fix(fs): align symlinked parent traversal

packages/fs/tool-fs/src/edit.ts

@@ -95,7 +95,7 @@ export function applyEditTool(ctx: Context, sandbox: FsSandboxSurface): void {
       // Resolve the per-call sandbox policy (approved mode > session override
       // > backend default, plus the session cwd root) BEFORE anything executes.
       const sandboxPolicy = await sandbox.resolvePolicy('edit', args, exec)
-      const target = await ctx.fs.resolve(input.filePath, sessionResolveOptions(exec, sandboxPolicy?.workspaceRoot))
+      const target = await ctx.fs.resolve(input.filePath, sessionResolveOptions(exec, input.filePath, sandboxPolicy?.workspaceRoot))
       // Single-slot decision: the policy plugin returns { version: vObserved } or
       // throws FS_NOT_OBSERVED; the bare default is undefined (unconditional edit).
       // No stat — the bare default never manufactures a version basis.

packages/fs/tool-fs/src/read.ts

@@ -88,7 +88,7 @@ export function applyReadTool(ctx: Context, caps: ReadToolCaps): void {
     isConcurrencySafe: () => true,
     async execute(args, exec): Promise<ContentBlock[]> {
       const input = parseReadArgs(args, caps.limit)
-      const target = await ctx.fs.resolve(input.filePath, sessionResolveOptions(exec))
+      const target = await ctx.fs.resolve(input.filePath, sessionResolveOptions(exec, input.filePath))
 
       // One stat: type check + size routing + the version recorded as observed.
       // A concurrent write can only make a later guarded mutation fail stale and require reread.

packages/fs/tool-fs/src/session-cwd.ts

@@ -16,22 +16,29 @@ const PARENT_PATH_SEGMENT = /(?:^|[\\/])\.\.(?:[\\/]|$)/
 /**
  * The session workspace cwd for this call, or `undefined` when none applies.
  * @param exec - the tool-execution context; only its optional `agent` is read.
+ * @param requestedPath - the path the provider will resolve; parent traversal
+ *   makes a symlinked cwd's filesystem identity observable.
  * @returns the calling agent's session cwd, or undefined for a non-agent caller (the backend then applies its own default).
  */
-export function sessionCwd(exec: ToolExecution): string | undefined {
+export function sessionCwd(exec: ToolExecution, requestedPath: string): string | undefined {
   const cwd = exec.agent?.session.header.cwd
-  if (cwd === undefined || !PARENT_PATH_SEGMENT.test(cwd)) return cwd
+  if (cwd === undefined || (!PARENT_PATH_SEGMENT.test(cwd) && !PARENT_PATH_SEGMENT.test(requestedPath))) return cwd
   return canonicalPath(cwd)
 }
 
 /**
  * Resolution options shared by all model-facing filesystem tools.
  * @param exec - the tool-execution context supplying session cwd and cancellation.
+ * @param requestedPath - the path the provider will resolve.
  * @param policyWorkspaceRoot - resolved per-call root, when a mutation carries sandbox policy.
  * @returns provider resolution options for the current tool call.
  */
-export function sessionResolveOptions(exec: ToolExecution, policyWorkspaceRoot?: string): { cwd?: string; signal?: AbortSignal } {
-  const cwd = policyWorkspaceRoot ?? sessionCwd(exec)
+export function sessionResolveOptions(
+  exec: ToolExecution,
+  requestedPath: string,
+  policyWorkspaceRoot?: string,
+): { cwd?: string; signal?: AbortSignal } {
+  const cwd = policyWorkspaceRoot ?? sessionCwd(exec, requestedPath)
   return {
     ...cwd !== undefined ? { cwd } : {},
     ...exec.signal !== undefined ? { signal: exec.signal } : {},

packages/fs/tool-fs/src/write.ts

@@ -80,7 +80,7 @@ export function applyWriteTool(ctx: Context, sandbox: FsSandboxSurface): void {
       // > backend default, plus the session cwd root) BEFORE anything executes;
       // an escalating call throws its distinct text on any non-grant.
       const sandboxPolicy = await sandbox.resolvePolicy('write', args, exec)
-      const target = await ctx.fs.resolve(input.filePath, sessionResolveOptions(exec, sandboxPolicy?.workspaceRoot))
+      const target = await ctx.fs.resolve(input.filePath, sessionResolveOptions(exec, input.filePath, sandboxPolicy?.workspaceRoot))
       // Single-slot decision: the policy plugin produces createIfAbsent/
       // replaceIfVersion; the bare default is undefined (unconditional). No stat.
       const intent = await ctx.waterfall('fs/write-intent', target, exec, () => undefined)