feat(web): expose get_diff on MCP server (#1142)

* feat(web): expose get_diff on MCP server

Add a new get_diff MCP tool backed by the existing git diff API so agents can request structured diffs between refs. Document the tool in MCP docs with parameters aligned to the public API spec.

Made-with: Cursor

* chore: add changelog entry for MCP get_diff tool

Document the new MCP get_diff capability under Unreleased using the repository changelog format and PR link.

Made-with: Cursor

* feat(chat): expose get_diff tool in Ask agent

Register get_diff in the Ask agent toolset and render its output in the details panel so chat workflows can compare refs directly.

Made-with: Cursor

Author: msukkari

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Added a `get_diff` MCP tool to return structured diffs between two git refs. [#1142](https://github.com/sourcebot-dev/sourcebot/pull/1142)
+
 ### Changed
 - Synced `vendor/zoekt` with upstream `sourcegraph/zoekt`. [#1140](https://github.com/sourcebot-dev/sourcebot/pull/1140)
 

docs/docs/features/mcp-server.mdx

@@ -388,6 +388,17 @@ Parameters:
 | `ref` | no | Commit SHA, branch or tag name to search on. If not provided, defaults to the default branch. |
 | `limit` | no | Maximum number of files to return (default: 100). |
 
+### `get_diff`
+
+Returns a structured diff between two refs in a repository using a two-dot comparison.
+
+Parameters:
+| Name | Required | Description |
+|:----------|:---------|:--------------------------------------------------------------------------------------------------------------|
+| `repo` | yes | The fully-qualified repository name. |
+| `base` | yes | The base git ref (branch, tag, or commit SHA) to diff from. |
+| `head` | yes | The head git ref (branch, tag, or commit SHA) to diff to. |
+
 ### `find_symbol_definitions`
 
 Finds where a symbol (function, class, variable, etc.) is defined in a repository.

packages/web/src/features/chat/agent.ts

@@ -271,7 +271,7 @@ const createPrompt = ({
         The user has explicitly selected the following repositories for analysis:
         ${repos.map(repo => `- ${repo}`).join('\n')}
 
-        When calling tools that accept a \`repo\` parameter (e.g. \`read_file\`, \`list_commits\`, \`list_tree\`, \`grep\`), use these repository names exactly as listed above, including the full host prefix (e.g. \`github.com/org/repo\`).
+        When calling tools that accept a \`repo\` parameter (e.g. \`read_file\`, \`list_commits\`, \`list_tree\`, \`get_diff\`, \`grep\`), use these repository names exactly as listed above, including the full host prefix (e.g. \`github.com/org/repo\`).
 
         When using \`grep\` to search across ALL selected repositories (e.g. "which repos have X?"), omit the \`repo\` parameter entirely — the tool will automatically search across all selected repositories in a single call. Do NOT call \`grep\` once per repository when a single broad search would suffice.
         </selected_repositories>

packages/web/src/features/chat/components/chatThread/detailsCard.tsx

@@ -19,6 +19,7 @@ import { FindSymbolDefinitionsToolComponent } from './tools/findSymbolDefinition
 import { FindSymbolReferencesToolComponent } from './tools/findSymbolReferencesToolComponent';
 import { GlobToolComponent } from './tools/globToolComponent';
 import { GrepToolComponent } from './tools/grepToolComponent';
+import { GetDiffToolComponent } from './tools/getDiffToolComponent';
 import { ListCommitsToolComponent } from './tools/listCommitsToolComponent';
 import { ListReposToolComponent } from './tools/listReposToolComponent';
 import { ListTreeToolComponent } from './tools/listTreeToolComponent';
@@ -289,6 +290,15 @@ export const StepPartRenderer = ({ part }: { part: SBChatMessagePart }) => {
                     {(output) => <ListCommitsToolComponent {...output} />}
                 </ToolOutputGuard>
             )
+        case 'tool-get_diff':
+            return (
+                <ToolOutputGuard
+                    part={part}
+                    loadingText="Comparing revisions..."
+                >
+                    {(output) => <GetDiffToolComponent {...output} />}
+                </ToolOutputGuard>
+            )
         case 'tool-list_tree':
             return (
                 <ToolOutputGuard

packages/web/src/features/chat/components/chatThread/tools/getDiffToolComponent.tsx

@@ -0,0 +1,30 @@
+'use client';
+
+import { Separator } from '@/components/ui/separator';
+import { GetDiffMetadata, ToolResult } from '@/features/tools';
+
+export const GetDiffToolComponent = ({ metadata }: ToolResult<GetDiffMetadata>) => {
+    const fileCount = metadata.files.length;
+
+    return (
+        <div className="flex items-center gap-2 select-none cursor-default text-sm text-muted-foreground">
+            <span className="flex-shrink-0">Compared</span>
+            <span className="text-xs font-mono bg-muted px-1.5 py-0.5 rounded text-foreground">
+                {metadata.base}
+            </span>
+            <span className="flex-shrink-0">to</span>
+            <span className="text-xs font-mono bg-muted px-1.5 py-0.5 rounded text-foreground">
+                {metadata.head}
+            </span>
+            <span className="flex-shrink-0">in</span>
+            <span className="text-xs font-mono bg-muted px-1.5 py-0.5 rounded text-foreground truncate">
+                {metadata.repo}
+            </span>
+            <span className="flex-1" />
+            <span className="text-xs flex-shrink-0">
+                {fileCount} changed {fileCount === 1 ? 'file' : 'files'}
+            </span>
+            <Separator orientation="vertical" className="h-3 flex-shrink-0" />
+        </div>
+    );
+};

packages/web/src/features/chat/tools.ts

@@ -3,6 +3,7 @@ import {
     readFileDefinition,
     listCommitsDefinition,
     listReposDefinition,
+    getDiffDefinition,
     grepDefinition,
     globDefinition,
     findSymbolReferencesDefinition,
@@ -17,6 +18,7 @@ export const createTools = (context: ToolContext) => ({
     [readFileDefinition.name]: toVercelAITool(readFileDefinition, context),
     [listCommitsDefinition.name]: toVercelAITool(listCommitsDefinition, context),
     [listReposDefinition.name]: toVercelAITool(listReposDefinition, context),
+    [getDiffDefinition.name]: toVercelAITool(getDiffDefinition, context),
     [grepDefinition.name]: toVercelAITool(grepDefinition, context),
     [globDefinition.name]: toVercelAITool(globDefinition, context),
     [findSymbolReferencesDefinition.name]: toVercelAITool(findSymbolReferencesDefinition, context),
@@ -27,6 +29,7 @@ export const createTools = (context: ToolContext) => ({
 export type ReadFileToolUIPart = ToolUIPart<{ read_file: SBChatMessageToolTypes['read_file'] }>;
 export type ListCommitsToolUIPart = ToolUIPart<{ list_commits: SBChatMessageToolTypes['list_commits'] }>;
 export type ListReposToolUIPart = ToolUIPart<{ list_repos: SBChatMessageToolTypes['list_repos'] }>;
+export type GetDiffToolUIPart = ToolUIPart<{ get_diff: SBChatMessageToolTypes['get_diff'] }>;
 export type GrepToolUIPart = ToolUIPart<{ grep: SBChatMessageToolTypes['grep'] }>;
 export type GlobToolUIPart = ToolUIPart<{ glob: SBChatMessageToolTypes['glob'] }>;
 export type FindSymbolReferencesToolUIPart = ToolUIPart<{ find_symbol_references: SBChatMessageToolTypes['find_symbol_references'] }>;

packages/web/src/features/mcp/server.ts

@@ -13,6 +13,7 @@ import { getConfiguredLanguageModelsInfo } from "../chat/utils.server";
 import {
     findSymbolDefinitionsDefinition,
     findSymbolReferencesDefinition,
+    getDiffDefinition,
     listCommitsDefinition,
     listReposDefinition,
     listTreeDefinition,
@@ -40,6 +41,7 @@ export async function createMcpServer(): Promise<McpServer> {
 
     registerMcpTool(server, grepDefinition, toolContext);
     registerMcpTool(server, globDefinition, toolContext);
+    registerMcpTool(server, getDiffDefinition, toolContext);
     registerMcpTool(server, listCommitsDefinition, toolContext);
     registerMcpTool(server, listReposDefinition, toolContext);
     registerMcpTool(server, readFileDefinition, toolContext);

packages/web/src/features/tools/getDiff.ts

@@ -0,0 +1,40 @@
+import { getDiff, GetDiffResult } from '@/features/git';
+import { getDiffRequestSchema } from '@/features/git/schemas';
+import { isServiceError } from '@/lib/utils';
+import description from './getDiff.txt';
+import { logger } from './logger';
+import { ToolDefinition } from './types';
+
+export type GetDiffMetadata = GetDiffResult & {
+    repo: string;
+    base: string;
+    head: string;
+};
+
+export const getDiffDefinition: ToolDefinition<'get_diff', typeof getDiffRequestSchema.shape, GetDiffMetadata> = {
+    name: 'get_diff',
+    title: 'Get diff',
+    isReadOnly: true,
+    isIdempotent: true,
+    description,
+    inputSchema: getDiffRequestSchema,
+    execute: async ({ repo, base, head }, _context) => {
+        logger.debug('get_diff', { repo, base, head });
+
+        const response = await getDiff({ repo, base, head });
+
+        if (isServiceError(response)) {
+            throw new Error(response.message);
+        }
+
+        return {
+            output: JSON.stringify(response),
+            metadata: {
+                ...response,
+                repo,
+                base,
+                head,
+            },
+        };
+    },
+};

packages/web/src/features/tools/getDiff.txt

@@ -0,0 +1,4 @@
+Returns a structured git diff between two refs in a repository.
+
+Use this tool when you need patch details (added, removed, and context lines) between a base and head revision.
+The output includes changed files and hunks with line ranges and diff body content.

packages/web/src/features/tools/index.ts

@@ -3,6 +3,7 @@ export * from './listCommits';
 export * from './listRepos';
 export * from './grep';
 export * from './glob';
+export * from './getDiff';
 export * from './findSymbolReferences';
 export * from './findSymbolDefinitions';
 export * from './listTree';