## Summary
Adds a new line-based file reading endpoint to the workspace agent,
replacing the unbounded byte-based approach for the `read_file` chat
tool and `coder_workspace_read_file` MCP tool.
**Problem**: The current `read_file` tool returns the entire file
contents with no limits, which can blow up LLM context windows and cause
OOM issues with large files.
**Solution**: Inspired by [`coder/mux`](https://github.com/coder/mux)
and [`openai/codex`](https://github.com/openai/codex), implement a
line-based reader with safety limits.
## Changes
### Agent (`agent/agentfiles/`)
- New `/read-file-lines` endpoint with `HandleReadFileLines` handler
- Line-based `offset` (1-based line number, default: 1) and `limit`
(line count, default: 2000)
- Safety constants:
| Constant | Value | Purpose |
|---|---|---|
| `MaxFileSize` | 1 MB | Reject files larger than this at stat |
| `MaxLineBytes` | 1,024 | Per-line truncation with `... [truncated]`
marker |
| `MaxResponseLines` | 2,000 | Max lines per response |
| `MaxResponseBytes` | 32 KB | Max total response size |
| `DefaultLineLimit` | 2,000 | Default when no limit specified |
- Line numbering format: `1\tcontent` (tab-separated)
- Structured JSON response: `{ success, file_size, total_lines,
lines_read, content, error }`
- Hard errors when limits exceeded — tells the LLM to use
`offset`/`limit`
- Existing byte-based `/read-file` endpoint preserved (used by
`instruction.go`)
### SDK (`codersdk/workspacesdk/`)
- `ReadFileLinesResponse` type added
- `ReadFileLines` method added to `AgentConn` interface
- Mock regenerated
### Chat tool (`coderd/chatd/chattool/`)
- `read_file` tool now uses `conn.ReadFileLines()` instead of
`conn.ReadFile()`
- Updated tool description to document line-based parameters
- Response includes `file_size`, `total_lines`, `lines_read` metadata
### MCP tool (`codersdk/toolsdk/`)
- `coder_workspace_read_file` updated to use line-based reading
- Schema descriptions updated for line-based offset/limit
- Removed `maxFileLimit` constant (agent handles limits now)
### Tests
- 13 new test cases for `TestReadFileLines`:
- Path validation (empty, relative, non-existent, directory, no
permissions)
- Empty file handling
- Basic read, offset, limit, offset+limit combinations
- Offset beyond file length
- Long line truncation (>1024 bytes)
- Large file rejection (>1MB)
- All existing tests pass unchanged
## Design decisions
| Decision | Rationale |
|---|---|
| Line-based, not byte-based | Both coder/mux and openai/codex use
line-based — matches how LLMs reason about code |
| Default limit of 2000 | Matches codex; prevents accidental full-file
dumps while being generous |
| 32 KB response cap | Compromise between mux (16 KB) and codex (no cap)
|
| 1024 byte/line truncation with marker | More generous than codex
(500), marker helps LLM know data is missing |
| Hard errors on overflow | Matches mux; forces LLM to paginate rather
than getting partial data |
| Preserve byte-based endpoint | `instruction.go` needs raw byte access
for AGENTS.md |
Kyle Carberry