diff --git a/packages/protocol/src/agent-types.ts b/packages/protocol/src/agent-types.ts
index 9f90cc65e8..fc95ae4eb9 100644
--- a/packages/protocol/src/agent-types.ts
+++ b/packages/protocol/src/agent-types.ts
@@ -77,6 +77,7 @@ export interface AgentModelDefinition {
   label: string;
   description?: string;
   isDefault?: boolean;
+  contextWindowMaxTokens?: number;
   metadata?: AgentMetadata;
   thinkingOptions?: AgentSelectOption[];
   defaultThinkingOptionId?: string;
diff --git a/packages/protocol/src/messages.providers-snapshot.test.ts b/packages/protocol/src/messages.providers-snapshot.test.ts
index 1b066062d4..f58b3867c8 100644
--- a/packages/protocol/src/messages.providers-snapshot.test.ts
+++ b/packages/protocol/src/messages.providers-snapshot.test.ts
@@ -48,6 +48,7 @@ describe("provider snapshot message schemas", () => {
           id: "MiniMax-M2.7",
           label: "MiniMax-M2.7",
           isDefault: true,
+          contextWindowMaxTokens: 200_000,
           thinkingOptions: [
             { id: "off", label: "Off" },
             { id: "max", label: "Max", isDefault: true },
@@ -62,6 +63,7 @@ describe("provider snapshot message schemas", () => {
         id: "MiniMax-M2.7",
         label: "MiniMax-M2.7",
         isDefault: true,
+        contextWindowMaxTokens: 200_000,
         thinkingOptions: [
           { id: "off", label: "Off" },
           { id: "max", label: "Max", isDefault: true },
diff --git a/packages/protocol/src/messages.ts b/packages/protocol/src/messages.ts
index 7eb0edaa18..d720e82c68 100644
--- a/packages/protocol/src/messages.ts
+++ b/packages/protocol/src/messages.ts
@@ -242,6 +242,7 @@ const AgentModelDefinitionSchema: z.ZodType<AgentModelDefinition> = z
     label: z.string(),
     description: z.string().optional(),
     isDefault: z.boolean().optional(),
+    contextWindowMaxTokens: z.number().optional(),
     metadata: z.record(z.string(), z.unknown()).optional(),
     thinkingOptions: z.array(AgentSelectOptionSchema).optional(),
     defaultThinkingOptionId: z.string().optional(),
diff --git a/packages/server/src/server/agent/agent-sdk-types.ts b/packages/server/src/server/agent/agent-sdk-types.ts
index 56c437ef08..9f00a656bb 100644
--- a/packages/server/src/server/agent/agent-sdk-types.ts
+++ b/packages/server/src/server/agent/agent-sdk-types.ts
@@ -78,6 +78,7 @@ export interface AgentModelDefinition {
   label: string;
   description?: string;
   isDefault?: boolean;
+  contextWindowMaxTokens?: number;
   metadata?: AgentMetadata;
   thinkingOptions?: AgentSelectOption[];
   defaultThinkingOptionId?: string;
diff --git a/packages/server/src/server/agent/providers/claude/agent.ts b/packages/server/src/server/agent/providers/claude/agent.ts
index 9d4b6c4ba0..427a24ce59 100644
--- a/packages/server/src/server/agent/providers/claude/agent.ts
+++ b/packages/server/src/server/agent/providers/claude/agent.ts
@@ -29,7 +29,11 @@ import {
   mapTaskNotificationSystemRecordToToolCall,
   mapTaskNotificationUserContentToToolCall,
 } from "./task-notification-tool-call.js";
-import { getClaudeModelsWithSettings, normalizeClaudeRuntimeModelId } from "./models.js";
+import {
+  findClaudeModel,
+  getClaudeModelsWithSettings,
+  normalizeClaudeRuntimeModelId,
+} from "./models.js";
 import { parsePartialJsonObject } from "./partial-json.js";
 import { ClaudeSidechainTracker } from "./sidechain-tracker.js";
 import { buildClaudeFeatures, claudeModelSupportsFastMode } from "./feature-definitions.js";
@@ -1626,24 +1630,6 @@ function extractContextWindowSize(modelUsage: unknown): number | undefined {
   return maxContextWindow;
 }
 
-function resolveInitialContextWindowSize(modelId: string | null | undefined): number | undefined {
-  const normalized = typeof modelId === "string" ? modelId.trim().toLowerCase() : "";
-  if (!normalized) {
-    return undefined;
-  }
-  if (normalized.includes("[1m]") || normalized.includes("context-1m")) {
-    return 1_000_000;
-  }
-  const catalogModelId = normalizeClaudeRuntimeModelId(normalized);
-  if (catalogModelId === "claude-fable-5" || catalogModelId === "claude-sonnet-5") {
-    return 1_000_000;
-  }
-  if (/(?:^|[~/_-])(?:claude[-_ ]*)?(opus|sonnet|haiku)(?:$|[-_ ./])/.test(normalized)) {
-    return 200_000;
-  }
-  return undefined;
-}
-
 function readStreamRequestInputTokens(event: Record<string, unknown>): number | undefined {
   const messageUsage = toObjectRecord(toObjectRecord(event.message)?.usage);
   if (!messageUsage) {
@@ -1942,7 +1928,7 @@ class ClaudeAgentSession implements AgentSession {
     this.queryFactory = options.queryFactory;
     this.resolveBinary = options.resolveBinary;
     this.contextUsage = new ClaudeContextUsageState(
-      resolveInitialContextWindowSize(this.config.model),
+      findClaudeModel(this.config.model)?.contextWindowMaxTokens,
     );
     const handle = options.handle;
 
@@ -2188,7 +2174,7 @@ class ClaudeAgentSession implements AgentSession {
       await this.applyFastModeFeature(false, activeQuery);
     }
     this.contextUsage.setInitialContextWindowMaxTokens(
-      resolveInitialContextWindowSize(this.config.model),
+      findClaudeModel(this.config.model)?.contextWindowMaxTokens,
     );
     this.lastOptionsModel = normalizedModelId ?? this.lastOptionsModel;
     this.lastRuntimeModel = null;
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 500ca942c9..539d47bf15 100644
--- a/packages/server/src/server/agent/providers/claude/models.test.ts
+++ b/packages/server/src/server/agent/providers/claude/models.test.ts
@@ -5,7 +5,7 @@ import { afterEach, describe, expect, it, vi } from "vitest";
 
 import { createTestLogger } from "../../../../test-utils/test-logger.js";
 import { ClaudeAgentClient } from "./agent.js";
-import { getClaudeModels, normalizeClaudeRuntimeModelId } from "./models.js";
+import { findClaudeModel, getClaudeModels, normalizeClaudeRuntimeModelId } from "./models.js";
 
 const createdClaudeConfigDirs: string[] = [];
 
@@ -56,6 +56,22 @@ describe("getClaudeModels", () => {
     expect(defaults[0].id).toBe("claude-opus-4-8");
   });
 
+  it("keeps fixed context windows on the model entries", () => {
+    const modelsById = new Map(getClaudeModels().map((model) => [model.id, model]));
+
+    expect(modelsById.get("claude-fable-5")?.contextWindowMaxTokens).toBe(1_000_000);
+    expect(modelsById.get("claude-opus-4-8[1m]")?.contextWindowMaxTokens).toBe(1_000_000);
+    expect(modelsById.get("claude-opus-4-8")?.contextWindowMaxTokens).toBe(200_000);
+    expect(modelsById.get("claude-sonnet-5")?.contextWindowMaxTokens).toBe(1_000_000);
+    expect(modelsById.get("claude-sonnet-4-6")?.contextWindowMaxTokens).toBe(200_000);
+    expect(modelsById.get("claude-haiku-4-5")?.contextWindowMaxTokens).toBe(200_000);
+  });
+
+  it("finds models through the runtime normalizer", () => {
+    expect(findClaudeModel("claude-sonnet-5-20260101")?.id).toBe("claude-sonnet-5");
+    expect(findClaudeModel("claude-sonnet-5[1m]")?.contextWindowMaxTokens).toBe(1_000_000);
+  });
+
   it("returns fresh copies each call", () => {
     const a = getClaudeModels();
     const b = getClaudeModels();
diff --git a/packages/server/src/server/agent/providers/claude/models.ts b/packages/server/src/server/agent/providers/claude/models.ts
index b978b4641b..b240b36f84 100644
--- a/packages/server/src/server/agent/providers/claude/models.ts
+++ b/packages/server/src/server/agent/providers/claude/models.ts
@@ -31,6 +31,7 @@ const CLAUDE_MODELS: AgentModelDefinition[] = [
     id: "claude-fable-5",
     label: "Fable 5",
     description: "Fable 5 · Most powerful model",
+    contextWindowMaxTokens: 1_000_000,
     thinkingOptions: [...CLAUDE_ULTRACODE_THINKING_OPTIONS],
   },
   {
@@ -38,6 +39,7 @@ const CLAUDE_MODELS: AgentModelDefinition[] = [
     id: "claude-opus-4-8[1m]",
     label: "Opus 4.8 1M",
     description: "Opus 4.8 with 1M context window",
+    contextWindowMaxTokens: 1_000_000,
     thinkingOptions: [...CLAUDE_ULTRACODE_THINKING_OPTIONS],
   },
   {
@@ -46,6 +48,7 @@ const CLAUDE_MODELS: AgentModelDefinition[] = [
     label: "Opus 4.8",
     description: "Opus 4.8 · Latest release",
     isDefault: true,
+    contextWindowMaxTokens: 200_000,
     thinkingOptions: [...CLAUDE_ULTRACODE_THINKING_OPTIONS],
   },
   {
@@ -53,6 +56,7 @@ const CLAUDE_MODELS: AgentModelDefinition[] = [
     id: "claude-sonnet-5",
     label: "Sonnet 5",
     description: "Sonnet 5 · Efficient for routine tasks",
+    contextWindowMaxTokens: 1_000_000,
     thinkingOptions: [...CLAUDE_EXTENDED_THINKING_OPTIONS],
   },
   {
@@ -60,6 +64,7 @@ const CLAUDE_MODELS: AgentModelDefinition[] = [
     id: "claude-opus-4-7[1m]",
     label: "Opus 4.7 1M",
     description: "Opus 4.7 with 1M context window",
+    contextWindowMaxTokens: 1_000_000,
     thinkingOptions: [...CLAUDE_EXTENDED_THINKING_OPTIONS],
   },
   {
@@ -67,6 +72,7 @@ const CLAUDE_MODELS: AgentModelDefinition[] = [
     id: "claude-opus-4-7",
     label: "Opus 4.7",
     description: "Opus 4.7 · Previous release",
+    contextWindowMaxTokens: 200_000,
     thinkingOptions: [...CLAUDE_EXTENDED_THINKING_OPTIONS],
   },
   {
@@ -74,6 +80,7 @@ const CLAUDE_MODELS: AgentModelDefinition[] = [
     id: "claude-opus-4-6[1m]",
     label: "Opus 4.6 1M",
     description: "Opus 4.6 with 1M context window",
+    contextWindowMaxTokens: 1_000_000,
     thinkingOptions: [...CLAUDE_THINKING_OPTIONS],
   },
   {
@@ -81,6 +88,7 @@ const CLAUDE_MODELS: AgentModelDefinition[] = [
     id: "claude-opus-4-6",
     label: "Opus 4.6",
     description: "Opus 4.6 · Most capable for complex work",
+    contextWindowMaxTokens: 200_000,
     thinkingOptions: [...CLAUDE_THINKING_OPTIONS],
   },
   {
@@ -88,6 +96,7 @@ const CLAUDE_MODELS: AgentModelDefinition[] = [
     id: "claude-sonnet-4-6[1m]",
     label: "Sonnet 4.6 1M",
     description: "Sonnet 4.6 with 1M context window",
+    contextWindowMaxTokens: 1_000_000,
     thinkingOptions: [...CLAUDE_THINKING_OPTIONS],
   },
   {
@@ -95,6 +104,7 @@ const CLAUDE_MODELS: AgentModelDefinition[] = [
     id: "claude-sonnet-4-6",
     label: "Sonnet 4.6",
     description: "Sonnet 4.6 · Best for everyday tasks",
+    contextWindowMaxTokens: 200_000,
     thinkingOptions: [...CLAUDE_THINKING_OPTIONS],
   },
   {
@@ -102,6 +112,7 @@ const CLAUDE_MODELS: AgentModelDefinition[] = [
     id: "claude-haiku-4-5",
     label: "Haiku 4.5",
     description: "Haiku 4.5 · Fastest for quick answers",
+    contextWindowMaxTokens: 200_000,
   },
 ];
 
@@ -117,6 +128,14 @@ export function getClaudeModels(): AgentModelDefinition[] {
   return CLAUDE_MODELS.map((model) => ({ ...model }));
 }
 
+export function findClaudeModel(modelId: string | null | undefined): AgentModelDefinition | null {
+  const normalizedModelId = normalizeClaudeRuntimeModelId(modelId);
+  if (!normalizedModelId) {
+    return null;
+  }
+  return CLAUDE_MODELS.find((model) => model.id === normalizedModelId) ?? null;
+}
+
 export async function getClaudeModelsWithSettings(
   logger: Logger,
   configDir?: string,