diff --git a/actions/setup/js/setup_threat_detection.cjs b/actions/setup/js/setup_threat_detection.cjs
index febd94eb7b2..03401154ab9 100644
--- a/actions/setup/js/setup_threat_detection.cjs
+++ b/actions/setup/js/setup_threat_detection.cjs
@@ -31,15 +31,33 @@ async function main() {
     return;
   }
   const templateContent = fs.readFileSync(templatePath, "utf-8");
-  // Check if prompt file exists
+  // Check if prompt file exists (soft check; detection can continue with fallback context)
   // The agent artifact is downloaded to /tmp/gh-aw/threat-detection/
   // GitHub Actions preserves the directory structure from the uploaded artifact
   // (stripping the common /tmp/gh-aw/ prefix from the uploaded paths)
   // So /tmp/gh-aw/aw-prompts/prompt.txt becomes /tmp/gh-aw/threat-detection/aw-prompts/prompt.txt
   const threatDetectionDir = "/tmp/gh-aw/threat-detection";
   const promptPath = path.join(threatDetectionDir, "aw-prompts/prompt.txt");
-  if (!checkFileExists(promptPath, threatDetectionDir, "Prompt file", true)) {
-    return;
+  let promptFileInfo;
+  if (!fs.existsSync(promptPath)) {
+    promptFileInfo = `${promptPath} (unavailable)`;
+    core.warning(
+      `⚠️ ${ERR_VALIDATION}: Missing workflow prompt context at ${promptPath}. ` +
+        "Ensure the agent artifact includes /tmp/gh-aw/aw-prompts/prompt.txt. " +
+        "Threat detection will continue with fallback workflow context."
+    );
+  } else {
+    const promptStats = fs.statSync(promptPath);
+    if (promptStats.size === 0) {
+      promptFileInfo = `${promptPath} (unavailable)`;
+      core.warning(
+        `⚠️ ${ERR_VALIDATION}: Workflow prompt context is empty at ${promptPath}. ` +
+          "Threat detection will continue with fallback workflow context."
+      );
+    } else {
+      core.info(`Prompt file found: ${promptPath} (${promptStats.size} bytes)`);
+      promptFileInfo = `${promptPath} (${promptStats.size} bytes)`;
+    }
   }
 
   // Check if agent output file exists
@@ -73,7 +91,6 @@ async function main() {
   }
 
   // Get file info for template replacement
-  const promptFileInfo = promptPath + " (" + fs.statSync(promptPath).size + " bytes)";
   const agentOutputFileInfo = agentOutputPath + " (" + fs.statSync(agentOutputPath).size + " bytes)";
   const commentMemoryDir = path.join(threatDetectionDir, "comment-memory");
   let commentMemoryFileInfo = "No comment-memory files found";
diff --git a/actions/setup/js/setup_threat_detection.test.cjs b/actions/setup/js/setup_threat_detection.test.cjs
new file mode 100644
index 00000000000..8473d365066
--- /dev/null
+++ b/actions/setup/js/setup_threat_detection.test.cjs
@@ -0,0 +1,83 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import fs from "fs";
+import path from "path";
+
+const TMP_ROOT = "/tmp/gh-aw";
+const THREAT_DIR = path.join(TMP_ROOT, "threat-detection");
+const TEMPLATE_DIR = "/tmp/gh-aw-test-prompts";
+
+describe("setup_threat_detection", () => {
+  beforeEach(() => {
+    vi.resetModules();
+    fs.rmSync(TMP_ROOT, { recursive: true, force: true });
+    fs.rmSync(TEMPLATE_DIR, { recursive: true, force: true });
+    fs.mkdirSync(TEMPLATE_DIR, { recursive: true });
+
+    fs.writeFileSync(
+      path.join(TEMPLATE_DIR, "threat_detection.md"),
+      "name={WORKFLOW_NAME}\ndescription={WORKFLOW_DESCRIPTION}\nprompt={WORKFLOW_PROMPT_FILE}\noutput={AGENT_OUTPUT_FILE}\ncomment={COMMENT_MEMORY_FILES}\npatch={AGENT_PATCH_FILE}\n"
+    );
+
+    fs.mkdirSync(THREAT_DIR, { recursive: true });
+    fs.writeFileSync(path.join(THREAT_DIR, "agent_output.json"), '{"ok":true}');
+
+    process.env.GH_AW_PROMPTS_DIR = TEMPLATE_DIR;
+    process.env.WORKFLOW_NAME = "Test Workflow";
+    process.env.WORKFLOW_DESCRIPTION = "Test Description";
+  });
+
+  afterEach(() => {
+    fs.rmSync(TMP_ROOT, { recursive: true, force: true });
+    fs.rmSync(TEMPLATE_DIR, { recursive: true, force: true });
+    delete process.env.GH_AW_PROMPTS_DIR;
+    delete process.env.WORKFLOW_NAME;
+    delete process.env.WORKFLOW_DESCRIPTION;
+  });
+
+  function setupCoreMocks() {
+    const summary = {
+      addRaw: vi.fn().mockReturnThis(),
+      write: vi.fn().mockResolvedValue(undefined),
+    };
+    global.core = {
+      info: vi.fn(),
+      warning: vi.fn(),
+      error: vi.fn(),
+      setFailed: vi.fn(),
+      exportVariable: vi.fn(),
+      summary,
+    };
+  }
+
+  it("continues with fallback workflow context when prompt artifact is missing", async () => {
+    setupCoreMocks();
+
+    const module = await import("./setup_threat_detection.cjs");
+    await module.main();
+
+    expect(global.core.setFailed).not.toHaveBeenCalled();
+    expect(global.core.warning).toHaveBeenCalledWith(expect.stringContaining("Missing workflow prompt context"));
+    expect(global.core.exportVariable).toHaveBeenCalledWith("GH_AW_PROMPT", "/tmp/gh-aw/aw-prompts/prompt.txt");
+
+    const generatedPromptPath = "/tmp/gh-aw/aw-prompts/prompt.txt";
+    expect(fs.existsSync(generatedPromptPath)).toBe(true);
+    const generatedPrompt = fs.readFileSync(generatedPromptPath, "utf8");
+    expect(generatedPrompt).toContain("name=Test Workflow");
+    expect(generatedPrompt).toContain("description=Test Description");
+    expect(generatedPrompt).toContain("prompt=/tmp/gh-aw/threat-detection/aw-prompts/prompt.txt (unavailable)");
+  });
+
+  it("warns but continues when prompt artifact is empty", async () => {
+    setupCoreMocks();
+    const promptDir = path.join(THREAT_DIR, "aw-prompts");
+    fs.mkdirSync(promptDir, { recursive: true });
+    fs.writeFileSync(path.join(promptDir, "prompt.txt"), "");
+
+    const module = await import("./setup_threat_detection.cjs");
+    await module.main();
+
+    expect(global.core.setFailed).not.toHaveBeenCalled();
+    expect(global.core.warning).toHaveBeenCalledWith(expect.stringContaining("is empty"));
+    expect(global.core.exportVariable).toHaveBeenCalledWith("GH_AW_PROMPT", "/tmp/gh-aw/aw-prompts/prompt.txt");
+  });
+});
diff --git a/pkg/workflow/threat_detection.go b/pkg/workflow/threat_detection.go
index 6f1546d7081..bf93583a520 100644
--- a/pkg/workflow/threat_detection.go
+++ b/pkg/workflow/threat_detection.go
@@ -423,6 +423,9 @@ func (c *Compiler) buildPrepareDetectionFilesStep() []string {
 		"        run: |\n",
 		"          mkdir -p /tmp/gh-aw/threat-detection/aw-prompts\n",
 		"          cp /tmp/gh-aw/aw-prompts/prompt.txt /tmp/gh-aw/threat-detection/aw-prompts/prompt.txt 2>/dev/null || true\n",
+		"          if [ ! -s /tmp/gh-aw/threat-detection/aw-prompts/prompt.txt ]; then\n",
+		"            echo \"::warning::ERR_VALIDATION: Missing or empty detection context prompt at /tmp/gh-aw/threat-detection/aw-prompts/prompt.txt. Ensure the agent artifact includes /tmp/gh-aw/aw-prompts/prompt.txt. Detection will continue with fallback workflow context.\"\n",
+		"          fi\n",
 		"          cp /tmp/gh-aw/agent_output.json /tmp/gh-aw/threat-detection/agent_output.json 2>/dev/null || true\n",
 		"          for f in /tmp/gh-aw/aw-*.patch; do\n",
 		"            [ -f \"$f\" ] && cp \"$f\" /tmp/gh-aw/threat-detection/ 2>/dev/null || true\n",
diff --git a/pkg/workflow/threat_detection_test.go b/pkg/workflow/threat_detection_test.go
index 8577be2b04b..5c0ebba47bc 100644
--- a/pkg/workflow/threat_detection_test.go
+++ b/pkg/workflow/threat_detection_test.go
@@ -955,6 +955,26 @@ func TestDetectionGuardStepCondition(t *testing.T) {
 	}
 }
 
+func TestPrepareDetectionFilesStepWarnsWhenPromptContextMissingOrEmpty(t *testing.T) {
+	compiler := NewCompiler()
+
+	steps := compiler.buildPrepareDetectionFilesStep()
+	if len(steps) == 0 {
+		t.Fatal("Expected non-empty prepare detection files steps")
+	}
+
+	joined := strings.Join(steps, "")
+	if !strings.Contains(joined, "if [ ! -s /tmp/gh-aw/threat-detection/aw-prompts/prompt.txt ]; then") {
+		t.Error("Expected prepare step to check for missing or empty detection context prompt")
+	}
+	if !strings.Contains(joined, "ERR_VALIDATION: Missing or empty detection context prompt") {
+		t.Error("Expected prepare step to emit actionable ERR_VALIDATION warning when prompt context is missing")
+	}
+	if !strings.Contains(joined, "Detection will continue with fallback workflow context.") {
+		t.Error("Expected prepare step warning to document fallback behavior")
+	}
+}
+
 // TestDetectionJobLevelCondition verifies that the detection job-level `if:` condition
 // skips the job entirely when the agent produced no outputs and no patch.
 // This prevents the detection job from wasting a runner and ensures safe_outputs is