Add pruning fallback and character truncation for large context

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Summary

This PR addresses CON-4274 by adding two improvements to handle large context in the CLI:

1. Pruning fallback: When context length validation fails before sending chat messages, we now use the existing compileChatMessages pruning logic from core/llm/countTokens.js as a fallback. This removes older messages while preserving the system message, tools, and the most recent user/tool sequence.

2. Bash tool output truncation: The Bash tool now truncates output by both:

   • Lines: 5000 lines (existing)
   • Characters: 200,000 characters (new)

   Whichever limit is hit first will trigger truncation with an appropriate message.

Changes

• extensions/cli/src/stream/streamChatResponse.ts: Added pruning fallback in processStreamingResponse when context validation fails
• extensions/cli/src/tools/runTerminalCommand.ts: Added character-based truncation to Bash tool output

Testing

• The pruning logic reuses the well-tested compileChatMessages function from core
• Character truncation follows the same pattern as the existing line truncation

Fixes CON-4274

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This agent session was co-authored by nate and Continue.

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Summary by cubic

Prevents oversized contexts from breaking CLI requests by always pruning chat history and truncating Bash tool output. Fixes CON-4274 and improves reliability with large conversations and long command results.

• New Features
  • Always uses compileChatMessages to prune chat history; preserves system, tools, and the latest exchange.
  • Truncates Bash tool output to the last 5000 lines or last 200,000 characters (whichever comes first) with a clear truncation notice.

^{Written for commit fdcdcb6. Summary will update automatically on new commits.}

[chezsmithy]

I'd recommend a different truncation strategy for the bash output. Perhaps keep the last X lines from the end backwards to capture the outcome of tests and installs.

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[cubic-dev-ai]

No issues found across 2 files

[RomneyDa]

compile chat messages already supports unified history, no need to convert between the two. CLI should just always use compile chat messages, no need for a duplicate token counting validation step.

[RomneyDa]

@continue also implement @chezsmithy's truncation recommendation

[RomneyDa]

@TomIsion thanks for the contribution! could you check cubic feedback and rebase main?
EDIT ignore this, mistaken comment

[RomneyDa]

kicked off a session to address comments.

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I have read the CLA Document and I hereby sign the CLA

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1 out of 2 committers have signed the CLA.
✅ (sestinj)[https://github.com/sestinj]
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Changes Made

Addressed the review feedback:

1. Simplified Context Handling (per @RomneyDa)

• Removed duplicate token validation logic - no longer calling validateContextLength separately
• Use compileChatMessages directly - this function already handles unified history format properly
• Eliminated unnecessary format conversions - reduced back-and-forth between ChatHistoryItem[] and ChatMessage[] formats
• The pruning now happens in one clean pass through compileChatMessages

2. Improved Bash Tool Truncation (per @chezsmithy)

• Changed truncation strategy to keep LAST X lines instead of first X lines
• Uses .slice(-MAX_OUTPUT_LINES) to capture the end of output
• For character truncation, also keeps the last X characters
• This better captures test results, installation outcomes, and error messages that typically appear at the end of command output
• Updated truncation messages to clearly indicate we're showing the "last" portion

Technical Details

The changes reduce complexity by ~37 lines while improving functionality:

• Removed the validateContextLength pre-check and error handling
• Removed the convertToUnifiedHistory conversion step after pruning
• Simplified the flow: unified history → ChatMessage → compileChatMessages → OpenAI format
• Bash truncation now properly prioritizes the most relevant output (the end result)

These changes make the code more maintainable and align with how compileChatMessages was designed to be used.