diff --git a/open-sse/utils/modelMarkers.js b/open-sse/utils/modelMarkers.js
new file mode 100644
index 00000000..6429fca0
--- /dev/null
+++ b/open-sse/utils/modelMarkers.js
@@ -0,0 +1,20 @@
+// Claude Code appends a bracketed context marker to the model name when the
+// 1M-context beta is toggled on: `claude-opus-5` becomes `claude-opus-5[1m]`.
+// The marker is a client-side annotation, not part of any model id: it never
+// matches a combo name, an alias or a `provider/model` pair, so a request that
+// carries it dies at model resolution with "Invalid model format".
+//
+// The capability itself travels in the `anthropic-beta: context-1m-2025-08-07`
+// header, which is forwarded untouched — stripping the marker is enough to let
+// the request route normally and still reach the upstream as a 1M request.
+
+const CONTEXT_MARKER = /\[1m\]$/i;
+
+// Returns { model, contextMarker } — contextMarker is null when there is none.
+export function stripModelContextMarker(modelStr) {
+  if (typeof modelStr !== "string") return { model: modelStr, contextMarker: null };
+  const trimmed = modelStr.trim();
+  const match = trimmed.match(CONTEXT_MARKER);
+  if (!match) return { model: modelStr, contextMarker: null };
+  return { model: trimmed.slice(0, -match[0].length), contextMarker: match[0].slice(1, -1).toLowerCase() };
+}
diff --git a/src/sse/handlers/chat.js b/src/sse/handlers/chat.js
index 8ae048e4..19dc8167 100644
--- a/src/sse/handlers/chat.js
+++ b/src/sse/handlers/chat.js
@@ -23,6 +23,7 @@ import { detectFormatByEndpoint } from "open-sse/translator/formats.js";
 import * as log from "../utils/logger.js";
 import { updateProviderCredentials, checkAndRefreshToken } from "../services/tokenRefresh.js";
 import { getProjectIdForConnection } from "open-sse/services/projectId.js";
+import { stripModelContextMarker } from "open-sse/utils/modelMarkers.js";
 
 /**
  * Handle chat completion request
@@ -47,7 +48,11 @@ export async function handleChat(request, clientRawRequest = null) {
       headers: Object.fromEntries(request.headers.entries())
     };
   }
-  const modelStr = body.model;
+  // Claude Code marks a 1M-context request as `<model>[1m]`; the marker matches
+  // no combo, alias or provider/model pair, so it must not reach resolution.
+  // The capability travels in the anthropic-beta header, forwarded as-is.
+  const { model: modelStr, contextMarker } = stripModelContextMarker(body.model);
+  if (contextMarker) body.model = modelStr;
 
   // Request summary is emitted as the unified "▶" line in chatCore (has fmt/thinking/account)
 
diff --git a/tests/unit/model-context-marker.test.js b/tests/unit/model-context-marker.test.js
new file mode 100644
index 00000000..a2fcf314
--- /dev/null
+++ b/tests/unit/model-context-marker.test.js
@@ -0,0 +1,54 @@
+/**
+ * Regression: Claude Code appends `[1m]` to the model name when the
+ * 1M-context beta is on, so `/v1/messages` arrives with
+ * `model: "claude-opus-5[1m]"`. Nothing in 9router knows about the marker:
+ * it matches no combo, no alias and no `provider/model` pair, so the request
+ * is rejected at model resolution and the client reports
+ *
+ *   There's an issue with the selected model (claude-opus-5[1m]).
+ *   It may not exist or you may not have access to it.
+ *
+ * The capability itself rides in the `anthropic-beta` header, which is
+ * forwarded untouched, so stripping the marker is all that is needed.
+ */
+
+import { describe, it, expect } from "vitest";
+import { stripModelContextMarker } from "../../open-sse/utils/modelMarkers.js";
+
+describe("model context marker", () => {
+  it("strips the [1m] marker and reports it", () => {
+    expect(stripModelContextMarker("claude-opus-5[1m]")).toEqual({
+      model: "claude-opus-5",
+      contextMarker: "1m",
+    });
+  });
+
+  it("strips it from a provider-prefixed model too", () => {
+    expect(stripModelContextMarker("cc/claude-sonnet-4.5[1m]")).toEqual({
+      model: "cc/claude-sonnet-4.5",
+      contextMarker: "1m",
+    });
+  });
+
+  it("is case insensitive", () => {
+    expect(stripModelContextMarker("claude-opus-5[1M]").model).toBe("claude-opus-5");
+  });
+
+  it("leaves a plain model untouched", () => {
+    expect(stripModelContextMarker("claude-opus-5")).toEqual({
+      model: "claude-opus-5",
+      contextMarker: null,
+    });
+  });
+
+  it("only strips a trailing marker, never one inside the name", () => {
+    expect(stripModelContextMarker("weird[1m]name")).toEqual({
+      model: "weird[1m]name",
+      contextMarker: null,
+    });
+  });
+
+  it("tolerates a non-string model", () => {
+    expect(stripModelContextMarker(undefined)).toEqual({ model: undefined, contextMarker: null });
+  });
+});