[BUG]: Terminal streaming renderer corrupts output - characters doubled/truncated during streaming

[Richard-Marlow]

Description

The Copilot CLI terminal renderer corrupts streamed output, producing doubled characters, truncated tokens, and repeated lines. This affects both the thinking/reasoning phase output and the final assistant response output.

Steps to Reproduce

1. Ask Copilot CLI any question that produces a moderately long response
2. Observe the output as it streams to the terminal

Observed Behavior

Output is visibly corrupted during streaming. Examples seen:

• Partial tokens rendered before full token is available (e.g. apil instead of apiVersion)
• Characters doubled mid-word (e.g. obserrvability, permpermissions)
• Words repeated mid-sentence (e.g. New ReliNew Relic, useruser)
• Lines repeated (e.g. metadata: block appearing twice)
• Thinking/reasoning tokens streamed visibly with same corruption artifacts

Example corrupted output observed:

apil
apiVersion: rbac.authorization.k8s.io/v1
...
- apiGroups: [piGroups: [""]

Expected Behavior

Output should render cleanly with no corruption, doubled characters, or repeated lines.

Environment

• Copilot CLI version: 1.0.61
• OS: Darwin (macOS)

Notes

The underlying content being generated is correct — the issue is purely in how the CLI streams and renders tokens to the terminal.

[Richard-Marlow]

This issue was observed in iTerm2 Build 3.6.11

[will-j-wright]

I've been having the same issue on Windows and Linux for 3 or 4 days now.