client-compatibility

Context

Squad runs on multiple Copilot surfaces (CLI, VS Code, JetBrains, GitHub.com). The coordinator must detect its platform and adapt spawning behavior accordingly. Different tools are available on different platforms, requiring conditional logic for agent spawning, SQL usage, and response timing.

Patterns

Platform Detection

Before spawning agents, determine the platform by checking available tools:

1. CLI mode — task tool is available → full spawning control. Use task with agent_type, mode, model, description, prompt parameters. Collect results via read_agent.
2. VS Code mode — runSubagent or agent tool is available → conditional behavior. Use runSubagent with the task prompt. Drop agent_type, mode, and model parameters. Multiple subagents in one turn run concurrently (equivalent to background mode). Results return automatically — no read_agent needed.
3. Fallback mode — neither task nor runSubagent/agent available → work inline. Do not apologize or explain the limitation. Execute the task directly.

If both task and runSubagent are available, prefer task (richer parameter surface).

CopilotSession Lifecycle Invariant

• CopilotSession lifecycle management must stay deterministic across surfaces: no ambiguous ownership or state transitions.
• Session resources must be released on completion, cancellation, and error paths; compatibility work must not trade platform coverage for leaks.

VS Code Spawn Adaptations

When in VS Code mode, the coordinator changes behavior in these ways:

• Spawning tool: Use runSubagent instead of task. The prompt is the only required parameter — pass the full agent prompt (charter, identity, task, hygiene, response order) exactly as you would on CLI.
• Parallelism: Spawn ALL concurrent agents in a SINGLE turn. They run in parallel automatically. This replaces mode: "background" + read_agent polling.
• Model selection: Accept the session model. Do NOT attempt per-spawn model selection or fallback chains — they only work on CLI. In Phase 1, all subagents use whatever model the user selected in VS Code's model picker.
• Scribe: Cannot fire-and-forget. Batch Scribe as the LAST subagent in any parallel group. Scribe is light work (file ops only), so the blocking is tolerable.
• Launch table: Skip it. Results arrive with the response, not separately. By the time the coordinator speaks, the work is already done.
• read_agent: Skip entirely. Results return automatically when subagents complete.
• agent_type: Drop it. All VS Code subagents have full tool access by default. Subagents inherit the parent's tools.
• description: Drop it. The agent name is already in the prompt.
• Prompt content: Keep ALL prompt structure — charter, identity, task, hygiene, response order blocks are surface-independent.

Feature Degradation Table

SQL Tool Caveat

The sql tool is CLI-only. It does not exist on VS Code, JetBrains, or GitHub.com. Any coordinator logic or agent workflow that depends on SQL (todo tracking, batch processing, session state) will silently fail on non-CLI surfaces. Cross-platform code paths must not depend on SQL. Use filesystem-based state (.squad/ files) for anything that must work everywhere.

Examples

Example 1: CLI parallel spawn

// Coordinator detects task tool available → CLI mode
task({ agent_type: "general-purpose", mode: "background", model: "claude-sonnet-4.5", ... })
task({ agent_type: "general-purpose", mode: "background", model: "claude-haiku-4.5", ... })
// Later: read_agent for both

Example 2: VS Code parallel spawn

// Coordinator detects runSubagent available → VS Code mode
runSubagent({ prompt: "...Runtime Engineer charter + task..." })
runSubagent({ prompt: "...Quality Engineer charter + task..." })
runSubagent({ prompt: "...Scribe charter + task..." }) // Last in group
// Results return automatically, no read_agent

Example 3: Fallback mode

// Neither task nor runSubagent available → work inline
// Coordinator executes the task directly without spawning

Anti-Patterns

• ❌ Using SQL tool in cross-platform workflows (breaks on VS Code/JetBrains/GitHub.com)
• ❌ Attempting per-spawn model selection on VS Code (Phase 1 — only session model works)
• ❌ Fire-and-forget Scribe on VS Code (must batch as last subagent)
• ❌ Showing launch table on VS Code (results already inline)
• ❌ Apologizing or explaining platform limitations to the user
• ❌ Using task when only runSubagent is available
• ❌ Dropping prompt structure (charter/identity/task) on non-CLI platforms