feat(site): add expand-context support to diff viewer

Add lazy per-file context expansion for local diffs. When viewing a
diff in the Git panel, each file header shows an "Expand context"
button. Clicking it fetches the full old (HEAD) and new (working tree)
file contents via a new coderd proxy endpoint, re-parses the diff with
@pierre/diffs processFile (which sets isPartial: false), and swaps in
the enriched FileDiffMetadata. The library's native hunk separator
expansion then takes over.

Backend:
- agent/agentgit: new GET /api/v0/git/show endpoint returning file
  contents at a git ref (with binary/size guards)
- codersdk/workspacesdk: GitShowFile SDK method on AgentConn
- coderd: new GET /api/experimental/chats/{chat}/file-content proxy
  endpoint that dials the agent for old (git show HEAD) or new
  (working tree read) file contents

Frontend:
- expandFileDiff utility wrapping processFile with oldFile/newFile
- extractFilePatch utility to split multi-file diffs
- DiffViewer: expansion state, ExpandContextButton, effectiveFiles
  overlay
- LocalDiffPanel: wired to fetch via Promise.allSettled
- Updated @pierre/diffs 1.1.0-beta.19 -> 1.1.7
- Replaced deprecated enableHoverUtility with enableGutterUtility

agent/agentgit/api.go

@@ -1,8 +1,13 @@
 package agentgit
 
 import (
+	"bytes"
 	"context"
+	"encoding/json"
 	"net/http"
+	"os"
+	"os/exec"
+	"path/filepath"
 
 	"github.com/go-chi/chi/v5"
 	"github.com/google/uuid"
@@ -30,10 +35,15 @@ func NewAPI(logger slog.Logger, pathStore *PathStore, opts ...Option) *API {
 	}
 }
 
+// maxShowFileSize is the maximum file size returned by the show
+// endpoint. Files larger than this are rejected with 422.
+const maxShowFileSize = 512 * 1024 // 512 KB
+
 // Routes returns the chi router for mounting at /api/v0/git.
 func (a *API) Routes() http.Handler {
 	r := chi.NewRouter()
 	r.Get("/watch", a.handleWatch)
+	r.Get("/show", a.handleShow)
 	return r
 }
 
@@ -145,3 +155,74 @@ func (a *API) handleWatch(rw http.ResponseWriter, r *http.Request) {
 		}
 	}
 }
+
+// GitShowResponse is the JSON response for the show endpoint.
+type GitShowResponse struct {
+	Contents string `json:"contents"`
+}
+
+func (a *API) handleShow(rw http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+
+	repoRoot := r.URL.Query().Get("repo_root")
+	filePath := r.URL.Query().Get("path")
+	ref := r.URL.Query().Get("ref")
+
+	if repoRoot == "" || filePath == "" || ref == "" {
+		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+			Message: "Missing required query parameters.",
+			Detail:  "repo_root, path, and ref are required.",
+		})
+		return
+	}
+
+	// Validate that repo_root is a git repository by checking for
+	// a .git entry.
+	gitPath := filepath.Join(repoRoot, ".git")
+	if _, err := os.Stat(gitPath); err != nil {
+		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+			Message: "Not a git repository.",
+			Detail:  repoRoot + " does not contain a .git directory.",
+		})
+		return
+	}
+
+	// Run `git show ref:path` to retrieve the file at the given
+	// ref.
+	//nolint:gosec // ref and filePath are user-provided but we
+	// intentionally pass them to git.
+	cmd := exec.CommandContext(ctx, "git", "-C", repoRoot, "show", ref+":"+filePath)
+	out, err := cmd.Output()
+	if err != nil {
+		// git show exits non-zero when the path doesn't exist at
+		// the given ref.
+		httpapi.Write(ctx, rw, http.StatusNotFound, codersdk.Response{
+			Message: "File not found.",
+			Detail:  filePath + " does not exist at ref " + ref + ".",
+		})
+		return
+	}
+
+	// Check if the file is binary by looking for null bytes in
+	// the first 8 KB.
+	checkLen := min(len(out), 8*1024)
+	if bytes.ContainsRune(out[:checkLen], '\x00') {
+		httpapi.Write(ctx, rw, http.StatusUnprocessableEntity, codersdk.Response{
+			Message: "binary file",
+		})
+		return
+	}
+
+	if len(out) > maxShowFileSize {
+		httpapi.Write(ctx, rw, http.StatusUnprocessableEntity, codersdk.Response{
+			Message: "file too large",
+		})
+		return
+	}
+
+	rw.Header().Set("Content-Type", "application/json")
+	rw.WriteHeader(http.StatusOK)
+	_ = json.NewEncoder(rw).Encode(GitShowResponse{
+		Contents: string(out),
+	})
+}

agent/agentgit/api_test.go

@@ -0,0 +1,117 @@
+package agentgit_test
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"cdr.dev/slog/v3/sloggers/slogtest"
+	"github.com/coder/coder/v2/agent/agentgit"
+)
+
+func TestGitShow_ReturnsFileAtHEAD(t *testing.T) {
+	t.Parallel()
+
+	repoDir := initTestRepo(t)
+	targetFile := filepath.Join(repoDir, "hello.txt")
+
+	// Write and commit a file with known content.
+	require.NoError(t, os.WriteFile(targetFile, []byte("committed content\n"), 0o600))
+	gitCmd(t, repoDir, "add", "hello.txt")
+	gitCmd(t, repoDir, "commit", "-m", "add hello")
+
+	// Modify the working tree version so it differs from HEAD.
+	require.NoError(t, os.WriteFile(targetFile, []byte("working tree content\n"), 0o600))
+
+	logger := slogtest.Make(t, nil)
+	api := agentgit.NewAPI(logger, nil)
+
+	req := httptest.NewRequest(http.MethodGet, "/show?repo_root="+repoDir+"&path=hello.txt&ref=HEAD", nil)
+	rec := httptest.NewRecorder()
+	api.Routes().ServeHTTP(rec, req)
+
+	require.Equal(t, http.StatusOK, rec.Code)
+
+	var resp agentgit.GitShowResponse
+	require.NoError(t, json.NewDecoder(rec.Body).Decode(&resp))
+	require.Equal(t, "committed content\n", resp.Contents)
+}
+
+func TestGitShow_FileNotFound(t *testing.T) {
+	t.Parallel()
+
+	repoDir := initTestRepo(t)
+	logger := slogtest.Make(t, nil)
+	api := agentgit.NewAPI(logger, nil)
+
+	req := httptest.NewRequest(http.MethodGet, "/show?repo_root="+repoDir+"&path=nonexistent.txt&ref=HEAD", nil)
+	rec := httptest.NewRecorder()
+	api.Routes().ServeHTTP(rec, req)
+
+	require.Equal(t, http.StatusNotFound, rec.Code)
+}
+
+func TestGitShow_InvalidRepoRoot(t *testing.T) {
+	t.Parallel()
+
+	notARepo := t.TempDir()
+	logger := slogtest.Make(t, nil)
+	api := agentgit.NewAPI(logger, nil)
+
+	req := httptest.NewRequest(http.MethodGet, "/show?repo_root="+notARepo+"&path=file.txt&ref=HEAD", nil)
+	rec := httptest.NewRecorder()
+	api.Routes().ServeHTTP(rec, req)
+
+	require.Equal(t, http.StatusBadRequest, rec.Code)
+}
+
+func TestGitShow_BinaryFile(t *testing.T) {
+	t.Parallel()
+
+	repoDir := initTestRepo(t)
+
+	// Create a file with null bytes to simulate binary content.
+	binPath := filepath.Join(repoDir, "binary.dat")
+	require.NoError(t, os.WriteFile(binPath, []byte("hello\x00world"), 0o600))
+	gitCmd(t, repoDir, "add", "binary.dat")
+	gitCmd(t, repoDir, "commit", "-m", "add binary")
+
+	logger := slogtest.Make(t, nil)
+	api := agentgit.NewAPI(logger, nil)
+
+	req := httptest.NewRequest(http.MethodGet, "/show?repo_root="+repoDir+"&path=binary.dat&ref=HEAD", nil)
+	rec := httptest.NewRecorder()
+	api.Routes().ServeHTTP(rec, req)
+
+	require.Equal(t, http.StatusUnprocessableEntity, rec.Code)
+	require.Contains(t, rec.Body.String(), "binary file")
+}
+
+func TestGitShow_FileTooLarge(t *testing.T) {
+	t.Parallel()
+
+	repoDir := initTestRepo(t)
+
+	// Create a file exceeding 512 KB.
+	largePath := filepath.Join(repoDir, "large.txt")
+	content := strings.Repeat("x", 512*1024+1)
+	require.NoError(t, os.WriteFile(largePath, []byte(content), 0o600))
+	gitCmd(t, repoDir, "add", "large.txt")
+	gitCmd(t, repoDir, "commit", "-m", "add large file")
+
+	logger := slogtest.Make(t, nil)
+	api := agentgit.NewAPI(logger, nil)
+
+	req := httptest.NewRequest(http.MethodGet, "/show?repo_root="+repoDir+"&path=large.txt&ref=HEAD", nil)
+	rec := httptest.NewRecorder()
+	api.Routes().ServeHTTP(rec, req)
+
+	require.Equal(t, http.StatusUnprocessableEntity, rec.Code)
+	require.Contains(t, rec.Body.String(), "file too large")
+}

coderd/coderd.go

@@ -1236,6 +1236,7 @@ func New(options *Options) *API {
 				r.Post("/interrupt", api.interruptChat)
 				r.Post("/title/regenerate", api.regenerateChatTitle)
 				r.Get("/diff", api.getChatDiffContents)
+				r.Get("/file-content", api.getChatFileContent)
 				r.Route("/queue/{queuedMessage}", func(r chi.Router) {
 					r.Delete("/", api.deleteChatQueuedMessage)
 					r.Post("/promote", api.promoteChatQueuedMessage)

coderd/exp_chats.go

@@ -5218,3 +5218,133 @@ func (api *API) prInsights(rw http.ResponseWriter, r *http.Request) {
 		RecentPRs:  prEntries,
 	})
 }
+
+// maxFileContentBytes is the maximum size of file contents returned
+// by the file-content proxy endpoint. This prevents unbounded reads
+// when the agent serves very large files.
+const maxFileContentBytes = 10 << 20 // 10 MiB
+
+// EXPERIMENTAL: this endpoint is experimental and is subject to change.
+//
+//nolint:revive // HTTP handler writes to ResponseWriter.
+func (api *API) getChatFileContent(rw http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+	chat := httpmw.ChatParam(r)
+
+	repoRoot := r.URL.Query().Get("repo_root")
+	filePath := r.URL.Query().Get("path")
+	side := r.URL.Query().Get("side")
+
+	if repoRoot == "" || filePath == "" || side == "" {
+		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+			Message: "Missing required query parameters.",
+			Detail:  "repo_root, path, and side are required.",
+		})
+		return
+	}
+	if side != "old" && side != "new" {
+		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+			Message: "Invalid side parameter.",
+			Detail:  "side must be \"old\" or \"new\".",
+		})
+		return
+	}
+
+	if !chat.WorkspaceID.Valid {
+		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+			Message: "Chat has no workspace.",
+		})
+		return
+	}
+
+	agents, err := api.Database.GetWorkspaceAgentsInLatestBuildByWorkspaceID(ctx, chat.WorkspaceID.UUID)
+	if err != nil {
+		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Internal error fetching workspace agents.",
+			Detail:  err.Error(),
+		})
+		return
+	}
+	if len(agents) == 0 {
+		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+			Message: "Chat workspace has no agents.",
+		})
+		return
+	}
+
+	apiAgent, err := db2sdk.WorkspaceAgent(
+		api.DERPMap(),
+		*api.TailnetCoordinator.Load(),
+		agents[0],
+		nil,
+		nil,
+		nil,
+		api.AgentInactiveDisconnectTimeout,
+		api.DeploymentValues.AgentFallbackTroubleshootingURL.String(),
+	)
+	if err != nil {
+		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Internal error reading workspace agent.",
+			Detail:  err.Error(),
+		})
+		return
+	}
+	if apiAgent.Status != codersdk.WorkspaceAgentConnected {
+		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
+			Message: fmt.Sprintf("Agent state is %q, it must be in the %q state.", apiAgent.Status, codersdk.WorkspaceAgentConnected),
+		})
+		return
+	}
+
+	dialCtx, dialCancel := context.WithTimeout(ctx, 30*time.Second)
+	defer dialCancel()
+
+	agentConn, release, err := api.agentProvider.AgentConn(dialCtx, agents[0].ID)
+	if err != nil {
+		httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+			Message: "Internal error dialing workspace agent.",
+			Detail:  err.Error(),
+		})
+		return
+	}
+	defer release()
+
+	var contents string
+	if side == "old" {
+		// Retrieve the file at HEAD from git.
+		contents, err = agentConn.GitShowFile(ctx, repoRoot, filePath, "HEAD")
+		if err != nil {
+			httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+				Message: "Failed to read file from git.",
+				Detail:  err.Error(),
+			})
+			return
+		}
+	} else {
+		// Read the working tree copy of the file.
+		absPath := repoRoot + "/" + filePath
+		rc, _, err := agentConn.ReadFile(ctx, absPath, 0, maxFileContentBytes)
+		if err != nil {
+			httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+				Message: "Failed to read working tree file.",
+				Detail:  err.Error(),
+			})
+			return
+		}
+		defer rc.Close()
+
+		data, err := io.ReadAll(rc)
+		if err != nil {
+			httpapi.Write(ctx, rw, http.StatusInternalServerError, codersdk.Response{
+				Message: "Failed to read file body.",
+				Detail:  err.Error(),
+			})
+			return
+		}
+		contents = string(data)
+	}
+
+	httpapi.Write(ctx, rw, http.StatusOK, map[string]string{
+		"contents": contents,
+	})
+}

codersdk/workspacesdk/agentconn.go

@@ -10,6 +10,7 @@ import (
 	"net"
 	"net/http"
 	"net/netip"
+	"net/url"
 	"strconv"
 	"sync"
 	"time"
@@ -84,6 +85,7 @@ type AgentConn interface {
 	ReadFileLines(ctx context.Context, path string, offset, limit int64, limits ReadFileLinesLimits) (ReadFileLinesResponse, error)
 	WriteFile(ctx context.Context, path string, reader io.Reader) error
 	EditFiles(ctx context.Context, edits FileEditRequest) error
+	GitShowFile(ctx context.Context, repoRoot, path, ref string) (string, error)
 	SSH(ctx context.Context) (*gonet.TCPConn, error)
 	SSHClient(ctx context.Context) (*ssh.Client, error)
 	SSHClientOnPort(ctx context.Context, port uint16) (*ssh.Client, error)
@@ -1095,6 +1097,33 @@ func (c *agentConn) EditFiles(ctx context.Context, edits FileEditRequest) error
 	return nil
 }
 
+// GitShowFile retrieves a file's contents from a git repository at a
+// given ref (e.g. "HEAD").
+func (c *agentConn) GitShowFile(ctx context.Context, repoRoot, path, ref string) (string, error) {
+	ctx, span := tracing.StartSpan(ctx)
+	defer span.End()
+
+	res, err := c.apiRequest(ctx, http.MethodGet, fmt.Sprintf(
+		"/api/v0/git/show?repo_root=%s&path=%s&ref=%s",
+		url.QueryEscape(repoRoot), url.QueryEscape(path), url.QueryEscape(ref),
+	), nil)
+	if err != nil {
+		return "", xerrors.Errorf("do request: %w", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusOK {
+		return "", codersdk.ReadBodyAsError(res)
+	}
+
+	var resp struct {
+		Contents string `json:"contents"`
+	}
+	if err := json.NewDecoder(res.Body).Decode(&resp); err != nil {
+		return "", xerrors.Errorf("decode response: %w", err)
+	}
+	return resp.Contents, nil
+}
+
 // apiRequest makes a request to the workspace agent's HTTP API server.
 func (c *agentConn) apiRequest(ctx context.Context, method, path string, body interface{}) (*http.Response, error) {
 	ctx, span := tracing.StartSpan(ctx)

site/package.json

@@ -62,7 +62,7 @@
 		"@mui/system": "5.18.0",
 		"@mui/x-tree-view": "7.29.10",
 		"@novnc/novnc": "^1.5.0",
-		"@pierre/diffs": "1.1.0-beta.19",
+		"@pierre/diffs": "1.1.7",
 		"@radix-ui/react-avatar": "1.1.11",
 		"@radix-ui/react-checkbox": "1.3.3",
 		"@radix-ui/react-collapsible": "1.1.12",

site/pnpm-lock.yaml

@@ -86,8 +86,8 @@ importers:
         specifier: ^1.5.0
         version: 1.5.0
       '@pierre/diffs':
-        specifier: 1.1.0-beta.19
-        version: 1.1.0-beta.19(react-dom@19.2.2(react@19.2.2))(react@19.2.2)
+        specifier: 1.1.7
+        version: 1.1.7(react-dom@19.2.2(react@19.2.2))(react@19.2.2)
       '@radix-ui/react-avatar':
         specifier: 1.1.11
         version: 1.1.11(@types/react-dom@19.2.3(@types/react@19.2.7))(@types/react@19.2.7)(react-dom@19.2.2(react@19.2.2))(react@19.2.2)
@@ -1720,8 +1720,8 @@ packages:
     cpu: [x64]
     os: [win32]
 
-  '@pierre/diffs@1.1.0-beta.19':
-    resolution: {integrity: sha512-XxGPKkVW+1t2KJQfgjmSnS+93nI9+ACJl1XjhF3Lo4BdQJOxV3pHeyix31ySn/m/1llq6O/7bXucE0OYCK6Kog==, tarball: https://registry.npmjs.org/@pierre/diffs/-/diffs-1.1.0-beta.19.tgz}
+  '@pierre/diffs@1.1.7':
+    resolution: {integrity: sha512-FWs2hHrjZPXmJl6ewnfFzOjNEM3aeSH1CB8ynZg4SOg95Wc5AxomeyJJhXf44PK9Cc+PNm1CgsJ1IvOdfgHyHA==, tarball: https://registry.npmjs.org/@pierre/diffs/-/diffs-1.1.7.tgz}
     peerDependencies:
       react: ^18.3.1 || ^19.0.0
       react-dom: ^18.3.1 || ^19.0.0
@@ -2400,26 +2400,26 @@ packages:
     cpu: [x64]
     os: [win32]
 
-  '@shikijs/core@3.22.0':
-    resolution: {integrity: sha512-iAlTtSDDbJiRpvgL5ugKEATDtHdUVkqgHDm/gbD2ZS9c88mx7G1zSYjjOxp5Qa0eaW0MAQosFRmJSk354PRoQA==, tarball: https://registry.npmjs.org/@shikijs/core/-/core-3.22.0.tgz}
+  '@shikijs/core@3.23.0':
+    resolution: {integrity: sha512-NSWQz0riNb67xthdm5br6lAkvpDJRTgB36fxlo37ZzM2yq0PQFFzbd8psqC2XMPgCzo1fW6cVi18+ArJ44wqgA==, tarball: https://registry.npmjs.org/@shikijs/core/-/core-3.23.0.tgz}
 
-  '@shikijs/engine-javascript@3.22.0':
-    resolution: {integrity: sha512-jdKhfgW9CRtj3Tor0L7+yPwdG3CgP7W+ZEqSsojrMzCjD1e0IxIbwUMDDpYlVBlC08TACg4puwFGkZfLS+56Tw==, tarball: https://registry.npmjs.org/@shikijs/engine-javascript/-/engine-javascript-3.22.0.tgz}
+  '@shikijs/engine-javascript@3.23.0':
+    resolution: {integrity: sha512-aHt9eiGFobmWR5uqJUViySI1bHMqrAgamWE1TYSUoftkAeCCAiGawPMwM+VCadylQtF4V3VNOZ5LmfItH5f3yA==, tarball: https://registry.npmjs.org/@shikijs/engine-javascript/-/engine-javascript-3.23.0.tgz}
 
-  '@shikijs/engine-oniguruma@3.22.0':
-    resolution: {integrity: sha512-DyXsOG0vGtNtl7ygvabHd7Mt5EY8gCNqR9Y7Lpbbd/PbJvgWrqaKzH1JW6H6qFkuUa8aCxoiYVv8/YfFljiQxA==, tarball: https://registry.npmjs.org/@shikijs/engine-oniguruma/-/engine-oniguruma-3.22.0.tgz}
+  '@shikijs/engine-oniguruma@3.23.0':
+    resolution: {integrity: sha512-1nWINwKXxKKLqPibT5f4pAFLej9oZzQTsby8942OTlsJzOBZ0MWKiwzMsd+jhzu8YPCHAswGnnN1YtQfirL35g==, tarball: https://registry.npmjs.org/@shikijs/engine-oniguruma/-/engine-oniguruma-3.23.0.tgz}
 
-  '@shikijs/langs@3.22.0':
-    resolution: {integrity: sha512-x/42TfhWmp6H00T6uwVrdTJGKgNdFbrEdhaDwSR5fd5zhQ1Q46bHq9EO61SCEWJR0HY7z2HNDMaBZp8JRmKiIA==, tarball: https://registry.npmjs.org/@shikijs/langs/-/langs-3.22.0.tgz}
+  '@shikijs/langs@3.23.0':
+    resolution: {integrity: sha512-2Ep4W3Re5aB1/62RSYQInK9mM3HsLeB91cHqznAJMuylqjzNVAVCMnNWRHFtcNHXsoNRayP9z1qj4Sq3nMqYXg==, tarball: https://registry.npmjs.org/@shikijs/langs/-/langs-3.23.0.tgz}
 
-  '@shikijs/themes@3.22.0':
-    resolution: {integrity: sha512-o+tlOKqsr6FE4+mYJG08tfCFDS+3CG20HbldXeVoyP+cYSUxDhrFf3GPjE60U55iOkkjbpY2uC3It/eeja35/g==, tarball: https://registry.npmjs.org/@shikijs/themes/-/themes-3.22.0.tgz}
+  '@shikijs/themes@3.23.0':
+    resolution: {integrity: sha512-5qySYa1ZgAT18HR/ypENL9cUSGOeI2x+4IvYJu4JgVJdizn6kG4ia5Q1jDEOi7gTbN4RbuYtmHh0W3eccOrjMA==, tarball: https://registry.npmjs.org/@shikijs/themes/-/themes-3.23.0.tgz}
 
-  '@shikijs/transformers@3.22.0':
-    resolution: {integrity: sha512-E7eRV7mwDBjueLF6852n2oYeJYxBq3NSsDk+uyruYAXONv4U8holGmIrT+mPRJQ1J1SNOH6L8G19KRzmBawrFw==, tarball: https://registry.npmjs.org/@shikijs/transformers/-/transformers-3.22.0.tgz}
+  '@shikijs/transformers@3.23.0':
+    resolution: {integrity: sha512-F9msZVxdF+krQNSdQ4V+Ja5QemeAoTQ2jxt7nJCwhDsdF1JWS3KxIQXA3lQbyKwS3J61oHRUSv4jYWv3CkaKTQ==, tarball: https://registry.npmjs.org/@shikijs/transformers/-/transformers-3.23.0.tgz}
 
-  '@shikijs/types@3.22.0':
-    resolution: {integrity: sha512-491iAekgKDBFE67z70Ok5a8KBMsQ2IJwOWw3us/7ffQkIBCyOQfm/aNwVMBUriP02QshIfgHCBSIYAl3u2eWjg==, tarball: https://registry.npmjs.org/@shikijs/types/-/types-3.22.0.tgz}
+  '@shikijs/types@3.23.0':
+    resolution: {integrity: sha512-3JZ5HXOZfYjsYSk0yPwBrkupyYSLpAE26Qc0HLghhZNGTZg/SKxXIIgoxOpmmeQP0RRSDJTk1/vPfw9tbw+jSQ==, tarball: https://registry.npmjs.org/@shikijs/types/-/types-3.23.0.tgz}
 
   '@shikijs/vscode-textmate@10.0.2':
     resolution: {integrity: sha512-83yeghZ2xxin3Nj8z1NMd/NCuca+gsYXswywDy5bHvwlWL8tpTQmzGeUuHd9FC3E/SBEMvzJRwWEOz5gGes9Qg==, tarball: https://registry.npmjs.org/@shikijs/vscode-textmate/-/vscode-textmate-10.0.2.tgz}
@@ -5601,8 +5601,8 @@ packages:
   oniguruma-parser@0.12.1:
     resolution: {integrity: sha512-8Unqkvk1RYc6yq2WBYRj4hdnsAxVze8i7iPfQr8e4uSP3tRv0rpZcbGUDvxfQQcdwHt/e9PrMvGCsa8OqG9X3w==, tarball: https://registry.npmjs.org/oniguruma-parser/-/oniguruma-parser-0.12.1.tgz}
 
-  oniguruma-to-es@4.3.4:
-    resolution: {integrity: sha512-3VhUGN3w2eYxnTzHn+ikMI+fp/96KoRSVK9/kMTcFqj1NRDh2IhQCKvYxDnWePKRXY/AqH+Fuiyb7VHSzBjHfA==, tarball: https://registry.npmjs.org/oniguruma-to-es/-/oniguruma-to-es-4.3.4.tgz}
+  oniguruma-to-es@4.3.5:
+    resolution: {integrity: sha512-Zjygswjpsewa0NLTsiizVuMQZbp0MDyM6lIt66OxsF21npUDlzpHi1Mgb/qhQdkb+dWFTzJmFbEWdvZgRho8eQ==, tarball: https://registry.npmjs.org/oniguruma-to-es/-/oniguruma-to-es-4.3.5.tgz}
 
   open@10.2.0:
     resolution: {integrity: sha512-YgBpdJHPyQ2UE5x+hlSXcnejzAvD0b22U2OuAP+8OnlJT+PjWPxtgmGqKKc+RgTM63U9gN0YzrYc71R2WT/hTA==, tarball: https://registry.npmjs.org/open/-/open-10.2.0.tgz}
@@ -6298,8 +6298,8 @@ packages:
     resolution: {integrity: sha512-7++dFhtcx3353uBaq8DDR4NuxBetBzC7ZQOhmTQInHEd6bSrXdiEyzCvG07Z44UYdLShWUyXt5M/yhz8ekcb1A==, tarball: https://registry.npmjs.org/shebang-regex/-/shebang-regex-3.0.0.tgz}
     engines: {node: '>=8'}
 
-  shiki@3.22.0:
-    resolution: {integrity: sha512-LBnhsoYEe0Eou4e1VgJACes+O6S6QC0w71fCSp5Oya79inkwkm15gQ1UF6VtQ8j/taMDh79hAB49WUk8ALQW3g==, tarball: https://registry.npmjs.org/shiki/-/shiki-3.22.0.tgz}
+  shiki@3.23.0:
+    resolution: {integrity: sha512-55Dj73uq9ZXL5zyeRPzHQsK7Nbyt6Y10k5s7OjuFZGMhpp4r/rsLBH0o/0fstIzX1Lep9VxefWljK/SKCzygIA==, tarball: https://registry.npmjs.org/shiki/-/shiki-3.23.0.tgz}
 
   side-channel-list@1.0.0:
     resolution: {integrity: sha512-FCLHtRD/gnpCiCHEiJLOwdmFP+wzCmDEkc9y7NsYxeF4u7Btsn1ZuwgwJGxImImHicJArLP4R0yX4c2KCrMrTA==, tarball: https://registry.npmjs.org/side-channel-list/-/side-channel-list-1.0.0.tgz}
@@ -8641,16 +8641,16 @@ snapshots:
   '@oxc-resolver/binding-win32-x64-msvc@11.14.0':
     optional: true
 
-  '@pierre/diffs@1.1.0-beta.19(react-dom@19.2.2(react@19.2.2))(react@19.2.2)':
+  '@pierre/diffs@1.1.7(react-dom@19.2.2(react@19.2.2))(react@19.2.2)':
     dependencies:
       '@pierre/theme': 0.0.22
-      '@shikijs/transformers': 3.22.0
+      '@shikijs/transformers': 3.23.0
       diff: 8.0.3
       hast-util-to-html: 9.0.5
       lru_map: 0.4.1
       react: 19.2.2
       react-dom: 19.2.2(react@19.2.2)
-      shiki: 3.22.0
+      shiki: 3.23.0
 
   '@pierre/theme@0.0.22': {}
 
@@ -9278,38 +9278,38 @@ snapshots:
   '@rollup/rollup-win32-x64-msvc@4.53.3':
     optional: true
 
-  '@shikijs/core@3.22.0':
+  '@shikijs/core@3.23.0':
     dependencies:
-      '@shikijs/types': 3.22.0
+      '@shikijs/types': 3.23.0
       '@shikijs/vscode-textmate': 10.0.2
       '@types/hast': 3.0.4
       hast-util-to-html: 9.0.5
 
-  '@shikijs/engine-javascript@3.22.0':
+  '@shikijs/engine-javascript@3.23.0':
     dependencies:
-      '@shikijs/types': 3.22.0
+      '@shikijs/types': 3.23.0
       '@shikijs/vscode-textmate': 10.0.2
-      oniguruma-to-es: 4.3.4
+      oniguruma-to-es: 4.3.5
 
-  '@shikijs/engine-oniguruma@3.22.0':
+  '@shikijs/engine-oniguruma@3.23.0':
     dependencies:
-      '@shikijs/types': 3.22.0
+      '@shikijs/types': 3.23.0
       '@shikijs/vscode-textmate': 10.0.2
 
-  '@shikijs/langs@3.22.0':
+  '@shikijs/langs@3.23.0':
     dependencies:
-      '@shikijs/types': 3.22.0
+      '@shikijs/types': 3.23.0
 
-  '@shikijs/themes@3.22.0':
+  '@shikijs/themes@3.23.0':
     dependencies:
-      '@shikijs/types': 3.22.0
+      '@shikijs/types': 3.23.0
 
-  '@shikijs/transformers@3.22.0':
+  '@shikijs/transformers@3.23.0':
     dependencies:
-      '@shikijs/core': 3.22.0
-      '@shikijs/types': 3.22.0
+      '@shikijs/core': 3.23.0
+      '@shikijs/types': 3.23.0
 
-  '@shikijs/types@3.22.0':
+  '@shikijs/types@3.23.0':
     dependencies:
       '@shikijs/vscode-textmate': 10.0.2
       '@types/hast': 3.0.4
@@ -11654,7 +11654,7 @@ snapshots:
       comma-separated-tokens: 2.0.3
       hast-util-whitespace: 3.0.0
       html-void-elements: 3.0.0
-      mdast-util-to-hast: 13.2.0
+      mdast-util-to-hast: 13.2.1
       property-information: 7.1.0
       space-separated-tokens: 2.0.2
       stringify-entities: 4.0.4
@@ -13223,7 +13223,7 @@ snapshots:
 
   oniguruma-parser@0.12.1: {}
 
-  oniguruma-to-es@4.3.4:
+  oniguruma-to-es@4.3.5:
     dependencies:
       oniguruma-parser: 0.12.1
       regex: 6.1.0
@@ -14041,14 +14041,14 @@ snapshots:
 
   shebang-regex@3.0.0: {}
 
-  shiki@3.22.0:
+  shiki@3.23.0:
     dependencies:
-      '@shikijs/core': 3.22.0
-      '@shikijs/engine-javascript': 3.22.0
-      '@shikijs/engine-oniguruma': 3.22.0
-      '@shikijs/langs': 3.22.0
-      '@shikijs/themes': 3.22.0
-      '@shikijs/types': 3.22.0
+      '@shikijs/core': 3.23.0
+      '@shikijs/engine-javascript': 3.23.0
+      '@shikijs/engine-oniguruma': 3.23.0
+      '@shikijs/langs': 3.23.0
+      '@shikijs/themes': 3.23.0
+      '@shikijs/types': 3.23.0
       '@shikijs/vscode-textmate': 10.0.2
       '@types/hast': 3.0.4
 

site/src/api/api.ts

@@ -3220,6 +3220,19 @@ class ExperimentalApiMethods {
 		return response.data;
 	};
 
+	getChatFileContent = async (
+		chatId: string,
+		repoRoot: string,
+		path: string,
+		side: "old" | "new",
+	): Promise<{ contents: string }> => {
+		const params = new URLSearchParams({ repo_root: repoRoot, path, side });
+		const response = await this.axios.get<{ contents: string }>(
+			`/api/experimental/chats/${chatId}/file-content?${params}`,
+		);
+		return response.data;
+	};
+
 	getChatModels = async (): Promise<TypesGen.ChatModelsResponse> => {
 		const response = await this.axios.get<TypesGen.ChatModelsResponse>(
 			"/api/experimental/chats/models",

site/src/pages/AgentsPage/AgentChatPageView.tsx

@@ -370,9 +370,9 @@ export const AgentChatPageView: FC<AgentChatPageViewProps> = ({
 								id: "git",
 								label: "Git",
 								content: (
-									<GitPanel
-										prTab={
-											prNumber && agentId
+										<GitPanel
+											chatId={agentId}
+											prTab={											prNumber && agentId
 												? { prNumber, chatId: agentId }
 												: undefined
 										}

site/src/pages/AgentsPage/components/DiffViewer/CommentableDiffViewer.tsx

@@ -193,6 +193,12 @@ interface CommentableDiffViewerProps {
 	scrollToFile?: string | null;
 	/** Called after scrollToFile has been processed. */
 	onScrollToFileComplete?: () => void;
+	/** Callback to fetch file contents for context expansion. */
+	onRequestFileContents?: (fileName: string) => Promise<{
+		oldContents: string | null;
+		newContents: string | null;
+		patchString: string;
+	} | null>;
 }
 
 /**

site/src/pages/AgentsPage/components/DiffViewer/DiffViewer.stories.tsx

@@ -1,5 +1,5 @@
 import type { DiffLineAnnotation, SelectedLineRange } from "@pierre/diffs";
-import { parsePatchFiles } from "@pierre/diffs";
+import { parsePatchFiles, processFile } from "@pierre/diffs";
 import type { Meta, StoryObj } from "@storybook/react-vite";
 import { expect, fn, waitFor } from "storybook/test";
 import type { DiffStyle } from "../DiffViewer/DiffViewer";
@@ -429,3 +429,86 @@ export const RenameWithLongPaths: Story = {
 		parsedFiles: renameFiles,
 	},
 };
+
+// -------------------------------------------------------------------
+// Expandable context stories
+// -------------------------------------------------------------------
+
+// A diff with full file contents provided via processFile so the
+// library has isPartial: false and renders native expand buttons.
+// biome-ignore format: raw diff string must preserve exact whitespace
+const expandableFilePatch = [
+"diff --git a/src/config.ts b/src/config.ts",
+"index abc1234..def5678 100644",
+"--- a/src/config.ts",
+"+++ b/src/config.ts",
+"@@ -5,3 +5,4 @@ const config = {",
+"   host: \"localhost\",",
+"   debug: false,",
+"+  verbose: true,",
+" };",
+].join("\n");
+
+const expandableOldContents = [
+	"// Configuration file",
+	"import { defaults } from './defaults';",
+	"",
+	"const config = {",
+	'  host: "localhost",',
+	"  debug: false,",
+	"};",
+	"",
+	"export default config;",
+].join("\n");
+
+const expandableNewContents = [
+	"// Configuration file",
+	"import { defaults } from './defaults';",
+	"",
+	"const config = {",
+	'  host: "localhost",',
+	"  debug: false,",
+	"  verbose: true,",
+	"};",
+	"",
+	"export default config;",
+].join("\n");
+
+const expandableFile = processFile(expandableFilePatch, {
+	oldFile: { name: "src/config.ts", contents: expandableOldContents },
+	newFile: { name: "src/config.ts", contents: expandableNewContents },
+	isGitDiff: true,
+});
+
+const expandableFiles = expandableFile ? [expandableFile] : [];
+
+export const WithExpandableContext: Story = {
+	args: {
+		parsedFiles: expandableFiles,
+	},
+	play: async ({ canvasElement }) => {
+		// The enriched diff should have isPartial: false, which means
+		// the library renders native expand buttons in the separator.
+		await waitFor(() => {
+			const container = canvasElement.querySelector("diffs-container");
+			expect(container).not.toBeNull();
+		});
+	},
+};
+
+// Story simulating the loading state while expansion is in progress.
+// The onRequestFileContents callback never resolves so the button
+// stays in its loading state.
+export const WithExpansionLoading: Story = {
+	args: {
+		parsedFiles: multiHunkFiles,
+		onRequestFileContents: () => new Promise(() => {}),
+	},
+	play: async ({ canvasElement }) => {
+		// The "Expand context" button should be visible in the header.
+		await waitFor(() => {
+			const btn = canvasElement.querySelector("button");
+			expect(btn).not.toBeNull();
+		});
+	},
+};

site/src/pages/AgentsPage/components/DiffViewer/DiffViewer.tsx

@@ -7,7 +7,7 @@ import type {
 } from "@pierre/diffs";
 import { Virtualizer } from "@pierre/diffs";
 import { FileDiff, VirtualizerContext } from "@pierre/diffs/react";
-import { ChevronRightIcon } from "lucide-react";
+import { ChevronDownIcon, ChevronRightIcon, LoaderIcon } from "lucide-react";
 import {
 	type ComponentProps,
 	type FC,
@@ -23,6 +23,7 @@ import { ScrollArea } from "#/components/ScrollArea/ScrollArea";
 import { Skeleton } from "#/components/Skeleton/Skeleton";
 import { cn } from "#/utils/cn";
 import { changeColor, changeLabel } from "../../utils/diffColors";
+import { expandFileDiff } from "../../utils/expandFileDiff";
 import {
 	DIFFS_FONT_STYLE,
 	getDiffViewerOptions,
@@ -91,6 +92,18 @@ interface DiffViewerProps {
 	scrollToFile?: string | null;
 	/** Called after scrollToFile has been processed. */
 	onScrollToFileComplete?: () => void;
+	/**
+	 * Callback to fetch file contents for context expansion. When
+	 * provided, partial diffs show an "Expand context" button in
+	 * the file header. Clicking it fetches the full file contents
+	 * and re-parses the diff so the library's native hunk
+	 * expansion takes over.
+	 */
+	onRequestFileContents?: (fileName: string) => Promise<{
+		oldContents: string | null;
+		newContents: string | null;
+		patchString: string;
+	} | null>;
 }
 
 // -------------------------------------------------------------------
@@ -432,6 +445,7 @@ interface LazyFileDiffProps {
 	lineAnnotations?: DiffLineAnnotation<string>[];
 	renderAnnotation?: (annotation: DiffLineAnnotation<string>) => ReactNode;
 	selectedLines?: SelectedLineRange | null;
+	renderHeaderMetadata?: (fileDiff: FileDiffMetadata) => ReactNode;
 }
 
 const LazyFileDiff: FC<LazyFileDiffProps> = ({
@@ -440,6 +454,7 @@ const LazyFileDiff: FC<LazyFileDiffProps> = ({
 	lineAnnotations,
 	renderAnnotation: renderAnnotationProp,
 	selectedLines,
+	renderHeaderMetadata: renderHeaderMetadataProp,
 }) => {
 	const placeholderRef = useRef<HTMLDivElement>(null);
 	const [visible, setVisible] = useState(false);
@@ -489,10 +504,57 @@ const LazyFileDiff: FC<LazyFileDiffProps> = ({
 			lineAnnotations={lineAnnotations}
 			renderAnnotation={renderAnnotationProp}
 			selectedLines={selectedLines}
+			renderHeaderMetadata={renderHeaderMetadataProp}
 		/>
 	);
 };
 
+// -------------------------------------------------------------------
+// Expand context button for file headers
+// -------------------------------------------------------------------
+
+/**
+ * Small button rendered in the file header via `renderHeaderMetadata`.
+ * When clicked it triggers the expansion flow that fetches full file
+ * contents and re-parses the diff with context expansion enabled.
+ */
+const ExpandContextButton: FC<{
+	fileName: string;
+	isLoading: boolean;
+	onClick: (fileName: string) => void;
+}> = ({ fileName, isLoading, onClick }) => {
+	return (
+		<button
+			type="button"
+			disabled={isLoading}
+			onClick={(e) => {
+				e.stopPropagation();
+				onClick(fileName);
+			}}
+			className={cn(
+				"flex items-center gap-1 rounded px-1.5 py-0.5 text-[11px] font-medium",
+				"border border-solid border-border-default bg-transparent",
+				"text-content-secondary transition-colors",
+				"hover:bg-surface-secondary hover:text-content-primary",
+				"cursor-pointer disabled:cursor-default disabled:opacity-60",
+				"outline-none focus-visible:ring-2 focus-visible:ring-content-link",
+			)}
+		>
+			{isLoading ? (
+				<>
+					<LoaderIcon className="size-3 animate-spin" />
+					Loading…
+				</>
+			) : (
+				<>
+					<ChevronDownIcon className="size-3" />
+					Expand context
+				</>
+			)}
+		</button>
+	);
+};
+
 // -------------------------------------------------------------------
 // Main component
 // -------------------------------------------------------------------
@@ -511,15 +573,88 @@ export const DiffViewer: FC<DiffViewerProps> = ({
 	renderAnnotation,
 	scrollToFile,
 	onScrollToFileComplete,
+	onRequestFileContents,
 }) => {
 	const theme = useTheme();
 	const isDark = theme.palette.mode === "dark";
 
+	// ---------------------------------------------------------------
+	// Expansion state: track which files have been enriched with
+	// full contents so the library's native hunk expansion works.
+	// ---------------------------------------------------------------
+	const [expandedFiles, setExpandedFiles] = useState<
+		Map<string, FileDiffMetadata>
+	>(new Map());
+	const [loadingFiles, setLoadingFiles] = useState<Set<string>>(new Set());
+
+	// Reset expansion state when the upstream parsedFiles change
+	// (e.g. the user switches to a different diff).
+	const prevFilesRef = useRef(parsedFiles);
+	if (prevFilesRef.current !== parsedFiles) {
+		prevFilesRef.current = parsedFiles;
+		if (expandedFiles.size > 0) setExpandedFiles(new Map());
+		if (loadingFiles.size > 0) setLoadingFiles(new Set());
+	}
+
+	const requestExpansion = (fileName: string) => {
+		if (
+			!onRequestFileContents ||
+			loadingFiles.has(fileName) ||
+			expandedFiles.has(fileName)
+		) {
+			return;
+		}
+
+		setLoadingFiles((prev) => new Set(prev).add(fileName));
+
+		onRequestFileContents(fileName)
+			.then((result) => {
+				if (!result) return;
+				const enriched = expandFileDiff(
+					fileName,
+					result.patchString,
+					result.oldContents,
+					result.newContents,
+				);
+				if (enriched) {
+					setExpandedFiles((prev) => {
+						const next = new Map(prev);
+						next.set(fileName, enriched);
+						return next;
+					});
+				}
+			})
+			.catch(() => {
+				// Silently ignore — the button just stops loading
+				// and the user can retry.
+			})
+			.finally(() => {
+				setLoadingFiles((prev) => {
+					const next = new Set(prev);
+					next.delete(fileName);
+					return next;
+				});
+			});
+	};
+
+	// Build effective file list: overlay expanded files on top of
+	// the original parsedFiles so enriched diffs with full context
+	// replace the partial originals.
+	const effectiveFiles: readonly FileDiffMetadata[] =
+		expandedFiles.size > 0
+			? parsedFiles.map((f) => expandedFiles.get(f.name) ?? f)
+			: parsedFiles;
+
 	const diffOptions = (() => {
 		const base = getDiffViewerOptions(isDark);
 		return {
 			...base,
 			diffStyle,
+			// Enable the library's native expand-context feature so
+			// that enriched (isPartial: false) files render expand
+			// buttons in their hunk separators.
+			expandUnchanged: true,
+			hunkSeparators: "line-info" as const,
 			// Extend the base CSS to make file headers sticky so they
 			// remain visible while scrolling through long diffs.
 			unsafeCSS: `${base.unsafeCSS ?? ""} ${STICKY_HEADER_CSS}`,
@@ -530,7 +665,7 @@ export const DiffViewer: FC<DiffViewerProps> = ({
 		...diffOptions,
 		overflow: "wrap" as const,
 		enableLineSelection: true,
-		enableHoverUtility: true,
+		enableGutterUtility: true,
 		onLineSelected() {
 			// TODO: Make this add context to the input so the
 			// user can type.
@@ -546,7 +681,7 @@ export const DiffViewer: FC<DiffViewerProps> = ({
 		...diffOptions,
 		overflow: "wrap" as const,
 		enableLineSelection: true,
-		enableHoverUtility: true,
+		enableGutterUtility: true,
 		...(onLineNumberClick && {
 			onLineNumberClick: (props: {
 				lineNumber: number;
@@ -567,7 +702,7 @@ export const DiffViewer: FC<DiffViewerProps> = ({
 				},
 	});
 
-	const fileTree = buildFileTree(parsedFiles);
+	const fileTree = buildFileTree(effectiveFiles);
 
 	// Sort diff blocks in the same order the file tree displays them
 	// (directories first, then alphabetical) so the rendering is
@@ -584,7 +719,7 @@ export const DiffViewer: FC<DiffViewerProps> = ({
 			}
 		};
 		walk(fileTree);
-		return [...parsedFiles].sort(
+		return [...effectiveFiles].sort(
 			(a, b) => (order.get(a.name) ?? 0) - (order.get(b.name) ?? 0),
 		);
 	})();
@@ -835,6 +970,22 @@ export const DiffViewer: FC<DiffViewerProps> = ({
 					>
 						{sortedFiles.map((fileDiff, i) => {
 							const isLast = i === sortedFiles.length - 1;
+							const isFileExpanded = expandedFiles.has(fileDiff.name);
+							const isFileLoading = loadingFiles.has(fileDiff.name);
+
+							// Show the expand button for partial files when
+							// the parent provides a content fetcher.
+							const headerMetadata =
+								onRequestFileContents && fileDiff.isPartial && !isFileExpanded
+									? () => (
+											<ExpandContextButton
+												fileName={fileDiff.name}
+												isLoading={isFileLoading}
+												onClick={requestExpansion}
+											/>
+										)
+									: undefined;
+
 							return (
 								<div
 									key={fileDiff.name}
@@ -853,6 +1004,7 @@ export const DiffViewer: FC<DiffViewerProps> = ({
 										selectedLines={
 											perFileSelectedLines?.get(fileDiff.name) ?? null
 										}
+										renderHeaderMetadata={headerMetadata}
 									/>
 									{isLast && (
 										<div className="flex items-center justify-center py-4 text-xs text-content-secondary">

site/src/pages/AgentsPage/components/DiffViewer/LocalDiffPanel.tsx

@@ -1,11 +1,14 @@
 import { parsePatchFiles } from "@pierre/diffs";
 import type { FC, RefObject } from "react";
+import { API } from "#/api/api";
 import type { WorkspaceAgentRepoChanges } from "#/api/typesGenerated";
+import { extractFilePatch } from "../../utils/extractFilePatch";
 import type { ChatMessageInputRef } from "../AgentChatInput";
 import { CommentableDiffViewer } from "../DiffViewer/CommentableDiffViewer";
 import type { DiffStyle } from "../DiffViewer/DiffViewer";
 
 interface LocalDiffPanelProps {
+	chatId: string;
 	repo: WorkspaceAgentRepoChanges;
 	isExpanded?: boolean;
 	diffStyle: DiffStyle;
@@ -13,13 +16,15 @@ interface LocalDiffPanelProps {
 }
 
 export const LocalDiffPanel: FC<LocalDiffPanelProps> = ({
+	chatId,
 	repo,
 	isExpanded,
 	diffStyle,
 	chatInputRef,
 }) => {
+	const diff = repo.unified_diff ?? "";
+
 	const parsedFiles = (() => {
-		const diff = repo.unified_diff;
 		if (!diff) {
 			return [];
 		}
@@ -31,6 +36,43 @@ export const LocalDiffPanel: FC<LocalDiffPanelProps> = ({
 		}
 	})();
 
+	const handleRequestFileContents = diff
+		? async (fileName: string) => {
+				const patchString = extractFilePatch(diff, fileName);
+				if (!patchString) return null;
+
+				try {
+					const [oldResult, newResult] = await Promise.allSettled([
+						API.experimental.getChatFileContent(
+							chatId,
+							repo.repo_root,
+							fileName,
+							"old",
+						),
+						API.experimental.getChatFileContent(
+							chatId,
+							repo.repo_root,
+							fileName,
+							"new",
+						),
+					]);
+
+					const oldContents =
+						oldResult.status === "fulfilled"
+							? oldResult.value.contents
+							: null;
+					const newContents =
+						newResult.status === "fulfilled"
+							? newResult.value.contents
+							: null;
+
+					return { oldContents, newContents, patchString };
+				} catch {
+					return { oldContents: null, newContents: null, patchString };
+				}
+			}
+		: undefined;
+
 	return (
 		<CommentableDiffViewer
 			parsedFiles={parsedFiles}
@@ -38,6 +80,7 @@ export const LocalDiffPanel: FC<LocalDiffPanelProps> = ({
 			emptyMessage="No file changes."
 			diffStyle={diffStyle}
 			chatInputRef={chatInputRef}
+			onRequestFileContents={handleRequestFileContents}
 		/>
 	);
 };

site/src/pages/AgentsPage/components/GitPanel/GitPanel.stories.tsx

@@ -99,6 +99,7 @@ const meta: Meta<typeof GitPanel> = {
 	title: "pages/AgentsPage/GitPanel",
 	component: GitPanel,
 	args: {
+		chatId: "test-chat",
 		onRefresh: fn().mockReturnValue(true),
 		onCommit: fn(),
 		repositories: new Map(),

site/src/pages/AgentsPage/components/GitPanel/GitPanel.tsx

@@ -38,6 +38,8 @@ interface DiffStats {
 }
 
 interface GitPanelProps {
+	/** Chat ID for agent API access. */
+	chatId: string;
 	/** PR tab data. Omitted if no PR is associated. */
 	prTab?: {
 		prNumber: number;
@@ -63,6 +65,7 @@ function repoTabLabel(repoRoot: string): string {
 }
 
 export const GitPanel: FC<GitPanelProps> = ({
+	chatId,
 	prTab,
 	repositories,
 	onRefresh,
@@ -285,6 +288,7 @@ export const GitPanel: FC<GitPanelProps> = ({
 					/>
 				) : (
 					<LocalRepoContent
+						chatId={chatId}
 						repoRoot={view.repoRoot}
 						repo={repositories.get(view.repoRoot)}
 						diffStats={
@@ -343,6 +347,7 @@ const RemoteContent: FC<{
 // ---------------------------------------------------------------
 
 const LocalRepoContent: FC<{
+	chatId: string;
 	repoRoot: string;
 	repo: WorkspaceAgentRepoChanges | undefined;
 	diffStats: DiffStats;
@@ -351,6 +356,7 @@ const LocalRepoContent: FC<{
 	diffStyle: DiffStyle;
 	chatInputRef?: RefObject<ChatMessageInputRef | null>;
 }> = ({
+	chatId,
 	repoRoot,
 	repo,
 	diffStats,
@@ -372,6 +378,7 @@ const LocalRepoContent: FC<{
 				onCommit={() => onCommit(repoRoot)}
 			/>
 			<LocalDiffPanel
+				chatId={chatId}
 				repo={repo}
 				isExpanded={isExpanded}
 				diffStyle={diffStyle}

site/src/pages/AgentsPage/utils/expandFileDiff.test.ts

@@ -0,0 +1,122 @@
+import {
+	expandFileDiff,
+	isExpandable,
+	MAX_EXPANDABLE_FILE_SIZE,
+} from "./expandFileDiff";
+
+const simplePatch = `diff --git a/test.ts b/test.ts
+index abc1234..def5678 100644
+--- a/test.ts
++++ b/test.ts
+@@ -1,5 +1,5 @@
+ line 1
+ line 2
+-old line 3
++new line 3
+ line 4
+ line 5
+`;
+
+const oldContents = "line 1\nline 2\nold line 3\nline 4\nline 5\n";
+const newContents = "line 1\nline 2\nnew line 3\nline 4\nline 5\n";
+
+const newFilePatch = `diff --git a/newfile.ts b/newfile.ts
+new file mode 100644
+index 0000000..abc1234
+--- /dev/null
++++ b/newfile.ts
+@@ -0,0 +1,3 @@
++line 1
++line 2
++line 3
+`;
+
+const deletedFilePatch = `diff --git a/deleted.ts b/deleted.ts
+deleted file mode 100644
+index abc1234..0000000
+--- a/deleted.ts
++++ /dev/null
+@@ -1,3 +0,0 @@
+-line 1
+-line 2
+-line 3
+`;
+
+describe("expandFileDiff", () => {
+	it("returns FileDiffMetadata with isPartial false", () => {
+		const result = expandFileDiff(
+			"test.ts",
+			simplePatch,
+			oldContents,
+			newContents,
+		);
+		expect(result).not.toBeNull();
+		expect(result!.isPartial).toBe(false);
+	});
+
+	it("handles new file (null old contents)", () => {
+		const result = expandFileDiff(
+			"newfile.ts",
+			newFilePatch,
+			null,
+			"line 1\nline 2\nline 3\n",
+		);
+		expect(result).not.toBeNull();
+		expect(result!.isPartial).toBe(false);
+	});
+
+	it("handles deleted file (null new contents)", () => {
+		const result = expandFileDiff(
+			"deleted.ts",
+			deletedFilePatch,
+			"line 1\nline 2\nline 3\n",
+			null,
+		);
+		expect(result).not.toBeNull();
+	});
+
+	it("returns empty diff on invalid patch (processFile still succeeds)", () => {
+		// processFile always succeeds when file contents are provided,
+		// even if the patch string is garbage — it diffs the files directly.
+		const result = expandFileDiff(
+			"test.ts",
+			"this is not a valid patch at all",
+			null,
+			null,
+		);
+		expect(result).not.toBeNull();
+		expect(result!.hunks).toHaveLength(0);
+		expect(result!.isPartial).toBe(false);
+	});
+
+	it("preserves file name", () => {
+		const result = expandFileDiff(
+			"test.ts",
+			simplePatch,
+			oldContents,
+			newContents,
+		);
+		expect(result).not.toBeNull();
+		expect(result!.name).toContain("test.ts");
+	});
+});
+
+describe("isExpandable", () => {
+	it("returns true for small files", () => {
+		expect(isExpandable("small content", "small content")).toBe(true);
+	});
+
+	it("returns false when old file exceeds limit", () => {
+		const huge = "x".repeat(MAX_EXPANDABLE_FILE_SIZE + 1);
+		expect(isExpandable(huge, "small")).toBe(false);
+	});
+
+	it("returns false when new file exceeds limit", () => {
+		const huge = "x".repeat(MAX_EXPANDABLE_FILE_SIZE + 1);
+		expect(isExpandable("small", huge)).toBe(false);
+	});
+
+	it("returns true when both are null", () => {
+		expect(isExpandable(null, null)).toBe(true);
+	});
+});

site/src/pages/AgentsPage/utils/expandFileDiff.ts

@@ -0,0 +1,64 @@
+import {
+	type FileContents,
+	type FileDiffMetadata,
+	processFile,
+} from "@pierre/diffs";
+
+export const MAX_EXPANDABLE_FILE_SIZE = 512 * 1024; // 512 KB
+
+/**
+ * Takes a single file's patch string and the full old/new file contents,
+ * returns an enriched FileDiffMetadata with isPartial: false.
+ * This enables the @pierre/diffs library's native expand-context feature.
+ *
+ * @param fileName - The file name (used for language detection)
+ * @param patchString - The single-file unified diff string (the portion
+ *   of the full diff that belongs to this file, including the diff --git header)
+ * @param oldContents - Full old file contents, or null for new files
+ * @param newContents - Full new file contents, or null for deleted files
+ * @param cacheKey - Optional cache key for the worker pool
+ * @returns Enriched FileDiffMetadata, or null if parsing fails
+ */
+export function expandFileDiff(
+	fileName: string,
+	patchString: string,
+	oldContents: string | null,
+	newContents: string | null,
+	cacheKey?: string,
+): FileDiffMetadata | null {
+	// For new files (null old), use empty string so processFile sees both
+	// sides and can set isPartial: false. Same for deleted files (null new).
+	const oldFile: FileContents = {
+		name: fileName,
+		contents: oldContents ?? "",
+	};
+	const newFile: FileContents = {
+		name: fileName,
+		contents: newContents ?? "",
+	};
+
+	const result = processFile(patchString, {
+		oldFile,
+		newFile,
+		cacheKey,
+		isGitDiff: true,
+	});
+
+	return result ?? null;
+}
+
+/**
+ * Checks if a file is eligible for expansion based on content size.
+ */
+export function isExpandable(
+	oldContents: string | null,
+	newContents: string | null,
+): boolean {
+	if (oldContents !== null && oldContents.length > MAX_EXPANDABLE_FILE_SIZE) {
+		return false;
+	}
+	if (newContents !== null && newContents.length > MAX_EXPANDABLE_FILE_SIZE) {
+		return false;
+	}
+	return true;
+}

site/src/pages/AgentsPage/utils/extractFilePatch.ts

@@ -0,0 +1,37 @@
+/**
+ * Extracts the unified diff section for a single file from a
+ * multi-file unified diff string. The returned string includes
+ * the `diff --git` header through to the end of the last hunk
+ * for that file.
+ *
+ * Returns null if the file isn't found in the diff.
+ */
+export function extractFilePatch(
+	fullDiff: string,
+	fileName: string,
+): string | null {
+	// Split the full diff into per-file sections at each
+	// `diff --git` boundary.
+	const diffHeaders = [...fullDiff.matchAll(/^diff --git /gm)];
+	if (diffHeaders.length === 0) return null;
+
+	for (let i = 0; i < diffHeaders.length; i++) {
+		const start = diffHeaders[i].index;
+		if (start === undefined) continue;
+		const end =
+			i + 1 < diffHeaders.length ? diffHeaders[i + 1].index : fullDiff.length;
+		const section = fullDiff.slice(start, end);
+
+		// Check whether this section belongs to the requested file.
+		// The diff header looks like: diff --git a/path b/path
+		// We also check --- and +++ lines for renamed files.
+		if (
+			section.includes(`a/${fileName}`) ||
+			section.includes(`b/${fileName}`)
+		) {
+			return section;
+		}
+	}
+
+	return null;
+}