diff --git a/src/ida/IdaInput.ts b/src/ida/IdaInput.ts
index 832b80c9cc..02eb68821c 100644
--- a/src/ida/IdaInput.ts
+++ b/src/ida/IdaInput.ts
@@ -2,6 +2,7 @@ import { TOOL_CONTRACTS } from "../contracts/toolContracts.js";
 import { AnalysisInputError } from "../domain/analysisErrorCore.js";
 import { jsonObjectSchema, type JsonValue } from "../domain/jsonValue.js";
 import { err, ok } from "../domain/result.js";
+import { invalidRegexPatternError } from "../process/RegexMatcher.js";
 import { idaAddressSchema } from "./IdaProtocolValues.js";
 import { analysisInputErrorFromIssues } from "../domain/inputIssueProjection.js";
 
@@ -65,13 +66,11 @@ export const parseIdaInput = (
       );
     } catch (cause: unknown) {
       return err(
-        new AnalysisInputError(operation, { cause }, [
-          {
-            path: ["pattern"],
-            reason: "invalid_format",
-            message: "Supply a valid regular expression.",
-          },
-        ]),
+        invalidRegexPatternError(
+          operation,
+          cause instanceof Error ? cause.message : String(cause),
+          cause,
+        ),
       );
     }
   }
diff --git a/src/ida/IdaSessionClient.test.ts b/src/ida/IdaSessionClient.test.ts
index e5f8d6be85..91d0f3ad29 100644
--- a/src/ida/IdaSessionClient.test.ts
+++ b/src/ida/IdaSessionClient.test.ts
@@ -662,10 +662,26 @@ describe("attached IDA function observations", () => {
       const found = await client.execute("search_strings", { pattern, mode });
       expect(found.ok && found.value.result).toHaveLength(count);
     }
-    expect(
-      (await client.execute("search_strings", { pattern: "[", mode: "regex" }))
-        .ok,
-    ).toBe(false);
+    const invalid = await client.execute("search_strings", {
+      pattern: "[",
+      mode: "regex",
+    });
+    if (invalid.ok) throw new Error("Expected invalid-pattern refusal");
+    expect(projectAnalysisError(invalid.error)).toMatchObject({
+      code: "invalid_request",
+      message: expect.stringContaining("Invalid regex pattern"),
+      remediation: { action: expect.stringMatching(/pattern.*mode literal/u) },
+      details: {
+        operation: "search_strings",
+        issues: [
+          {
+            path: ["pattern"],
+            reason: "invalid_format",
+            message: expect.stringContaining("Invalid regular expression"),
+          },
+        ],
+      },
+    });
     await client.close();
   });
 });
diff --git a/src/process/RegexMatcher.ts b/src/process/RegexMatcher.ts
index d33fc4baad..dccae9340c 100644
--- a/src/process/RegexMatcher.ts
+++ b/src/process/RegexMatcher.ts
@@ -518,6 +518,27 @@ export const regexWorkerHeapError = (
       remediationAction: `${heapBound(heap, request)} Use update_analysis_settings with setting regex.worker_heap_request_mib to change the request for the next worker; launch heap flags can override it. Restart REA after changing --max-old-space-size or ${REGEX_WORKER_HEAP_REQUEST_SETTING} in the launch environment. A launch --max-old-space-size-percentage takes precedence over that flag in either order. Simplify the pattern or use literal mode.`,
     },
   );
+/**
+ * A pattern the regex engine rejects. The caller sees the engine's syntax
+ * error on `pattern` and can retry in literal mode. Providers that check the
+ * pattern before starting share this result with the worker.
+ */
+export const invalidRegexPatternError = (
+  operation: string,
+  reason: string,
+  cause?: unknown,
+): AnalysisInputError =>
+  new AnalysisInputError(
+    operation,
+    {
+      cause,
+      userMessage: `Invalid regex pattern: ${reason}`,
+      remediationAction:
+        "Correct the pattern using the reported regex syntax error, or select mode literal to search for the text literally, then retry.",
+    },
+    [{ path: ["pattern"], reason: "invalid_format", message: reason }],
+  );
+
 const replySchema = z.discriminatedUnion("status", [
   z.strictObject({
     status: z.literal("matched"),
@@ -621,23 +642,7 @@ export const matchRegexValues = async (input: {
           );
         if (reply.data.status === "invalid_pattern")
           return finish(
-            err(
-              new AnalysisInputError(
-                input.operation,
-                {
-                  userMessage: `Invalid regex pattern: ${reply.data.reason}`,
-                  remediationAction:
-                    "Correct the pattern using the reported regex syntax error, or select mode literal to search for the text literally, then retry.",
-                },
-                [
-                  {
-                    path: ["pattern"],
-                    reason: "invalid_format",
-                    message: reply.data.reason,
-                  },
-                ],
-              ),
-            ),
+            err(invalidRegexPatternError(input.operation, reply.data.reason)),
           );
         if (
           reply.data.indices.some(