From b92f045f704daae648985ba9b7089321fae6537b Mon Sep 17 00:00:00 2001
From: Diego Rodrigues de Sa e Souza
 <8016841+diegosouzapw@users.noreply.github.com>
Date: Tue, 23 Jun 2026 22:01:00 -0300
Subject: [PATCH] fix(opencode): preserve DeepSeek reasoning content in
 streamed responses (#4631)
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Integrated into release/v3.8.36 — DeepSeek reasoning_content injection (port #1099); release-green
---
 open-sse/executors/opencode.ts                | 14 ++-
 open-sse/utils/reasoningContentInjector.ts    | 73 ++++++++++++++
 ...encode-deepseek-reasoning-injected.test.ts | 94 +++++++++++++++++++
 3 files changed, 180 insertions(+), 1 deletion(-)
 create mode 100644 open-sse/utils/reasoningContentInjector.ts
 create mode 100644 tests/unit/opencode-deepseek-reasoning-injected.test.ts

diff --git a/open-sse/executors/opencode.ts b/open-sse/executors/opencode.ts
index af0cf431b3..28666efb76 100644
--- a/open-sse/executors/opencode.ts
+++ b/open-sse/executors/opencode.ts
@@ -7,6 +7,10 @@ import {
 } from "./base.ts";
 import { PROVIDERS } from "../config/constants.ts";
 import { getModelTargetFormat } from "../config/providerModels.ts";
+import {
+  injectReasoningContentForThinkingModel,
+  isThinkingMessageModel,
+} from "../utils/reasoningContentInjector.ts";
 
 export class OpencodeExecutor extends BaseExecutor {
   _requestFormat: string | null = null;
@@ -138,7 +142,7 @@ export class OpencodeExecutor extends BaseExecutor {
     stream: boolean,
     credentials: ProviderCredentials
   ): any {
-    const modifiedBody = super.transformRequest(model, body, stream, credentials);
+    let modifiedBody = super.transformRequest(model, body, stream, credentials);
     if (
       modifiedBody &&
       typeof modifiedBody === "object" &&
@@ -162,6 +166,14 @@ export class OpencodeExecutor extends BaseExecutor {
         }
       }
     }
+    // #1543 / upstream PR #1099: thinking-mode upstreams routed through OpenCode
+    // (DeepSeek V4 Flash, Kimi, MiniMax, ...) require reasoning_content echoed
+    // back on assistant messages, or they 400 with "reasoning_content must be
+    // passed back". OpenAI clients drop it across turns, so we inject a
+    // placeholder for the affected model families.
+    if (isThinkingMessageModel(model)) {
+      modifiedBody = injectReasoningContentForThinkingModel(modifiedBody);
+    }
     return modifiedBody;
   }
 }
diff --git a/open-sse/utils/reasoningContentInjector.ts b/open-sse/utils/reasoningContentInjector.ts
new file mode 100644
index 0000000000..8fd9b13bd4
--- /dev/null
+++ b/open-sse/utils/reasoningContentInjector.ts
@@ -0,0 +1,73 @@
+/**
+ * Thinking-mode upstreams (DeepSeek V4 Flash, Kimi, MiniMax, ...) require
+ * `reasoning_content` to be echoed back on every assistant message in the
+ * conversation history. Standard OpenAI clients do not preserve that field
+ * across turns, so we inject a non-empty placeholder before forwarding.
+ *
+ * Without the placeholder these upstreams return:
+ *   400 Bad Request — reasoning_content must be passed back
+ *
+ * Ported from decolua/9router#1099 (issue #1543). Pure helper — no I/O, no
+ * cross-module deps — kept narrow so it can be reused by other meta-providers
+ * that proxy to thinking-mode models.
+ */
+
+const PLACEHOLDER = " ";
+
+type JsonRecord = Record<string, unknown>;
+
+/**
+ * Model-id predicates for thinking-mode families that need the echo.
+ * Matched case-insensitively against the resolved model id (post upstream
+ * routing, e.g. `oc/deepseek-v4-flash-free` for the OpenCode meta-provider).
+ */
+const THINKING_MODEL_PATTERNS: RegExp[] = [
+  /deepseek/i,
+  /\bkimi\b/i,
+  /\bk2\b/i, // moonshot kimi k2 family alias
+  /\bminimax\b/i,
+];
+
+export function isThinkingMessageModel(model: string | undefined | null): boolean {
+  if (!model || typeof model !== "string") return false;
+  return THINKING_MODEL_PATTERNS.some((re) => re.test(model));
+}
+
+function hasNonEmptyReasoningContent(message: JsonRecord): boolean {
+  return (
+    typeof message.reasoning_content === "string" &&
+    (message.reasoning_content as string).trim().length > 0
+  );
+}
+
+function isAssistantMessage(value: unknown): value is JsonRecord {
+  return (
+    !!value &&
+    typeof value === "object" &&
+    !Array.isArray(value) &&
+    (value as JsonRecord).role === "assistant"
+  );
+}
+
+/**
+ * Inject a placeholder `reasoning_content` on every assistant message in
+ * `body.messages` that lacks one. Returns the original object if no mutation
+ * was needed, or a shallow-copied body with a new messages array otherwise.
+ *
+ * No-op when the body shape is unexpected (defensive).
+ */
+export function injectReasoningContentForThinkingModel(body: unknown): unknown {
+  if (!body || typeof body !== "object" || Array.isArray(body)) return body;
+  const record = body as JsonRecord;
+  if (!Array.isArray(record.messages)) return body;
+
+  let modified = false;
+  const messages = record.messages.map((message) => {
+    if (!isAssistantMessage(message)) return message;
+    if (hasNonEmptyReasoningContent(message)) return message;
+    modified = true;
+    return { ...message, reasoning_content: PLACEHOLDER };
+  });
+
+  return modified ? { ...record, messages } : body;
+}
diff --git a/tests/unit/opencode-deepseek-reasoning-injected.test.ts b/tests/unit/opencode-deepseek-reasoning-injected.test.ts
new file mode 100644
index 0000000000..9e98a6c601
--- /dev/null
+++ b/tests/unit/opencode-deepseek-reasoning-injected.test.ts
@@ -0,0 +1,94 @@
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+
+const { OpencodeExecutor } = await import("../../open-sse/executors/opencode.ts");
+
+/**
+ * Regression test for upstream decolua/9router#1099 (issue #1543):
+ *
+ * OpenCode is a meta-provider that proxies multi-turn chats to thinking-mode
+ * upstreams (DeepSeek V4 Flash, Kimi, MiniMax, ...). Those upstreams require
+ * `reasoning_content` to be echoed back on every assistant message in the
+ * conversation history; otherwise they return:
+ *
+ *     400 Bad Request — reasoning_content must be passed back
+ *
+ * Standard OpenAI clients do not preserve `reasoning_content` across turns, so
+ * OpencodeExecutor.transformRequest must inject a non-empty placeholder on any
+ * assistant message that lacks it when routing to a thinking-mode model.
+ */
+describe("OpencodeExecutor — DeepSeek reasoning_content injection (#1099)", () => {
+  const executor = new OpencodeExecutor("opencode-zen");
+
+  function buildBody(extraAssistant: Record<string, unknown>) {
+    return {
+      model: "oc/deepseek-v4-flash-free",
+      stream: true,
+      messages: [
+        { role: "user", content: "hi" },
+        { role: "assistant", content: "previous answer", ...extraAssistant },
+        { role: "user", content: "follow up" },
+      ],
+    };
+  }
+
+  it("injects reasoning_content on an assistant message that lacks it (deepseek model)", () => {
+    const out = executor.transformRequest(
+      "oc/deepseek-v4-flash-free",
+      buildBody({}),
+      true,
+      { apiKey: "test" } as never
+    );
+    const assistant = out.messages.find((m: { role: string }) => m.role === "assistant");
+    assert.equal(typeof assistant.reasoning_content, "string");
+    assert.ok(
+      assistant.reasoning_content.length > 0,
+      "expected non-empty reasoning_content placeholder injected"
+    );
+  });
+
+  it("preserves an existing non-empty reasoning_content instead of overwriting", () => {
+    const original = "prior chain of thought";
+    const out = executor.transformRequest(
+      "oc/deepseek-v4-flash-free",
+      buildBody({ reasoning_content: original }),
+      true,
+      { apiKey: "test" } as never
+    );
+    const assistant = out.messages.find((m: { role: string }) => m.role === "assistant");
+    assert.equal(assistant.reasoning_content, original);
+  });
+
+  it("injects for kimi-family models routed through opencode", () => {
+    const body = {
+      model: "oc/kimi-k2-thinking",
+      stream: false,
+      messages: [
+        { role: "user", content: "hi" },
+        { role: "assistant", content: "answer" },
+      ],
+    };
+    const out = executor.transformRequest("oc/kimi-k2-thinking", body, false, {
+      apiKey: "test",
+    } as never);
+    const assistant = out.messages.find((m: { role: string }) => m.role === "assistant");
+    assert.equal(typeof assistant.reasoning_content, "string");
+    assert.ok(assistant.reasoning_content.length > 0);
+  });
+
+  it("does not inject for non-thinking opencode models (no false positives)", () => {
+    const body = {
+      model: "oc/gpt-5",
+      stream: false,
+      messages: [
+        { role: "user", content: "hi" },
+        { role: "assistant", content: "answer" },
+      ],
+    };
+    const out = executor.transformRequest("oc/gpt-5", body, false, {
+      apiKey: "test",
+    } as never);
+    const assistant = out.messages.find((m: { role: string }) => m.role === "assistant");
+    assert.equal(assistant.reasoning_content, undefined);
+  });
+});