🐛 fix(opencode): preserve thinking block signatures + configurable strategy UI

Root cause fix (from PR anomalyco#14393):
- Always pass providerMetadata for reasoning parts (removed differentModel guard)
- Always pass callProviderMetadata for tool parts
- Fix asymmetric compaction buffer (use maxOutputTokens consistently)

Configurable thinking strategy (none/strip/compact):
- Settings > General: Thinking Strategy dropdown
- Context tab: Always-visible strategy selector
- Error card: Retry buttons for thinking block errors
- Processor: Auto-compact on thinking error with compact strategy

Default 'none' preserves original behavior.

Author: praxstack

docs/09-temp/thinking-block-fix-design.md

@@ -1,7 +1,7 @@
-# Design: Fix Thinking Block Error (Option D)
+# Design: Fix Thinking Block Error
 
 **Date:** 2026-02-25
-**Status:** Approved — Ready to implement
+**Status:** Implemented
 
 ## Problem
 When using Claude models with extended thinking, the API returns `thinking`/`redacted_thinking` blocks. When OpenCode replays these back (on next message or compaction), if they're modified during storage/retrieval, Claude rejects them:
@@ -11,93 +11,34 @@ messages.3.content.1: `thinking` or `redacted_thinking` blocks in the latest ass
 
 Session becomes stuck — even compaction triggers the same error.
 
-## Root Cause
-`MessageV2.toModelMessages()` stores reasoning parts as `{type: "reasoning", text: part.text}` but the original API response had `{type: "thinking", thinking: "..."}`. The reconstruction is not byte-identical. Claude's constraint only applies to the LAST assistant message.
-
-## Approach: Strip reasoning from last assistant message (user-controlled)
-
-### Component 1: Backend Strip Logic
-**File:** `packages/opencode/src/session/message-v2.ts`
-
-In `toModelMessages()`, add optional `stripLastReasoning` parameter:
-```typescript
-export function toModelMessages(input: WithParts[], model: Provider.Model, opts?: { stripLastReasoning?: boolean }): ModelMessage[] {
-  // ... existing code ...
-  
-  // Before return, if stripLastReasoning:
-  if (opts?.stripLastReasoning) {
-    const lastAssistantIdx = result.findLastIndex((msg) => msg.role === "assistant")
-    if (lastAssistantIdx !== -1) {
-      result[lastAssistantIdx].parts = result[lastAssistantIdx].parts.filter((p) => p.type !== "reasoning")
-      if (result[lastAssistantIdx].parts.length === 0 || result[lastAssistantIdx].parts.every((p) => p.type === "step-start")) {
-        result.splice(lastAssistantIdx, 1)
-      }
-    }
-  }
-  
-  return convertToModelMessages(...)
-}
-```
-
-### Component 2: Config Setting
-**File:** `packages/opencode/src/config/config.ts`
-
-Add to appearance/compaction config:
-```typescript
-strip_thinking_on_error: z.boolean().optional().default(false).describe("Automatically strip thinking blocks when API error occurs")
-```
-
-### Component 3: Auto-Retry in Processor
-**File:** `packages/opencode/src/session/processor.ts`
-
-In the catch block (~line 350), detect the specific error:
-```typescript
-const isThinkingError = e?.message?.includes("thinking") && e?.message?.includes("cannot be modified")
-if (isThinkingError) {
-  const config = await Config.get()
-  if (config.strip_thinking_on_error) {
-    // Auto-retry with stripped thinking
-    // Set a flag that toModelMessages should strip
-    continue // retry the loop
-  }
-  // Otherwise, throw the error (UI will show "Retry without thinking" button)
-}
-```
-
-### Component 4: Error Card Button
-**File:** `packages/ui/src/components/message-part.tsx`
-
-In the error rendering section (~line 1040), detect thinking error:
-```tsx
-<Match when={cleaned.includes("thinking") && cleaned.includes("cannot be modified")}>
-  <Card variant="error">
-    <div>{cleaned}</div>
-    <Button onClick={() => retryWithoutThinking()} variant="secondary">
-      Retry without thinking blocks
-    </Button>
-  </Card>
-</Match>
-```
-
-### Component 5: Settings Toggle
-**File:** `packages/app/src/components/settings-general.tsx`
-
-Add toggle in Appearance section:
-```
-Strip Thinking on Error: [Toggle]
-Description: "Automatically retry without thinking blocks when API rejects modified thinking content"
-```
-
-## Implementation Order
-1. Backend strip logic (message-v2.ts)
-2. Config setting (config.ts)
-3. Auto-retry logic (processor.ts)
-4. Error card button (message-part.tsx)
-5. Settings toggle (settings-general.tsx)
-
-## Testing
-- Reproduce with Claude Opus in long conversation
-- Verify error → button appears
-- Click button → retries successfully
-- Enable auto-mode → errors auto-recover
-- Compaction still works after fix
+## Root Cause (verified via PR #14393)
+1. **Bug 1:** `toModelMessages()` strips `providerMetadata` (including Bedrock thinking signatures) when `differentModel` is true — which always happens during compaction due to model ID format mismatch.
+2. **Bug 2:** Asymmetric compaction buffer (20K vs 32K) causes compaction to trigger too late for some models.
+
+## Solution: Root Fix + Configurable Strategy
+
+### Root Fix (from PR #14393)
+- Always pass `providerMetadata` for reasoning parts and `callProviderMetadata` for tool parts (removed `differentModel` guard)
+- Symmetric compaction buffer using `maxOutputTokens()` consistently
+
+### Configurable Thinking Strategy
+Three options available in Settings and Context tab:
+- **"none" (default):** Original behavior — send thinking blocks as-is. With the root fix, signatures are now preserved correctly.
+- **"strip":** Proactively remove thinking from last assistant message before sending. Prevents errors but loses thinking context.
+- **"compact":** Preserve thinking but auto-compact on error. First message may fail, then auto-recovers.
+
+### Error Recovery UI
+- Chat error card shows "Retry (strip thinking)" and "Retry (compact session)" buttons
+- Context tab shows error alert with recovery buttons when thinking error detected
+
+## Files Modified
+1. `message-v2.ts` — Root fix: always pass providerMetadata/callProviderMetadata + conditional strip logic
+2. `compaction.ts` — Root fix: symmetric buffer calculation
+3. `config.ts` — `thinking_strategy: "none" | "strip" | "compact"` config option
+4. `prompt.ts` — Reads config, passes stripLastReasoning flag
+5. `processor.ts` — Detects thinking errors, auto-compacts with "compact" strategy
+6. `session-turn.tsx` — Error card with retry buttons
+7. `session-turn.css` — Error button styles
+8. `message-timeline.tsx` — Retry handler wiring
+9. `settings-general.tsx` — Thinking Strategy dropdown
+10. `session-context-tab.tsx` — Always-visible strategy selector + error recovery