diff --git a/crates/tinyagents-harness/src/agent_loop/mod_tests.rs b/crates/tinyagents-harness/src/agent_loop/mod_tests.rs
index 61f801b0a..d8dc41532 100644
--- a/crates/tinyagents-harness/src/agent_loop/mod_tests.rs
+++ b/crates/tinyagents-harness/src/agent_loop/mod_tests.rs
@@ -3693,6 +3693,76 @@ async fn provider_invalid_arguments_recoverable_by_relaxed_json_are_repaired_and
     );
 }
 
+/// The host stack installs `ArgRecoveryMiddleware`, whose `before_tool` runs
+/// before admission's relaxed-JSON repair. It used to coerce a provider-invalid
+/// call's raw string to `{}` (the tool's schema is unknown to it here, so it
+/// counts as permissive), which left repair nothing to fix and answered a
+/// repairable call with the bare parse error.
+#[tokio::test]
+async fn arg_recovery_middleware_leaves_provider_invalid_arguments_for_repair() {
+    let tool = Arc::new(crate::testkit::FakeTool::returning("lookup", "found it"));
+    let mut harness: AgentHarness<()> = AgentHarness::new();
+    harness.with_model_resolver(Arc::new(crate::host::FixedModelResolver::named(
+        "mock",
+        Arc::new(MockModel::with_responses(vec![
+            invalid_tool_call_response("call-x", "lookup", "{query:\"weather\"}"),
+            text_response("found it", 1, 1),
+        ])),
+    )));
+    harness.register_tool(tool.clone());
+    harness.push_middleware(Arc::new(crate::middleware::ArgRecoveryMiddleware::new(
+        Vec::new(),
+    )));
+
+    let run = harness
+        .invoke_in_context(
+            &(),
+            RunContext::new(RunConfig::new("arg-recovery-repair"), ()),
+            vec![Message::user("lookup the weather")],
+        )
+        .await
+        .expect("repaired arguments let the call execute");
+
+    assert_eq!(run.text().as_deref(), Some("found it"));
+    assert_eq!(tool.calls(), vec![json!({"query": "weather"})]);
+}
+
+/// An unrepairable call is answered with the parse detail plus a corrective
+/// the model can act on, not the bare serde error alone.
+#[tokio::test]
+async fn unrepairable_arguments_are_answered_with_an_actionable_error() {
+    let tool = Arc::new(crate::testkit::FakeTool::returning("lookup", "found it"));
+    let mut harness: AgentHarness<()> = AgentHarness::new();
+    harness.with_model_resolver(Arc::new(crate::host::FixedModelResolver::named(
+        "mock",
+        Arc::new(MockModel::with_responses(vec![
+            invalid_tool_call_response("call-x", "lookup", "{\"query\": [1 2]}"),
+            text_response("gave up", 1, 1),
+        ])),
+    )));
+    harness.register_tool(tool.clone());
+
+    let run = harness
+        .invoke_in_context(
+            &(),
+            RunContext::new(RunConfig::new("unrepairable-args"), ()),
+            vec![Message::user("lookup")],
+        )
+        .await
+        .expect("unparseable arguments do not fail the run");
+
+    assert!(tool.calls().is_empty());
+    let transcript = format!("{:?}", run.messages);
+    assert!(
+        transcript.contains("invalid JSON arguments"),
+        "{transcript}"
+    );
+    assert!(
+        transcript.contains("matches the tool's argument schema"),
+        "{transcript}"
+    );
+}
+
 #[tokio::test]
 async fn structured_output_is_extracted() {
     let mut harness: AgentHarness<()> = AgentHarness::new();
diff --git a/crates/tinyagents-harness/src/agent_loop/tools.rs b/crates/tinyagents-harness/src/agent_loop/tools.rs
index a34177577..4fae1acc6 100644
--- a/crates/tinyagents-harness/src/agent_loop/tools.rs
+++ b/crates/tinyagents-harness/src/agent_loop/tools.rs
@@ -869,7 +869,7 @@ impl<State: Send + Sync, Ctx: Send + Sync> AgentHarness<State, Ctx> {
             });
             status.set_last_event(record.id);
             return Ok(ResolvedToolCall::Answered(tinytools::ToolResult::error(
-                detail,
+                unparseable_tool_call_message(&call.name, &detail),
             )));
         }
 
@@ -2342,6 +2342,21 @@ fn repeated_truncated_tool_call_message(tool_name: &str) -> String {
     )
 }
 
+/// The corrective for a call whose arguments the provider could not parse and
+/// relaxed-JSON repair could not fix. A bare parse error ("expected `,` or `]`
+/// at line 1 column 911") gave the model nothing to act on: production traces
+/// show it re-sending the same large inline document until the failure breaker
+/// halted the run.
+fn unparseable_tool_call_message(tool_name: &str, detail: &str) -> String {
+    format!(
+        "Tool call `{tool_name}` was not run: its arguments are not valid JSON ({detail}). \
+         Re-send it as well-formed JSON that matches the tool's argument schema. If the \
+         arguments carry a large document, do not inline it again: build it in smaller \
+         incremental calls (for example with an editing tool that returns a handle or id) and \
+         pass that reference instead."
+    )
+}
+
 /// Appends a batch's follow-up user messages (B2) after its last tool row,
 /// in the calls' original order.
 ///
diff --git a/crates/tinyagents-harness/src/middleware/library/arg_recovery.rs b/crates/tinyagents-harness/src/middleware/library/arg_recovery.rs
index 85e44a276..9850e2117 100644
--- a/crates/tinyagents-harness/src/middleware/library/arg_recovery.rs
+++ b/crates/tinyagents-harness/src/middleware/library/arg_recovery.rs
@@ -24,10 +24,12 @@ use tinytools::Tool;
 /// 1. Already a JSON object → leave it (the common, valid case).
 /// 2. A JSON-encoded string (optionally fenced) that decodes to an object →
 ///    decode and use it.
-/// 3. Otherwise a non-object whose tool schema declares **no** required fields →
+/// 3. A call the provider marked invalid (`ToolCall::invalid`) keeps its raw
+///    string, so admission can run its relaxed-JSON repair on it.
+/// 4. Otherwise a non-object whose tool schema declares **no** required fields →
 ///    coerce to `{}` (legacy-engine parity: the tool runs and produces its own
 ///    recoverable error).
-/// 4. Otherwise (non-object + schema has required fields) → leave the arguments
+/// 5. Otherwise (non-object + schema has required fields) → leave the arguments
 ///    untouched so the crate's `InvalidArgsPolicy::ReturnToolError` path reports
 ///    the original validation failure. Coercing to `{}` would discard the raw
 ///    malformed value without improving recovery.
@@ -66,7 +68,9 @@ impl<C: Send + Sync> Middleware<(), C> for ArgRecoveryMiddleware {
         }
 
         // (2) JSON-encoded-string arguments (optionally markdown-fenced): decode
-        // and adopt the inner object.
+        // and adopt the inner object. A provider marks a fenced object invalid,
+        // so the decode also clears that mark; otherwise admission would answer
+        // the now-valid call with the stale parse error.
         if let Some(raw) = call.arguments.as_str()
             && let Some(obj) = recover_object_from_json_string(raw)
         {
@@ -75,10 +79,23 @@ impl<C: Send + Sync> Middleware<(), C> for ArgRecoveryMiddleware {
                 "[tinyagents::mw] arg_recovery: decoded JSON-encoded-string tool arguments to object"
             );
             call.arguments = obj;
+            call.invalid = None;
             return Ok(());
         }
 
-        // (3) Non-object with a permissive schema (no required fields): coerce to
+        // (3) A call the provider marked unparseable keeps its raw string: admission
+        // runs the relaxed-JSON repair on it next, and otherwise answers with the
+        // parse error. Coercing it to `{}` here destroyed the input that repair
+        // needs while the call was still answered as invalid.
+        if call.invalid.is_some() {
+            tracing::debug!(
+                tool = call.name.as_str(),
+                "[tinyagents::mw] arg_recovery: leaving provider-invalid tool arguments for admission repair"
+            );
+            return Ok(());
+        }
+
+        // (4) Non-object with a permissive schema (no required fields): coerce to
         // `{}` so the tool runs and produces its own recoverable error — engine
         // parity for tools that predate the schema gate.
         let has_required = self
@@ -94,7 +111,7 @@ impl<C: Send + Sync> Middleware<(), C> for ArgRecoveryMiddleware {
             return Ok(());
         }
 
-        // (4) Non-object + schema has required fields: leave untouched. The
+        // (5) Non-object + schema has required fields: leave untouched. The
         // crate admission policy surfaces a descriptive, recoverable tool
         // result without executing the real tool.
         tracing::debug!(
diff --git a/crates/tinyagents-harness/src/middleware/library/arg_recovery_tests.rs b/crates/tinyagents-harness/src/middleware/library/arg_recovery_tests.rs
index 0d8df8306..a7b0a1f4d 100644
--- a/crates/tinyagents-harness/src/middleware/library/arg_recovery_tests.rs
+++ b/crates/tinyagents-harness/src/middleware/library/arg_recovery_tests.rs
@@ -97,3 +97,23 @@ async fn a_non_object_for_a_schema_with_required_fields_is_left_for_the_crate_ga
 async fn an_unknown_tool_is_treated_as_permissive() {
     assert_eq!(recover("nope", json!(3)).await, json!({}));
 }
+
+#[tokio::test]
+async fn provider_invalid_arguments_keep_their_raw_string_for_admission_repair() {
+    let mut ctx: RunContext = RunContext::new(RunConfig::new("mw-test"), ());
+    let raw = "{query:\"weather\"}";
+    let mut call = ToolCall::invalid("c1", "loose", raw, "key must be a string");
+    mw().before_tool(&mut ctx, &(), &mut call).await.unwrap();
+    assert_eq!(call.arguments, json!(raw));
+    assert!(call.invalid.is_some());
+}
+
+#[tokio::test]
+async fn a_provider_invalid_fenced_object_is_decoded_and_cleared() {
+    let mut ctx: RunContext = RunContext::new(RunConfig::new("mw-test"), ());
+    let raw = "```json\n{\"q\": \"x\"}\n```";
+    let mut call = ToolCall::invalid("c1", "strict", raw, "expected value");
+    mw().before_tool(&mut ctx, &(), &mut call).await.unwrap();
+    assert_eq!(call.arguments, json!({"q": "x"}));
+    assert!(call.invalid.is_none());
+}