diff --git a/open-sse/translator/request/claude-to-openai.ts b/open-sse/translator/request/claude-to-openai.ts
index 9b8b70f3c7..ab50607e75 100644
--- a/open-sse/translator/request/claude-to-openai.ts
+++ b/open-sse/translator/request/claude-to-openai.ts
@@ -349,7 +349,15 @@ function fixMissingToolResponses(messages) {
 
 // Convert single Claude message - returns single message or array of messages
 function convertClaudeMessage(msg, preserveCacheControl = false) {
-  const role = msg.role === "user" || msg.role === "tool" ? "user" : "assistant";
+  // Preserve system role for mid-conversation system turns (#6954).
+  // Previously any role that wasn't "user" or "tool" was mapped to "assistant",
+  // which misattributed system messages as assistant output.
+  const role =
+    msg.role === "user" || msg.role === "tool"
+      ? "user"
+      : msg.role === "system"
+        ? "system"
+        : "assistant";
 
   // Simple string content
   if (typeof msg.content === "string") {
@@ -411,9 +419,7 @@ function convertClaudeMessage(msg, preserveCacheControl = false) {
             function: {
               name: block.name,
               arguments:
-                typeof block.input === "string"
-                  ? block.input
-                  : JSON.stringify(block.input || {}),
+                typeof block.input === "string" ? block.input : JSON.stringify(block.input || {}),
             },
           });
           break;
diff --git a/open-sse/translator/request/openai-to-claude.ts b/open-sse/translator/request/openai-to-claude.ts
index ff5dc4b6d5..1cfa99ddff 100644
--- a/open-sse/translator/request/openai-to-claude.ts
+++ b/open-sse/translator/request/openai-to-claude.ts
@@ -592,7 +592,24 @@ function getContentBlocksFromMessage(
         if (part.type === "text" && part.text) {
           blocks.push({ type: "text", text: part.text });
         } else if (part.type === "thinking" || part.type === "redacted_thinking") {
-          // Preserve thinking blocks with signature
+          // #6953 — thinking blocks with signature:"" (empty string) come from non-Anthropic
+          // providers (codex/gpt-5.x).  Anthropic rejects replayed `thinking` blocks that
+          // carry a foreign or fabricated signature with HTTP 400.  Fabricating a default
+          // signature (the old behaviour) made the poisoning permanent: once a codex-served
+          // turn introduced a `signature:""` thinking block, every subsequent Anthropic leg
+          // attempt 400'd and the router silently fell back to codex forever.
+          //
+          // Fix: strip thinking blocks whose signature is the empty string — that explicit
+          // empty value is the hallmark of a synthesized block from a non-Anthropic provider.
+          // Thinking blocks with `signature: undefined` (field absent) are legitimate Claude-
+          // format messages and fall through to the DEFAULT_THINKING_CLAUDE_SIGNATURE fallback
+          // as before.
+          if (part.type === "thinking" && part.signature === "") {
+            continue; // drop — synthesized by non-Anthropic provider, no valid signature
+          }
+          if (part.type === "redacted_thinking" && part.data === "") {
+            continue; // drop — same: empty data from non-Anthropic provider
+          }
           blocks.push({
             ...part,
             signature: part.signature || DEFAULT_THINKING_CLAUDE_SIGNATURE,
diff --git a/tests/unit/claude-to-openai-system-role-6954.test.ts b/tests/unit/claude-to-openai-system-role-6954.test.ts
new file mode 100644
index 0000000000..d9c48ac085
--- /dev/null
+++ b/tests/unit/claude-to-openai-system-role-6954.test.ts
@@ -0,0 +1,103 @@
+/**
+ * Tests for #6954 — mid-conversation system turns misattributed as assistant.
+ *
+ * `convertClaudeMessage` mapped any role that wasn't "user" or "tool" to
+ * "assistant", so a Claude message with `role: "system"` (e.g. an injected
+ * system reminder mid-conversation) was forwarded to OpenAI-format upstreams
+ * as an assistant turn — polluting the conversation history.
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { claudeToOpenAIRequest } =
+  await import("../../open-sse/translator/request/claude-to-openai.ts");
+
+// ---------------------------------------------------------------------------
+// 1. system message mid-conversation keeps role: "system"
+// ---------------------------------------------------------------------------
+test("mid-conversation system message preserves role:system (not assistant)", () => {
+  const result = claudeToOpenAIRequest(
+    "gpt-4o",
+    {
+      messages: [
+        { role: "user", content: "hello" },
+        { role: "assistant", content: "hi" },
+        { role: "system", content: "Reminder: be concise." },
+        { role: "user", content: "ok" },
+      ],
+    },
+    false
+  );
+
+  const roles = result.messages.map((m: { role: string }) => m.role);
+  assert.deepEqual(roles, ["user", "assistant", "system", "user"]);
+});
+
+// ---------------------------------------------------------------------------
+// 2. system message with array content keeps role: "system"
+// ---------------------------------------------------------------------------
+test("system message with array content preserves role:system", () => {
+  const result = claudeToOpenAIRequest(
+    "gpt-4o",
+    {
+      messages: [
+        { role: "user", content: "hello" },
+        {
+          role: "system",
+          content: [{ type: "text", text: "System reminder text" }],
+        },
+      ],
+    },
+    false
+  );
+
+  const sysMsg = result.messages.find((m: { role: string }) => m.role === "system");
+  assert.ok(sysMsg, "expected a system message in output");
+  // Array content with text blocks is flattened to a string for system role
+  assert.equal(
+    typeof sysMsg.content === "string" ? sysMsg.content : JSON.stringify(sysMsg.content),
+    "System reminder text"
+  );
+});
+
+// ---------------------------------------------------------------------------
+// 3. top-level body.system still produces role: "system" (regression check)
+// ---------------------------------------------------------------------------
+test("body.system still produces role:system at index 0", () => {
+  const result = claudeToOpenAIRequest(
+    "gpt-4o",
+    {
+      system: "You are helpful.",
+      messages: [{ role: "user", content: "hi" }],
+    },
+    false
+  );
+
+  assert.equal(result.messages[0].role, "system");
+  assert.equal(result.messages[1].role, "user");
+});
+
+// ---------------------------------------------------------------------------
+// 4. assistant with tool_use still maps to assistant (regression check)
+// ---------------------------------------------------------------------------
+test("assistant role still maps to assistant (no regression)", () => {
+  const result = claudeToOpenAIRequest(
+    "gpt-4o",
+    {
+      messages: [
+        { role: "user", content: "use the tool" },
+        {
+          role: "assistant",
+          content: [
+            { type: "text", text: "calling tool" },
+            { type: "tool_use", id: "t1", name: "foo", input: {} },
+          ],
+        },
+      ],
+    },
+    false
+  );
+
+  const roles = result.messages.map((m: { role: string }) => m.role);
+  assert.ok(roles.includes("assistant"), "assistant role must be preserved");
+});
diff --git a/tests/unit/openai-to-claude-strip-empty-signature-6953.test.ts b/tests/unit/openai-to-claude-strip-empty-signature-6953.test.ts
new file mode 100644
index 0000000000..f129a64f70
--- /dev/null
+++ b/tests/unit/openai-to-claude-strip-empty-signature-6953.test.ts
@@ -0,0 +1,202 @@
+/**
+ * TDD regression for #6953 — thinking blocks with empty signatures poison the
+ * Anthropic leg of combo/blend routes.
+ *
+ * Non-Anthropic providers (codex/gpt-5.x) synthesize Anthropic-format `thinking`
+ * blocks with `signature: ""`. When the client replays these in the next
+ * request's history, the Anthropic leg rejects them with HTTP 400 "Invalid
+ * signature in thinking block", and the router silently falls back to codex
+ * permanently.
+ *
+ * The old code fabricated a DEFAULT_THINKING_CLAUDE_SIGNATURE to fill the empty
+ * signature — but that fabricated signature is equally foreign to Anthropic, so
+ * it also 400'd.
+ *
+ * Fix (#6953): strip thinking blocks with empty/missing signatures entirely.
+ * They carry no replayable cryptographic value. For `redacted_thinking`, strip
+ * if `data` is empty/missing for the same reason.
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { openaiToClaudeRequest } =
+  await import("../../open-sse/translator/request/openai-to-claude.ts");
+
+test('#6953: thinking block with signature:"" is stripped, not fabricated', () => {
+  const result = openaiToClaudeRequest(
+    "claude-opus-4-8",
+    {
+      messages: [
+        { role: "user", content: "hello" },
+        {
+          role: "assistant",
+          content: [
+            { type: "text", text: "I will help you." },
+            { type: "thinking", thinking: "reasoning here", signature: "" },
+            {
+              type: "text",
+              text: "Let me use a tool.",
+            },
+          ],
+        },
+        { role: "user", content: "ok go ahead" },
+      ],
+    },
+    false
+  );
+
+  const assistant = result.messages.find((m) => m.role === "assistant");
+  assert.ok(assistant, "expected assistant message");
+
+  // The thinking block with empty signature must be DROPPED, not preserved
+  // with a fabricated signature.
+  const thinkingBlocks = assistant.content.filter((b) => b && b.type === "thinking");
+  assert.equal(
+    thinkingBlocks.length,
+    0,
+    "thinking block with empty signature must be stripped, not fabricated"
+  );
+
+  // Text blocks must survive
+  const textBlocks = assistant.content.filter((b) => b && b.type === "text");
+  assert.ok(textBlocks.length >= 1, "text blocks must be preserved");
+});
+
+test("#6953: thinking block with valid signature is preserved verbatim", () => {
+  const realSig = "EuY2xhdWRlLXNpZ25hdHVyZS0xNzA5...";
+  const result = openaiToClaudeRequest(
+    "claude-opus-4-8",
+    {
+      messages: [
+        { role: "user", content: "hello" },
+        {
+          role: "assistant",
+          content: [
+            { type: "thinking", thinking: "real reasoning", signature: realSig },
+            { type: "text", text: "response" },
+          ],
+        },
+        { role: "user", content: "ok" },
+      ],
+    },
+    false
+  );
+
+  const assistant = result.messages.find((m) => m.role === "assistant");
+  assert.ok(assistant);
+
+  const thinkingBlocks = assistant.content.filter((b) => b && b.type === "thinking");
+  assert.equal(thinkingBlocks.length, 1, "valid thinking block must be preserved");
+  assert.equal(thinkingBlocks[0].signature, realSig, "valid signature must be preserved verbatim");
+});
+
+test("#6953: thinking block with undefined signature (Claude-format) is preserved with fallback", () => {
+  // Claude-format messages may have thinking blocks without a signature field at all.
+  // These are legitimate and must NOT be stripped — only signature:"" (empty string)
+  // indicates a non-Anthropic synthesized block.
+  const result = openaiToClaudeRequest(
+    "claude-opus-4-8",
+    {
+      messages: [
+        { role: "user", content: "hello" },
+        {
+          role: "assistant",
+          content: [
+            { type: "thinking", thinking: "I already have this" },
+            { type: "text", text: "response" },
+          ],
+        },
+        { role: "user", content: "ok" },
+      ],
+    },
+    false
+  );
+
+  const assistant = result.messages.find((m) => m.role === "assistant");
+  assert.ok(assistant);
+
+  const thinkingBlocks = assistant.content.filter((b) => b && b.type === "thinking");
+  assert.equal(
+    thinkingBlocks.length,
+    1,
+    "thinking block with undefined signature must be preserved"
+  );
+  assert.equal(thinkingBlocks[0].thinking, "I already have this", "thinking content must match");
+  assert.ok(thinkingBlocks[0].signature, "fallback signature must be applied");
+});
+
+test("#6953: redacted_thinking with empty data is stripped", () => {
+  const result = openaiToClaudeRequest(
+    "claude-opus-4-8",
+    {
+      messages: [
+        { role: "user", content: "hello" },
+        {
+          role: "assistant",
+          content: [
+            { type: "redacted_thinking", data: "" },
+            { type: "text", text: "response" },
+          ],
+        },
+        { role: "user", content: "ok" },
+      ],
+    },
+    false
+  );
+
+  const assistant = result.messages.find((m) => m.role === "assistant");
+  assert.ok(assistant);
+
+  const redactedBlocks = assistant.content.filter((b) => b && b.type === "redacted_thinking");
+  assert.equal(redactedBlocks.length, 0, "redacted_thinking with empty data must be stripped");
+});
+
+test("#6953: combo scenario — codex-sourced thinking block does not block Anthropic leg", () => {
+  // Simulates a combo route: turn 1 served by codex produced a thinking block
+  // with signature:"". Turn 2 should be able to route to Anthropic without
+  // the poisoned block causing a 400.
+  const result = openaiToClaudeRequest(
+    "claude-opus-4-8",
+    {
+      messages: [
+        { role: "user", content: "write a function" },
+        {
+          role: "assistant",
+          content: [
+            {
+              type: "thinking",
+              thinking: "**Reviewing the request**\n\nI need to write a function...",
+              signature: "", // codex-sourced, no real signature
+            },
+            { type: "text", text: "Here's the function:" },
+            {
+              type: "tool_use",
+              id: "toolu_01abc",
+              name: "write_file",
+              input: { path: "main.rs", content: "fn main() {}" },
+            },
+          ],
+        },
+        { role: "user", content: "looks good, now add tests" },
+      ],
+    },
+    false
+  );
+
+  const assistant = result.messages.find((m) => m.role === "assistant");
+  assert.ok(assistant);
+
+  // No thinking block with empty-string signature should survive
+  const badThinking = assistant.content.find(
+    (b) => b && b.type === "thinking" && b.signature === ""
+  );
+  assert.equal(
+    badThinking,
+    undefined,
+    "no thinking block with empty-string signature should survive"
+  );
+
+  // Tool use must survive
+  const toolUse = assistant.content.find((b) => b && b.type === "tool_use");
+  assert.ok(toolUse, "tool_use block must be preserved");
+});