextWindow: 272000, maxOutput: 128000 };

open-sse/providers/registry/opencode.js

@@ -20,10 +20,10 @@ export default {
     noAuth: true,
   },
   models: [
-    // Muse Spark models are served by /zen/v1/responses; the rest stay on
-    // /chat/completions, so the format is declared per-model, not per-provider.
+    // Endpoint formats differ per model, so declare non-chat models explicitly.
     { id: "muse-spark-1.2-contributor-free", name: "Muse Spark 1.2 Contributor Free", targetFormat: "openai-responses" },
     { id: "muse-spark-1.3-contributor-free", name: "Muse Spark 1.3 Contributor Free", targetFormat: "openai-responses" },
+    { id: "union-alpha", name: "Union Alpha Free", targetFormat: "claude" },
   ],
   modelsFetcher: { url: "https://opencode.ai/zen/v1/models", type: "opencode-free" },
   passthroughModels: true,

tests/unit/opencode-muse-spark-thinking.test.js

@@ -63,6 +63,39 @@ describe("OpenCode Free Muse Spark thinking", () => {
     expect(out.max_tokens).toBeUndefined();
   });
 
+  it("routes Union Alpha through Anthropic Messages", () => {
+    const caps = getCapabilitiesForModel(PROVIDER, "union-alpha");
+    expect(caps.vision).toBe(true);
+    expect(caps.contextWindow).toBe(262144);
+    expect(caps.maxOutput).toBe(131072);
+
+    const executor = new OpenCodeExecutor();
+
+    expect(getModelTargetFormat("oc", "union-alpha")).toBe(FORMATS.CLAUDE);
+    const url = executor.buildUrl("union-alpha");
+    expect(url).toBe("https://opencode.ai/zen/v1/messages");
+    expect(executor.buildHeaders({}, true, url)).toMatchObject({
+      "anthropic-version": "2023-06-01",
+    });
+    expect(executor.buildHeaders({}, true, executor.buildUrl("big-pickle")))
+      .not.toHaveProperty("anthropic-version");
+
+    const translated = translateRequest(
+      FORMATS.OPENAI,
+      FORMATS.CLAUDE,
+      "union-alpha",
+      { messages: [{ role: "user", content: "ping" }], max_tokens: 1 },
+      false,
+      {},
+      PROVIDER,
+    );
+    expect(translated).toMatchObject({
+      model: "union-alpha",
+      messages: [{ role: "user", content: [{ type: "text", text: "ping" }] }],
+      max_tokens: 1,
+    });
+  });
+
   it("leaves the other free models on Chat Completions", () => {
     const executor = new OpenCodeExecutor();
     const body = { messages: [{ role: "user", content: "hi" }], max_tokens: 1024 };