Built-in view reports Path does not exist for existing files in 1.0.73; 1.0.71 succeeds

Description

The built-in view tool reports Path does not exist for an existing text file with Copilot CLI 1.0.73. The same isolated Vally/Copilot SDK probe succeeds consistently with CLI 1.0.71.

This appears to have started in 1.0.72 and remains present in 1.0.73.

Controlled reproduction

The probe uses:

• Official public npm packages @github/copilot@1.0.71 and @github/copilot@1.0.73
• @microsoft/vally-cli@0.7.0
• The Copilot SDK's supported COPILOT_CLI_PATH override, pointing to each package's platform index.js
• The same model, prompt, skill, marker file, and grader for both versions

Minimal skill:

---
name: view-probe
description: Verify that Copilot can read a staged text file with its built-in view tool.
---

1. Use the built-in `view` tool exactly once to read `references/marker.txt`.
2. Do not use another file-reading tool.
3. Reply with the exact marker value.

references/marker.txt:

VIEW_PROBE_MARKER_8A4F21

The skill is staged into the agent workspace, and the prompt invokes it. Each version was run three times.

Results

Copilot CLI	Result	view outcome
1.0.71	3/3 passed	Returned VIEW_PROBE_MARKER_8A4F21
1.0.73	0/3 passed	Path does not exist

A final one-trial run preserved both workspaces. Immediately after each run, the exact absolute path supplied to view existed:

Copilot CLI 1.0.71
FileExists: true
tool.execution_complete.success: true
result: VIEW_PROBE_MARKER_8A4F21

Copilot CLI 1.0.73
FileExists: true
tool.execution_complete.success: false
error: Path does not exist

Sanitized failing event:

{
  "type": "tool.execution_start",
  "data": {
    "toolName": "view",
    "arguments": {
      "path": "C:\\probe\\workspace\\skill\\references\\marker.txt"
    }
  }
}
{
  "type": "tool.execution_complete",
  "data": {
    "success": false,
    "error": {
      "message": "Path does not exist",
      "code": "failure"
    }
  }
}

Additional CI evidence

We also observed the version transition on GitHub-hosted Ubuntu runners:

• CLI 1.0.71: 175 of 208 view calls succeeded.
• CLI 1.0.72: all 193 observed view calls failed with Path does not exist.
• CLI 1.0.73: all three captured view calls in another run failed with the same error.

The affected paths were accessible in the same agent sessions through glob, PowerShell Get-ChildItem, and Get-Content.

Expected behavior

view should return the contents of an existing readable file, as it does in 1.0.71.

Impact

Agents cannot read staged skill references and other workspace files. They may improvise without the reference, retry with additional tools, exceed turn/tool/token budgets, or produce incorrect results. This causes broad false failures in automated evaluations.

Environment

• Copilot CLI: 1.0.71 (working), 1.0.72/1.0.73 (affected)
• Copilot SDK: 1.0.6
• Vally CLI: 0.7.0
• Controlled reproduction: Windows x64
• Additional observations: GitHub-hosted Ubuntu runners
• Model: claude-sonnet-4.6

Start with the built-in view entry point and the platform index.js selected through COPILOT_CLI_PATH, then compare the path handling in CLI 1.0.71 with 1.0.73 using the supplied controlled probe. Trace why an existing absolute path is rejected and add regression coverage; done means the probe reads marker.txt successfully on affected versions and preserves the 1.0.71 behavior.