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Go SDK: Deadlock in client.Start() when starting CLI via stdio #19

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Go SDK: Deadlock in client.Start() when starting CLI via stdio#19
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devm33
Labels
buggopriority/highsdk/go

Description

@djdefi
djdefi
opened on Jan 15, 2026

Summary

The Go SDK deadlocks when calling client.Start() on macOS. The CLI works correctly when invoked directly, but the SDK's JSON-RPC communication over stdio fails.

Environment

  • OS: macOS (Apple Silicon)
  • Go version: 1.24
  • SDK version: v0.0.0-20260115160841-b8a836b2c6a3 (pseudo-version from main)
  • CLI version: 0.0.354 (Commit: 076bd172b)
  • CLI path: /Users/djdefi/Library/Application Support/Code - Insiders/User/globalStorage/github.copilot-chat/copilotCli/copilot

Steps to Reproduce

package main

import (
    "log"
    copilot "github.com/github/copilot-sdk/go"
)

func main() {
    client := copilot.NewClient(&copilot.ClientOptions{
        LogLevel: "error",
    })
    
    // This deadlocks - never returns
    if err := client.Start(); err != nil {
        log.Fatal(err)
    }
    defer client.Stop()
    
    log.Println("Client started") // Never reached
}

Expected Behavior

client.Start() should either:

  1. Successfully start the CLI and establish JSON-RPC communication
  2. Return an error if communication fails

Actual Behavior

Program deadlocks with:

fatal error: all goroutines are asleep - deadlock!

goroutine 1 [select]:
github.com/github/copilot-sdk/go.(*JSONRPCClient).Request(...)
        jsonrpc.go:147
github.com/github/copilot-sdk/go.(*Client).Ping(...)
        client.go:730
github.com/github/copilot-sdk/go.(*Client).verifyProtocolVersion(...)
        client.go:753
github.com/github/copilot-sdk/go.(*Client).Start(...)
        client.go:239

Working CLI Test

The CLI works correctly when invoked directly:

$ copilot --log-level debug --allow-all-tools -p "hello"
Hello! I'm GitHub Copilot CLI, ready to help you with software engineering tasks...

Analysis

The deadlock occurs in verifyProtocolVersion() → Ping() → JSONRPCClient.Request(), suggesting the JSON-RPC channel setup over stdio isn't working correctly. The CLI starts but either:

  1. Doesn't output the expected JSON-RPC responses
  2. The SDK isn't reading them correctly
  3. There's a timing issue in the startup sequence

Workaround

Currently using the mock client while investigating. Would appreciate any guidance on:

  1. Required CLI flags for JSON-RPC mode
  2. Environment variables for SDK communication
  3. Alternative TCP mode configuration

Related

This is a follow-up discovery from #18 (Go submodule version tags). Found while trying to integrate the SDK into a project.

Activity

  1. djdefi
    added
    bug
    go
    on Jan 15, 2026
  2. github-actions
    added
    sdk/go
    priority/high
    on Jan 15, 2026

  3. github-actions commented on Jan 15, 2026

    @github-actions
    github-actionsbot
    on Jan 15, 2026 – with GitHub Actions
    Contributor

    Thanks for reporting this issue, @djdefi!

    This is a well-documented bug report with excellent reproduction steps and stack traces. I've labeled this as sdk/go, bug, and priority/high since it's a blocking deadlock preventing SDK usage on macOS.

    The issue appears to be in the JSON-RPC communication setup over stdio during client.Start(). The CLI works standalone but the SDK's JSON-RPC handshake isn't completing, causing the deadlock in verifyProtocolVersion().

    This will need investigation into:

    • Whether the CLI requires specific flags to enable JSON-RPC mode over stdio
    • How the SDK is spawning and communicating with the CLI process
    • Platform-specific stdio handling differences on macOS

    The team will take a look at this soon!

    AI generated by Issue Triage Agent

  4. djdefi commented on Jan 15, 2026

    @djdefi
    djdefi
    on Jan 15, 2026
    MemberAuthor

    Update: Root cause identified - Documentation gap, not a bug

    After further investigation, I believe this is not a SDK bug but a documentation issue.

    What I discovered

    The npm @github/copilot package contains two different CLIs:

    File Supports --server --stdio
    index.js (14MB bundled Node.js) ✅ YES
    copilot-darwin-arm64/copilot (binary) ❌ NO

    The confusion

    I was using the binary from VS Code Insiders (copilot-chat/copilotCli/copilot) which lacks the --server --stdio flags. The SDK requires the index.js version.

    Verified working

    # Install the npm package
npm install @github/copilot@0.0.382

# Set CLI path to index.js (NOT the binary)
export COPILOT_CLI_PATH="$(pwd)/node_modules/@github/copilot/index.js"

# SDK now works!
go test -tags=integration -v ./pkg/copilot/...
# PASS!

    Documentation gap

    The SDK READMEs all say:

    CLIPath (string): Path to CLI executable (default: "copilot")

    But all SDK test harnesses explicitly use index.js:

    • Go: ../nodejs/node_modules/@github/copilot/index.js
    • Node.js: node_modules/@github/copilot/index.js
    • Python: nodejs/node_modules/@github/copilot/index.js
    • .NET: nodejs/node_modules/@github/copilot/index.js

    Suggestion

    Could this issue be converted to a documentation request to clarify:

    1. SDK requires index.js, not the binary
    2. The VS Code Insiders CLI binary is not compatible
    3. Users should npm install @github/copilot and point to index.js

    Apologies for the false positive! Happy to close this if you'd prefer to track documentation separately.

  5. anowardear062-svg commented on Jan 15, 2026

    @anowardear062-svg
    anowardear062-svg
    on Jan 15, 2026

    Sure

  6. friggeri
    assigned
    devm33
    on Jan 16, 2026

  7. friggeri commented on Jan 16, 2026

    @friggeri
    friggeri
    on Jan 16, 2026
    Collaborator

    Thanks for reporting! @devm33 can you take a look?

  8. devm33 commented on Jan 20, 2026

    @devm33
    devm33
    on Jan 20, 2026
    Member

    Closing since this was just a version issue

  9. devm33
    closed this as completedon Jan 20, 2026
  10. jmoseley
    added a commit that references this issue on Jan 21, 2026

    Go APIs for defining tools with automatic schema (#19)

    4cc779b
  11. jeremiahjordanisaacson
    added a commit that references this issue on Apr 29, 2026

    Add Delphi/Object Pascal SDK for GitHub Copilot CLI

    9df7930
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