Summary
The model picker fails to populate (clicking it does nothing) because the app's JSON-RPC read loop to the Copilot CLI core dies at startup with a parse error, cancelling all in-flight requests — including list_models. This reproduces on essentially every app launch.
Impact
- Model picker renders empty / does not open on launch.
- Same failure intermittently takes out
list_global_skills and get_account_quota (whatever requests are in-flight when the read loop dies).
- Self-recovers later (a subsequent on-demand fetch returns the full model list), so it presents as flaky rather than permanently broken.
Root cause (evidence)
The reader in github_copilot_sdk::jsonrpc fails with:
ERROR jsonrpc_read_loop: github_copilot_sdk::jsonrpc: error reading from CLI
error=unexpected end of hex escape at line 1 column N
ERROR handlers::misc: failed to list models error=request cancelled
ERROR frontend: [WebSocket] Server error: Failed to list models: request cancelled
The failure column N varies across launches — observed values: 23940, 23972, 23973, 27207, 27570, 27571, 28537, 28538, 29217, 30877, 31480, 32952 (all in the ~24K–33K range).
A varying "unexpected end of hex escape" at a large, non-fixed column is the classic signature of a message-framing/buffering bug: a large JSON-RPC response line is split across read chunks, and when the chunk boundary lands inside a \uXXXX escape sequence, the reader attempts to JSON-parse the partial buffer instead of first accumulating a complete line/frame. Parsing then fails on the truncated escape and the whole read loop tears down, cancelling every pending request.
Reproduction
- Launch the app.
- Observe the read-loop error in the app log near startup.
- Open the model picker — it is empty / unresponsive.
Reproduced on 21 of 21 recent launch logs.
Observed in
- App version: 0.2.25 and 0.2.26 (also present with 0.2.27 staged)
- CLI core: GitHub Copilot CLI 1.0.58
- Platform: macOS (darwin-arm64)
Suggested fix
In the github_copilot_sdk::jsonrpc read loop, accumulate a full delimited line/frame from the CLI stream before JSON-parsing, rather than parsing partial read chunks. Additionally, a read-loop parse error should fail only the offending message (or trigger a reconnect/retry) instead of cancelling all concurrently in-flight requests.
Workaround
Relaunching the app re-establishes the connection and the picker repopulates.
Summary
The model picker fails to populate (clicking it does nothing) because the app's JSON-RPC read loop to the Copilot CLI core dies at startup with a parse error, cancelling all in-flight requests — including
list_models. This reproduces on essentially every app launch.Impact
list_global_skillsandget_account_quota(whatever requests are in-flight when the read loop dies).Root cause (evidence)
The reader in
github_copilot_sdk::jsonrpcfails with:The failure column
Nvaries across launches — observed values: 23940, 23972, 23973, 27207, 27570, 27571, 28537, 28538, 29217, 30877, 31480, 32952 (all in the ~24K–33K range).A varying "unexpected end of hex escape" at a large, non-fixed column is the classic signature of a message-framing/buffering bug: a large JSON-RPC response line is split across read chunks, and when the chunk boundary lands inside a
\uXXXXescape sequence, the reader attempts to JSON-parse the partial buffer instead of first accumulating a complete line/frame. Parsing then fails on the truncated escape and the whole read loop tears down, cancelling every pending request.Reproduction
Reproduced on 21 of 21 recent launch logs.
Observed in
Suggested fix
In the
github_copilot_sdk::jsonrpcread loop, accumulate a full delimited line/frame from the CLI stream before JSON-parsing, rather than parsing partial read chunks. Additionally, a read-loop parse error should fail only the offending message (or trigger a reconnect/retry) instead of cancelling all concurrently in-flight requests.Workaround
Relaunching the app re-establishes the connection and the picker repopulates.