deepseek-harness/packages/fs/tool-fs

@deepseek-ai/dsh-tool-fs

The model-facing filesystem tools — read, write, edit — and their executor. This is the consumer layer of the filesystem stack: it owns tool names, JSON schemas, argument validation, prompt sections, read windowing, and result formatting. It reads/writes/edits through the ctx.fs provider seam (@deepseek-ai/dsh-fs) directly — it injects fs (plus tools/systemPrompt), not a policy service. The freshness/observation policy is contributed by a separate plugin (@deepseek-ai/dsh-fs-policy) through the fs/* event gate; the tool is not method-coupled to it.

// Default deployment: a ctx.fs provider, the policy plugin, then the tools.
await ctx.plugin(LocalFileSystem, { cwd: process.cwd() }) // @deepseek-ai/dsh-fs-local
await ctx.plugin(FsPolicy)                             // @deepseek-ai/dsh-fs-policy (policy gate)
await ctx.plugin(ToolFs)                                  // this package — registers read/write/edit

@deepseek-ai/dsh-fs-policy is optional: omit it and the tools run against the bare provider (unconditional write/overwrite/edit, no observed-state). A deployment that loads these tools is expected to also load it, so the behavior is read-before-write/edit.

Config

All keys are optional; the defaults are the shipped read caps.

Key	Default	Meaning
readLimit	2000	Default and maximum lines returned by one read call (the tool schema advertises it as the limit default).
readMaxLineLength	2000	Characters kept per line before truncation (the suffix names the cap).
readMaxBytes	51200	Byte cap on one read call's selected lines; overflow ends the window with a "capped" footer.
readStreamMinSize	10485760	Files at or above this size (or with unknown size) stream instead of loading whole into memory.

Tools (schemas per the filesystem tool schemas Agent Note)

Tool	Arguments	Behavior
read	file_path, offset?, limit?	Line-numbered UTF-8 content with a pagination footer. offset is 1-based; limit defaults to and caps at the configured readLimit (2000).
write	file_path, content	Create or fully replace a file. With the policy plugin: overwriting an existing file requires a prior read at the unchanged version; creating a new file does not. Without it: unconditional.
edit	file_path, non-empty old_string, new_string, replace_all?	Literal replacement; unique match required unless replace_all is true. With the policy plugin: requires a prior read (any window) and the file unchanged since. Without it: unconditional.

Field names are snake_case to match Claude Code and existing harness tool schemas.

The tool is the executor; policy is an event gate

The tools do not inject a policy service or inspect any cache. Each tool resolves the path via ctx.fs.resolve(path, { cwd, signal }) — passing the calling agent's session cwd (exec.agent.session.header.cwd) so a relative path resolves against the session's workspace, matching dsh-tool-bash, and forwarding tool cancellation through resolution (see the per-session cwd Agent Note) — then:

• read — one ctx.fs.stat (type + size routing + version), then readText/streamText, then builds the line window, then emits fs/observed with a plain ctx.emit. (1 stat.)
• write — ctx.waterfall('fs/write-intent', target, exec, () => undefined) for the optional guard, then ctx.fs.writeText(target, content, intent), then fs/observed. (0 stat.)
• edit — ctx.waterfall('fs/edit-intent', target, exec, () => undefined) for the optional guard, then ctx.fs.editText(target, edit, intent), then fs/observed. (0 stat.)

The tool passes exec (the tool-execution context) as the opaque actor on every dispatch. The default thunks return undefined (the unconstrained bare provider). When @deepseek-ai/dsh-fs-policy is loaded it occupies the single decision slot — returning createIfAbsent/replaceIfVersion/{ version } or throwing FS_NOT_OBSERVED — and records on fs/observed. Backend errors (FsError) and a thrown FS_NOT_OBSERVED flow through ToolRegistry.execute() and become isError tool results with their { name, code } attached.

fs/observed is fire-and-forget

fs/observed fires AFTER the read/write/edit already succeeded, via a plain ctx.emit. A listener is contractually a synchronous, side-effect-only recorder (@deepseek-ai/dsh-fs-policy's is a WeakMap.set); the tool does not guard the emit, so a listener that throws would surface as the tool's isError result — async or fallible observation does not belong on this event.

read opts into concurrent scheduling because its only mutation is the synchronous version recorder. Recorder races fail closed when a later write or edit re-checks the version under its target lock; both mutation tools remain exclusive. See the parallel tool-call Agent Note.

The package root exports only the Cordis plugin contract (name, inject, Config, and apply). Read rendering (line windowing + output formatting) lives in src/read-render.ts (Cordis-free, independently unit-tested); src/read.ts/write.ts/edit.ts are the tool executors and src/index.ts composes them.

Model Experience

System prompt

What the model sees

Every request in this plugin's registration scope receives the independently registered read, write, and edit guidance below. Scoped tool restrictions can hide schemas without removing these sections.

Read guidance

Use the read tool — not shell commands like cat — to inspect text files. Results include line numbers. Use offset and limit to continue reading large files.

Write guidance

Use the write tool to create files or completely replace file contents. Existing files are overwritten, so read an existing file first (the default fs-policy requires it) and prefer edit for targeted changes.

Edit guidance

Use the edit tool for targeted changes to existing UTF-8 text files. It replaces literal old_string with new_string; by default old_string must appear exactly once. If old_string appears multiple times, provide a more specific old_string or set replace_all to true. Read the file first (the default fs-policy requires it), unless you just created or edited it in this session.

Token effect

Fixed guidance cost per request while the plugin is active, even when a restriction hides one or more tools.

KV Cache effect

Prefix-stable while the plugin scope and guidance text are unchanged. Tool restrictions do not remove this section, but plugin activation or disposal may invalidate reuse from it.

Tool schemas

What the model sees

The model sees the generated read, write, and edit schemas, with snake_case arguments. Scoped tool restrictions can remove any definition for one agent.

Token effect

Fixed schema cost on every request in that tool view.

KV Cache effect

Prefix-stable while the visible tool definitions and order are unchanged. Registration lifecycle or scoped restrictions may invalidate reuse from the first changed schema token.

Read result

What the model sees

A successful read is exactly <path><displayPath></path>, newline, <type>file</type>, newline, <content>, numbered lines as <lineNumber>: <text>, a blank line, one footer, and </content>. The footer is exactly (Output capped. Showing lines <start>-<end>. Use offset=<next> to continue.), (Showing lines <start>-<end> of <total>. Use offset=<next> to continue.), or (End of file - total <total> lines). A long line ends exactly ... (line truncated to <max> chars).

Token effect

Read output is capped by readLimit, readMaxLineLength, and readMaxBytes; the retained call and result are resent until compaction.

KV Cache effect

Append-only; newly visible content follows the reusable request prefix and does not invalidate existing KV-cache entries.

Write and edit results

What the model sees

Write returns the exact five-line envelope <path><displayPath></path>, <type>file</type>, <content>, Created file or Updated file, then </content>. Edit returns exactly The file <displayPath> has been updated successfully. or, for replace_all, The file <displayPath> has been updated. All occurrences were successfully replaced. The full write or replacement text remains in the assistant tool-call arguments.

Token effect

Success text is small, but large mutation arguments and any result are resent until compaction.

KV Cache effect

Append-only; newly visible content follows the reusable request prefix and does not invalidate existing KV-cache entries.

Tool errors

What the model sees

Failures are normalized as Error: <message>. This package's stable validation and read messages are file_path must be a non-empty string, limit must be less than or equal to <max>, old_string must be a non-empty string, old_string and new_string must differ, cannot read "<path>": not found, cannot read "<path>": not a regular file, and offset <offset> is out of range for "<path>" (<total> lines); provider and policy templates are quoted in their package READMEs.

Token effect

Only a failing call adds these retained tokens.

KV Cache effect

Append-only; newly visible content follows the reusable request prefix and does not invalidate existing KV-cache entries.

Known Limitations and Deferred Work

• No model-facing directory listing ships — ctx.fs.listDir serves provider code such as skill discovery, while the sibling dsh-tool-fs-search package supplies bash-backed glob and grep rather than extending the filesystem seam.
• read handles UTF-8 text files only — binary-safe reads and PDF/image/multimodal content are deferred; a directory target is FS_NOT_REGULAR_FILE.
• No timeout surface — read/write/edit take no timeout argument and declare no timeout-policy budget; cancellation rides exec.signal only (the deliberate fs-family stance).