fix(providers): switch minimax from claude to openai format so images work

The Anthropic-compatible /anthropic/v1/messages endpoint rejects image
input with 403. MiniMax's OpenAI-compatible /v1/chat/completions endpoint
supports image_url natively for MiniMax-M3.

- minimax + minimax-cn: format claude→openai, baseUrl→/v1/chat/completions
- Remove Anthropic-Version header + ?beta=true suffix (not needed for openai)
- Remove minimax/minimax-cn from ?beta=true executor case
- Update cache-control tests (openai format uses different caching path)
- Fix reasoning-split test names (no longer claude format)

TDD: 2 registry tests assert format=openai (red→green).
Refs: Hermes Agent #15715, MiniMax OpenAI-compatible API docs.

open-sse/config/providers/registry/minimax/cn/index.ts

@@ -1,17 +1,14 @@
 import type { RegistryEntry } from "../../../shared.ts";
-import { getAnthropicCompatHeaders, ANTHROPIC_VERSION_HEADER } from "../../../shared.ts";
 
 export const minimax_cnProvider: RegistryEntry = {
   id: "minimax-cn",
   alias: "minimax-cn", // unique alias (was colliding with minimax)
-  format: "claude",
+  format: "openai",
   executor: "default",
-  baseUrl: "https://api.minimaxi.com/anthropic/v1/messages",
+  baseUrl: "https://api.minimaxi.com/v1/chat/completions",
   modelsUrl: "https://api.minimaxi.com/v1/models",
-  urlSuffix: "?beta=true",
   authType: "apikey",
   authHeader: "bearer",
-  headers: getAnthropicCompatHeaders(),
   models: [
     // Keep parity with minimax to ensure model discovery works for minimax-cn connections.
     // #3110: MiniMax M3 — frontier coding model with 1M context

open-sse/config/providers/registry/minimax/index.ts

@@ -1,17 +1,14 @@
 import type { RegistryEntry } from "../../shared.ts";
-import { getAnthropicCompatHeaders, ANTHROPIC_VERSION_HEADER } from "../../shared.ts";
 
 export const minimaxProvider: RegistryEntry = {
   id: "minimax",
   alias: "minimax",
-  format: "claude",
+  format: "openai",
   executor: "default",
-  baseUrl: "https://api.minimax.io/anthropic/v1/messages",
+  baseUrl: "https://api.minimax.io/v1/chat/completions",
   modelsUrl: "https://api.minimax.io/v1/models",
-  urlSuffix: "?beta=true",
   authType: "apikey",
   authHeader: "bearer",
-  headers: getAnthropicCompatHeaders(),
   models: [
     // T12/T28: MiniMax default upgraded from M2.5 to M2.7
     // #3110: MiniMax M3 — frontier coding model with 1M context

open-sse/executors/default.ts

@@ -289,8 +289,6 @@ export class DefaultExecutor extends BaseExecutor {
       case "glm":
       case "glmt":
       case "kimi-coding":
-      case "minimax":
-      case "minimax-cn":
         return `${this.config.baseUrl}?beta=true`;
       case "agentrouter":
         return this.usesClaudeCodeProtocol(credentials)

tests/unit/cache-control-claude-providers.test.ts

@@ -17,8 +17,10 @@ describe("Cache Control Policy - Claude Protocol Providers", () => {
     // These should be detected via targetFormat
     assert.equal(providerSupportsCaching("bailian-coding-plan", "claude"), true);
     assert.equal(providerSupportsCaching("glm", "claude"), true);
-    assert.equal(providerSupportsCaching("minimax", "claude"), true);
-    assert.equal(providerSupportsCaching("minimax-cn", "claude"), true);
+    // minimax/minimax-cn use openai format (#3110 / image 403 fix);
+    // caching support for their OpenAI-compatible endpoint is TBD
+    assert.equal(providerSupportsCaching("minimax", "openai"), false);
+    assert.equal(providerSupportsCaching("minimax-cn", "openai"), false);
     assert.equal(providerSupportsCaching("kimi-coding", "claude"), true);
 
     // #3955 — OpenAI / Codex use automatic prefix caching (no cache_control needed).
@@ -66,15 +68,17 @@ describe("Cache Control Policy - Claude Protocol Providers", () => {
       true
     );
 
+    // minimax now uses openai format — caching behavior may differ;
+    // cache_control preservation depends on whether it joins CACHING_PROVIDERS
     assert.equal(
       shouldPreserveCacheControl({
         userAgent: claudeCodeUA,
         isCombo: false,
         targetProvider: "minimax",
-        targetFormat: "claude",
+        targetFormat: "openai",
         settings: { alwaysPreserveClientCache: "auto" },
       }),
-      true
+      false
     );
   });
 

tests/unit/minimax-m3-model-registry.test.ts

@@ -73,6 +73,26 @@ describe("MiniMax M3 model registration (#3110)", () => {
     assert.equal(m3.contextLength, 1_048_576);
   });
 
+  it("minimax uses openai format (not claude) so images work via /v1/chat/completions", () => {
+    const entry = REGISTRY.minimax;
+    assert.ok(entry, "minimax registry entry must exist");
+    assert.equal(
+      entry.format,
+      "openai",
+      "minimax must use openai format — the Anthropic-compatible /anthropic/v1/messages endpoint rejects images with 403; images work on the OpenAI-compatible /v1/chat/completions endpoint. See Hermes Agent #15715."
+    );
+  });
+
+  it("minimax-cn uses openai format (not claude) so images work via /v1/chat/completions", () => {
+    const entry = REGISTRY["minimax-cn"];
+    assert.ok(entry, "minimax-cn registry entry must exist");
+    assert.equal(
+      entry.format,
+      "openai",
+      "minimax-cn must use openai format — parity with minimax; the Anthropic endpoint on api.minimaxi.com also rejects images."
+    );
+  });
+
   it("nvidia provider does NOT list minimaxai/minimax-m3 (removed in #3329 — 404 upstream)", () => {
     const entry = REGISTRY.nvidia;
     assert.ok(entry, "nvidia registry entry must exist");

tests/unit/responsesanitizer-reasoning-split.test.ts

@@ -137,12 +137,12 @@ describe("responseSanitizer/reasoning — MiniMax M3 textual reasoning-tag route
 });
 
 describe("responseSanitizer/reasoning — MiniMax M3 fix regression guards", () => {
-  it("direct minimax tier (claude format) stays unaffected", () => {
+  it("direct minimax tier (openai format) stays unaffected for textual reasoning tags", () => {
     assert.equal(isTextualReasoningTagNativeRoute("minimax", "minimax-m3"), false);
     assert.equal(shouldParseTextualReasoningTags("minimax", "MiniMax-M3"), false);
   });
 
-  it("direct minimax-cn tier (claude format) stays unaffected", () => {
+  it("direct minimax-cn tier (openai format) stays unaffected for textual reasoning tags", () => {
     assert.equal(isTextualReasoningTagNativeRoute("minimax-cn", "minimax-m3"), false);
     assert.equal(shouldParseTextualReasoningTags("minimax-cn", "MiniMax-M3"), false);
   });