fix(web): correct context window calculation for local-mode sessions

Remote sessions get an authoritative context_window injected into usage by
the CLI (from SDK result modelUsage), but local-mode sessions forward raw
transcript JSONL whose usage has no context_window, so the web fell back to
getContextBudgetTokens — which could not tell Fable's 1M window from its
bare id (only the "[1m]" suffix was recognized) and received session.model,
which is usually null for local sessions, defaulting to a 200k budget. A 1M
Fable session with ~256k of context showed 135% used with an over-limit
warning instead of ~26%.

- modelConfig: recognize Fable ids (fable, fable[1m], claude-fable-*) as 1M
- reducer: LatestUsage carries the usage-bearing message's own model
- StatusBar: new contextModel prop feeds the fallback heuristic only;
  the model prop's other semantics are unchanged
- HappyComposer forwards it; SessionChat passes
  latestUsage.model ?? session.model

web/src/chat/modelConfig.test.ts

@@ -14,6 +14,12 @@ describe('getContextBudgetTokens', () => {
         expect(getContextBudgetTokens('claude-opus-4-8[1m]', 'claude')).toBe(990_000)
     })
 
+    it('uses the large budget for Fable even under its bare id (1M window)', () => {
+        expect(getContextBudgetTokens('claude-fable-5', 'claude')).toBe(990_000)
+        expect(getContextBudgetTokens('fable', 'claude')).toBe(990_000)
+        expect(getContextBudgetTokens('fable[1m]', 'claude')).toBe(990_000)
+    })
+
     it('uses Codex app-server context window with headroom', () => {
         expect(getContextBudgetTokens('gpt-5.4', 'codex')).toBe(248_400)
     })

web/src/chat/modelConfig.ts

@@ -75,7 +75,15 @@ export function getContextBudgetTokens(model: string | null | undefined, flavor?
             return DEFAULT_CLAUDE_CONTEXT_WINDOW_TOKENS
         }
         if (isClaudeModelPreset(trimmedModel) || trimmedModel.startsWith('claude-')) {
-            return trimmedModel.endsWith('[1m]')
+            // Fable ships with a 1M window even under its bare id: the SDK
+            // result message reports modelUsage["claude-fable-5"].contextWindow
+            // = 1,000,000, so the "[1m]" suffix check alone would undercount
+            // local-mode sessions (their transcript usage carries no
+            // context_window and falls through to this heuristic).
+            const isFable = trimmedModel === 'fable'
+                || trimmedModel === 'fable[1m]'
+                || trimmedModel.startsWith('claude-fable')
+            return trimmedModel.endsWith('[1m]') || isFable
                 ? LARGE_CLAUDE_CONTEXT_WINDOW_TOKENS
                 : DEFAULT_CLAUDE_CONTEXT_WINDOW_TOKENS
         }

web/src/chat/reducer.test.ts

@@ -213,6 +213,36 @@ describe('reduceChatBlocks', () => {
         })
     })
 
+    it('carries the usage message model for the context-window heuristic', () => {
+        // Local-mode Claude transcripts have no context_window in usage and
+        // session.model is often null, so latestUsage.model is the only
+        // signal the status bar has to resolve a plausible window.
+        const messages: NormalizedMessage[] = [
+            {
+                id: 'local-turn',
+                localId: null,
+                createdAt: 1_700_000_000_000,
+                role: 'agent',
+                content: [],
+                isSidechain: false,
+                model: 'claude-fable-5',
+                usage: {
+                    input_tokens: 2,
+                    output_tokens: 50,
+                    cache_read_input_tokens: 250_000
+                }
+            }
+        ] as NormalizedMessage[]
+
+        const reduced = reduceChatBlocks(messages, null)
+
+        expect(reduced.latestUsage).toMatchObject({
+            contextSize: 250_002,
+            contextWindow: null,
+            model: 'claude-fable-5'
+        })
+    })
+
     it('keeps active goals visible across later normal user messages', () => {
         const reduced = reduceChatBlocks([
             goalMessage('goal-active', 'active', 1),

web/src/chat/reducer.ts

@@ -37,6 +37,13 @@ export type LatestUsage = {
     cacheRead: number
     contextSize: number
     contextWindow: number | null
+    /**
+     * Model reported by the usage-bearing message itself. Local-mode Claude
+     * sessions often have session.model = null (the model is picked inside the
+     * TUI), so this is the only model signal available for the context-window
+     * heuristic when the usage carries no explicit context_window.
+     */
+    model: string | null
     timestamp: number
 }
 
@@ -179,6 +186,7 @@ export function reduceChatBlocks(
                 cacheRead: msg.usage.cache_read_input_tokens ?? 0,
                 contextSize: calculateContextSize(msg.usage),
                 contextWindow: msg.usage.context_window ?? null,
+                model: msg.model ?? null,
                 timestamp: msg.createdAt
             }
             break

web/src/components/AssistantChat/HappyComposer.tsx

@@ -156,6 +156,8 @@ export function HappyComposer(props: {
     contextSize?: number
     contextCacheRead?: number
     contextWindow?: number | null
+    /** Model for the context-window heuristic; see StatusBar.contextModel. */
+    contextModel?: string | null
     controlledByUser?: boolean
     agentFlavor?: string | null
     availableModelOptions?: Array<{ value: string | null; label: string }>
@@ -229,6 +231,7 @@ export function HappyComposer(props: {
         contextSize,
         contextCacheRead,
         contextWindow,
+        contextModel,
         controlledByUser = false,
         agentFlavor,
         availableModelOptions,
@@ -1330,6 +1333,7 @@ export function HappyComposer(props: {
                         contextSize={contextSize}
                         contextCacheRead={contextCacheRead}
                         contextWindow={contextWindow}
+                        contextModel={contextModel}
                         model={model}
                         modelReasoningEffort={modelReasoningEffort}
                         serviceTier={serviceTier}

web/src/components/AssistantChat/StatusBar.tsx

@@ -196,6 +196,13 @@ export function StatusBar(props: {
     contextSize?: number
     contextCacheRead?: number
     contextWindow?: number | null
+    /**
+     * Model to use for the context-window fallback heuristic when
+     * contextWindow is absent. Falls back to `model`. Callers pass the
+     * usage-bearing message's own model here so local Claude sessions (whose
+     * session.model is often null) still resolve a plausible window.
+     */
+    contextModel?: string | null
     model?: string | null
     modelReasoningEffort?: string | null
     serviceTier?: string | null
@@ -212,30 +219,31 @@ export function StatusBar(props: {
         [props.active, props.thinking, props.agentState, props.voiceStatus, props.backgroundTaskCount, t]
     )
 
+    const contextHeuristicModel = props.contextModel ?? props.model
     const contextWarning = useMemo(
         () => {
             if (props.contextSize === undefined) return null
-            const maxContextSize = props.contextWindow ?? getContextBudgetTokens(props.model, props.agentFlavor)
+            const maxContextSize = props.contextWindow ?? getContextBudgetTokens(contextHeuristicModel, props.agentFlavor)
             if (!maxContextSize) return null
             return getContextWarning(props.contextSize, maxContextSize)
         },
-        [props.contextSize, props.contextWindow, props.model, props.agentFlavor]
+        [props.contextSize, props.contextWindow, contextHeuristicModel, props.agentFlavor]
     )
     const contextUsageLabel = useMemo(() => {
         if (props.contextSize === undefined) return null
-        const maxContextSize = props.contextWindow ?? getContextBudgetTokens(props.model, props.agentFlavor)
+        const maxContextSize = props.contextWindow ?? getContextBudgetTokens(contextHeuristicModel, props.agentFlavor)
         return formatContextUsageLabel(props.contextSize, maxContextSize)
-    }, [props.contextSize, props.contextWindow, props.model, props.agentFlavor])
+    }, [props.contextSize, props.contextWindow, contextHeuristicModel, props.agentFlavor])
     const compactContextUsageLabel = useMemo(() => {
         if (props.contextSize === undefined) return null
-        const maxContextSize = props.contextWindow ?? getContextBudgetTokens(props.model, props.agentFlavor)
+        const maxContextSize = props.contextWindow ?? getContextBudgetTokens(contextHeuristicModel, props.agentFlavor)
         return formatCompactContextUsageLabel(props.contextSize, maxContextSize)
-    }, [props.contextSize, props.contextWindow, props.model, props.agentFlavor])
+    }, [props.contextSize, props.contextWindow, contextHeuristicModel, props.agentFlavor])
     const contextUsageDetails = useMemo(() => {
         if (props.contextSize === undefined) return null
-        const maxContextSize = props.contextWindow ?? getContextBudgetTokens(props.model, props.agentFlavor)
+        const maxContextSize = props.contextWindow ?? getContextBudgetTokens(contextHeuristicModel, props.agentFlavor)
         return getContextUsageDetails(props.contextSize, maxContextSize, props.contextCacheRead)
-    }, [props.contextSize, props.contextCacheRead, props.contextWindow, props.model, props.agentFlavor])
+    }, [props.contextSize, props.contextCacheRead, props.contextWindow, contextHeuristicModel, props.agentFlavor])
     const contextUsedPercentage = contextUsageDetails?.usedPercentage ?? null
 
     const permissionMode = props.permissionMode

web/src/components/SessionChat.tsx

@@ -1467,6 +1467,7 @@ function SessionChatInner(props: SessionChatProps) {
                         contextSize={reduced.latestUsage?.contextSize}
                         contextCacheRead={reduced.latestUsage?.cacheRead}
                         contextWindow={reduced.latestUsage?.contextWindow ?? piContextWindow}
+                        contextModel={reduced.latestUsage?.model ?? props.session.model}
                         controlledByUser={controlledByUser}
                         onCollaborationModeChange={
                             codexCollaborationModeSupported && props.session.active && !controlledByUser