task: append environment details into existing blocks

[hannesrudolph]

Problem

Task messages currently append <environment_details> as a standalone trailing text block. Some interleaved-thinking models are sensitive to message content “shape” and can fail when an extra trailing block is introduced.

Desired behavior

Append <environment_details> into the final existing user text block (or tool result block) rather than adding a new standalone block.

Acceptance criteria

• If the last user content item is a text block, append <environment_details> to that block.
• If the last item is a tool result, append <environment_details> to that tool result text.
• If there is no suitable final block (empty content), create a single text block containing <environment_details>.
• Preserve existing non-text blocks (e.g. images) and keep original content immutable.

[roomote]

I've analyzed this issue and here's my proposed implementation plan:

Problem Analysis

In src/core/task/Task.ts, environment details is currently appended as a new standalone text block in two locations:

1. Line ~2574 in resumeAfterDelegation():

   lastUserMsg.content = [...contentWithoutEnvDetails, { type: "text" as const, text: environmentDetails }]

2. Line ~2757 in the main task loop:

   let finalUserContent = [...contentWithoutEnvDetails, { type: "text" as const, text: environmentDetails }]

This creates a separate trailing text block that can cause issues with interleaved-thinking models.

Proposed Solution

Create a helper function appendEnvironmentDetails() that:

1. If the last content item is a text block: Append <environment_details> to that block's text (separated by newlines)
2. If the last item is a tool_result: Append <environment_details> to the tool_result's text content
3. If content is empty or has no suitable final block: Create a single text block containing <environment_details>
4. Preserve non-text blocks (images) and keep original content immutable by creating new block objects

Implementation Details

• Add the helper function in src/core/task/Task.ts (or extract to a utility file)
• Replace the two locations where environment details is added with calls to the helper
• Add unit tests for the helper function covering all acceptance criteria
• Ensure the helper properly handles edge cases (empty arrays, only images, mixed content)

This approach ensures a consistent message content "shape" that interleaved-thinking models can handle reliably, while maintaining backward compatibility for other providers.

Please mention @roomote in a comment saying that you approve this plan, and I'll proceed with the implementation.