diff --git a/changelog.d/fixes/13922-deepseek-flash-1m-context.md b/changelog.d/fixes/13922-deepseek-flash-1m-context.md
new file mode 100644
index 0000000000..e9b0c8df4e
--- /dev/null
+++ b/changelog.d/fixes/13922-deepseek-flash-1m-context.md
@@ -0,0 +1 @@
+- fix(sse): deepseek provider registry now declares `defaultContextLength: 1_000_000` and an explicit `deepseek-flash` model entry, so unlisted/new DeepSeek models (like the new DeepSeek V4.1 Flash) no longer fall back to the generic 128k context limit (#13922)
diff --git a/open-sse/config/providers/registry/deepseek/index.ts b/open-sse/config/providers/registry/deepseek/index.ts
index e45f471bb8..38d7e23eeb 100644
--- a/open-sse/config/providers/registry/deepseek/index.ts
+++ b/open-sse/config/providers/registry/deepseek/index.ts
@@ -7,6 +7,7 @@ export const deepseekProvider: RegistryEntry = {
   executor: "default",
   baseUrl: "https://api.deepseek.com/responses",
   authType: "apikey",
+  defaultContextLength: 1_000_000,
   authHeader: "bearer",
   alternateFormats: [
     {
@@ -36,5 +37,14 @@ export const deepseekProvider: RegistryEntry = {
       supportedThinkingEfforts: ["none", "low", "high", "max"],
       toolCalling: true,
     },
+    {
+      id: "deepseek-flash",
+      name: "DeepSeek V4.1 Flash",
+      contextLength: 1_000_000,
+      maxOutputTokens: 384_000,
+      supportsReasoning: true,
+      supportedThinkingEfforts: ["none", "low", "high", "max"],
+      toolCalling: true,
+    },
   ],
 };
diff --git a/tests/unit/deepseek-flash-context-limit-13922.test.ts b/tests/unit/deepseek-flash-context-limit-13922.test.ts
new file mode 100644
index 0000000000..9fed4ffb17
--- /dev/null
+++ b/tests/unit/deepseek-flash-context-limit-13922.test.ts
@@ -0,0 +1,15 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { resolveTokenLimit } = await import("../../open-sse/services/contextManager.ts");
+
+test("#13922: deepseek-flash (new 1M model, no registry/synced metadata) resolves to DeepSeek's 1M window", () => {
+  const resolved = resolveTokenLimit("deepseek", "deepseek-flash");
+  assert.equal(resolved.limit, 1_000_000);
+  assert.equal(resolved.specific, true);
+});
+
+test("#13922: any unlisted deepseek model id inherits the provider's 1M defaultContextLength", () => {
+  const resolved = resolveTokenLimit("deepseek", "deepseek-some-future-model-not-yet-in-registry");
+  assert.equal(resolved.limit, 1_000_000);
+});