diff --git a/apps/web/src/chat/Markdown.tsx b/apps/web/src/chat/Markdown.tsx
index 5fe1a89a4..0e1934e38 100644
--- a/apps/web/src/chat/Markdown.tsx
+++ b/apps/web/src/chat/Markdown.tsx
@@ -14,12 +14,14 @@ export function Markdown({
 	className = CHAT_PROSE,
 	remarkPlugins,
 	rehypePlugins,
+	urlTransform,
 	components,
 }: {
 	text: string;
 	className?: string;
 	remarkPlugins?: ComponentProps<typeof ReactMarkdown>["remarkPlugins"];
 	rehypePlugins?: ComponentProps<typeof ReactMarkdown>["rehypePlugins"];
+	urlTransform?: ComponentProps<typeof ReactMarkdown>["urlTransform"];
 	components?: ComponentProps<typeof ReactMarkdown>["components"];
 }) {
 	return (
@@ -27,6 +29,7 @@ export function Markdown({
 			<ReactMarkdown
 				remarkPlugins={remarkPlugins ? [remarkGfm, ...remarkPlugins] : [remarkGfm]}
 				rehypePlugins={rehypePlugins}
+				urlTransform={urlTransform}
 				components={{ code: CodeBlock, a: Anchor, table: Table, ...components }}
 			>
 				{text}
diff --git a/apps/web/src/chat/SPEC.md b/apps/web/src/chat/SPEC.md
index fe00634d5..1cf787ea5 100644
--- a/apps/web/src/chat/SPEC.md
+++ b/apps/web/src/chat/SPEC.md
@@ -79,7 +79,21 @@ blocks in order into rows; `ChatTurnView` dispatches on row kind:
   summarized disappear immediately rather than only after reload. Its `summary` opens on click
   (`data-testid="chat-compaction"`). Hydration/reopen starts directly from that same durable form. Both forms
   share the **"Context compacted"** title; only facts unavailable after reload disappear.
-- `markdown` — a non-empty assistant text block (react-markdown + remark-gfm + shiki). A fenced
+- `markdown` — a non-empty assistant text block (react-markdown + remark-gfm + shiki). Safe
+  worktree-relative links in assistant prose open the target in ThinkRail through `ChatTurnView`'s existing
+  workspace-file callback; absolute paths are accepted only when they normalize inside the active worktree,
+  while URL schemes, protocol-relative URLs, fragments, and unsafe/outside paths retain ordinary safe
+  new-tab anchor behavior. Percent-encoded file paths are decoded once before validation, so encoded
+  separators and traversal cannot bypass containment. A narrow assistant-only URL transform preserves
+  recognized Windows drive-letter anchor paths, including Markdown's percent-encoded backslash form, until
+  validation; drive-rooted containment compares case-insensitively while preserving the linked path's casing.
+  Every other value delegates to react-markdown's default sanitizer, and a rejected Windows path
+  is re-sanitized before fallback anchor rendering. The generic `Markdown` primitive remains props-driven and
+  receives this behavior only as an `a` component override at the assistant-turn integration edge;
+  accepted workspace targets render as button controls without a raw browser `href`, so alternate native
+  anchor activation cannot escape into the SPA fallback. That override keeps a stable component identity
+  while its workspace inputs are unchanged: workbench focus can rerender a chat row between pointer-down and
+  click, and replacing the control in that interval cancels activation. A fenced
   ```mermaid block renders as a themed diagram via `tools/visualize`'s `MermaidView` (fullscreen
   pan-zoom, error → source fallback) — uniform across every `Markdown` surface (chat, file/specs
   preview); until mounted it renders as highlighted source, so static contexts (`RenderedDiff`'s
@@ -879,14 +893,15 @@ from their `toolCall` args and reply through **`ChatActions`** (see below). Work
 
 ## Boundary
 
-- **Public surface:** the registry API (`toolRegistry`), the props-driven slash-completion primitive, and
-  the renderers (incl. the presentational `Markdown` — GFM + shiki, no store/transport; the rendering is fixed but the **prose skin** is the
+- **Public surface:** the registry API (`toolRegistry`), the shared workspace-file target canonicalizer
+  (`fileTargets`), the props-driven slash-completion primitive, and the renderers (incl. the presentational
+  `Markdown` — GFM + shiki, no store/transport; the rendering is fixed but the **prose skin** is the
   caller's via an optional `className` — chat uses the compact bubble skin (`tr-prose-chat`),
   `panels/MarkdownPreview` the document skin (`tr-prose-doc`). A skin names exactly one generated
   `tr-prose-*` system and then carries only spacing/measure/chrome — no size, weight, leading or
   tracking (see `styles/TYPOGRAPHY.md`); a caller may
-  also **extend** the render with extra `remarkPlugins` + `components`, e.g. the file view's GitHub
-  alert callouts), the view types
+  also **extend** the render with an optional `urlTransform`, extra `remarkPlugins`, and `components`, e.g.
+  the file view's GitHub alert callouts), the view types
   (`types.ts`,
   incl. `ToolResultState` + `ExtUiDialogRequest`), and `ChatView` (lazy-mounted by the shell workbench
   resource renderer;
diff --git a/apps/web/src/chat/assistantLinks.test.tsx b/apps/web/src/chat/assistantLinks.test.tsx
new file mode 100644
index 000000000..6b825eea7
--- /dev/null
+++ b/apps/web/src/chat/assistantLinks.test.tsx
@@ -0,0 +1,91 @@
+import { describe, expect, test } from "bun:test";
+import { renderToStaticMarkup } from "react-dom/server";
+import { AssistantMarkdown, assistantFileTarget, assistantUrlTransform } from "./assistantLinks";
+
+describe("assistantFileTarget", () => {
+	test("accepts normalized relative paths and absolute paths inside the workspace", () => {
+		expect(assistantFileTarget(".thinkrail/context/report.md", "/repo")).toBe(
+			".thinkrail/context/report.md",
+		);
+		expect(assistantFileTarget("docs/../README.md#usage", "/repo")).toBe("README.md");
+		expect(assistantFileTarget("reports/manual%20report.md", "/repo")).toBe(
+			"reports/manual report.md",
+		);
+		expect(assistantFileTarget("/repo/docs/report.md?raw=1", "/repo")).toBe("docs/report.md");
+		expect(assistantFileTarget("c:/repo/docs/Report.md", "C:/Repo")).toBe("docs/Report.md");
+	});
+
+	test("rejects external, fragment, outside-workspace, and context-free targets", () => {
+		for (const href of [
+			"https://example.com/report",
+			"mailto:owner@example.com",
+			"//example.com/report",
+			"#report",
+			"../report.md",
+			"%2e%2e/report.md",
+			"/tmp/report.md",
+			"report%E0%A4%A.md",
+		]) {
+			expect(assistantFileTarget(href, "/repo")).toBeNull();
+		}
+		expect(assistantFileTarget("README.md", undefined)).toBeNull();
+	});
+});
+
+test("assistant URL transforms preserve only anchor-shaped Windows paths", () => {
+	const anchor = { tagName: "a" };
+	expect(assistantUrlTransform("C:/repo/report.md", "href", anchor)).toBe("C:/repo/report.md");
+	expect(assistantUrlTransform("C:%5Crepo%5Creport.md", "href", anchor)).toBe(
+		"C:%5Crepo%5Creport.md",
+	);
+	expect(assistantUrlTransform("C:/repo/image.png", "src", { tagName: "img" })).toBe("");
+	expect(assistantUrlTransform("C:/repo/style.css", "href", { tagName: "link" })).toBe("");
+	expect(assistantUrlTransform("javascript:alert(1)", "href", anchor)).toBe("");
+	expect(assistantUrlTransform("https://example.com/report", "href", anchor)).toBe(
+		"https://example.com/report",
+	);
+});
+
+test("assistant Markdown distinguishes workspace files from ordinary anchors", () => {
+	const html = renderToStaticMarkup(
+		<AssistantMarkdown
+			text={[
+				"[report](<.thinkrail/context/manual report.md>)",
+				"[site](https://example.com/report)",
+				"[outside](../report.md)",
+			].join("\n\n")}
+			workspaceRoot="/repo"
+			onOpenFile={() => {}}
+		/>,
+	);
+
+	expect(html).toContain('<button type="button" data-testid="chat-file-link"');
+	expect(html).toContain('data-path=".thinkrail/context/manual report.md"');
+	expect(html).not.toContain('href=".thinkrail/context/manual%20report.md"');
+	expect(html.match(/target="_blank"/g)).toHaveLength(2);
+	expect(html).toContain('href="https://example.com/report"');
+	expect(html).toContain('href="../report.md"');
+});
+
+test("assistant Markdown opens contained Windows paths without exposing unsafe URLs", () => {
+	const html = renderToStaticMarkup(
+		<AssistantMarkdown
+			text={[
+				"[inside](c:/repo/docs/Report.md)",
+				"[inside backslash](c:\\REPO\\docs\\backslash.md)",
+				"[outside](D:/other/report.md)",
+				"[unsafe](javascript:alert(1))",
+			].join("\n\n")}
+			workspaceRoot="C:/Repo"
+			onOpenFile={() => {}}
+		/>,
+	);
+
+	expect(html.match(/data-testid="chat-file-link"/g)).toHaveLength(2);
+	expect(html).toContain('data-path="docs/Report.md"');
+	expect(html).toContain('data-path="docs/backslash.md"');
+	expect(html).not.toContain('href="c:/repo/docs/Report.md"');
+	expect(html).not.toContain('href="c:%5CREPO%5Cdocs%5Cbackslash.md"');
+	expect(html).not.toContain('href="D:/other/report.md"');
+	expect(html).not.toContain('href="javascript:alert(1)"');
+});
diff --git a/apps/web/src/chat/assistantLinks.tsx b/apps/web/src/chat/assistantLinks.tsx
new file mode 100644
index 000000000..a0736d9fe
--- /dev/null
+++ b/apps/web/src/chat/assistantLinks.tsx
@@ -0,0 +1,91 @@
+import { type ReactNode, useMemo } from "react";
+import { type Components, defaultUrlTransform } from "react-markdown";
+import { isWindowsAbsolutePath, workspaceFileTarget } from "./fileTargets";
+import { Markdown } from "./Markdown";
+
+export function assistantUrlTransform(
+	value: string,
+	property: string,
+	node: { tagName: string },
+): string {
+	if (property === "href" && node.tagName === "a") {
+		try {
+			if (isWindowsAbsolutePath(decodeURIComponent(value))) return value;
+		} catch {
+			return defaultUrlTransform(value);
+		}
+	}
+	return defaultUrlTransform(value);
+}
+
+export function assistantFileTarget(
+	href: string | undefined,
+	workspaceRoot: string | undefined,
+): string | null {
+	const candidate = href?.trim();
+	if (!candidate || !workspaceRoot || candidate.startsWith("#") || candidate.startsWith("//")) {
+		return null;
+	}
+	const encodedPath = candidate.split(/[?#]/, 1)[0];
+	if (!encodedPath) return null;
+	try {
+		return workspaceFileTarget(decodeURIComponent(encodedPath), workspaceRoot);
+	} catch {
+		return null;
+	}
+}
+
+function AssistantLink({
+	href,
+	children,
+	workspaceRoot,
+	onOpenFile,
+}: {
+	href?: string | undefined;
+	children?: ReactNode;
+	workspaceRoot?: string | undefined;
+	onOpenFile?: ((path: string) => void) | undefined;
+}) {
+	const target = assistantFileTarget(href, workspaceRoot);
+	if (!target || !onOpenFile) {
+		const safeHref = href === undefined ? undefined : defaultUrlTransform(href);
+		return (
+			<a href={safeHref} target="_blank" rel="noopener noreferrer">
+				{children}
+			</a>
+		);
+	}
+	return (
+		<button
+			type="button"
+			data-testid="chat-file-link"
+			data-path={target}
+			onClick={() => onOpenFile(target)}
+			className="cursor-pointer text-left text-primary underline"
+		>
+			{children}
+		</button>
+	);
+}
+
+export function AssistantMarkdown({
+	text,
+	workspaceRoot,
+	onOpenFile,
+}: {
+	text: string;
+	workspaceRoot?: string | undefined;
+	onOpenFile?: ((path: string) => void) | undefined;
+}) {
+	const components = useMemo<Components>(
+		() => ({
+			a: ({ href, children }) => (
+				<AssistantLink href={href} workspaceRoot={workspaceRoot} onOpenFile={onOpenFile}>
+					{children}
+				</AssistantLink>
+			),
+		}),
+		[workspaceRoot, onOpenFile],
+	);
+	return <Markdown text={text} urlTransform={assistantUrlTransform} components={components} />;
+}
diff --git a/apps/web/src/chat/fileTargets.test.ts b/apps/web/src/chat/fileTargets.test.ts
new file mode 100644
index 000000000..9e2211c4b
--- /dev/null
+++ b/apps/web/src/chat/fileTargets.test.ts
@@ -0,0 +1,19 @@
+import { describe, expect, it } from "bun:test";
+import { workspaceFileTarget } from "./fileTargets";
+
+describe("workspaceFileTarget", () => {
+	it("canonicalizes only paths contained by the active worktree", () => {
+		for (const [path, root, expected] of [
+			["module-a/../README.md", "/repo", "README.md"],
+			["/repo/module-a/SPEC.md", "/repo", "module-a/SPEC.md"],
+			["C:\\repo\\module-a\\SPEC.md", "C:\\repo", "module-a/SPEC.md"],
+			["/repo-other/SPEC.md", "/repo", null],
+			["../outside.md", "/repo", null],
+			["https://example.com/a.md", "/repo", null],
+			["file:///repo/a.md", "/repo", null],
+			["", "/repo", null],
+		] as const) {
+			expect(workspaceFileTarget(path, root)).toBe(expected);
+		}
+	});
+});
diff --git a/apps/web/src/chat/fileTargets.ts b/apps/web/src/chat/fileTargets.ts
new file mode 100644
index 000000000..96bbf7ad6
--- /dev/null
+++ b/apps/web/src/chat/fileTargets.ts
@@ -0,0 +1,26 @@
+import { isAbsolutePath, projectRelativePath } from "@/lib";
+
+const URI_SCHEME = /^[A-Za-z][A-Za-z0-9+.-]*:/;
+const WINDOWS_ABSOLUTE_PATH = /^[A-Za-z]:[\\/]/;
+
+export function hasUriScheme(value: string): boolean {
+	return URI_SCHEME.test(value);
+}
+
+export function isWindowsAbsolutePath(value: string): boolean {
+	return WINDOWS_ABSOLUTE_PATH.test(value);
+}
+
+export function workspaceFileTarget(
+	path: string,
+	workspaceRoot?: string | undefined,
+): string | null {
+	const candidate = path.trim();
+	if (!candidate) return null;
+	if (!isAbsolutePath(candidate) && hasUriScheme(candidate)) return null;
+	const relative = projectRelativePath(candidate, workspaceRoot);
+	if (!relative || isAbsolutePath(relative) || relative === ".." || relative.startsWith("../")) {
+		return null;
+	}
+	return relative;
+}
diff --git a/apps/web/src/chat/tools/SPEC.md b/apps/web/src/chat/tools/SPEC.md
index 291e3fd9d..f0b65a0ed 100644
--- a/apps/web/src/chat/tools/SPEC.md
+++ b/apps/web/src/chat/tools/SPEC.md
@@ -200,8 +200,9 @@ registration runs once when the chat module mounts. Unregistered tools fall back
   also registered for `get_subagent_result`, routine), the `SubagentCompletionCard` turn card, and the
   pure `runDetails` readers/formatters; own child spec ([subagent/SPEC.md](subagent/SPEC.md)).
 - **Shared pieces** — `CodeBlock` (shiki), `Collapsible` ("Show all N lines" fold for long output),
-  `ToolFileLink` + exact-reference linked text, pure `toolHelpers` (arg readers, `resultText`,
-  `languageFromPath`) + `lib`'s `projectRelativePath`. `resultText` delegates canonical result parsing to
+  `ToolFileLink` + exact-reference linked text backed by the parent chat module's shared
+  `workspaceFileTarget`, pure `toolHelpers` (arg readers, `resultText`, `languageFromPath`) + `lib`'s
+  `projectRelativePath`. `resultText` delegates canonical result parsing to
   the parent chat primitive, so text extraction and the common image layer cannot disagree about what
   constitutes a valid content block.
 
@@ -209,7 +210,8 @@ registration runs once when the chat module mounts. Unregistered tools fall back
 
 - **Public surface:** the side-effect `register` import + the shared `CodeBlock`/`Collapsible`/
   `toolHelpers` for sibling renderers + `visualize/MermaidView` for the parent `Markdown` primitive. No barrel (chat pulls shiki — per-file imports, as in the parent).
-- **Allowed deps:** parent chat primitives (`toolRegistry`, `Markdown`, `ChatActions`, `askState`);
+- **Allowed deps:** parent chat primitives (`toolRegistry`, `Markdown`, `ChatActions`, `askState`,
+  `fileTargets`);
   `contracts` (type-only + the `ASK_USER_ANSWERS_CUSTOM_TYPE` constant); `components/ui`; `lib`;
   `@remixicon/react`; `mermaid` (**lazy, `visualize/` only**).
 - **Forbidden:** value-importing any `pi` package; `store`/`transport` (renderers stay presentational —
diff --git a/apps/web/src/chat/tools/ToolFileLink.tsx b/apps/web/src/chat/tools/ToolFileLink.tsx
index 2bfba67e0..7fe08b873 100644
--- a/apps/web/src/chat/tools/ToolFileLink.tsx
+++ b/apps/web/src/chat/tools/ToolFileLink.tsx
@@ -1,17 +1,5 @@
 import { cn, isAbsolutePath, projectRelativePath } from "@/lib";
-
-const URI_SCHEME = /^[A-Za-z][A-Za-z0-9+.-]*:/;
-
-export function toolFileTarget(path: string, workspaceRoot?: string | undefined): string | null {
-	const candidate = path.trim();
-	if (!candidate) return null;
-	if (!isAbsolutePath(candidate) && URI_SCHEME.test(candidate)) return null;
-	const relative = projectRelativePath(candidate, workspaceRoot);
-	if (!relative || isAbsolutePath(relative) || relative === ".." || relative.startsWith("../")) {
-		return null;
-	}
-	return relative;
-}
+import { hasUriScheme, workspaceFileTarget } from "../fileTargets";
 
 export function ToolFileLink({
 	path,
@@ -31,10 +19,10 @@ export function ToolFileLink({
 	const candidate = path.trim();
 	const displayPath =
 		label ??
-		(candidate && !isAbsolutePath(candidate) && URI_SCHEME.test(candidate)
+		(candidate && !isAbsolutePath(candidate) && hasUriScheme(candidate)
 			? candidate
 			: projectRelativePath(candidate, workspaceRoot));
-	const target = disabled ? null : toolFileTarget(candidate, workspaceRoot);
+	const target = disabled ? null : workspaceFileTarget(candidate, workspaceRoot);
 	if (!target || !onOpenFile) {
 		return (
 			<span
diff --git a/apps/web/src/chat/tools/ToolFileLinks.test.ts b/apps/web/src/chat/tools/ToolFileLinks.test.ts
index 408a8fb54..0f5d024ac 100644
--- a/apps/web/src/chat/tools/ToolFileLinks.test.ts
+++ b/apps/web/src/chat/tools/ToolFileLinks.test.ts
@@ -4,7 +4,6 @@ import { renderToStaticMarkup } from "react-dom/server";
 import type { ToolRenderProps } from "../toolRegistry";
 import { EditCard } from "./EditCard";
 import { ReadCard } from "./ReadCard";
-import { toolFileTarget } from "./ToolFileLink";
 import { WriteCard } from "./WriteCard";
 
 const result = { content: [{ type: "text", text: "ok" }] };
@@ -36,21 +35,6 @@ function markup(
 }
 
 describe("structured tool file links", () => {
-	it("canonicalizes only paths contained by the active worktree", () => {
-		for (const [path, root, expected] of [
-			["module-a/../README.md", "/repo", "README.md"],
-			["/repo/module-a/SPEC.md", "/repo", "module-a/SPEC.md"],
-			["C:\\repo\\module-a\\SPEC.md", "C:\\repo", "module-a/SPEC.md"],
-			["/repo-other/SPEC.md", "/repo", null],
-			["../outside.md", "/repo", null],
-			["https://example.com/a.md", "/repo", null],
-			["file:///repo/a.md", "/repo", null],
-			["", "/repo", null],
-		] as const) {
-			expect(toolFileTarget(path, root)).toBe(expected);
-		}
-	});
-
 	it("opens relative and in-worktree absolute read paths under one canonical relative label", () => {
 		const relative = markup(ReadCard, "read", "module-a/../README.md");
 		const absolute = markup(ReadCard, "read", "/repo/module-a/SPEC.md");
diff --git a/apps/web/src/chat/turns.tsx b/apps/web/src/chat/turns.tsx
index 3d79e7ad8..e74a4cdba 100644
--- a/apps/web/src/chat/turns.tsx
+++ b/apps/web/src/chat/turns.tsx
@@ -22,6 +22,7 @@ import {
 	userText,
 } from "@/lib";
 import { ActivityGroup } from "./ActivityGroup";
+import { AssistantMarkdown } from "./assistantLinks";
 import { FileChip } from "./FileChip";
 import { useFold, useSelection } from "./foldState";
 import { Markdown } from "./Markdown";
@@ -91,7 +92,11 @@ export function ChatTurnView({
 					data-role="assistant"
 					className="tr-text-reading text-text-default"
 				>
-					<Markdown text={row.text} />
+					<AssistantMarkdown
+						text={row.text}
+						workspaceRoot={workspaceRoot}
+						onOpenFile={onOpenFile}
+					/>
 				</div>
 			);
 		case "subagentCompletion":
diff --git a/apps/web/src/lib/SPEC.md b/apps/web/src/lib/SPEC.md
index 0ceda0112..8182a20f2 100644
--- a/apps/web/src/lib/SPEC.md
+++ b/apps/web/src/lib/SPEC.md
@@ -54,7 +54,8 @@ Tiny UI helpers shared across components.
   semantic resource key, so delimiters and stable noncanonical placement ids cannot split or alias identities.
 - **Public surface (barrel):** `cn`, `isMarkdownPath`, `stripFrontmatter`, `cssColorToHex`,
   `normalizePath`, `isAbsolutePath`, `projectRelativePath` (canonical worktree-relative POSIX identity;
-  collapses in-root `.`/`..` aliases but preserves an attempted leading escape for host rejection),
+  collapses in-root `.`/`..` aliases but preserves an attempted leading escape for host rejection; Windows
+  drive-rooted containment compares path/root case-insensitively while preserving the candidate's casing),
   `shallowEqualArrays`, `userText`, `parseSkillInvocation`, `matchesSkillInvocationCommand`,
   `relativeTime`, `platformShortcutLabel`, `hasPlatformModifier`, `copyText`, `randomId`,
   `DOUBLE_CLICK_SETTLE_MS`, `tupleKey`, `parseTupleKey`, `layoutResourceIdentity`,
diff --git a/apps/web/src/lib/utils.test.ts b/apps/web/src/lib/utils.test.ts
index c27ce6ad7..fdd323033 100644
--- a/apps/web/src/lib/utils.test.ts
+++ b/apps/web/src/lib/utils.test.ts
@@ -121,6 +121,10 @@ test("projectRelativePath yields the worktree-relative tab identity from every r
 	expect(projectRelativePath("src/./nested/../foo.ts", root)).toBe("src/foo.ts");
 	expect(projectRelativePath("../outside.ts", root)).toBe("../outside.ts");
 	expect(projectRelativePath("C:\\wt\\ws\\src\\..\\foo.ts", "C:\\wt\\ws")).toBe("foo.ts");
+	expect(projectRelativePath("c:/WT/ws/Src/Foo.ts", "C:/wt/WS")).toBe("Src/Foo.ts");
+	expect(projectRelativePath("C:/Repository/foo.ts", "c:/repo")).toBe("C:/Repository/foo.ts");
+	expect(projectRelativePath("D:/Repo/foo.ts", "c:/repo")).toBe("D:/Repo/foo.ts");
+	expect(projectRelativePath("/WT/ws/src/foo.ts", root)).toBe("/WT/ws/src/foo.ts");
 	expect(projectRelativePath("/src/foo.ts", "/")).toBe("src/foo.ts");
 	expect(projectRelativePath("C:/src/foo.ts", "C:/")).toBe("src/foo.ts");
 	expect(projectRelativePath("/elsewhere/foo.ts", root)).toBe("/elsewhere/foo.ts");
diff --git a/apps/web/src/lib/utils.ts b/apps/web/src/lib/utils.ts
index 45aa93b0f..b8ea21f10 100644
--- a/apps/web/src/lib/utils.ts
+++ b/apps/web/src/lib/utils.ts
@@ -94,9 +94,13 @@ export function normalizePath(path: string): string {
 	return path.replaceAll("\\", "/").replace(/^\.\/+/, "");
 }
 
+function hasWindowsDriveRoot(path: string): boolean {
+	return /^[A-Za-z]:\//.test(normalizePath(path));
+}
+
 export function isAbsolutePath(path: string): boolean {
 	const normalized = normalizePath(path);
-	return normalized.startsWith("/") || /^[A-Za-z]:\//.test(normalized);
+	return normalized.startsWith("/") || hasWindowsDriveRoot(normalized);
 }
 
 export function shallowEqualArrays(
@@ -142,8 +146,14 @@ export function projectRelativePath(path: string, workspaceRoot?: string | undef
 	if (!canonical || !isAbsolutePath(canonical)) return canonical;
 
 	const root = workspaceRoot ? trimTrailingSlashes(canonicalPosixPath(workspaceRoot)) : "";
-	const rootPrefix = root.endsWith("/") ? root : `${root}/`;
-	if (root && (canonical === root || canonical.startsWith(rootPrefix))) {
+	const ignoreCase = hasWindowsDriveRoot(canonical) && hasWindowsDriveRoot(root);
+	const comparableCanonical = ignoreCase ? canonical.toLowerCase() : canonical;
+	const comparableRoot = ignoreCase ? root.toLowerCase() : root;
+	const rootPrefix = comparableRoot.endsWith("/") ? comparableRoot : `${comparableRoot}/`;
+	if (
+		comparableRoot &&
+		(comparableCanonical === comparableRoot || comparableCanonical.startsWith(rootPrefix))
+	) {
 		return canonical.slice(root.length).replace(/^\/+/, "") || fileName(canonical);
 	}
 
diff --git a/e2e/tool-file-links.spec.ts b/e2e/tool-file-links.spec.ts
index 5febd01d4..3c18f0d59 100644
--- a/e2e/tool-file-links.spec.ts
+++ b/e2e/tool-file-links.spec.ts
@@ -170,6 +170,52 @@ async function returnToChat(page: Page): Promise<void> {
 	await expect(page.getByTestId("chat-view")).toBeVisible();
 }
 
+test("assistant Markdown opens safe relative files without navigating the browser", async ({
+	page,
+}) => {
+	await openFixtureProject(page);
+	seedWorkspaceSession(repoCwd(), {
+		name: "assistant file links",
+		messages: [
+			{ role: "user", text: "show the evidence", timestamp: BASE_TS },
+			{
+				role: "assistant",
+				text: [
+					"[Open README](README.md)",
+					"[Open docs](https://example.com/docs)",
+					"[Outside workspace](../outside.md)",
+				].join("\n\n"),
+				timestamp: BASE_TS + 1_000,
+			},
+		],
+	});
+
+	await expect(defaultWorkspaceRow(page)).toBeVisible();
+	await enterDefaultWorkspace(page);
+	await expect(page.locator('[data-testid="editor-tab"][data-kind="chat"]')).toHaveCount(1);
+
+	const message = page.locator('[data-testid="chat-message"][data-role="assistant"]').last();
+	const local = message.getByRole("button", { name: "Open README" });
+	await expect(local).toHaveAttribute("data-testid", "chat-file-link");
+	await expect(local).toHaveAttribute("data-path", "README.md");
+	await expect(local).not.toHaveAttribute("href", "README.md");
+	await expect(local).not.toHaveAttribute("target", "_blank");
+	await expect(message.getByRole("link", { name: "Open docs" })).toHaveAttribute(
+		"target",
+		"_blank",
+	);
+	await expect(message.getByRole("link", { name: "Outside workspace" })).toHaveAttribute(
+		"target",
+		"_blank",
+	);
+
+	await local.click();
+	const fileTab = page.locator('[data-testid="editor-tab"][data-kind="file"]');
+	await expect(fileTab).toHaveCount(1);
+	await expect(fileTab).toContainText("README.md");
+	await expect(fileTab).toHaveAttribute("data-preview", "true");
+});
+
 test("structured tool paths reuse the preview tab while rich tool results stay intentional", async ({
 	page,
 }) => {