diff --git a/packages/core/src/models.ts b/packages/core/src/models.ts
index 1efeb68..d5b2256 100644
--- a/packages/core/src/models.ts
+++ b/packages/core/src/models.ts
@@ -50,6 +50,25 @@ export function isClaudeFableOrMythos5Model(model: unknown) {
   )
 }
 
+export const CLAUDE_SONNET_5_MODEL_ID = 'claude-sonnet-5'
+
+/**
+ * Sonnet 5 enables adaptive thinking by default but ships `display: "omitted"`,
+ * so the `thinking` field returns empty and the user sees nothing. Injecting
+ * this makes the adaptive thinking summary visible. Reuses the Fable/Mythos
+ * shape because the API contract is identical.
+ */
+export const CLAUDE_SONNET_5_ADAPTIVE_THINKING =
+  CLAUDE_FABLE_MYTHOS_5_SUMMARIZED_THINKING
+
+export function isClaudeSonnet5Model(model: unknown) {
+  return (
+    typeof model === 'string' &&
+    (model === CLAUDE_SONNET_5_MODEL_ID ||
+      model.startsWith(`${CLAUDE_SONNET_5_MODEL_ID}-`))
+  )
+}
+
 export function isOpenAIReasoningSignature(value: unknown): boolean {
   if (typeof value !== 'string') return false
   if (value.startsWith('gAAAA')) return true
diff --git a/packages/core/src/tests/models.test.ts b/packages/core/src/tests/models.test.ts
new file mode 100644
index 0000000..c648216
--- /dev/null
+++ b/packages/core/src/tests/models.test.ts
@@ -0,0 +1,30 @@
+import { describe, expect, test } from 'bun:test'
+import { isClaudeSonnet5Model } from '../models'
+
+describe('isClaudeSonnet5Model', () => {
+  test('matches the bare claude-sonnet-5 id', () => {
+    expect(isClaudeSonnet5Model('claude-sonnet-5')).toBe(true)
+  })
+
+  test('matches a dated claude-sonnet-5 snapshot', () => {
+    expect(isClaudeSonnet5Model('claude-sonnet-5-20260630')).toBe(true)
+  })
+
+  test('does not match claude-sonnet-4-6', () => {
+    expect(isClaudeSonnet5Model('claude-sonnet-4-6')).toBe(false)
+  })
+
+  test('does not match the Fable/Mythos ids', () => {
+    expect(isClaudeSonnet5Model('claude-fable-5')).toBe(false)
+    expect(isClaudeSonnet5Model('claude-mythos-5')).toBe(false)
+  })
+
+  test('does not match a non-dash prefix collision', () => {
+    expect(isClaudeSonnet5Model('claude-sonnet-5x')).toBe(false)
+  })
+
+  test('does not match non-string input', () => {
+    expect(isClaudeSonnet5Model(undefined)).toBe(false)
+    expect(isClaudeSonnet5Model(42)).toBe(false)
+  })
+})
diff --git a/packages/opencode/src/tests/transform.test.ts b/packages/opencode/src/tests/transform.test.ts
index d1517e9..7321ef2 100644
--- a/packages/opencode/src/tests/transform.test.ts
+++ b/packages/opencode/src/tests/transform.test.ts
@@ -938,6 +938,66 @@ describe('rewriteRequestBody', () => {
     expect(result.output_config).toEqual({ effort: 'xhigh' })
   })
 
+  test('requests summarized adaptive thinking for Sonnet 5 without thinking', async () => {
+    const body = JSON.stringify({
+      model: 'claude-sonnet-5',
+      messages: [{ role: 'user', content: 'hi' }],
+    })
+
+    const result = JSON.parse(await rewriteRequestBody(body))
+
+    expect(result.thinking).toEqual({ type: 'adaptive', display: 'summarized' })
+  })
+
+  test('replaces manual enabled thinking with adaptive summarized for Sonnet 5', async () => {
+    const body = JSON.stringify({
+      model: 'claude-sonnet-5-20260630',
+      messages: [{ role: 'user', content: 'hi' }],
+      thinking: { type: 'enabled', budget_tokens: 10_000 },
+    })
+
+    const result = JSON.parse(await rewriteRequestBody(body))
+
+    expect(result.thinking).toEqual({ type: 'adaptive', display: 'summarized' })
+    expect(result.thinking.budget_tokens).toBeUndefined()
+  })
+
+  test('preserves explicitly disabled thinking for Sonnet 5', async () => {
+    const body = JSON.stringify({
+      model: 'claude-sonnet-5',
+      messages: [{ role: 'user', content: 'hi' }],
+      thinking: { type: 'disabled' },
+    })
+
+    const result = JSON.parse(await rewriteRequestBody(body))
+
+    expect(result.thinking).toEqual({ type: 'disabled' })
+  })
+
+  test('strips display from disabled thinking for Sonnet 5', async () => {
+    const body = JSON.stringify({
+      model: 'claude-sonnet-5',
+      messages: [{ role: 'user', content: 'hi' }],
+      thinking: { type: 'disabled', display: 'summarized' },
+    })
+
+    const result = JSON.parse(await rewriteRequestBody(body))
+
+    expect(result.thinking).toEqual({ type: 'disabled' })
+  })
+
+  test('does not touch thinking for non-Sonnet5 models', async () => {
+    const body = JSON.stringify({
+      model: 'claude-sonnet-4-6',
+      messages: [{ role: 'user', content: 'hi' }],
+      thinking: { type: 'enabled', budget_tokens: 5_000 },
+    })
+
+    const result = JSON.parse(await rewriteRequestBody(body))
+
+    expect(result.thinking).toEqual({ type: 'enabled', budget_tokens: 5_000 })
+  })
+
   test('strips OpenAI encrypted reasoning blocks before sending to Anthropic', async () => {
     const body = JSON.stringify({
       model: 'claude-opus-4-8',
diff --git a/packages/opencode/src/transform.ts b/packages/opencode/src/transform.ts
index 5dc088e..5b45cfd 100644
--- a/packages/opencode/src/transform.ts
+++ b/packages/opencode/src/transform.ts
@@ -6,9 +6,11 @@ import {
   CLAUDE_CODE_ENTRYPOINT,
   CLAUDE_CODE_IDENTITY,
   CLAUDE_FABLE_MYTHOS_5_SUMMARIZED_THINKING,
+  CLAUDE_SONNET_5_ADAPTIVE_THINKING,
   type ClaudeCodeIdentity,
   FAST_MODE_BETA,
   isClaudeFableOrMythos5Model,
+  isClaudeSonnet5Model,
   isFastModeSupportedModel,
   isOpenAIReasoningSignature,
   mergeAnthropicBetas,
@@ -776,6 +778,31 @@ function normalizeFableMythosRequest(
   return { replacedExisting: hadThinking }
 }
 
+/**
+ * Sonnet 5 has adaptive thinking on by default but defaults `display` to
+ * "omitted", so the thinking field returns empty. Inject
+ * `{type:"adaptive",display:"summarized"}` to make it visible — UNLESS the
+ * caller explicitly disabled thinking. Unlike Fable/Mythos (which reject a
+ * disable), Sonnet 5 accepts `{type:"disabled"}`, so an intentional opt-out is
+ * preserved rather than force-enabled. A manual `{type:"enabled",budget_tokens}`
+ * would 400 on Sonnet 5, so it is overwritten with the adaptive form.
+ */
+function normalizeSonnet5Request(
+  parsed: Record<string, unknown>,
+): { replacedExisting: boolean; display: 'summarized' | 'disabled' } | null {
+  if (!isClaudeSonnet5Model(parsed.model)) return null
+  const hadThinking = Object.hasOwn(parsed, 'thinking')
+  const thinking = parsed.thinking
+  if (isRecord(thinking) && thinking.type === 'disabled') {
+    // `display` is invalid alongside `type:"disabled"` (nothing to display) and
+    // can 400; canonicalize to a bare disabled object while keeping thinking off.
+    parsed.thinking = { type: 'disabled' }
+    return { replacedExisting: hadThinking, display: 'disabled' }
+  }
+  parsed.thinking = { ...CLAUDE_SONNET_5_ADAPTIVE_THINKING }
+  return { replacedExisting: hadThinking, display: 'summarized' }
+}
+
 function applyCache1hStrategy(
   parsed: Record<string, unknown>,
   options: { enabled: boolean; mode: Cache1hMode },
@@ -902,6 +929,7 @@ export async function rewriteRequestBody(
     const modelNormalizeStart = rewriteNowMs()
     const removedNonAnthropicThinking = stripNonAnthropicThinkingBlocks(parsed)
     const fableMythosThinking = normalizeFableMythosRequest(parsed)
+    const sonnet5Thinking = normalizeSonnet5Request(parsed)
     options.perf?.('model_normalize', {
       ms: rewriteRoundMs(rewriteNowMs() - modelNormalizeStart),
       model: typeof parsed.model === 'string' ? parsed.model : undefined,
@@ -910,6 +938,8 @@ export async function rewriteRequestBody(
         : undefined,
       replacedFableMythosThinking:
         fableMythosThinking?.replacedExisting ?? false,
+      sonnet5ThinkingDisplay: sonnet5Thinking?.display,
+      replacedSonnet5Thinking: sonnet5Thinking?.replacedExisting ?? false,
       removedNonAnthropicThinking,
       hasOutputConfig: Object.hasOwn(parsed, 'output_config'),
     })
diff --git a/packages/pi/src/convert.ts b/packages/pi/src/convert.ts
index 9dcb1c9..60fc17d 100644
--- a/packages/pi/src/convert.ts
+++ b/packages/pi/src/convert.ts
@@ -5,8 +5,10 @@ import {
   CLAUDE_CODE_ENTRYPOINT,
   CLAUDE_CODE_IDENTITY,
   CLAUDE_FABLE_MYTHOS_5_SUMMARIZED_THINKING,
+  CLAUDE_SONNET_5_ADAPTIVE_THINKING,
   type ClaudeCodeIdentity,
   isClaudeFableOrMythos5Model,
+  isClaudeSonnet5Model,
   isFastModeSupportedModel,
   isOpenAIReasoningSignature,
   orderClaudeCodeBody,
@@ -406,12 +408,19 @@ export async function buildAnthropicRequest(
   }
 
   const isFableOrMythos5 = isClaudeFableOrMythos5Model(modelId)
+  const isSonnet5 = isClaudeSonnet5Model(modelId)
+  // Sonnet 5 shares Fable/Mythos's adaptive-summarized contract: make adaptive
+  // thinking visible (display defaults to "omitted") and map reasoning to
+  // output_config effort. Pi's typed options cannot express thinking-disabled,
+  // so there is no disable case here (see transform.ts for the raw-body path).
   if (isFableOrMythos5) {
     body.thinking = { ...CLAUDE_FABLE_MYTHOS_5_SUMMARIZED_THINKING }
+  } else if (isSonnet5) {
+    body.thinking = { ...CLAUDE_SONNET_5_ADAPTIVE_THINKING }
   }
 
   if (options?.reasoning) {
-    if (isFableOrMythos5) {
+    if (isFableOrMythos5 || isSonnet5) {
       body.output_config = { effort: options.reasoning }
     } else {
       const budgets: Record<string, number> = {
diff --git a/packages/pi/src/tests/convert.test.ts b/packages/pi/src/tests/convert.test.ts
index e12ddcf..0a86de9 100644
--- a/packages/pi/src/tests/convert.test.ts
+++ b/packages/pi/src/tests/convert.test.ts
@@ -364,6 +364,42 @@ describe('buildAnthropicRequest — Fable/Mythos thinking', () => {
   })
 })
 
+describe('buildAnthropicRequest — Sonnet 5 thinking', () => {
+  test('requests summarized adaptive thinking for Sonnet 5 without reasoning', async () => {
+    const { body } = await buildAnthropicRequest(
+      'claude-sonnet-5',
+      { messages: [userMsg('hello')], systemPrompt: 'test', tools: [] } as any,
+      {} as any,
+      defaultCache,
+    )
+
+    expect(body.thinking).toEqual({ type: 'adaptive', display: 'summarized' })
+  })
+
+  test('replaces manual reasoning budget with adaptive summarized for Sonnet 5', async () => {
+    const { body } = await buildAnthropicRequest(
+      'claude-sonnet-5-20260630',
+      { messages: [userMsg('hello')], systemPrompt: 'test', tools: [] } as any,
+      { reasoning: 'high' } as any,
+      defaultCache,
+    )
+
+    expect(body.thinking).toEqual({ type: 'adaptive', display: 'summarized' })
+    expect(body.output_config).toEqual({ effort: 'high' })
+  })
+
+  test('sets display summarized (not omitted) so Sonnet 5 thinking is visible', async () => {
+    const { body } = await buildAnthropicRequest(
+      'claude-sonnet-5',
+      { messages: [userMsg('hello')], systemPrompt: 'test', tools: [] } as any,
+      {} as any,
+      defaultCache,
+    )
+
+    expect((body.thinking as { display?: string }).display).toBe('summarized')
+  })
+})
+
 describe('convertMessages — empty error tool_result guard', () => {
   test('injects Error placeholder when is_error=true and content is empty', async () => {
     const messages = await buildMessages([