diff --git a/src/services/acp/bridge/paths.ts b/src/services/acp/bridge/paths.ts
index fb04ef3d8..95bcd5a23 100644
--- a/src/services/acp/bridge/paths.ts
+++ b/src/services/acp/bridge/paths.ts
@@ -1,5 +1,12 @@
 // Pure path-normalisation helper used by toolInfo / toolResults / forwarding.
-import { isAbsolute, resolve } from 'node:path'
+//
+// POSIX semantics are used so that emitted paths are platform-independent:
+// ACP v1 spec (tool-calls.mdx:304-306) requires ToolCallLocation.path /
+// Diff.path to be absolute, and the wire format is POSIX-style regardless of
+// the host OS. Using the platform-specific `node:path` here would prepend the
+// Windows drive letter (e.g. "D:\...") to POSIX-style inputs like
+// "/Users/test/project" — silently corrupting paths emitted to ACP clients.
+import { isAbsolute, resolve } from 'node:path/posix'
 
 /**
  * Normalises an emitted file path against the session cwd so that
diff --git a/src/services/acp/utils.ts b/src/services/acp/utils.ts
index 9e7ab92df..47979d089 100644
--- a/src/services/acp/utils.ts
+++ b/src/services/acp/utils.ts
@@ -172,7 +172,9 @@ export function sanitizeTitle(text: string): string {
 
 // ── Path display helpers ──────────────────────────────────────────
 
-import * as path from 'node:path'
+// POSIX semantics so paths are normalised consistently regardless of host OS.
+// ACP paths are always POSIX-style (see bridge/paths.ts for the same rationale).
+import * as path from 'node:path/posix'
 
 /**
  * Convert an absolute file path to a project-relative path for display.
@@ -186,7 +188,7 @@ export function toDisplayPath(filePath: string, cwd?: string): string {
     resolvedFile.startsWith(resolvedCwd + path.sep) ||
     resolvedFile === resolvedCwd
   ) {
-    return path.relative(resolvedCwd, resolvedFile).replaceAll('\\', '/')
+    return path.relative(resolvedCwd, resolvedFile)
   }
   return filePath
 }