feat: support Claude Opus 5 across core, opencode, and pi (#143)

- claude-opus-5 model support: adaptive thinking made visible (display
  defaults to omitted), explicit thinking disable preserved and
  canonicalized, and the disabled+xhigh/max effort combination lowered
  to high so requests do not 400
- fast mode support for claude-opus-5
- refusal recovery parity with Fable 5: Opus 5 content-filter refusals
  downgrade the session to claude-opus-4-8 for 10 responses with the
  same cache-anchor, prewarm, and bridge machinery; notices, sidebar
  text, and recovery logs name the actual requested model
- Pi provider maps Opus 5 to adaptive summarized thinking with its own
  per-family constant

Author: iceteaSA

README.md

@@ -481,7 +481,7 @@ Both OpenCode and Pi packages can persistently request Anthropic fast mode for s
 /claude-fast off
 ```
 
-When enabled, supported requests add `speed: "fast"` to the Anthropic JSON body and include the `fast-mode-2026-02-01` beta header. Unsupported models are left at standard speed. Anthropic currently documents fast mode for `claude-opus-4-6`, `claude-opus-4-7`, and `claude-opus-4-8`; Claude Fable 5 and Mythos 5 are not fast-mode models.
+When enabled, supported requests add `speed: "fast"` to the Anthropic JSON body and include the `fast-mode-2026-02-01` beta header. Unsupported models are left at standard speed. Anthropic currently documents fast mode for `claude-opus-4-6`, `claude-opus-4-7`, `claude-opus-4-8`, and `claude-opus-5`; Claude Fable 5 and Mythos 5 are not fast-mode models.
 
 Fast and standard speeds do not share prompt-cache prefixes, so switching this setting can cause cache misses.
 

packages/core/src/constants.ts

@@ -48,7 +48,8 @@ export function isFastModeSupportedModel(model: unknown) {
     typeof model === 'string' &&
     (model.startsWith('claude-opus-4-6') ||
       model.startsWith('claude-opus-4-7') ||
-      model.startsWith('claude-opus-4-8'))
+      model.startsWith('claude-opus-4-8') ||
+      model.startsWith('claude-opus-5'))
   )
 }
 

packages/core/src/fast.ts

@@ -47,7 +47,7 @@ export function buildFastModeStatusSummary(input?: { enabled?: boolean }) {
     `- Enabled: ${enabled ? 'enabled' : 'disabled'}`,
     '- Persisted: ~/.config/opencode/anthropic-auth.json',
     '- Scope: adds Anthropic fast mode to supported Opus requests',
-    '- Supported models: claude-opus-4-6, claude-opus-4-7, and claude-opus-4-8',
+    '- Supported models: claude-opus-4-6, claude-opus-4-7, claude-opus-4-8, and claude-opus-5',
     '- Request changes: adds `speed: "fast"` and the `fast-mode-2026-02-01` beta header',
     '- Note: fast and standard speeds do not share prompt-cache prefixes',
   ].join('\n')

packages/core/src/models.ts

@@ -69,6 +69,27 @@ export function isClaudeSonnet5Model(model: unknown) {
   )
 }
 
+export const CLAUDE_OPUS_5_MODEL_ID = 'claude-opus-5'
+
+/**
+ * Opus 5 has the same adaptive-thinking-by-default + `display: "omitted"`
+ * shape as Sonnet 5, so the injected thinking must also be summarized to be
+ * visible. Kept as its own constant per the PR #100 lesson — hook callers
+ * should reference the per-family name (`CLAUDE_OPUS_5_ADAPTIVE_THINKING`,
+ * not the Fable/Mythos alias) so a future divergence between the families
+ * only changes this re-export, not every call site.
+ */
+export const CLAUDE_OPUS_5_ADAPTIVE_THINKING =
+  CLAUDE_FABLE_MYTHOS_5_SUMMARIZED_THINKING
+
+export function isClaudeOpus5Model(model: unknown) {
+  return (
+    typeof model === 'string' &&
+    (model === CLAUDE_OPUS_5_MODEL_ID ||
+      model.startsWith(`${CLAUDE_OPUS_5_MODEL_ID}-`))
+  )
+}
+
 export function isOpenAIReasoningSignature(value: unknown): boolean {
   if (typeof value !== 'string') return false
   if (value.startsWith('gAAAA')) return true

packages/core/src/tests/models.test.ts

@@ -1,5 +1,10 @@
 import { describe, expect, test } from 'bun:test'
-import { isClaudeSonnet5Model } from '../models'
+import {
+  CLAUDE_OPUS_5_ADAPTIVE_THINKING,
+  CLAUDE_OPUS_5_MODEL_ID,
+  isClaudeOpus5Model,
+  isClaudeSonnet5Model,
+} from '../models'
 
 describe('isClaudeSonnet5Model', () => {
   test('matches the bare claude-sonnet-5 id', () => {
@@ -28,3 +33,58 @@ describe('isClaudeSonnet5Model', () => {
     expect(isClaudeSonnet5Model(42)).toBe(false)
   })
 })
+
+describe('isClaudeOpus5Model', () => {
+  test('exposes the bare id constant', () => {
+    expect(CLAUDE_OPUS_5_MODEL_ID).toBe('claude-opus-5')
+  })
+
+  test('matches the bare claude-opus-5 id', () => {
+    expect(isClaudeOpus5Model('claude-opus-5')).toBe(true)
+  })
+
+  test('matches the catalog claude-opus-5-fast variant', () => {
+    expect(isClaudeOpus5Model('claude-opus-5-fast')).toBe(true)
+  })
+
+  test('matches a dated claude-opus-5 snapshot', () => {
+    expect(isClaudeOpus5Model('claude-opus-5-20260701')).toBe(true)
+  })
+
+  test('matches the dated fast snapshot', () => {
+    expect(isClaudeOpus5Model('claude-opus-5-fast-20260701')).toBe(true)
+  })
+
+  test('does not match claude-opus-4-8', () => {
+    expect(isClaudeOpus5Model('claude-opus-4-8')).toBe(false)
+  })
+
+  test('does not match a non-dash prefix collision', () => {
+    expect(isClaudeOpus5Model('claude-opus-5x')).toBe(false)
+  })
+
+  test('does not match the Fable/Mythos ids', () => {
+    expect(isClaudeOpus5Model('claude-fable-5')).toBe(false)
+    expect(isClaudeOpus5Model('claude-mythos-5')).toBe(false)
+  })
+
+  test('does not match Sonnet 5', () => {
+    expect(isClaudeOpus5Model('claude-sonnet-5')).toBe(false)
+  })
+
+  test('does not match non-string input', () => {
+    expect(isClaudeOpus5Model(undefined)).toBe(false)
+    expect(isClaudeOpus5Model(42)).toBe(false)
+    expect(isClaudeOpus5Model(null)).toBe(false)
+    expect(isClaudeOpus5Model({})).toBe(false)
+  })
+})
+
+describe('CLAUDE_OPUS_5_ADAPTIVE_THINKING', () => {
+  test('matches the shared adaptive+summarized shape (alias of Fable/Mythos)', () => {
+    expect(CLAUDE_OPUS_5_ADAPTIVE_THINKING).toEqual({
+      type: 'adaptive',
+      display: 'summarized',
+    })
+  })
+})

packages/opencode/README.md

@@ -387,7 +387,7 @@ Both OpenCode and Pi packages can persistently request Anthropic fast mode for s
 /claude-fast off
 ```
 
-When enabled, supported requests add `speed: "fast"` to the Anthropic JSON body and include the `fast-mode-2026-02-01` beta header. Unsupported models are left at standard speed. Anthropic currently documents fast mode for `claude-opus-4-6`, `claude-opus-4-7`, and `claude-opus-4-8`; Claude Fable 5 and Mythos 5 are not fast-mode models.
+When enabled, supported requests add `speed: "fast"` to the Anthropic JSON body and include the `fast-mode-2026-02-01` beta header. Unsupported models are left at standard speed. Anthropic currently documents fast mode for `claude-opus-4-6`, `claude-opus-4-7`, `claude-opus-4-8`, and `claude-opus-5`; Claude Fable 5 and Mythos 5 are not fast-mode models.
 
 Fast and standard speeds do not share prompt-cache prefixes, so switching this setting can cause cache misses.
 

packages/opencode/src/fable-fallback.ts

@@ -1,4 +1,7 @@
-import { CLAUDE_FABLE_5_MODEL_ID } from '@cortexkit/anthropic-auth-core'
+import {
+  CLAUDE_FABLE_5_MODEL_ID,
+  isClaudeOpus5Model,
+} from '@cortexkit/anthropic-auth-core'
 
 export const FABLE_FALLBACK_MODEL_ID = 'claude-opus-4-8'
 export const FABLE_FALLBACK_TURNS = 10
@@ -37,11 +40,21 @@ type FableFallbackState = {
   updatedAt: number
 }
 
-function isFableModel(model: unknown): model is string {
+/**
+ * Models whose content-filter refusal we recover from by downgrading to Opus 4.8
+ * for a short window. Fable 5 is the original case; Opus 5 is identical in
+ * shape (cyber safety classifiers that can return `stop_reason: "refusal"`,
+ * Anthropic's own fallback default is `claude-opus-4-8`, same pricing). Mythos
+ * has no classifiers and Sonnet 5 is not flagged — both stay outside this
+ * gate. The single shared fallback id matches Anthropic's default for both
+ * source models, so the recovery period is uniform.
+ */
+function isRecoverableRefusalModel(model: unknown): model is string {
+  if (typeof model !== 'string') return false
+  if (isClaudeOpus5Model(model)) return true
   return (
-    typeof model === 'string' &&
-    (model === CLAUDE_FABLE_5_MODEL_ID ||
-      model.startsWith(`${CLAUDE_FABLE_5_MODEL_ID}-`))
+    model === CLAUDE_FABLE_5_MODEL_ID ||
+    model.startsWith(`${CLAUDE_FABLE_5_MODEL_ID}-`)
   )
 }
 
@@ -79,7 +92,7 @@ export class FableFallbackManager {
   ): FableFallbackPlan | null {
     if (!sessionId || typeof bodyText !== 'string') return null
     const parsed = parseBody(bodyText)
-    if (!parsed || !isFableModel(parsed.model)) return null
+    if (!parsed || !isRecoverableRefusalModel(parsed.model)) return null
 
     this.prune()
     const state = this.sessions.get(sessionId)
@@ -135,7 +148,7 @@ export class FableFallbackManager {
   }
 
   activate(plan: FableFallbackPlan, cacheAccountId?: string): number {
-    if (plan.downgraded || !isFableModel(plan.requestedModel)) {
+    if (plan.downgraded || !isRecoverableRefusalModel(plan.requestedModel)) {
       return this.remaining(plan.sessionId)
     }
     const state: FableFallbackState = {
@@ -179,9 +192,9 @@ export class FableFallbackManager {
       state.standbyAnchorSequence = plan.requestSequence
     }
     state.updatedAt = this.now()
-    // Retain the zero-remaining state so a later Fable refusal can bridge back
+    // Retain the zero-remaining state so a later refusal can bridge back
     // to this model-specific Opus cache boundary without continuously warming
-    // Opus while Fable is healthy.
+    // Opus while the original model is healthy.
     this.sessions.delete(plan.sessionId)
     this.sessions.set(plan.sessionId, state)
     return { counted: true, remaining: state.remaining }

packages/opencode/src/index.ts

@@ -64,6 +64,7 @@ import {
   isCacheKeepHybridActive,
   isCacheKeepPersistentlyEnabled,
   isCacheKeepSubagentsEnabled,
+  isClaudeOpus5Model,
   isCostZeroingEnabled,
   isDumpPersistentlyEnabled,
   isFastModeEnabled,
@@ -631,6 +632,26 @@ const FABLE_SWITCHED_TO_OPUS_NOTICE =
 const FABLE_RESTORED_NOTICE =
   'Fable recovery window complete. Returning to Fable 5.'
 
+/**
+ * Compose the recovery-window notice text for the requested model. Fable 5 and
+ * Opus 5 follow the same Anthropic-recommended fallback (Opus 4.8) for the
+ * same reason (cyber safety classifiers returning `stop_reason: "refusal"`),
+ * so the message structure is shared; only the model name moves.
+ */
+function buildSwitchedToOpusNotice(modelId: string): string {
+  if (isClaudeOpus5Model(modelId)) {
+    return 'Opus 5 content filter detected. Switched to Opus 4.8 for a 10-response recovery window while keeping the Opus 5 cache warm.'
+  }
+  return FABLE_SWITCHED_TO_OPUS_NOTICE
+}
+
+function buildRestoredNotice(modelId: string): string {
+  if (isClaudeOpus5Model(modelId)) {
+    return 'Opus 5 recovery window complete. Returning to Opus 5.'
+  }
+  return FABLE_RESTORED_NOTICE
+}
+
 type AnthropicProviderModel = {
   id?: string
   name?: string
@@ -1148,6 +1169,9 @@ export const AnthropicAuthPlugin: Plugin = async (ctx) => {
 
   function warmFableAfterOpus(context: FableRequestContext) {
     const sessionId = context.plan.sessionId
+    const modelLabel = isClaudeOpus5Model(context.plan.requestedModel)
+      ? 'Opus 5'
+      : 'Fable'
     const run = async () => {
       const target = context.warmTarget
       if (!target) {
@@ -1180,20 +1204,20 @@ export const AnthropicAuthPlugin: Plugin = async (ctx) => {
             target.oauthAccountId,
         })
         if (result.ok) {
-          logger.debug('fable-fallback', 'Fable cache warmed', {
+          logger.debug('fable-fallback', `${modelLabel} cache warmed`, {
             session: sessionId,
             remaining: fableFallbackManager.remaining(sessionId),
             ...(result.usage && { usage: result.usage }),
           })
           return
         }
-        logger.warn('fable-fallback', 'Fable cache warm skipped', {
+        logger.warn('fable-fallback', `${modelLabel} cache warm skipped`, {
           session: sessionId,
           status: result.status,
           reason: result.reason,
         })
       } catch (error) {
-        logger.warn('fable-fallback', 'Fable cache warm failed', {
+        logger.warn('fable-fallback', `${modelLabel} cache warm failed`, {
           session: sessionId,
           error: error instanceof Error ? error.message : String(error),
         })
@@ -3898,6 +3922,7 @@ export const AnthropicAuthPlugin: Plugin = async (ctx) => {
               const wrapResponse = (response: Response) =>
                 createStrippedStream(response, {
                   perf: (stage, data) => trace.mark(stage, data),
+                  contentFilterModel: fablePlan?.requestedModel,
                   ...(!fablePlan?.downgraded && fablePlan
                     ? {
                         onContentFilter: () => {
@@ -3916,17 +3941,22 @@ export const AnthropicAuthPlugin: Plugin = async (ctx) => {
                           logger.info(
                             'fable-fallback',
                             'content filter detected; switching session to Opus 4.8',
-                            { session: fablePlan.sessionId, remaining },
+                            {
+                              session: fablePlan.sessionId,
+                              requestedModel: fablePlan.requestedModel,
+                              remaining,
+                            },
                           )
                           publishFableRecoveryNotice(
                             {
                               sessionId: fablePlan.sessionId,
                               mode: 'opus',
                               remaining,
+                              requestedModelId: fablePlan.requestedModel,
                             },
                             storage,
                             auth,
-                            FABLE_SWITCHED_TO_OPUS_NOTICE,
+                            buildSwitchedToOpusNotice(fablePlan.requestedModel),
                           )
                         },
                       }
@@ -3944,6 +3974,7 @@ export const AnthropicAuthPlugin: Plugin = async (ctx) => {
                             'Opus 4.8 turn completed',
                             {
                               session: fablePlan.sessionId,
+                              requestedModel: fablePlan.requestedModel,
                               finishReason,
                               remaining: completed.remaining,
                             },
@@ -3953,6 +3984,7 @@ export const AnthropicAuthPlugin: Plugin = async (ctx) => {
                               sessionId: fablePlan.sessionId,
                               mode: 'opus',
                               remaining: completed.remaining,
+                              requestedModelId: fablePlan.requestedModel,
                             },
                             storage,
                             auth,
@@ -3971,10 +4003,11 @@ export const AnthropicAuthPlugin: Plugin = async (ctx) => {
                                   sessionId: fablePlan.sessionId,
                                   mode: 'fable',
                                   remaining: 0,
+                                  requestedModelId: fablePlan.requestedModel,
                                 },
                                 storage,
                                 auth,
-                                FABLE_RESTORED_NOTICE,
+                                buildRestoredNotice(fablePlan.requestedModel),
                               )
                             }
                             void warm.then(notifyRestored, notifyRestored)

packages/opencode/src/sidebar-state.ts

@@ -42,6 +42,12 @@ export interface FableRecoverySidebarState {
   mode: 'opus' | 'fable'
   remaining: number
   changedAt: number
+  /**
+   * The model id the user originally requested (e.g. 'claude-fable-5' or
+   * 'claude-opus-5'). Optional for backward compatibility with pre-Opus-5
+   * state files; older recoveries default to Fable 5 in the summary.
+   */
+  requestedModelId?: string
 }
 
 export interface SidebarState {
@@ -287,6 +293,10 @@ export function normalizeSidebarState(raw: unknown): SidebarState {
               mode: recovery.mode,
               remaining: Math.max(0, Math.floor(recovery.remaining)),
               changedAt: recovery.changedAt,
+              requestedModelId:
+                typeof recovery.requestedModelId === 'string'
+                  ? recovery.requestedModelId
+                  : undefined,
             },
           ]
         })
@@ -677,6 +687,19 @@ export function formatScopedQuotaLabel(title: string) {
   return /^fable$/i.test(label) ? 'Fa' : label
 }
 
+/**
+ * Pretty label for a recoverable-refusal source model (Fable 5 or Opus 5).
+ * Falls back to the raw id for unrecognized ids so the sidebar never goes
+ * blank on a future model that has not yet been cataloged here.
+ */
+export function formatFallbackModelLabel(modelId: string | undefined): string {
+  if (modelId === 'claude-fable-5' || modelId?.startsWith('claude-fable-5-'))
+    return 'Fable 5'
+  if (modelId === 'claude-opus-5' || modelId?.startsWith('claude-opus-5-'))
+    return 'Opus 5'
+  return modelId ?? 'Fable 5'
+}
+
 export function getFableRecoverySummary(
   state: SidebarState,
   sessionId: string,
@@ -685,7 +708,9 @@ export function getFableRecoverySummary(
     (candidate) => candidate.sessionId === sessionId,
   )
   if (!recovery) return undefined
-  if (recovery.mode === 'fable') return 'Fable 5 · restored'
+  if (recovery.mode === 'fable') {
+    return `${formatFallbackModelLabel(recovery.requestedModelId)} · restored`
+  }
   return `Opus 4.8 · ${recovery.remaining} left`
 }