diff --git a/.changeset/fix-invalid-tool-call-input.md b/.changeset/fix-invalid-tool-call-input.md
new file mode 100644
index 00000000..826337c9
--- /dev/null
+++ b/.changeset/fix-invalid-tool-call-input.md
@@ -0,0 +1,6 @@
+---
+"@perstack/core": patch
+"@perstack/runtime": patch
+---
+
+fix: reject invalid tool call inputs and retry instead of sending to API
diff --git a/packages/core/src/schemas/message-part.ts b/packages/core/src/schemas/message-part.ts
index 789a8384..4a5e52ad 100644
--- a/packages/core/src/schemas/message-part.ts
+++ b/packages/core/src/schemas/message-part.ts
@@ -128,14 +128,14 @@ export interface ToolCallPart extends BasePart {
   /** Name of the tool to call */
   toolName: string
   /** Arguments to pass to the tool */
-  args: unknown
+  args: Record<string, unknown>
 }
 
 export const toolCallPartSchema = basePartSchema.extend({
   type: z.literal("toolCallPart"),
   toolCallId: z.string(),
   toolName: z.string(),
-  args: z.unknown(),
+  args: z.record(z.string(), z.unknown()),
 })
 toolCallPartSchema satisfies z.ZodType<ToolCallPart>
 
diff --git a/packages/runtime/src/state-machine/states/generating-tool-call.ts b/packages/runtime/src/state-machine/states/generating-tool-call.ts
index 1f7fd8a7..0bcdf011 100644
--- a/packages/runtime/src/state-machine/states/generating-tool-call.ts
+++ b/packages/runtime/src/state-machine/states/generating-tool-call.ts
@@ -176,6 +176,40 @@ export async function generatingToolCallLogic({
   const thinkingParts = extractThinkingParts(reasoning as ReasoningPart[] | undefined)
   const thinkingText = extractThinkingText(reasoning as ReasoningPart[] | undefined)
 
+  // Check for invalid tool calls (e.g., malformed input from the model)
+  // When the AI SDK fails to parse tool call input, it marks the tool call as invalid.
+  // These must be rejected and retried to avoid sending non-dict input to the API.
+  const invalidToolCalls = toolCalls.filter((tc: { invalid?: boolean }) => tc.invalid === true)
+  if (invalidToolCalls.length > 0) {
+    const currentRetryCount = checkpoint.retryCount ?? 0
+    if (currentRetryCount >= setting.maxRetries) {
+      return stopRunByError(setting, checkpoint, {
+        checkpoint: {
+          ...checkpoint,
+          status: "stoppedByError",
+        },
+        step: {
+          ...step,
+          finishedAt: Date.now(),
+        },
+        error: {
+          name: "InvalidToolCallInput",
+          message: `Max retries (${setting.maxRetries}) exceeded: Invalid tool call input for ${invalidToolCalls.map((tc) => tc.toolName).join(", ")}`,
+          isRetryable: false,
+        },
+      })
+    }
+    const reason = JSON.stringify({
+      error: "Error: Invalid tool call input",
+      message: `The following tool call(s) had invalid input: ${invalidToolCalls.map((tc) => tc.toolName).join(", ")}. Ensure tool call arguments are valid JSON objects matching the tool schema. Try again.`,
+    })
+    return retry(setting, checkpoint, {
+      reason,
+      newMessages: [createUserMessage([{ type: "textPart", text: reason }])],
+      usage,
+    })
+  }
+
   // Text only = implicit completion (no tool calls)
   if (toolCalls.length === 0 && text) {
     const contents: Array<