diff --git a/src/client.ts b/src/client.ts
index 7a5000f..a05c4dd 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -500,6 +500,16 @@ export interface JobDetails {
     branch_name?: string;
     commit_login: string;
     commit_email: string;
+    /**
+     * Login for the `Co-authored-by` trailer on agent commits. Distinct from the
+     * commit author (`commit_login`): by default the agent authors the commit and
+     * the user is credited as co-author, but under act-as-user the roles swap so
+     * the user authors and the agent is the co-author. Optional for backward
+     * compatibility with platforms that do not populate it.
+     */
+    commit_coauthor_login?: string;
+    /** Email for the `Co-authored-by` trailer. See {@link commit_coauthor_login}. */
+    commit_coauthor_email?: string;
     mcp_proxy_url?: string;
     /** Model selected by the platform for this run. Present when model selection is enabled. */
     selected_model?: string;
diff --git a/src/git.ts b/src/git.ts
index 30d768c..0c3f709 100644
--- a/src/git.ts
+++ b/src/git.ts
@@ -16,6 +16,13 @@ import { existsSync } from "fs";
 
 const DEFAULT_CLONE_DIR = "/tmp/workspace";
 
+// Environment variables carrying the platform-served commit co-author, set by
+// the ccav3 app from job details. Used to append a `Co-authored-by` trailer to
+// commits this SDK creates (the finalize safety-net commit), mirroring the
+// trailer the runtime emits for report_progress commits.
+const COAUTHOR_LOGIN_ENV = "GITHUB_COPILOT_COMMIT_COAUTHOR_LOGIN";
+const COAUTHOR_EMAIL_ENV = "GITHUB_COPILOT_COMMIT_COAUTHOR_EMAIL";
+
 // =============================================================================
 // Helpers
 // =============================================================================
@@ -33,6 +40,29 @@ function branchExistsOnRemote(repoLocation: string, branchName: string): boolean
     return output.length > 0;
 }
 
+/**
+ * Appends a `Co-authored-by` trailer built from the platform-served commit
+ * co-author (exposed via GITHUB_COPILOT_COMMIT_COAUTHOR_LOGIN/GITHUB_COPILOT_COMMIT_COAUTHOR_EMAIL)
+ * to the commit message. The served email is used verbatim so it matches the
+ * trailer the runtime emits and the server-side signing check. Returns the
+ * message unchanged when the variables are unset or the trailer is already
+ * present.
+ */
+export function withCoAuthorTrailer(commitMessage: string): string {
+    const login = process.env[COAUTHOR_LOGIN_ENV];
+    const email = process.env[COAUTHOR_EMAIL_ENV];
+    if (!login || !email) {
+        return commitMessage;
+    }
+    const trailer = `Co-authored-by: ${login} <${email}>`;
+    const normalizedMessage = commitMessage.trimEnd();
+    const lastParagraph = normalizedMessage.split(/\r?\n(?:\r?\n)+/).pop() ?? "";
+    if (lastParagraph.split(/\r?\n/).includes(trailer)) {
+        return commitMessage;
+    }
+    return `${normalizedMessage}\n\n${trailer}`;
+}
+
 // =============================================================================
 // Types
 // =============================================================================
@@ -266,7 +296,7 @@ export function commitAndPush(repoLocation: string, commitMessage: string): Comm
     if (status) {
         hadChanges = true;
         git(["add", "."], repoLocation);
-        git(["commit", "-m", commitMessage], repoLocation);
+        git(["commit", "-m", withCoAuthorTrailer(commitMessage)], repoLocation);
     }
 
     pushWithRebaseFallback(repoLocation);
diff --git a/src/mcp-server.ts b/src/mcp-server.ts
index c5f7ec1..d2967c9 100644
--- a/src/mcp-server.ts
+++ b/src/mcp-server.ts
@@ -25,7 +25,7 @@ import { appendFileSync } from "fs";
 import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
 import { StdioServerTransport } from "@modelcontextprotocol/sdk/server/stdio.js";
 import { z } from "zod";
-import { commitAndPush } from "./git.js";
+import { commitAndPush, withCoAuthorTrailer } from "./git.js";
 import type { PlatformClient } from "./client.js";
 
 // =============================================================================
@@ -135,7 +135,7 @@ async function executeReportProgress(
 
             if (status) {
                 execFileSync("git", ["add", "."], { cwd: config.workingDir });
-                execFileSync("git", ["commit", "-m", commitMessage], { cwd: config.workingDir });
+                execFileSync("git", ["commit", "-m", withCoAuthorTrailer(commitMessage)], { cwd: config.workingDir });
                 log("git commit complete (local only)", { message: commitMessage });
                 results.push(`Committed locally: ${commitMessage}`);
             } else {