diff --git a/apps/mobile/src/features/pull-requests/PullRequestMarkdown.tsx b/apps/mobile/src/features/pull-requests/PullRequestMarkdown.tsx
index 63946016d591f..b1ff0f10ee781 100644
--- a/apps/mobile/src/features/pull-requests/PullRequestMarkdown.tsx
+++ b/apps/mobile/src/features/pull-requests/PullRequestMarkdown.tsx
@@ -10,27 +10,42 @@ import {
 } from "./PullRequestMedia";
 import { videoMimeType } from "@t3tools/shared/video";
 import { createContext, useContext, useMemo, useState } from "react";
-import { changeRequestReferenceAutolinker } from "@t3tools/client-runtime/pull-request-markdown";
+import {
+  changeRequestLinkResolver,
+  changeRequestReferenceAutolinker,
+} from "@t3tools/client-runtime/pull-request-markdown";
 import type { ReferenceAutolink } from "@t3tools/client-runtime/source-control-clients";
+import { assetUrlStateFromResult } from "@t3tools/client-runtime/state/assets";
 import { linkReferenceAutolinks } from "./pullRequestAutolinks";
+import { assetEnvironment } from "../../state/assets";
+import { usePreparedConnection } from "../../state/session";
+import { useAtomQueryRunner } from "../../state/use-atom-query-runner";
 
 import { useOpenPullRequest } from "../../native/useOpenPullRequest";
 import { SelectableMarkdownText } from "../../native/SelectableMarkdownText";
 import { useMarkdownPreviewStyles } from "../files/FileMarkdownPreview";
 import { markdownImageSources } from "../../lib/markdownImageSources";
 import { FilePreviewGallery, useFilePreviewGallery } from "../../components/FilePreviewGallery";
-import { pullRequestImageSource } from "./pullRequestMediaSource";
+import { pullRequestImageSource, pullRequestMediaSource } from "./pullRequestMediaSource";
 import {
   FilePreviewModal,
   type FilePreviewSource,
   type FilePreviewItem,
 } from "../../components/FilePreviewModal";
 import { ThreadMarkdownImageView } from "../threads/ThreadMarkdownImage";
-import type { MarkdownImageRenderer } from "../../native/SelectableMarkdownText";
+import type {
+  MarkdownImageRenderer,
+  MarkdownImageSourceResolver,
+} from "../../native/SelectableMarkdownText";
 
 const AutolinksContext = createContext<((text: string) => ReadonlyArray<ReferenceAutolink>) | null>(
   null,
 );
+/** `base` is the change request URL `resolve` reads relative destinations against. */
+const LinkResolverContext = createContext<{
+  readonly resolve: (url: string) => string;
+  readonly base: string;
+} | null>(null);
 
 export function PullRequestMarkdownRepository(props: {
   /** The change request the markdown belongs to, whose host decides what its text links. */
@@ -44,6 +59,8 @@ export function PullRequestMarkdownRepository(props: {
     provider === undefined || url === undefined
       ? null
       : changeRequestReferenceAutolinker({ provider, url });
+  const resolveUrl = changeRequestLinkResolver(props.changeRequest);
+  const linkResolver = resolveUrl && url !== undefined ? { resolve: resolveUrl, base: url } : null;
   const target = pullRequestTarget(props.params);
   const project = useProject({
     environmentId: target.environmentId,
@@ -53,11 +70,46 @@ export function PullRequestMarkdownRepository(props: {
   const media = cwd ? { environmentId: target.environmentId, cwd } : null;
   return (
     <AutolinksContext value={autolinks}>
-      <PullRequestMediaContext value={media}>{props.children}</PullRequestMediaContext>
+      <LinkResolverContext value={linkResolver}>
+        <PullRequestMediaContext value={media}>{props.children}</PullRequestMediaContext>
+      </LinkResolverContext>
     </AutolinksContext>
   );
 }
 
+/**
+ * Enriched draws an image in a table, list or link itself, from this source, not `renderImage`.
+ * It keeps a resolved source for the life of the view, so the view is keyed to everything an
+ * answer depends on: the change request it resolves against, the media location and the origin.
+ */
+function usePullRequestImageSource(resolve: (url: string) => string): {
+  readonly resolveImageSource: MarkdownImageSourceResolver;
+  readonly sourceKey: string;
+} {
+  const location = useContext(PullRequestMediaContext);
+  const connection = usePreparedConnection(location?.environmentId ?? null);
+  const httpBaseUrl = connection._tag === "Some" ? connection.value.httpBaseUrl : null;
+  const createUrl = useAtomQueryRunner(assetEnvironment.createUrl, { reportFailure: false });
+  const resolveImageSource: MarkdownImageSourceResolver = async (image) => {
+    const url = resolve(image.href);
+    const source = pullRequestMediaSource(url, location);
+    if ("uri" in source) return /^https?:\/\//i.test(url) ? { uri: url } : null;
+    const state = assetUrlStateFromResult(
+      await createUrl({
+        environmentId: source.environmentId,
+        input: { resource: source.resource },
+      }),
+      httpBaseUrl,
+    );
+    if (state._tag !== "Success") throw new Error("Pull request image is unavailable");
+    return { uri: state.url };
+  };
+  return {
+    resolveImageSource,
+    sourceKey: `${location?.environmentId ?? ""}\n${location?.cwd ?? ""}\n${httpBaseUrl ?? ""}`,
+  };
+}
+
 function MarkdownImage(props: {
   readonly href: string;
   readonly alt: string | null;
@@ -107,20 +159,32 @@ export function PullRequestMarkdown(props: {
   );
 }
 
+/** The agent writes the guide about the checkout, so its relative links name workspace files. */
+export function PullRequestGuideMarkdown(props: { readonly markdown: string }) {
+  return (
+    <LinkResolverContext value={null}>
+      <PullRequestMarkdown markdown={props.markdown} />
+    </LinkResolverContext>
+  );
+}
+
 function PullRequestMarkdownContent(props: {
   readonly markdown: string;
   readonly imageGroupMarkdown?: string;
 }) {
   const autolinks = useContext(AutolinksContext);
+  const linkResolver = useContext(LinkResolverContext);
   const mediaContext = useContext(PullRequestMediaContext);
   const markdown = linkReferenceAutolinks(props.markdown, autolinks);
   const openPullRequest = useOpenPullRequest();
+  const resolve = (href: string) => linkResolver?.resolve(href) ?? href;
   const openLink = (href: string) => {
-    void openPullRequest(href);
+    void openPullRequest(resolve(href));
   };
   const gallery: FilePreviewItem[] = markdownImageSources(
     props.imageGroupMarkdown ?? props.markdown,
   )
+    .map(resolve)
     .filter(
       (uri) =>
         /^https?:\/\//i.test(uri) &&
@@ -129,17 +193,21 @@ function PullRequestMarkdownContent(props: {
     .map((uri) => pullRequestImageSource(uri, mediaContext));
   const [preview, setPreview] = useState<FilePreviewSource | null>(null);
   const renderImage: MarkdownImageRenderer = (image) => (
-    <MarkdownImage href={image.href} alt={image.alt} onPreview={setPreview} />
+    <MarkdownImage href={resolve(image.href)} alt={image.alt} onPreview={setPreview} />
   );
+  const { resolveImageSource, sourceKey } = usePullRequestImageSource(resolve);
+  const nativeKey = `${linkResolver?.base ?? ""}\n${sourceKey}`;
   const styles = useMarkdownPreviewStyles();
   return (
     <FilePreviewGallery value={gallery}>
       <SelectableMarkdownText
+        key={nativeKey}
         markdown={markdown}
         textStyle={styles.nativeTextStyle}
         onLinkPress={openLink}
         renderImage={renderImage}
-        onImagePress={(href) => setPreview(pullRequestImageSource(href, mediaContext))}
+        resolveImageSource={resolveImageSource}
+        onImagePress={(href) => setPreview(pullRequestImageSource(resolve(href), mediaContext))}
       />
       <FilePreviewModal source={preview} onRequestClose={() => setPreview(null)} />
     </FilePreviewGallery>
diff --git a/apps/mobile/src/features/pull-requests/guide/GuideOutline.tsx b/apps/mobile/src/features/pull-requests/guide/GuideOutline.tsx
index 1308d4caee799..4301a84be72c2 100644
--- a/apps/mobile/src/features/pull-requests/guide/GuideOutline.tsx
+++ b/apps/mobile/src/features/pull-requests/guide/GuideOutline.tsx
@@ -4,7 +4,7 @@ import { stopFilePaths } from "@t3tools/client-runtime/review-guide-tour";
 import { Pressable, View } from "react-native";
 import { AppText as Text } from "../../../components/ui/AppText";
 import { SymbolView } from "../../../components/ui/AppSymbol";
-import { PullRequestMarkdown } from "../PullRequestMarkdown";
+import { PullRequestGuideMarkdown } from "../PullRequestMarkdown";
 import { Menu } from "../../../components/ui/Menu";
 
 import { useAppearancePreferences } from "../../settings/appearance/AppearancePreferencesProvider";
@@ -59,7 +59,7 @@ export function GuideOutline({
       </View>
       {guide.stops[0]?.summary ? (
         <View className="mb-6">
-          <PullRequestMarkdown markdown={guide.stops[0].summary} />
+          <PullRequestGuideMarkdown markdown={guide.stops[0].summary} />
         </View>
       ) : null}
       {guide.stops.map((stop, position) => {
diff --git a/apps/mobile/src/features/pull-requests/guide/PullRequestGuideScreen.tsx b/apps/mobile/src/features/pull-requests/guide/PullRequestGuideScreen.tsx
index caa1e72938c87..9e9eedc6d6041 100644
--- a/apps/mobile/src/features/pull-requests/guide/PullRequestGuideScreen.tsx
+++ b/apps/mobile/src/features/pull-requests/guide/PullRequestGuideScreen.tsx
@@ -21,7 +21,7 @@ import { IconButton } from "../../../components/ui/IconButton";
 import { useAppearancePreferences } from "../../settings/appearance/AppearancePreferencesProvider";
 import { NativeStackScreenOptions } from "../../../native/StackHeader";
 import { PullRequestCardGroup, PullRequestCardRow } from "../PullRequestControls";
-import { PullRequestMarkdown, PullRequestMarkdownRepository } from "../PullRequestMarkdown";
+import { PullRequestGuideMarkdown, PullRequestMarkdownRepository } from "../PullRequestMarkdown";
 import { usePullRequestDiff, type PullRequestParams } from "../usePullRequestDiff";
 import { ReviewGuideExcerpt } from "./ReviewGuideExcerpt";
 import { reviewGuideState } from "./reviewGuideState";
@@ -188,7 +188,7 @@ export function PullRequestGuideStopScreen({ route }: StaticScreenProps<GuideSto
               <>
                 <GuideNotice state={state} />
                 <Text className="text-2xl leading-8 font-t3-semibold">{stop.title}</Text>
-                <PullRequestMarkdown markdown={stop.summary} />
+                <PullRequestGuideMarkdown markdown={stop.summary} />
                 {stop.attention ? (
                   <View className="gap-3 rounded-xl border border-border bg-grouped-card p-4">
                     <View className="flex-row items-center gap-2">
@@ -197,7 +197,7 @@ export function PullRequestGuideStopScreen({ route }: StaticScreenProps<GuideSto
                         {attentionLabels[stop.attention.kind]}
                       </Text>
                     </View>
-                    <PullRequestMarkdown markdown={stop.attention.note} />
+                    <PullRequestGuideMarkdown markdown={stop.attention.note} />
                   </View>
                 ) : null}
                 {stale ? (
@@ -237,7 +237,7 @@ export function PullRequestGuideStopScreen({ route }: StaticScreenProps<GuideSto
                           <Text className="text-xs text-foreground-muted">
                             {note.path.split("/").at(-1)}:{note.line}
                           </Text>
-                          <PullRequestMarkdown markdown={note.body} />
+                          <PullRequestGuideMarkdown markdown={note.body} />
                         </View>
                       </View>
                     ))}
@@ -266,7 +266,7 @@ export function PullRequestGuideStopScreen({ route }: StaticScreenProps<GuideSto
                   <View className="gap-3">
                     <Text className="font-t3-semibold">Questions to consider</Text>
                     {stop.questions.map((question) => (
-                      <PullRequestMarkdown key={question} markdown={question} />
+                      <PullRequestGuideMarkdown key={question} markdown={question} />
                     ))}
                   </View>
                 ) : null}
diff --git a/apps/mobile/src/features/pull-requests/guide/ReviewGuideExcerpt.tsx b/apps/mobile/src/features/pull-requests/guide/ReviewGuideExcerpt.tsx
index aa5f9e8d87f34..4700a15520b3a 100644
--- a/apps/mobile/src/features/pull-requests/guide/ReviewGuideExcerpt.tsx
+++ b/apps/mobile/src/features/pull-requests/guide/ReviewGuideExcerpt.tsx
@@ -11,7 +11,7 @@ import { buildReviewParsedDiff } from "../../review/reviewModel";
 import { useNativeReviewDiffBridge } from "../../review/useNativeReviewDiffBridge";
 import { useAppearanceCodeSurface } from "../../settings/appearance/useAppearanceCodeSurface";
 import { useAppearancePreferences } from "../../settings/appearance/AppearancePreferencesProvider";
-import { PullRequestMarkdown } from "../PullRequestMarkdown";
+import { PullRequestGuideMarkdown } from "../PullRequestMarkdown";
 
 const EMPTY_IDS: readonly string[] = [];
 
@@ -55,7 +55,7 @@ export function ReviewGuideExcerpt({
   const [NativeDiff] = useState(() => resolveNativeReviewDiffView());
   return (
     <View className="gap-3">
-      {excerpt.caption.trim() ? <PullRequestMarkdown markdown={excerpt.caption} /> : null}
+      {excerpt.caption.trim() ? <PullRequestGuideMarkdown markdown={excerpt.caption} /> : null}
       <View className="overflow-hidden rounded-lg border border-border-subtle">
         <Pressable
           accessibilityRole="button"
diff --git a/apps/web/src/components/ChatMarkdown.browser.test.tsx b/apps/web/src/components/ChatMarkdown.browser.test.tsx
index 745e69087cb1e..416201d280e29 100644
--- a/apps/web/src/components/ChatMarkdown.browser.test.tsx
+++ b/apps/web/src/components/ChatMarkdown.browser.test.tsx
@@ -1,3 +1,4 @@
+import { changeRequestLinkResolver } from "@t3tools/client-runtime/pull-request-markdown";
 import { EnvironmentId, ThreadId } from "@t3tools/contracts";
 import { AsyncResult } from "effect/reactivity";
 import { type ReactElement } from "react";
@@ -139,6 +140,39 @@ it("loads a host image through the page's own route when there is no environment
   );
 });
 
+it("resolves destinations in parsed html and reference links, but not code", async () => {
+  setup();
+  const resolveUrl = changeRequestLinkResolver({
+    provider: "github",
+    url: "https://github.com/team/repo/pull/8",
+  })!;
+  await using screen = await renderText(
+    [
+      '<img alt="before > after" src="/team/repo/blob/abc/a.png">',
+      "",
+      `<img alt="b" title="src='example'" src="../blob/abc/b.png">`,
+      "",
+      '<a href="../blob/abc/docs.md">docs</a> [ref][r] `![c](../blob/abc/c.png)`',
+      "",
+      "[r]: ../blob/abc/ref.md",
+    ].join("\n"),
+    { resolveUrl },
+  );
+  await expect
+    .element(screen.getByAltText("before > after"))
+    .toHaveAttribute("src", "https://github.com/team/repo/blob/abc/a.png");
+  await expect
+    .element(screen.getByAltText("b"))
+    .toHaveAttribute("src", "https://github.com/team/repo/blob/abc/b.png");
+  await expect
+    .element(screen.getByRole("link", { name: "docs" }))
+    .toHaveAttribute("href", "https://github.com/team/repo/blob/abc/docs.md");
+  await expect
+    .element(screen.getByRole("link", { name: "ref" }))
+    .toHaveAttribute("href", "https://github.com/team/repo/blob/abc/ref.md");
+  await expect.element(screen.getByText("![c](../blob/abc/c.png)", { exact: true })).toBeVisible();
+});
+
 describe("ChatMarkdown favicon privacy", () => {
   it("suppresses private link images while preserving public links across updates", async () => {
     setup();
diff --git a/apps/web/src/components/ChatMarkdown.tsx b/apps/web/src/components/ChatMarkdown.tsx
index 99c2ebbc8f40f..5b8c4e82a42eb 100644
--- a/apps/web/src/components/ChatMarkdown.tsx
+++ b/apps/web/src/components/ChatMarkdown.tsx
@@ -245,6 +245,8 @@ interface ChatMarkdownProps {
       `cwd`'s; without it a private repository's uploads load unauthenticated and 404. */
   sourceControlMedia?: boolean | undefined;
   headingLevelOffset?: number | undefined;
+  /** Rewrites every destination, raw HTML's included, before the renderer reads it. */
+  resolveUrl?: ((url: string) => string) | undefined;
 }
 
 export interface ChatMarkdownContextReference {
@@ -491,6 +493,7 @@ function useChatMarkdownState({
   renderContextReference,
   headingLevelOffset = 0,
   sourceControlMedia = false,
+  resolveUrl,
 }: ChatMarkdownProps) {
   const resolvedTheme = useRenderedAppearance();
   const markdownRef = useRef<HTMLElement>(null);
@@ -644,7 +647,8 @@ function useChatMarkdownState({
     });
   };
   const diffThemeName = resolveDiffThemeName(resolvedTheme);
-  const markdownUrlTransform = (href: string) => {
+  const markdownUrlTransform = (url: string) => {
+    const href = resolveUrl?.(url) ?? url;
     if (parseAssistantCitationHref(href)) return href;
     if (parseComposerContextHref(href)) return href;
     if (parseThreadLinkHref(href)) return href;
diff --git a/apps/web/src/components/pullRequest/PullRequestDetailPanel.tsx b/apps/web/src/components/pullRequest/PullRequestDetailPanel.tsx
index 135ae2970ceb9..25eb1c84c58c6 100644
--- a/apps/web/src/components/pullRequest/PullRequestDetailPanel.tsx
+++ b/apps/web/src/components/pullRequest/PullRequestDetailPanel.tsx
@@ -1,6 +1,9 @@
 import { projectChangeRequestUrl } from "@t3tools/client-runtime/change-request-links";
 import { normalizeThreadPullRequestKey } from "@t3tools/client-runtime/thread-pull-requests";
-import { changeRequestReferenceAutolinker } from "@t3tools/client-runtime/pull-request-markdown";
+import {
+  changeRequestLinkResolver,
+  changeRequestReferenceAutolinker,
+} from "@t3tools/client-runtime/pull-request-markdown";
 import { useAtomValue } from "@effect/atom-react";
 import { usePullRequestStack } from "~/state/usePullRequestStack";
 import {
@@ -239,7 +242,11 @@ function pullRequestMarkdownContext(
   detail: Pick<PullRequestDetail, "provider" | "url"> | null,
   threadRef: ScopedThreadRef | null,
 ) {
-  return { autolinks: changeRequestReferenceAutolinker(detail), threadRef };
+  return {
+    autolinks: changeRequestReferenceAutolinker(detail),
+    resolveUrl: changeRequestLinkResolver(detail),
+    threadRef,
+  };
 }
 
 function pullRequestStackReference(
diff --git a/apps/web/src/components/pullRequest/PullRequestMarkdown.tsx b/apps/web/src/components/pullRequest/PullRequestMarkdown.tsx
index 7086d5441bc5a..3ca14b9932ee5 100644
--- a/apps/web/src/components/pullRequest/PullRequestMarkdown.tsx
+++ b/apps/web/src/components/pullRequest/PullRequestMarkdown.tsx
@@ -23,6 +23,7 @@ import { remarkPullRequestAutolinks, splitPullRequestBody } from "./pullRequestM
 
 export const PullRequestMarkdownContext = createContext<{
   autolinks: ((text: string) => ReadonlyArray<ReferenceAutolink>) | null;
+  resolveUrl: ((url: string) => string) | null;
   threadRef: ScopedThreadRef | null;
 } | null>(null);
 
@@ -93,6 +94,7 @@ export function PullRequestMarkdown({
               pullRequestPanelRef={resolvedThreadRef ?? PULL_REQUESTS_PANEL_REF}
               environmentId={environmentId}
               extraRemarkPlugins={extraRemarkPlugins}
+              resolveUrl={context?.resolveUrl ?? undefined}
               sourceControlMedia
             />
           );
diff --git a/apps/web/src/components/pullRequest/guide/GuideProse.tsx b/apps/web/src/components/pullRequest/guide/GuideProse.tsx
index e117191fb97a6..f2643046a1360 100644
--- a/apps/web/src/components/pullRequest/guide/GuideProse.tsx
+++ b/apps/web/src/components/pullRequest/guide/GuideProse.tsx
@@ -3,7 +3,7 @@ import { createContext, useContext } from "react";
 
 import { cn } from "~/lib/utils";
 
-import { PullRequestMarkdown } from "../PullRequestMarkdown";
+import { PullRequestMarkdown, PullRequestMarkdownContext } from "../PullRequestMarkdown";
 
 export const GuideProseContext = createContext<{
   readonly environmentId: EnvironmentId;
@@ -20,13 +20,19 @@ export function GuideProse({
   className?: string;
 }) {
   const context = useContext(GuideProseContext);
+  const markdownContext = useContext(PullRequestMarkdownContext);
   if (plain || context === null) return text;
   return (
-    <PullRequestMarkdown
-      text={text}
-      cwd={context.cwd}
-      environmentId={context.environmentId}
-      className={cn("space-y-0 [&_.chat-markdown]:text-inherit", className)}
-    />
+    // The agent writes the guide about the checkout, so its relative links name workspace files.
+    <PullRequestMarkdownContext
+      value={markdownContext === null ? null : { ...markdownContext, resolveUrl: null }}
+    >
+      <PullRequestMarkdown
+        text={text}
+        cwd={context.cwd}
+        environmentId={context.environmentId}
+        className={cn("space-y-0 [&_.chat-markdown]:text-inherit", className)}
+      />
+    </PullRequestMarkdownContext>
   );
 }
diff --git a/packages/client-runtime/src/pullRequestMarkdown.ts b/packages/client-runtime/src/pullRequestMarkdown.ts
index 1e03c32509e9a..63f9974d85dd3 100644
--- a/packages/client-runtime/src/pullRequestMarkdown.ts
+++ b/packages/client-runtime/src/pullRequestMarkdown.ts
@@ -132,6 +132,18 @@ export function splitPullRequestBody(body: string): ReadonlyArray<PullRequestBod
   return segments;
 }
 
+/**
+ * Makes absolute a link destination the change request's host writes relative to its page, and
+ * returns any other destination as it is. Null when the host writes none.
+ */
+export function changeRequestLinkResolver(
+  changeRequest: { readonly provider: string; readonly url: string } | null | undefined,
+): ((url: string) => string) | null {
+  if (!changeRequest) return null;
+  const resolve = sourceControlClients.get(changeRequest.provider).resolveRelativeUrl;
+  return resolve === undefined ? null : (url) => resolve(url, changeRequest.url) ?? url;
+}
+
 /**
  * Finds the references the change request's host links in plain text, resolved against the change
  * request's repository. Null when the host links none.
diff --git a/packages/source-control-core/src/shared/definition.ts b/packages/source-control-core/src/shared/definition.ts
index e9e68325b7418..4c51de24a08cd 100644
--- a/packages/source-control-core/src/shared/definition.ts
+++ b/packages/source-control-core/src/shared/definition.ts
@@ -286,6 +286,12 @@ export interface SourceControlDefinition {
     text: string,
     repositoryUrl: string,
   ) => ReadonlyArray<ReferenceAutolink>;
+  /**
+   * The absolute URL for a link destination this host's bodies write relative to the change
+   * request's page at `changeRequestUrl`, or null for any other destination. Absent for a host
+   * whose bodies link absolutely.
+   */
+  readonly resolveRelativeUrl?: (url: string, changeRequestUrl: string) => string | null;
   /** Where the host lists a change request's checks. Absent means the change request's own page. */
   readonly changeRequestChecksUrl?: (changeRequestUrl: string) => string;
   /**
diff --git a/packages/source-control-github/src/shared/definition.test.ts b/packages/source-control-github/src/shared/definition.test.ts
index 3aadd4697b6fe..3fde741aebf30 100644
--- a/packages/source-control-github/src/shared/definition.test.ts
+++ b/packages/source-control-github/src/shared/definition.test.ts
@@ -159,3 +159,26 @@ describe("GitHub media URLs", () => {
     }
   });
 });
+
+describe("GitHub relative URLs", () => {
+  const resolve = (url: string) =>
+    definition.resolveRelativeUrl?.(url, "https://github.com/o/r/pull/5");
+
+  it("resolves a pull request body's relative destinations against the pull request page", () => {
+    expect(resolve("../blob/dd60e3cd/docs/shot.png?raw=true")).toBe(
+      "https://github.com/o/r/blob/dd60e3cd/docs/shot.png?raw=true",
+    );
+    expect(resolve("/other/r/blob/801d91ff/docs/shot.webp?raw=true")).toBe(
+      "https://github.com/other/r/blob/801d91ff/docs/shot.webp?raw=true",
+    );
+    for (const url of [
+      "https://example.com/shot.png",
+      "#x",
+      "mailto:a@example.com",
+      "//evil.example/shot.png",
+      "\\\\evil.example\\shot.png",
+    ]) {
+      expect(resolve(url), url).toBeNull();
+    }
+  });
+});
diff --git a/packages/source-control-github/src/shared/definition.ts b/packages/source-control-github/src/shared/definition.ts
index 6ac8aa305f3cd..9d9170750c2eb 100644
--- a/packages/source-control-github/src/shared/definition.ts
+++ b/packages/source-control-github/src/shared/definition.ts
@@ -44,6 +44,21 @@ function isRepositoryPath(repository: string): boolean {
   );
 }
 
+const ABSOLUTE_OR_PAGE_LOCAL = /^(?:[a-z][a-z\d+.-]*:|[#?]|$)/iu;
+
+/** As github.com's browser reads a body's destinations: relative to the pull request's page. */
+function resolveRelativeUrl(url: string, changeRequestUrl: string): string | null {
+  if (ABSOLUTE_OR_PAGE_LOCAL.test(url)) return null;
+  try {
+    const page = new URL(changeRequestUrl);
+    const resolved = new URL(url, page);
+    // `//host/x`, and the `\\host/x` a browser reads the same way, leave the page's origin.
+    return resolved.origin === page.origin ? resolved.href : null;
+  } catch {
+    return null;
+  }
+}
+
 /** Signing in through the T3 Code for GitHub app lifts the per-user API rate limit to the app's. */
 export const APP_SIGN_IN: SourceControlAppSignIn = {
   appName: "T3 Code for GitHub",
@@ -127,6 +142,7 @@ export const definition = defineSourceControl({
       ? null
       : new URL(`/${encodeURIComponent(login)}`, repositoryUrl).toString(),
   referenceAutolinks: gitHubReferenceAutolinks,
+  resolveRelativeUrl,
   changeRequestChecksUrl: (changeRequestUrl) => `${changeRequestUrl.replace(/\/+$/u, "")}/checks`,
   // Before servers reported their actions, GitHub was the only host they acted on from a list.
   changeRequestActionsWhenUnreported: new Set(["close", "merge", "ready", "reopen"] as const),