fix(minimax): echo reasoning_content on follow-up turns to avoid 400

MiniMax requires reasoning_content echoed back on assistant messages in
multi-turn/tool-call conversations. Add minimax and minimax-cn to
PROVIDER_RULES (scope all), same fix as DeepSeek (#1543).

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/utils/reasoningContentInjector.js

@@ -1,4 +1,4 @@
-// Some thinking-mode providers (DeepSeek, Kimi, ...) require reasoning_content
+// Some thinking-mode providers (DeepSeek, Kimi, MiniMax, ...) require reasoning_content
 // to be echoed back on assistant messages. Clients in OpenAI format don't send it,
 // so we inject a non-empty placeholder to satisfy upstream validation.
 
@@ -6,7 +6,9 @@ const PLACEHOLDER = " ";
 
 // Provider-level rules: keyed by executor.provider
 const PROVIDER_RULES = {
-  deepseek: { scope: "all" }
+  deepseek: { scope: "all" },
+  minimax: { scope: "all" },
+  "minimax-cn": { scope: "all" }
 };
 
 // Model-level rules: matched by predicate against model id

tests/unit/reasoningContentInjector.test.js

@@ -84,6 +84,69 @@ describe("injectReasoningContent — DeepSeek thinking round-trip", () => {
   });
 });
 
+describe("injectReasoningContent — MiniMax thinking round-trip", () => {
+  const minimaxAssistantMsg = { role: "assistant", content: "here is a response", reasoning_content: "" };
+  const minimaxAssistantWithToolCall = {
+    role: "assistant",
+    content: "",
+    tool_calls: [{ id: "call_x", type: "function", function: { name: "get_weather", arguments: "{}" } }],
+    reasoning_content: "",
+  };
+
+  it("injects reasoning_content on a minimax assistant message that lacks it", () => {
+    const out = injectReasoningContent({
+      provider: "minimax",
+      model: "MiniMax-M2.7",
+      body: bodyWith([{ role: "user", content: "hi" }, minimaxAssistantMsg]),
+    });
+    const assistant = out.messages.find((m) => m.role === "assistant");
+    expect(typeof assistant.reasoning_content).toBe("string");
+    expect(assistant.reasoning_content.length).toBeGreaterThan(0);
+  });
+
+  it("injects reasoning_content on minimax assistant message with tool_calls but no reasoning_content", () => {
+    const out = injectReasoningContent({
+      provider: "minimax",
+      model: "MiniMax-M2.7",
+      body: bodyWith([{ role: "user", content: "hi" }, minimaxAssistantWithToolCall]),
+    });
+    const assistant = out.messages.find((m) => m.role === "assistant");
+    expect(typeof assistant.reasoning_content).toBe("string");
+    expect(assistant.reasoning_content.length).toBeGreaterThan(0);
+  });
+
+  it("applies provider-level rule for provider 'minimax-cn' (scope all)", () => {
+    const out = injectReasoningContent({
+      provider: "minimax-cn",
+      model: "MiniMax-M2.5",
+      body: bodyWith([{ role: "assistant", content: "answer" }]),
+    });
+    expect(out.messages[0].reasoning_content).toBeDefined();
+  });
+
+  it("preserves an existing reasoning_content on minimax instead of overwriting", () => {
+    const original = "MiniMax chain of thought reasoning";
+    const out = injectReasoningContent({
+      provider: "minimax",
+      model: "MiniMax-M2.7",
+      body: bodyWith([{ ...minimaxAssistantMsg, reasoning_content: original }]),
+    });
+    expect(out.messages[0].reasoning_content).toBe(original);
+  });
+
+  it("DefaultExecutor transformRequest runs the injector for minimax", () => {
+    const { DefaultExecutor } = require("../../open-sse/executors/default.js");
+    const executor = new DefaultExecutor("minimax");
+    const out = executor.transformRequest(
+      "MiniMax-M2.7",
+      bodyWith([{ role: "user", content: "hi" }, minimaxAssistantMsg]),
+    );
+    const assistant = out.messages.find((m) => m.role === "assistant");
+    expect(typeof assistant.reasoning_content).toBe("string");
+    expect(assistant.reasoning_content.length).toBeGreaterThan(0);
+  });
+});
+
 describe("OpenCodeExecutor — issue #1543 regression", () => {
   it("runs the injector so deepseek-v4-flash-free round-trips reasoning_content", () => {
     const executor = new OpenCodeExecutor();