fix: reasoning text trimEnd() in processor.ts invalidates Bedrock extended thinking signature

[wonjun3991]

Bug Report

Error

undefined: The model returned the following errors:
messages.3.content.6: `thinking` or `redacted_thinking` blocks in the latest assistant message 
cannot be modified. These blocks must remain as they were in the original response.

Root Cause

In packages/opencode/src/session/processor.ts, the reasoning-end handler trims the reasoning text before storing it in SQLite:

case "reasoning-end":
  if (value.id in reasoningMap) {
    const part = reasoningMap[value.id]
    part.text = part.text.trimEnd()  // ← BUG: trims text permanently
    // ...
    if (value.providerMetadata) part.metadata = value.providerMetadata  // signature is for ORIGINAL text
    await Session.updatePart(part)
  }

The Amazon Bedrock API computes the signature cryptographically over the exact original bytes of the reasoning text (including trailing whitespace). When opencode calls trimEnd():

• Stored text: "Let me think..." (trimmed)
• Stored signature: computed over "Let me think...\n\n" (original)

On the next request, Bedrock receives text ≠ signature → ValidationException: "cannot be modified".

Data Flow

Bedrock response: reasoning.text = "thinking...\n\n", signature = "sig123"
    ↓
processor.ts reasoning-end: part.text = "thinking..." (trimEnd applied)
    ↓
SQLite: { text: "thinking...", metadata: { bedrock: { signature: "sig123" } } }
    ↓
Next request: text = "thinking...", signature = "sig123"
    ↓
Bedrock validates: sig123 was for "thinking...\n\n" ≠ "thinking..." → ERROR ❌

Fix

In packages/opencode/src/session/processor.ts, remove trimEnd() from the reasoning-end case:

case "reasoning-end":
  if (value.id in reasoningMap) {
    const part = reasoningMap[value.id]
    // REMOVED: part.text = part.text.trimEnd()
    // Reasoning text must be preserved byte-for-byte — the API-provided signature
    // validates the exact content. Trimming invalidates the signature and causes
    // "thinking blocks cannot be modified" on the next Bedrock request.
    part.time = { ...part.time, end: Date.now() }
    if (value.providerMetadata) part.metadata = value.providerMetadata
    await Session.updatePart(part)
    delete reasoningMap[value.id]
  }

Related Bug in @ai-sdk/amazon-bedrock

A secondary bug exists in packages/amazon-bedrock/src/convert-to-bedrock-chat-messages.ts where trimIfLast() is applied to reasoning blocks that have a signature:

// BUGGY (line 298):
text: trimIfLast(isLastBlock, isLastMessage, isLastContentPart, part.text),
signature: reasoningMetadata.signature,

// FIX: never trim when signature is present
text: reasoningMetadata.signature != null
  ? part.text
  : trimIfLast(isLastBlock, isLastMessage, isLastContentPart, part.text),
signature: reasoningMetadata.signature,

This should be reported separately to vercel/ai.

Reproduction

1. Configure OpenCode with Claude Opus 4 via Amazon Bedrock (extended thinking enabled)
2. Have a multi-turn conversation where extended thinking is used
3. Error appears on the turn after extended thinking was used (typically turn 2-3)

Environment

• OpenCode: v1.2.27
• Provider: amazon-bedrock
• Model: global.anthropic.claude-opus-4-6-v1 (variant: max)
• Confirmed in oh-my-opencode logs: ValidationException on converse-stream

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Claude API error: thinking/redacted_thinking blocks modified in conversation history #18073: Same error ("thinking/redacted_thinking blocks cannot be modified") reported by the same author — this issue appears to be a more specific follow-up identifying the root cause as trimEnd() in processor.ts
• Claude Opus 4.5 (latest) eventually fails: thinking block cannot be modified #13286: Same "thinking block cannot be modified" error on Claude Opus with extended thinking

If #18078 is intended as the fix/root-cause clarification for #18073, please consider closing one of them or linking them explicitly.