diff --git a/actions/setup/js/mcp_cli_bridge.cjs b/actions/setup/js/mcp_cli_bridge.cjs
index 4c96401d929..11c0a7ef0fa 100644
--- a/actions/setup/js/mcp_cli_bridge.cjs
+++ b/actions/setup/js/mcp_cli_bridge.cjs
@@ -622,6 +622,63 @@ function readStdinSync() {
   return Buffer.concat(chunks).toString("utf8");
 }
 
+/**
+ * Try to extract a specific field value from a JSON object string.
+ *
+ * When per-field stdin mode (`--key .`) is used and stdin contains a JSON
+ * object, this function checks whether that object has the target key.  If so,
+ * it returns the field value, allowing agents to pipe a full JSON payload and
+ * extract individual fields without the entire JSON string ending up as the
+ * field value.
+ *
+ * Example: an agent may construct a full payload JSON and still pass individual
+ * fields via `--key .`:
+ *   printf '{"title":"Fix bug","body":"Details here"}' \
+ *     | safeoutputs create_pull_request --title "Fix bug" --body .
+ * Without this helper the body would be the raw JSON string.  With it, the
+ * body is correctly extracted as "Details here".
+ *
+ * Returns `undefined` when stdin is not a JSON object, when the key is absent
+ * from the object, or when JSON parsing fails — in all those cases the caller
+ * falls back to using the raw stdin content.
+ *
+ * @param {string} trimmedStdin - Pre-trimmed stdin content.  Must have no
+ *   leading whitespace — the function uses a `startsWith('{')` fast-path to
+ *   avoid parsing non-JSON input, so leading whitespace will cause a JSON
+ *   object to be treated as non-JSON and return `undefined`.  All callers
+ *   must pass `stdinContent.trim()` before invoking this function.
+ * @param {string} canonicalKey - Canonical schema key to look up
+ * @param {Record<string, {type?: string|string[]}>} schemaProperties - Tool input schema properties
+ * @param {Map<string, string>} normalizedSchemaKeyMap - Map from normalized key to canonical schema key
+ * @param {Set<string>} ambiguousNormalizedSchemaKeys - Normalized keys that map to multiple schema keys
+ * @returns {unknown} The extracted field value, or `undefined` if not found
+ */
+function tryExtractJsonFieldFromStdin(trimmedStdin, canonicalKey, schemaProperties, normalizedSchemaKeyMap, ambiguousNormalizedSchemaKeys) {
+  if (!trimmedStdin.startsWith("{")) {
+    return undefined;
+  }
+  try {
+    const parsed = JSON.parse(trimmedStdin);
+    if (parsed === null || typeof parsed !== "object" || Array.isArray(parsed)) {
+      return undefined;
+    }
+    // Try the canonical key first, then any schema-aliased form.
+    if (Object.prototype.hasOwnProperty.call(parsed, canonicalKey)) {
+      return parsed[canonicalKey];
+    }
+    // Search for a key in the JSON that resolves to the same canonical key.
+    for (const jsonKey of Object.keys(parsed)) {
+      const resolved = resolveSchemaPropertyKey(jsonKey, schemaProperties, normalizedSchemaKeyMap, ambiguousNormalizedSchemaKeys);
+      if (resolved === canonicalKey) {
+        return parsed[jsonKey];
+      }
+    }
+    return undefined;
+  } catch {
+    return undefined;
+  }
+}
+
 /**
  * Parse user-provided --key value pairs into a tool arguments object.
  * Supports both --key value and --key=value styles.
@@ -634,10 +691,15 @@ function readStdinSync() {
  *   printf '{"issue_number":42,"body":"hello"}' | safeoutputs add_comment .
  *
  * When `stdinContent` is provided and non-empty, any '--key .' or '--key=.'
- * pair substitutes that field's value with the raw stdin text (per-field
- * stdin mode). This enables agents to pipe large text into a single field:
- *   printf 'Long issue body...' | safeoutputs create_issue --title "Bug" --body .
- * When stdin is empty, the '.' is passed through as a literal value.
+ * pair substitutes that field's value with stdin.  If stdin is a JSON object
+ * that contains the target key, the matching field value is extracted from the
+ * JSON instead of using the entire stdin string.  This prevents a common agent
+ * mistake where the whole JSON payload ends up as a string field value:
+ *   printf '{"title":"Fix bug","body":"Details"}' \
+ *     | safeoutputs create_pull_request --title "Fix bug" --body .
+ * When stdin is empty, the '.' is passed through as a literal value.  When
+ * stdin is non-empty but the key is absent from the JSON (or stdin is not a
+ * JSON object), the entire trimmed stdin string is used as the field value.
  *
  * @param {string[]} args - User arguments after the tool name
  * @param {Record<string, {type?: string|string[]}>} [schemaProperties] - Tool input schema properties
@@ -686,7 +748,8 @@ function parseToolArgs(args, schemaProperties = {}, stdinContent = null) {
           const canonicalKey = resolveSchemaPropertyKey(key, schemaProperties, normalizedSchemaKeyMap, ambiguousNormalizedSchemaKeys);
           const rawValue = raw.slice(eqIdx + 1);
           if (rawValue === "." && trimmedStdin) {
-            result[canonicalKey] = trimmedStdin;
+            const extracted = tryExtractJsonFieldFromStdin(trimmedStdin, canonicalKey, schemaProperties, normalizedSchemaKeyMap, ambiguousNormalizedSchemaKeys);
+            result[canonicalKey] = extracted !== undefined ? extracted : trimmedStdin;
           } else {
             result[canonicalKey] = coerceToolArgValue(canonicalKey, rawValue, schemaProperties[canonicalKey], result[canonicalKey], !hasSchemaProperties);
           }
@@ -697,7 +760,8 @@ function parseToolArgs(args, schemaProperties = {}, stdinContent = null) {
         const canonicalKey = resolveSchemaPropertyKey(raw, schemaProperties, normalizedSchemaKeyMap, ambiguousNormalizedSchemaKeys);
         const rawValue = args[i + 1];
         if (rawValue === "." && trimmedStdin) {
-          result[canonicalKey] = trimmedStdin;
+          const extracted = tryExtractJsonFieldFromStdin(trimmedStdin, canonicalKey, schemaProperties, normalizedSchemaKeyMap, ambiguousNormalizedSchemaKeys);
+          result[canonicalKey] = extracted !== undefined ? extracted : trimmedStdin;
         } else {
           result[canonicalKey] = coerceToolArgValue(canonicalKey, rawValue, schemaProperties[canonicalKey], result[canonicalKey], !hasSchemaProperties);
         }
@@ -1441,6 +1505,7 @@ module.exports = {
   parseToolArgs,
   coerceToolArgValue,
   unescapeCliStringArg,
+  tryExtractJsonFieldFromStdin,
   extractJSONRPCMessages,
   renderProgressMessages,
   formatResponse,
diff --git a/actions/setup/js/mcp_cli_bridge.test.cjs b/actions/setup/js/mcp_cli_bridge.test.cjs
index de91e64368a..5e682df02a0 100644
--- a/actions/setup/js/mcp_cli_bridge.test.cjs
+++ b/actions/setup/js/mcp_cli_bridge.test.cjs
@@ -13,6 +13,7 @@ import {
   shouldShowToolHelpForEmptyArgs,
   showHelp,
   showToolHelp,
+  tryExtractJsonFieldFromStdin,
   unescapeCliStringArg,
   writeStdoutAndFlush,
 } from "./mcp_cli_bridge.cjs";
@@ -764,6 +765,162 @@ describe("mcp_cli_bridge.cjs", () => {
     });
   });
 
+  describe("per-field stdin mode with JSON stdin — field extraction", () => {
+    it("extracts matching field from JSON stdin when --body . is used (space-separated)", () => {
+      // Root cause of gh-aw-workshop#2118: agent piped JSON payload and used --body .
+      // expecting the body field to be extracted, but the entire JSON string ended up as body.
+      const schemaProperties = { title: { type: "string" }, body: { type: "string" } };
+      const stdinContent = '{"title":"Fix bug","body":"This PR fixes the issue."}';
+
+      const { args } = parseToolArgs(["--title", "Fix bug", "--body", "."], schemaProperties, stdinContent);
+
+      expect(args).toEqual({ title: "Fix bug", body: "This PR fixes the issue." });
+    });
+
+    it("extracts matching field from JSON stdin when --body=. is used (equals-separated)", () => {
+      const schemaProperties = { title: { type: "string" }, body: { type: "string" } };
+      const stdinContent = '{"title":"Fix bug","body":"Details here."}';
+
+      const { args } = parseToolArgs(["--title", "Fix bug", "--body=."], schemaProperties, stdinContent);
+
+      expect(args).toEqual({ title: "Fix bug", body: "Details here." });
+    });
+
+    it("extracts both title and body when --title . --body . used with JSON stdin", () => {
+      const schemaProperties = { title: { type: "string" }, body: { type: "string" } };
+      const stdinContent = '{"title":"Fix: Bug #123","body":"This PR fixes bug #123."}';
+
+      const { args } = parseToolArgs(["--title", ".", "--body", "."], schemaProperties, stdinContent);
+
+      expect(args).toEqual({ title: "Fix: Bug #123", body: "This PR fixes bug #123." });
+    });
+
+    it("falls back to raw stdin when JSON does not contain the target key", () => {
+      const schemaProperties = { body: { type: "string" } };
+      const stdinContent = '{"other_field":"value"}';
+
+      const { args } = parseToolArgs(["--body", "."], schemaProperties, stdinContent);
+
+      // No 'body' key in JSON → use raw stdin content
+      expect(args).toEqual({ body: stdinContent });
+    });
+
+    it("falls back to raw stdin when stdin is not a JSON object", () => {
+      const schemaProperties = { body: { type: "string" } };
+      const stdinContent = "This is a long body from stdin.";
+
+      const { args } = parseToolArgs(["--body", "."], schemaProperties, stdinContent);
+
+      expect(args).toEqual({ body: stdinContent });
+    });
+
+    it("falls back to raw stdin when stdin is a JSON array", () => {
+      const schemaProperties = { body: { type: "string" } };
+      const stdinContent = '["item1","item2"]';
+
+      const { args } = parseToolArgs(["--body", "."], schemaProperties, stdinContent);
+
+      expect(args).toEqual({ body: stdinContent });
+    });
+
+    it("falls back to raw stdin when stdin is invalid JSON", () => {
+      const schemaProperties = { body: { type: "string" } };
+      const stdinContent = '{"body": not-valid-json}';
+
+      const { args } = parseToolArgs(["--body", "."], schemaProperties, stdinContent);
+
+      expect(args).toEqual({ body: stdinContent });
+    });
+
+    it("resolves dash/underscore aliased JSON key to canonical schema key", () => {
+      const schemaProperties = { issue_number: { type: "integer" } };
+      const stdinContent = '{"issue-number":42}';
+
+      const { args } = parseToolArgs(["--issue_number", "."], schemaProperties, stdinContent);
+
+      expect(args).toEqual({ issue_number: 42 });
+    });
+
+    it("preserves non-string JSON values extracted from stdin (e.g. boolean, number)", () => {
+      const schemaProperties = { draft: { type: "boolean" }, count: { type: "integer" } };
+      const stdinContent = '{"draft":true,"count":5}';
+
+      const { args: draftArgs } = parseToolArgs(["--draft", "."], schemaProperties, stdinContent);
+      const { args: countArgs } = parseToolArgs(["--count", "."], schemaProperties, stdinContent);
+
+      expect(draftArgs).toEqual({ draft: true });
+      expect(countArgs).toEqual({ count: 5 });
+    });
+
+    it("handles multiline JSON payload with --body . correctly", () => {
+      const schemaProperties = { title: { type: "string" }, body: { type: "string" } };
+      const stdinContent = `{
+  "title": "Fix bug",
+  "body": "### Summary\\n\\nDetails here."
+}`;
+
+      const { args } = parseToolArgs(["--title", "Fix bug", "--body", "."], schemaProperties, stdinContent);
+
+      expect(args).toEqual({ title: "Fix bug", body: "### Summary\n\nDetails here." });
+    });
+  });
+
+  describe("tryExtractJsonFieldFromStdin", () => {
+    it("extracts a field value from a JSON object string", () => {
+      const schemaProperties = { body: { type: "string" } };
+      const result = tryExtractJsonFieldFromStdin(
+        '{"title":"Fix","body":"PR description"}',
+        "body",
+        schemaProperties,
+        new Map([
+          ["title", "title"],
+          ["body", "body"],
+        ]),
+        new Set()
+      );
+      expect(result).toBe("PR description");
+    });
+
+    it("returns undefined when the key is absent from the JSON", () => {
+      const schemaProperties = { body: { type: "string" } };
+      const result = tryExtractJsonFieldFromStdin('{"title":"Fix"}', "body", schemaProperties, new Map([["title", "title"]]), new Set());
+      expect(result).toBeUndefined();
+    });
+
+    it("returns undefined for non-object JSON (array)", () => {
+      const result = tryExtractJsonFieldFromStdin('["a","b"]', "body", {}, new Map(), new Set());
+      expect(result).toBeUndefined();
+    });
+
+    it("returns undefined for invalid JSON", () => {
+      const result = tryExtractJsonFieldFromStdin("{not valid}", "body", {}, new Map(), new Set());
+      expect(result).toBeUndefined();
+    });
+
+    it("returns undefined for plain text (not starting with {)", () => {
+      const result = tryExtractJsonFieldFromStdin("plain text", "body", {}, new Map(), new Set());
+      expect(result).toBeUndefined();
+    });
+
+    it("preserves null value extracted from JSON stdin (does not fall back to raw stdin)", () => {
+      const schemaProperties = { body: { type: "string" } };
+      const result = tryExtractJsonFieldFromStdin('{"body":null}', "body", schemaProperties, new Map([["body", "body"]]), new Set());
+      expect(result).toBeNull();
+    });
+
+    it("preserves false boolean extracted from JSON stdin", () => {
+      const schemaProperties = { draft: { type: "boolean" } };
+      const result = tryExtractJsonFieldFromStdin('{"draft":false}', "draft", schemaProperties, new Map([["draft", "draft"]]), new Set());
+      expect(result).toBe(false);
+    });
+
+    it("preserves 0 number extracted from JSON stdin", () => {
+      const schemaProperties = { count: { type: "number" } };
+      const result = tryExtractJsonFieldFromStdin('{"count":0}', "count", schemaProperties, new Map([["count", "count"]]), new Set());
+      expect(result).toBe(0);
+    });
+  });
+
   describe("unescapeCliStringArg", () => {
     it("converts \\n to an actual newline", () => {
       expect(unescapeCliStringArg("Hello\\nWorld")).toBe("Hello\nWorld");