diff --git a/packages/server/src/server/agent/providers/claude/agent.test.ts b/packages/server/src/server/agent/providers/claude/agent.test.ts
index f57f4ab284..3c35988a8f 100644
--- a/packages/server/src/server/agent/providers/claude/agent.test.ts
+++ b/packages/server/src/server/agent/providers/claude/agent.test.ts
@@ -1892,6 +1892,45 @@ describe("ClaudeAgentSession context window usage", () => {
     }
   });
 
+  test("selected native-1M Claude models seed active context window usage with 1M max tokens", async () => {
+    const session = await createSessionForTurns(
+      [
+        [
+          createInitMessage(),
+          createMessageStartEvent(),
+          createSuccessResult({ modelUsage: undefined }),
+        ],
+      ],
+      { model: "claude-sonnet-5" },
+    );
+
+    try {
+      const events = await collectStreamEvents(session);
+
+      expect(events).toContainEqual(
+        expect.objectContaining({
+          type: "usage_updated",
+          provider: "claude",
+          usage: {
+            contextWindowMaxTokens: 1_000_000,
+            contextWindowUsedTokens: 150,
+          },
+        }),
+      );
+      expect(events).toContainEqual(
+        expect.objectContaining({
+          type: "turn_completed",
+          provider: "claude",
+          usage: expect.objectContaining({
+            contextWindowMaxTokens: 1_000_000,
+          }),
+        }),
+      );
+    } finally {
+      await session.close();
+    }
+  });
+
   test("message_delta stream events update per-request usage", async () => {
     const session = await createSessionForTurns([
       [
diff --git a/packages/server/src/server/agent/providers/claude/agent.ts b/packages/server/src/server/agent/providers/claude/agent.ts
index 62af2b0fa8..9d4b6c4ba0 100644
--- a/packages/server/src/server/agent/providers/claude/agent.ts
+++ b/packages/server/src/server/agent/providers/claude/agent.ts
@@ -1634,7 +1634,8 @@ function resolveInitialContextWindowSize(modelId: string | null | undefined): nu
   if (normalized.includes("[1m]") || normalized.includes("context-1m")) {
     return 1_000_000;
   }
-  if (normalized.includes("claude-fable-5")) {
+  const catalogModelId = normalizeClaudeRuntimeModelId(normalized);
+  if (catalogModelId === "claude-fable-5" || catalogModelId === "claude-sonnet-5") {
     return 1_000_000;
   }
   if (/(?:^|[~/_-])(?:claude[-_ ]*)?(opus|sonnet|haiku)(?:$|[-_ ./])/.test(normalized)) {
diff --git a/packages/server/src/server/agent/providers/claude/models.test.ts b/packages/server/src/server/agent/providers/claude/models.test.ts
index 7264052d23..500ca942c9 100644
--- a/packages/server/src/server/agent/providers/claude/models.test.ts
+++ b/packages/server/src/server/agent/providers/claude/models.test.ts
@@ -219,6 +219,9 @@ describe("normalizeClaudeRuntimeModelId", () => {
 
   it("preserves [1m] suffix from runtime model strings", () => {
     expect(normalizeClaudeRuntimeModelId("claude-opus-4-6[1m]")).toBe("claude-opus-4-6[1m]");
+  });
+
+  it("strips [1m] suffix for natively-1M models", () => {
     expect(normalizeClaudeRuntimeModelId("claude-sonnet-5[1m]")).toBe("claude-sonnet-5");
   });
 
diff --git a/packages/server/src/server/agent/providers/claude/models.ts b/packages/server/src/server/agent/providers/claude/models.ts
index df9d59a9b4..b978b4641b 100644
--- a/packages/server/src/server/agent/providers/claude/models.ts
+++ b/packages/server/src/server/agent/providers/claude/models.ts
@@ -225,15 +225,16 @@ export function normalizeClaudeRuntimeModelId(value: string | null | undefined):
     return trimmed;
   }
 
-  // Fable and Sonnet 5 use single-segment versions, not the {major}-{minor}
-  // scheme of older opus/sonnet/haiku models. They are natively 1M, so map
-  // dated or suffixed runtime strings back to the one catalog ID.
+  // Some new Claude model families use single-segment versions, not the
+  // {major}-{minor} scheme of older opus/sonnet/haiku models. Map dated or
+  // suffixed runtime strings back to the matching catalog ID.
   const singleSegmentMatch = trimmed.match(/(?:claude-)?(fable|sonnet)[-_ ]+(\d+)/i);
   if (singleSegmentMatch) {
     const family = singleSegmentMatch[1].toLowerCase();
     const major = singleSegmentMatch[2];
-    if (family === "fable" || (family === "sonnet" && major === "5")) {
-      return `claude-${family}-${major}`;
+    const modelId = `claude-${family}-${major}`;
+    if (CLAUDE_MODELS.some((model) => model.id === modelId)) {
+      return modelId;
     }
   }