diff --git a/changelog.d/fixes/13599-deepseek-bai-reasoning-content-echo.md b/changelog.d/fixes/13599-deepseek-bai-reasoning-content-echo.md
new file mode 100644
index 0000000000..9f7e923912
--- /dev/null
+++ b/changelog.d/fixes/13599-deepseek-bai-reasoning-content-echo.md
@@ -0,0 +1 @@
+- **fix(providers):** echo back `reasoning_content` on `bai` DeepSeek thinking-mode follow-up turns, fixing the upstream 400 "reasoning_content must be passed back" (#13599) — thanks @afonsoft
diff --git a/open-sse/config/providers/registry/bai/index.ts b/open-sse/config/providers/registry/bai/index.ts
index 574362693c..6423f1c2ec 100644
--- a/open-sse/config/providers/registry/bai/index.ts
+++ b/open-sse/config/providers/registry/bai/index.ts
@@ -11,4 +11,7 @@ export const baiProvider: RegistryEntry = {
   modelsUrl: "https://api.b.ai/v1/models",
   models: [],
   passthroughModels: true,
+  // #13599: bai resells DeepSeek's `deepseek-reasoner` thinking-mode models, which 400
+  // when a prior assistant turn is missing `reasoning_content` on a follow-up request.
+  requiresReasoningContentEcho: true,
 };
diff --git a/open-sse/config/providers/shared.ts b/open-sse/config/providers/shared.ts
index 250d631775..8007b12627 100644
--- a/open-sse/config/providers/shared.ts
+++ b/open-sse/config/providers/shared.ts
@@ -144,6 +144,16 @@ export interface RegistryEntry {
   responsesBaseUrl?: string;
   /** Provider-bound replay format; omitted providers accept portable plaintext reasoning. */
   reasoningTransport?: ReasoningTransport;
+  /**
+   * Thinking-mode upstreams proxied by this provider require the assistant's
+   * prior-turn `reasoning_content` to be echoed back on every follow-up request
+   * (e.g. DeepSeek-reselling gateways such as `bai`). Standard OpenAI-shaped
+   * clients do not preserve that field when replaying history, so when this is
+   * `true`, DefaultExecutor injects a placeholder via
+   * `open-sse/utils/reasoningContentInjector.ts` for model ids matching
+   * `isThinkingMessageModel()`. See issue #13599.
+   */
+  requiresReasoningContentEcho?: boolean;
   /** Anthropic-native /v1/messages endpoint (e.g. GitHub Copilot's shim) used
    *  for models tagged `targetFormat: "claude"` on an otherwise openai-format
    *  provider — see registry/github/index.ts. */
diff --git a/open-sse/executors/default.ts b/open-sse/executors/default.ts
index c267904d9f..3887515203 100644
--- a/open-sse/executors/default.ts
+++ b/open-sse/executors/default.ts
@@ -1013,18 +1013,19 @@ export class DefaultExecutor extends BaseExecutor {
       this.ensureThinkingBudget(withDefaults as Record<string, unknown>, model);
     }
 
-    // 9router#1480: native Moonshot providers 400 when a prior assistant turn
-    // lacks reasoning_content. OpencodeExecutor
-    // already injects a placeholder for OpenCode-routed thinking models; the
-    // direct connections hit neither injection path. Scope to Moonshot ids so
-    // gateway-served models that merely match the thinking-model name pattern
-    // (and may reject an extra field) are unaffected.
-    if (this.provider === "kimi" || this.provider === "moonshot") {
+    // 9router#1480: native Moonshot providers 400 when a prior assistant turn lacks
+    // reasoning_content. Scope to Moonshot ids, or a registry entry opting in via
+    // `requiresReasoningContentEcho` (e.g. `bai`'s DeepSeek resale, #13599).
+    const reasoningEcho =
+      this.provider === "kimi" ||
+      this.provider === "moonshot" ||
+      !!getRegistryEntry(this.provider)?.requiresReasoningContentEcho;
+    if (reasoningEcho) {
       const outboundModel =
         typeof (withDefaults as Record<string, unknown>)?.model === "string"
           ? ((withDefaults as Record<string, unknown>).model as string)
           : model;
-      if (shouldInjectReasoningContentPlaceholder(this.provider, outboundModel)) {
+      if (shouldInjectReasoningContentPlaceholder(reasoningEcho, this.provider, outboundModel)) {
         withDefaults = injectReasoningContentForThinkingModel(withDefaults);
       }
     }
diff --git a/open-sse/utils/reasoningContentInjector.ts b/open-sse/utils/reasoningContentInjector.ts
index a1d69ebf6d..7d3f48a9e7 100644
--- a/open-sse/utils/reasoningContentInjector.ts
+++ b/open-sse/utils/reasoningContentInjector.ts
@@ -55,16 +55,20 @@ export function isThinkingMessageModel(model: string | undefined | null): boolea
   return THINKING_MODEL_PATTERNS.some((re) => re.test(model));
 }
 
+/**
+ * `providerRequiresEcho` is resolved by the caller (Moonshot/Kimi legacy check,
+ * or a registry entry's `requiresReasoningContentEcho` capability flag — see
+ * `open-sse/config/providers/shared.ts`) so the provider allowlist lives in one
+ * place instead of being duplicated here. See issue #13599.
+ */
 export function shouldInjectReasoningContentPlaceholder(
+  providerRequiresEcho: boolean,
   provider: unknown,
   model: string | undefined | null
 ): boolean {
-  const normalizedProvider = String(provider ?? "")
-    .trim()
-    .toLowerCase();
   return (
-    (normalizedProvider === "moonshot" || normalizedProvider === "kimi") &&
-    !requiresAuthenticReasoningContent(normalizedProvider, model) &&
+    providerRequiresEcho &&
+    !requiresAuthenticReasoningContent(provider, model) &&
     isThinkingMessageModel(model)
   );
 }
diff --git a/tests/unit/issue-13599-bai-deepseek-reasoning-content-echo.test.ts b/tests/unit/issue-13599-bai-deepseek-reasoning-content-echo.test.ts
new file mode 100644
index 0000000000..80c56e9060
--- /dev/null
+++ b/tests/unit/issue-13599-bai-deepseek-reasoning-content-echo.test.ts
@@ -0,0 +1,70 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { DefaultExecutor } from "../../open-sse/executors/default.ts";
+
+// Issue #13599: follow-up requests to DeepSeek thinking-mode models served through the
+// `bai` provider (api.b.ai) are rejected upstream with
+//   400 The `reasoning_content` in the thinking mode must be passed back to the API
+// because standard OpenAI-shaped clients do not preserve `reasoning_content` on the prior
+// assistant turn when they replay conversation history. OmniRoute already has a mechanism
+// for exactly this requirement (open-sse/utils/reasoningContentInjector.ts, ported from
+// 9router#1480), but DefaultExecutor.transformRequest only invoked it when
+// `this.provider === "kimi" || this.provider === "moonshot"` (open-sse/executors/default.ts) —
+// the `bai` DeepSeek-reselling gateway was not covered, so a follow-up turn was forwarded to
+// DeepSeek with no `reasoning_content` on the prior assistant message.
+
+function priorTurnBody(model: string) {
+  return {
+    model,
+    stream: false,
+    messages: [
+      { role: "user", content: "What is 2+2?" },
+      // Standard OpenAI-shaped client history: the assistant turn carries only
+      // `content`. It does NOT echo back `reasoning_content` from the previous
+      // response, exactly like a normal ChatGPT-style client would replay it.
+      { role: "assistant", content: "4" },
+      { role: "user", content: "Now multiply that by 10." },
+    ],
+  };
+}
+
+test("DefaultExecutor injects reasoning_content for bai/deepseek follow-up turns (issue #13599)", () => {
+  const executor = new DefaultExecutor("bai");
+  const body = priorTurnBody("bai/deepseek-reasoner");
+
+  const transformed = executor.transformRequest("bai/deepseek-reasoner", body, false, {
+    apiKey: "sk-bai-test",
+  }) as { messages: Array<Record<string, unknown>> };
+
+  const assistantTurn = transformed.messages.find((m) => m.role === "assistant");
+  assert.ok(assistantTurn, "expected an assistant message in the transformed body");
+
+  // The injector's placeholder is a single space (matching the existing Moonshot/Kimi
+  // convention in reasoningContentInjector.ts — DeepSeek only requires the field to be
+  // present and non-empty, not semantically meaningful), so assert non-empty length
+  // directly rather than trimming.
+  assert.ok(
+    typeof assistantTurn!.reasoning_content === "string" &&
+      (assistantTurn!.reasoning_content as string).length > 0,
+    "expected DefaultExecutor to inject a non-empty reasoning_content placeholder on the " +
+      "assistant turn for a bai/deepseek thinking-mode follow-up (it did not — this is issue #13599)"
+  );
+});
+
+test("DefaultExecutor does not touch non-thinking-model bai follow-up turns", () => {
+  const executor = new DefaultExecutor("bai");
+  const body = priorTurnBody("bai/gpt-4o-mini");
+
+  const transformed = executor.transformRequest("bai/gpt-4o-mini", body, false, {
+    apiKey: "sk-bai-test",
+  }) as { messages: Array<Record<string, unknown>> };
+
+  const assistantTurn = transformed.messages.find((m) => m.role === "assistant");
+  assert.ok(assistantTurn, "expected an assistant message in the transformed body");
+  assert.equal(
+    "reasoning_content" in assistantTurn!,
+    false,
+    "a non-thinking model must not receive an injected reasoning_content field"
+  );
+});