diff --git a/docs/_static/llama-stack-spec.html b/docs/_static/llama-stack-spec.html
index 5575eb36e..31c637bbb 100644
--- a/docs/_static/llama-stack-spec.html
+++ b/docs/_static/llama-stack-spec.html
@@ -9985,11 +9985,11 @@
                         "description": "The prompt text content with variable placeholders."
                     },
                     "variables": {
-                        "type": "object",
-                        "additionalProperties": {
+                        "type": "array",
+                        "items": {
                             "type": "string"
                         },
-                        "description": "Dictionary of variable names to their default values."
+                        "description": "List of variable names that can be used in the prompt template."
                     }
                 },
                 "additionalProperties": false,
@@ -10014,8 +10014,8 @@
                         "description": "Unique identifier formatted as 'pmpt_<48-digit-hash>'"
                     },
                     "variables": {
-                        "type": "object",
-                        "additionalProperties": {
+                        "type": "array",
+                        "items": {
                             "type": "string"
                         },
                         "description": "Dictionary of prompt variable names and values"
@@ -10030,6 +10030,7 @@
                 "required": [
                     "version",
                     "prompt_id",
+                    "variables",
                     "is_default"
                 ],
                 "title": "Prompt",
@@ -17824,11 +17825,11 @@
                         "description": "The updated prompt text content."
                     },
                     "variables": {
-                        "type": "object",
-                        "additionalProperties": {
+                        "type": "array",
+                        "items": {
                             "type": "string"
                         },
-                        "description": "Updated dictionary of variable names to their default values."
+                        "description": "Updated list of variable names that can be used in the prompt template."
                     },
                     "version": {
                         "type": "string",
diff --git a/docs/_static/llama-stack-spec.yaml b/docs/_static/llama-stack-spec.yaml
index 2c0993a9e..03457ecf0 100644
--- a/docs/_static/llama-stack-spec.yaml
+++ b/docs/_static/llama-stack-spec.yaml
@@ -7373,11 +7373,11 @@ components:
           description: >-
             The prompt text content with variable placeholders.
         variables:
-          type: object
-          additionalProperties:
+          type: array
+          items:
             type: string
           description: >-
-            Dictionary of variable names to their default values.
+            List of variable names that can be used in the prompt template.
       additionalProperties: false
       required:
         - prompt
@@ -7399,8 +7399,8 @@ components:
           description: >-
             Unique identifier formatted as 'pmpt_<48-digit-hash>'
         variables:
-          type: object
-          additionalProperties:
+          type: array
+          items:
             type: string
           description: >-
             Dictionary of prompt variable names and values
@@ -7414,6 +7414,7 @@ components:
       required:
         - version
         - prompt_id
+        - variables
         - is_default
       title: Prompt
       description: >-
@@ -13226,11 +13227,11 @@ components:
           type: string
           description: The updated prompt text content.
         variables:
-          type: object
-          additionalProperties:
+          type: array
+          items:
             type: string
           description: >-
-            Updated dictionary of variable names to their default values.
+            Updated list of variable names that can be used in the prompt template.
         version:
           type: string
           description: >-
diff --git a/llama_stack/apis/prompts/prompts.py b/llama_stack/apis/prompts/prompts.py
index ca102d9c6..8d34aa5ca 100644
--- a/llama_stack/apis/prompts/prompts.py
+++ b/llama_stack/apis/prompts/prompts.py
@@ -4,10 +4,11 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+import re
 import secrets
 from typing import Protocol, runtime_checkable
 
-from pydantic import BaseModel, Field, field_validator
+from pydantic import BaseModel, Field, field_validator, model_validator
 
 from llama_stack.providers.utils.telemetry.trace_protocol import trace_protocol
 from llama_stack.schema_utils import json_schema_type, webmethod
@@ -27,8 +28,8 @@ class Prompt(BaseModel):
     prompt: str | None = Field(default=None, description="The system prompt with variable placeholders")
     version: str = Field(description="Version string (integer start at 1 cast as string)")
     prompt_id: str = Field(description="Unique identifier in format 'pmpt_<48-digit-hash>'")
-    variables: dict[str, str] | None = Field(
-        default_factory=dict, description="Variables for dynamic injection using {{variable}} syntax"
+    variables: list[str] = Field(
+        default_factory=list, description="List of variable names that can be used in the prompt template"
     )
     is_default: bool = Field(
         default=False, description="Boolean indicating whether this version is the default version"
@@ -66,6 +67,21 @@ class Prompt(BaseModel):
             raise
         return prompt_version
 
+    @model_validator(mode="after")
+    def validate_prompt_variables(self):
+        """Validate that all variables used in the prompt are declared in the variables list."""
+        if not self.prompt:
+            return self
+
+        prompt_variables = set(re.findall(r"{{\s*(\w+)\s*}}", self.prompt))
+        declared_variables = set(self.variables)
+
+        undeclared = prompt_variables - declared_variables
+        if undeclared:
+            raise ValueError(f"Prompt contains undeclared variables: {sorted(undeclared)}")
+
+        return self
+
     @classmethod
     def generate_prompt_id(cls) -> str:
         # Generate 48 hex characters (24 bytes)
@@ -78,14 +94,14 @@ class CreatePromptRequest(BaseModel):
     """Request model to create a prompt."""
 
     prompt: str = Field(description="The prompt text content")
-    variables: dict[str, str] = Field(default_factory=dict, description="Variables for dynamic injection")
+    variables: list[str] = Field(default_factory=list, description="List of variable names for dynamic injection")
 
 
 class UpdatePromptRequest(BaseModel):
     """Request model for updating a prompt."""
 
     prompt: str = Field(description="The prompt text content")
-    variables: dict[str, str] = Field(default_factory=dict, description="Variables for dynamic injection")
+    variables: list[str] = Field(default_factory=list, description="List of variable names for dynamic injection")
 
 
 class ListPromptsResponse(BaseModel):
@@ -137,12 +153,12 @@ class Prompts(Protocol):
     async def create_prompt(
         self,
         prompt: str,
-        variables: dict[str, str] | None = None,
+        variables: list[str] | None = None,
     ) -> Prompt:
         """Create a new prompt.
 
         :param prompt: The prompt text content with variable placeholders.
-        :param variables: Dictionary of variable names to their default values.
+        :param variables: List of variable names that can be used in the prompt template.
         :returns: The created Prompt resource.
         """
         ...
@@ -152,14 +168,14 @@ class Prompts(Protocol):
         self,
         prompt_id: str,
         prompt: str,
-        variables: dict[str, str] | None = None,
+        variables: list[str] | None = None,
         version: str | None = None,
     ) -> Prompt:
         """Update an existing prompt (increments version).
 
         :param prompt_id: The identifier of the prompt to update.
         :param prompt: The updated prompt text content.
-        :param variables: Updated dictionary of variable names to their default values.
+        :param variables: Updated list of variable names that can be used in the prompt template.
         :param version: The current version of the prompt being updated (as a string).
         :returns: The updated Prompt resource with incremented version.
         """
diff --git a/llama_stack/core/prompts/prompts.py b/llama_stack/core/prompts/prompts.py
index 730ce00e3..624764020 100644
--- a/llama_stack/core/prompts/prompts.py
+++ b/llama_stack/core/prompts/prompts.py
@@ -76,7 +76,7 @@ class PromptServiceImpl(Prompts):
                 "prompt_id": prompt.prompt_id,
                 "prompt": prompt.prompt,
                 "version": prompt.version,
-                "variables": prompt.variables or {},
+                "variables": prompt.variables or [],
             }
         )
 
@@ -84,7 +84,7 @@ class PromptServiceImpl(Prompts):
         """Deserialize a prompt from JSON string."""
         obj = json.loads(data)
         return Prompt(
-            prompt_id=obj["prompt_id"], prompt=obj["prompt"], version=obj["version"], variables=obj.get("variables", {})
+            prompt_id=obj["prompt_id"], prompt=obj["prompt"], version=obj["version"], variables=obj.get("variables", [])
         )
 
     async def list_prompts(self) -> ListPromptsResponse:
@@ -121,11 +121,11 @@ class PromptServiceImpl(Prompts):
     async def create_prompt(
         self,
         prompt: str,
-        variables: dict[str, str] | None = None,
+        variables: list[str] | None = None,
     ) -> Prompt:
         """Create a new prompt."""
         if variables is None:
-            variables = {}
+            variables = []
 
         prompt_obj = Prompt(prompt_id=Prompt.generate_prompt_id(), prompt=prompt, version="1", variables=variables)
 
@@ -142,12 +142,12 @@ class PromptServiceImpl(Prompts):
         self,
         prompt_id: str,
         prompt: str,
-        variables: dict[str, str] | None = None,
+        variables: list[str] | None = None,
         version: str | None = None,
     ) -> Prompt:
         """Update an existing prompt (increments version)."""
         if variables is None:
-            variables = {}
+            variables = []
 
         current_prompt = await self.get_prompt(prompt_id)
         if version and current_prompt.version != version:
diff --git a/tests/unit/prompts/prompts/test_prompts.py b/tests/unit/prompts/prompts/test_prompts.py
index 01395df84..93f443374 100644
--- a/tests/unit/prompts/prompts/test_prompts.py
+++ b/tests/unit/prompts/prompts/test_prompts.py
@@ -30,11 +30,11 @@ class TestPrompts:
             yield store
 
     async def test_create_and_get_prompt(self, store):
-        prompt = await store.create_prompt("Hello world!", {"name": "John"})
+        prompt = await store.create_prompt("Hello world!", ["name"])
         assert prompt.prompt == "Hello world!"
         assert prompt.version == "1"
         assert prompt.prompt_id.startswith("pmpt_")
-        assert prompt.variables == {"name": "John"}
+        assert prompt.variables == ["name"]
 
         retrieved = await store.get_prompt(prompt.prompt_id)
         assert retrieved.prompt_id == prompt.prompt_id
@@ -42,7 +42,7 @@ class TestPrompts:
 
     async def test_update_prompt(self, store):
         prompt = await store.create_prompt("Original")
-        updated = await store.update_prompt(prompt.prompt_id, "Updated", {"v": "2"})
+        updated = await store.update_prompt(prompt.prompt_id, "Updated", ["v"])
         assert updated.version == "2"
         assert updated.prompt == "Updated"
 
@@ -51,16 +51,16 @@ class TestPrompts:
 
         prompt = await store.create_prompt("Original")
         assert prompt.version == "1"
-        prompt = await store.update_prompt(prompt.prompt_id, "Updated", {"v": "2"}, version_for_update)
+        prompt = await store.update_prompt(prompt.prompt_id, "Updated", ["v"], version_for_update)
         assert prompt.version == "2"
 
         with pytest.raises(ValueError):
             # now this is a stale version
-            await store.update_prompt(prompt.prompt_id, "Another Update", {"v": "2"}, version_for_update)
+            await store.update_prompt(prompt.prompt_id, "Another Update", ["v"], version_for_update)
 
         with pytest.raises(ValueError):
             # this version does not exist
-            await store.update_prompt(prompt.prompt_id, "Another Update", {"v": "2"}, "99")
+            await store.update_prompt(prompt.prompt_id, "Another Update", ["v"], "99")
 
     async def test_delete_prompt(self, store):
         prompt = await store.create_prompt("to be deleted")
@@ -134,3 +134,13 @@ class TestPrompts:
 
         with pytest.raises(ValueError):
             await store.list_prompt_versions("nonexistent")
+
+    async def test_prompt_variable_validation(self, store):
+        prompt = await store.create_prompt("Hello {{ name }}, you live in {{ city }}!", ["name", "city"])
+        assert prompt.variables == ["name", "city"]
+
+        prompt_no_vars = await store.create_prompt("Hello world!", [])
+        assert prompt_no_vars.variables == []
+
+        with pytest.raises(ValueError, match="undeclared variables"):
+            await store.create_prompt("Hello {{ name }}, invalid {{ unknown }}!", ["name"])