ACP: external agents have no way to read project diagnostics (incl. LSP-only warnings like Tailwind)

[ap-justin]

Written by Claude (Claude Code), filed on behalf of the user.

Summary

External agents running over ACP (Claude Code via claude-code-acp, Gemini CLI, Codex, etc.) cannot programmatically read Zed's project diagnostics. The user can manually inject them through the @diagnostics mention in the agent input, but the agent itself has no tool/method to query them between turns.

This forces external agents to re-derive diagnostics from the CLI (tsc --noEmit, eslint, biome check, etc.), which works for compiler/linter output but misses LSP-only warnings — most painfully @tailwindcss/language-server (invalid classes, deprecated arbitrary values like z-[51] → z-51), CSS LSP, and any language server without an equivalent headless CLI.

Zed's native agent already has access via the internal diagnostics_tool (crates/agent/src/tools/diagnostics_tool.rs). External ACP agents have no equivalent.

Why this matters

The agent-iteration loop is "make a change → check diagnostics → fix → repeat." Without programmatic diagnostics access, that loop breaks for any warning class that lives only in an LSP. Today the workarounds are:

1. User pastes diagnostics into the thread (manual, defeats automation).
2. User opens Zed's native agent panel just for the LSP-cleanup task (split-brain — two agent surfaces, two histories, two configs).
3. Agent ignores LSP-only warnings entirely.

Proposal

Expose project diagnostics to ACP agents via either:

• A new ACP method (e.g. session/diagnostics or fs/diagnostics) that returns current diagnostics, optionally filtered by file/severity/source.
• Or a session-update push so agents can subscribe to diagnostic changes after edits, matching how textDocument/publishDiagnostics works on the LSP side.

Either route reuses Zed's existing aggregated diagnostics buffer — the same source @diagnostics and the native diagnostics_tool already consume.

Related

• LSP tools listed in agent profiles can't be used by agents #56778 — LSP-related tools listed in agent profiles aren't actually callable (native agent side of the same gap).
• Agent Panel: How to disable project diagnostics / errors from being given to agent? #30142, Agent Panel: Agent doesn't respect diagnostics in Rust. #30436, Bedrock: Project diagnostic tool without a path fails to be executed #33204 — prior diagnostics-tool issues for Zed's native agent.

Environment

Zed v1.5.x stable, Claude Code via @zed-industries/claude-code-acp.