diff --git a/.changeset/patch-fail-claude-no-log-entries.md b/.changeset/patch-fail-claude-no-log-entries.md
new file mode 100644
index 0000000000..589350451e
--- /dev/null
+++ b/.changeset/patch-fail-claude-no-log-entries.md
@@ -0,0 +1,5 @@
+---
+"gh-aw": patch
+---
+
+Fail Claude log parsing when no structured log entries appear so users see an execution failure instead of silent success.
diff --git a/actions/setup/js/log_parser_bootstrap.cjs b/actions/setup/js/log_parser_bootstrap.cjs
index 47753b8051..baeb49e42a 100644
--- a/actions/setup/js/log_parser_bootstrap.cjs
+++ b/actions/setup/js/log_parser_bootstrap.cjs
@@ -209,6 +209,12 @@ async function runLogParser(options) {
       core.error(`Failed to parse ${parserName} log`);
     }
 
+    // Claude-specific guardrail: if no structured log entries were parsed, treat as execution failure.
+    // This catches silent startup failures where Claude exits before producing JSON tool activity.
+    if (parserName === "Claude" && (!logEntries || logEntries.length === 0)) {
+      core.setFailed("Claude execution failed: no structured log entries were produced. This usually indicates a startup or configuration error before tool execution.");
+    }
+
     // Handle MCP server failures if present
     if (mcpFailures && mcpFailures.length > 0) {
       const failedServers = mcpFailures.join(", ");
diff --git a/actions/setup/js/log_parser_bootstrap.test.cjs b/actions/setup/js/log_parser_bootstrap.test.cjs
index e564586332..730e094b30 100644
--- a/actions/setup/js/log_parser_bootstrap.test.cjs
+++ b/actions/setup/js/log_parser_bootstrap.test.cjs
@@ -62,6 +62,22 @@ describe("log_parser_bootstrap.cjs", () => {
             fs.unlinkSync(logFile),
             fs.rmdirSync(tmpDir));
         }),
+        it("should fail Claude runs when no structured log entries are parsed", () => {
+          const tmpDir = fs.mkdtempSync(path.join(__dirname, "test-"));
+          const logFile = path.join(tmpDir, "test.log");
+          try {
+            fs.writeFileSync(logFile, "unstructured log output");
+            process.env.GH_AW_AGENT_OUTPUT = logFile;
+            const mockParseLog = vi.fn().mockReturnValue({ markdown: "## Result\n", mcpFailures: [], maxTurnsHit: false, logEntries: [] });
+            runLogParser({ parseLog: mockParseLog, parserName: "Claude" });
+            expect(mockCore.setFailed).toHaveBeenCalledWith(
+              "Claude execution failed: no structured log entries were produced. This usually indicates a startup or configuration error before tool execution."
+            );
+          } finally {
+            fs.unlinkSync(logFile);
+            fs.rmdirSync(tmpDir);
+          }
+        }),
         it("should generate plain text summary when logEntries are available", () => {
           const tmpDir = fs.mkdtempSync(path.join(__dirname, "test-")),
             logFile = path.join(tmpDir, "test.log");
diff --git a/actions/setup/js/parse_claude_log.test.cjs b/actions/setup/js/parse_claude_log.test.cjs
index 5eb2d04bc9..3c6a77c817 100644
--- a/actions/setup/js/parse_claude_log.test.cjs
+++ b/actions/setup/js/parse_claude_log.test.cjs
@@ -437,6 +437,11 @@ describe("parse_claude_log.cjs", () => {
       expect(mockCore.info).toHaveBeenCalledWith("No agent log file specified");
       expect(mockCore.setFailed).not.toHaveBeenCalled();
     });
+
+    it("should fail when Claude log has no structured entries", async () => {
+      await runScript("this is not structured Claude JSON output");
+      expect(mockCore.setFailed).toHaveBeenCalledWith("Claude execution failed: no structured log entries were produced. This usually indicates a startup or configuration error before tool execution.");
+    });
   });
 
   describe("helper function tests", () => {