diff --git a/CHANGELOG.md b/CHANGELOG.md
index fb1f69ad4b..ce49d3dfe9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@
 - **i18n**: Thai (th) + Persian (fa) translations / README
 
 ## Fixes
+- **MiniMax**: add the required empty signature field to unsigned Anthropic thinking block starts
 - **Providers**: bulk-add API keys no longer overwrite existing keys (gap-fill `Key N`)
 - **Anthropic**: lowercase `anthropic-version` header to prevent duplication on `/v1/messages`
 - **Alicode-intl**: use DashScope compatible-mode endpoint so standard keys work
diff --git a/open-sse/providers/registry/minimax-cn.js b/open-sse/providers/registry/minimax-cn.js
index 19aa3127c1..ab3109408b 100644
--- a/open-sse/providers/registry/minimax-cn.js
+++ b/open-sse/providers/registry/minimax-cn.js
@@ -22,6 +22,7 @@ export default {
     headers: { ...CLAUDE_API_HEADERS },
     quirks: {
       dropOutputConfig: true,
+      ensureThinkingSignature: true,
     },
     reasoningInject: {
       scope: "all",
diff --git a/open-sse/providers/registry/minimax.js b/open-sse/providers/registry/minimax.js
index 47b82c896f..481e3b07e2 100644
--- a/open-sse/providers/registry/minimax.js
+++ b/open-sse/providers/registry/minimax.js
@@ -22,6 +22,7 @@ export default {
     headers: { ...CLAUDE_API_HEADERS },
     quirks: {
       dropOutputConfig: true,
+      ensureThinkingSignature: true,
     },
     reasoningInject: {
       scope: "all",
diff --git a/open-sse/utils/stream.js b/open-sse/utils/stream.js
index 464acde467..f6020f44f2 100644
--- a/open-sse/utils/stream.js
+++ b/open-sse/utils/stream.js
@@ -1,5 +1,7 @@
 import { translateResponse, initState } from "../translator/index.js";
 import { FORMATS } from "../translator/formats.js";
+import { CLAUDE_BLOCK } from "../translator/schema/index.js";
+import { PROVIDERS } from "../config/providers.js";
 import { trackPendingRequest, appendRequestLog } from "@/lib/usageDb.js";
 import { extractUsage, mergeUsage, hasValidUsage, estimateUsage, logUsage, addBufferToUsage, filterUsageForFormat, COLORS } from "./usageTracking.js";
 import { parseSSELine, hasValuableContent, fixInvalidId, formatSSE } from "./streamHelpers.js";
@@ -107,10 +109,23 @@ export function createSSEStream(options = {}) {
             try {
               const parsed = JSON.parse(trimmed.slice(5).trim());
 
+              // Some Anthropic-compatible providers omit `signature` from the
+              // thinking block start. Strict Messages clients deserialize that
+              // field before later signature_delta events arrive.
+              let fieldsInjected = false;
+              if (
+                PROVIDERS[provider]?.quirks?.ensureThinkingSignature &&
+                parsed.type === "content_block_start" &&
+                parsed.content_block?.type === CLAUDE_BLOCK.THINKING &&
+                parsed.content_block.signature === undefined
+              ) {
+                parsed.content_block.signature = "";
+                fieldsInjected = true;
+              }
+
               const idFixed = fixInvalidId(parsed);
 
               // Ensure OpenAI-required fields are present on streaming chunks (Letta compat)
-              let fieldsInjected = false;
               if (parsed.choices !== undefined) {
                 if (!parsed.object) { parsed.object = "chat.completion.chunk"; fieldsInjected = true; }
                 if (!parsed.created) { parsed.created = Math.floor(Date.now() / 1000); fieldsInjected = true; }
diff --git a/tests/unit/minimax-thinking-signature.test.js b/tests/unit/minimax-thinking-signature.test.js
new file mode 100644
index 0000000000..d04464512e
--- /dev/null
+++ b/tests/unit/minimax-thinking-signature.test.js
@@ -0,0 +1,118 @@
+import { describe, expect, it } from "vitest";
+
+import { PROVIDERS } from "../../open-sse/config/providers.js";
+import { createPassthroughStreamWithLogger } from "../../open-sse/utils/stream.js";
+
+async function runPassthrough(provider, input, chunkSize = input.length) {
+  const encoder = new TextEncoder();
+  const inputStream = new ReadableStream({
+    start(controller) {
+      for (let index = 0; index < input.length; index += chunkSize) {
+        controller.enqueue(encoder.encode(input.slice(index, index + chunkSize)));
+      }
+      controller.close();
+    },
+  });
+  const outputStream = inputStream.pipeThrough(
+    createPassthroughStreamWithLogger(provider),
+  );
+  const reader = outputStream.getReader();
+  const decoder = new TextDecoder();
+  let output = "";
+
+  while (true) {
+    const { value, done } = await reader.read();
+    if (done) break;
+    output += decoder.decode(value, { stream: true });
+  }
+  output += decoder.decode();
+  return output;
+}
+
+function firstDataEvent(output) {
+  const line = output.split("\n").find((item) => item.startsWith("data: {"));
+  return JSON.parse(line.slice(6));
+}
+
+describe("MiniMax Anthropic thinking stream", () => {
+  it.each(["minimax", "minimax-cn"])(
+    "enables thinking signature normalization for %s",
+    (provider) => {
+      expect(PROVIDERS[provider].quirks.ensureThinkingSignature).toBe(true);
+    },
+  );
+
+  it.each(["minimax", "minimax-cn"])(
+    "adds a deserializable signature field for %s thinking block starts",
+    async (provider) => {
+      const event = {
+        type: "content_block_start",
+        index: 0,
+        content_block: { type: "thinking", thinking: "" },
+      };
+      const output = await runPassthrough(
+        provider,
+        `event: content_block_start\ndata: ${JSON.stringify(event)}\n\n`,
+      );
+
+      expect(firstDataEvent(output).content_block.signature).toBe("");
+    },
+  );
+
+  it("preserves real signature events across fragmented chunks", async () => {
+    const start = {
+      type: "content_block_start",
+      index: 0,
+      content_block: { type: "thinking", thinking: "" },
+    };
+    const signature = {
+      type: "content_block_delta",
+      index: 0,
+      delta: { type: "signature_delta", signature: "minimax-signature" },
+    };
+    const output = await runPassthrough(
+      "minimax",
+      `event: content_block_start\ndata: ${JSON.stringify(start)}\n\n` +
+        `event: content_block_delta\ndata: ${JSON.stringify(signature)}\n\n`,
+      7,
+    );
+    const events = output
+      .split("\n")
+      .filter((line) => line.startsWith("data: {"))
+      .map((line) => JSON.parse(line.slice(6)));
+
+    expect(events[0].content_block.signature).toBe("");
+    expect(events[1].delta).toEqual({
+      type: "signature_delta",
+      signature: "minimax-signature",
+    });
+  });
+
+  it("preserves a MiniMax signature already present on the block start", async () => {
+    const event = {
+      type: "content_block_start",
+      index: 0,
+      content_block: { type: "thinking", thinking: "", signature: "minimax-signature" },
+    };
+    const output = await runPassthrough(
+      "minimax",
+      `event: content_block_start\ndata: ${JSON.stringify(event)}\n\n`,
+    );
+
+    expect(firstDataEvent(output).content_block.signature).toBe("minimax-signature");
+  });
+
+  it("does not modify unsigned thinking starts from unrelated providers", async () => {
+    const event = {
+      type: "content_block_start",
+      index: 0,
+      content_block: { type: "thinking", thinking: "" },
+    };
+    const output = await runPassthrough(
+      "deepseek",
+      `event: content_block_start\ndata: ${JSON.stringify(event)}\n\n`,
+    );
+
+    expect(firstDataEvent(output).content_block).not.toHaveProperty("signature");
+  });
+});