fix(translator): drop replayed reasoning fields for Groq/Mistral/Cerebras (#4220)

Strict OpenAI-compatible validators reject unknown assistant-message
fields: Groq 400 ("property 'reasoning_content' is unsupported"),
Mistral 422 ("extra_forbidden"), Cerebras 400 ("wrong_api_format").

Clients driving reasoning models (Hermes Agent, and anything following
the DeepSeek/Kimi convention) echo the previous turn's reasoning_content
on every assistant message, so from the second turn on every request to
these providers fails and a fallback combo silently skips them.

Add a dropMessageFields rule to paramSupport.js that strips
reasoning_content / reasoning / reasoning_details from assistant turns
for groq, mistral, and cerebras.

open-sse/translator/concerns/paramSupport.js

@@ -24,6 +24,15 @@ const STRIP_RULES = [
   // "integer above maximum value, expected <= 32768". Pin an explicit endpoint cap;
   // min() with the model ceiling still applies if a variant's own limit is lower.
   { provider: "volcengine-ark", match: /kimi/i, maxOutputCap: 32768, clampToModelMaxOutput: true },
+  // Strict OpenAI-compatible validators reject unknown assistant-message fields.
+  // Clients that talk to reasoning models (e.g. Hermes) echo the prior turn's
+  // reasoning back on every assistant message; Groq answers 400 and Mistral 422
+  // ("extra_forbidden") on it, which knocks these providers out of every
+  // multi-turn combo. Providers that *require* the field (DeepSeek, Kimi) are
+  // handled by reasoningContentInjector and are not listed here.
+  { provider: "groq", dropMessageFields: ["reasoning_content", "reasoning", "reasoning_details"] },
+  { provider: "mistral", dropMessageFields: ["reasoning_content", "reasoning", "reasoning_details"] },
+  { provider: "cerebras", dropMessageFields: ["reasoning_content", "reasoning", "reasoning_details"] },
 ];
 
 // Test a rule's match (regex or predicate) against the model id.
@@ -47,6 +56,15 @@ export function stripUnsupportedParams(provider, model, body) {
     for (const key of rule.drop || []) {
       if (body[key] !== undefined) delete body[key];
     }
+    // Per-message field drop (assistant turns only — that is where clients replay reasoning).
+    if (Array.isArray(rule.dropMessageFields) && Array.isArray(body.messages)) {
+      for (const msg of body.messages) {
+        if (!msg || msg.role !== "assistant") continue;
+        for (const key of rule.dropMessageFields) {
+          if (msg[key] !== undefined) delete msg[key];
+        }
+      }
+    }
     // CF Workers AI oneOf root schema only accepts content as plain string (#1926)
     if (rule.flattenContent && Array.isArray(body.messages)) {
       for (const msg of body.messages) {

tests/unit/param-support.test.js

@@ -52,4 +52,48 @@ describe("stripUnsupportedParams", () => {
 
     expect(body.max_tokens).toBe(64000);
   });
+
+  it("drops replayed reasoning fields from assistant messages for strict providers", () => {
+    const makeBody = () => ({
+      messages: [
+        { role: "user", content: "hi" },
+        {
+          role: "assistant",
+          content: "hello",
+          reasoning_content: "thinking...",
+          reasoning: "thinking...",
+          reasoning_details: [{ text: "thinking..." }],
+          tool_calls: [{ id: "c1", type: "function", function: { name: "f", arguments: "{}" } }],
+        },
+        { role: "user", content: "again", reasoning_content: "user-side field stays" },
+      ],
+    });
+
+    for (const [provider, model] of [
+      ["groq", "openai/gpt-oss-120b"],
+      ["mistral", "codestral-latest"],
+      ["cerebras", "gpt-oss-120b"],
+    ]) {
+      const body = makeBody();
+      stripUnsupportedParams(provider, model, body);
+      expect(body.messages[1]).toEqual({
+        role: "assistant",
+        content: "hello",
+        tool_calls: [{ id: "c1", type: "function", function: { name: "f", arguments: "{}" } }],
+      });
+      // only assistant turns are touched
+      expect(body.messages[2].reasoning_content).toBe("user-side field stays");
+    }
+  });
+
+  it("leaves reasoning fields alone for providers that accept or require them", () => {
+    const body = {
+      messages: [{ role: "assistant", content: "hello", reasoning_content: "thinking..." }],
+    };
+
+    stripUnsupportedParams("deepseek", "deepseek-reasoner", body);
+    stripUnsupportedParams("openrouter", "nvidia/nemotron-3-ultra-550b-a55b:free", body);
+
+    expect(body.messages[0].reasoning_content).toBe("thinking...");
+  });
 });