diff --git a/src/openhuman/agent_orchestration/tools/dispatch.rs b/src/openhuman/agent_orchestration/tools/dispatch.rs
index 212199c48..5d9b2030e 100644
--- a/src/openhuman/agent_orchestration/tools/dispatch.rs
+++ b/src/openhuman/agent_orchestration/tools/dispatch.rs
@@ -119,11 +119,30 @@ pub(crate) async fn dispatch_subagent(
                 agent_id: definition.id.clone(),
                 error: message.clone(),
             });
-            Ok(ToolResult::error(format!("{tool_name} failed: {message}")))
+            // Make the failure unmistakable to the orchestrator: the delegated
+            // task did NOT run, so it must not be reported as success or have
+            // its output fabricated. Without this guardrail a weak orchestrator
+            // can narrate a plausible success from the bare error text — the
+            // "hallucinated success" half of #3193 (e.g. claiming `run_code`
+            // wrote a file when the coding model 404'd and nothing executed).
+            Ok(ToolResult::error(format_subagent_failure(
+                tool_name, &message,
+            )))
         }
     }
 }
 
+/// Format a subagent-delegation failure so the orchestrator cannot mistake it
+/// for success. Kept as a standalone, side-effect-free fn so the exact wording
+/// is unit-testable without standing up a registry + failing model (#3193).
+fn format_subagent_failure(tool_name: &str, message: &str) -> String {
+    format!(
+        "{tool_name} failed and did not complete — no work was performed and no \
+         results were produced. Do NOT treat this as success or fabricate an \
+         output; report the failure to the user. Error: {message}"
+    )
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -162,4 +181,31 @@ mod tests {
             "unexpected graceful-failure message: {out}"
         );
     }
+
+    #[test]
+    fn subagent_failure_envelope_forbids_fabricated_success() {
+        // #3193: a hard delegation failure (e.g. run_code's coding model
+        // 404ing) must be surfaced so the orchestrator cannot narrate a
+        // plausible success. The envelope states the task did not run, tells
+        // the model not to fabricate output, and preserves the root error.
+        let msg = format_subagent_failure(
+            "run_code",
+            "openhuman API error (404): model 'davinci-002' does not support \
+             the chat-completions API",
+        );
+        assert!(msg.contains("run_code failed"), "names the tool: {msg}");
+        assert!(
+            msg.contains("did not complete"),
+            "states no completion: {msg}"
+        );
+        assert!(
+            msg.to_lowercase().contains("do not treat this as success")
+                && msg.contains("fabricate"),
+            "warns against fabricated success: {msg}"
+        );
+        assert!(
+            msg.contains("davinci-002") && msg.contains("404"),
+            "preserves the root error: {msg}"
+        );
+    }
 }
diff --git a/src/openhuman/inference/provider/compatible.rs b/src/openhuman/inference/provider/compatible.rs
index a851f59e9..a82fa972f 100644
--- a/src/openhuman/inference/provider/compatible.rs
+++ b/src/openhuman/inference/provider/compatible.rs
@@ -137,6 +137,43 @@ impl OpenAiCompatibleProvider {
         }
     }
 
+    /// Build an actionable error for a completion-only model that was routed
+    /// to `/v1/chat/completions`. OpenHuman only speaks the chat-completions
+    /// API (with an optional `/v1/responses` fallback) — a completion-only /
+    /// base model 404s here and the responses fallback cannot rescue it, so we
+    /// surface the model name and concrete remediation instead of an opaque
+    /// "responses fallback failed" chain. See issue #3193.
+    fn completion_only_model_message(&self, model: &str, sanitized: &str) -> String {
+        format!(
+            "{name} API error (404): model '{model}' does not support the \
+             chat-completions API that OpenHuman uses — it appears to be a \
+             completion-only / base model. Assign a chat-capable model to this \
+             provider (e.g. in Settings → AI), or pick a different model. \
+             Provider detail: {sanitized}",
+            name = self.name,
+        )
+    }
+
+    /// Guard shared by every chat-completions 404 handler: if the body shows a
+    /// completion-only model, return the actionable error so the caller can
+    /// fail fast instead of attempting the futile `/v1/responses` fallback.
+    /// `None` means "not this case — proceed with normal fallback/enrich".
+    /// See issue #3193.
+    fn completion_only_404_guard(
+        &self,
+        status: reqwest::StatusCode,
+        sanitized: &str,
+        model: &str,
+    ) -> Option<anyhow::Error> {
+        if Self::is_completion_only_model_404(status, sanitized) {
+            Some(anyhow::anyhow!(
+                self.completion_only_model_message(model, sanitized)
+            ))
+        } else {
+            None
+        }
+    }
+
     /// Create a provider with a custom User-Agent header.
     ///
     /// Some providers (for example Kimi Code) require a specific User-Agent
@@ -818,6 +855,24 @@ impl OpenAiCompatibleProvider {
         Self::is_native_tool_schema_unsupported(reqwest::StatusCode::BAD_REQUEST, error)
     }
 
+    /// Detect a 404 whose body says the model is completion-only and cannot be
+    /// served from `/v1/chat/completions` (OpenAI: "This is not a chat model
+    /// and thus not supported in the v1/chat/completions endpoint. Did you
+    /// mean to use v1/completions?"). When this fires, attempting the
+    /// `/v1/responses` fallback is futile, so callers should fail fast with an
+    /// actionable message via [`completion_only_model_message`]. The match is
+    /// deliberately tight so ordinary "model does not exist" 404s are NOT
+    /// caught (those should keep their existing fallback / enrich behaviour).
+    /// See issue #3193.
+    fn is_completion_only_model_404(status: reqwest::StatusCode, error: &str) -> bool {
+        if status != reqwest::StatusCode::NOT_FOUND {
+            return false;
+        }
+        let lower = error.to_lowercase();
+        lower.contains("not a chat model")
+            || (lower.contains("v1/chat/completions") && lower.contains("v1/completions"))
+    }
+
     /// Streaming variant of the native-tools chat path.
     ///
     /// Sends the request with `stream: true`, consumes the upstream SSE
@@ -1363,6 +1418,12 @@ impl Provider for OpenAiCompatibleProvider {
             let error = response.text().await?;
             let sanitized = super::sanitize_api_error(&error);
 
+            // A completion-only model 404s here and the /v1/responses fallback
+            // cannot rescue it — fail fast with actionable guidance (#3193).
+            if let Some(err) = self.completion_only_404_guard(status, &sanitized, model) {
+                return Err(err);
+            }
+
             if status == reqwest::StatusCode::NOT_FOUND && self.supports_responses_fallback {
                 return self
                     .chat_via_responses(credential, &fallback_messages, model)
@@ -1530,18 +1591,38 @@ impl Provider for OpenAiCompatibleProvider {
         if !response.status().is_success() {
             let status = response.status();
 
-            // Mirror chat_with_system: 404 may mean this provider uses the Responses API
-            if status == reqwest::StatusCode::NOT_FOUND && self.supports_responses_fallback {
-                return self
-                    .chat_via_responses(credential, &effective_messages, model)
-                    .await
-                    .map_err(|responses_err| {
-                        let fb = super::format_anyhow_chain(&responses_err);
-                        anyhow::anyhow!(
-                            "{} API error (chat completions unavailable; responses fallback failed: {fb})",
-                            self.name
-                        )
-                    });
+            // A 404 may mean this provider uses the Responses API, OR that the
+            // model is completion-only. Read the body once so we can tell the
+            // two apart (#3193) — only the 404 branch needs it; the response is
+            // not used again here, so `api_error` below still owns the rest.
+            if status == reqwest::StatusCode::NOT_FOUND {
+                let error = response.text().await?;
+                let sanitized = super::sanitize_api_error(&error);
+
+                // Completion-only model: the responses fallback can't help —
+                // fail fast with actionable guidance.
+                if let Some(err) = self.completion_only_404_guard(status, &sanitized, model) {
+                    return Err(err);
+                }
+
+                if self.supports_responses_fallback {
+                    return self
+                        .chat_via_responses(credential, &effective_messages, model)
+                        .await
+                        .map_err(|responses_err| {
+                            let fb = super::format_anyhow_chain(&responses_err);
+                            anyhow::anyhow!(
+                                "{} API error ({status}): {sanitized} (chat completions unavailable; responses fallback failed: {fb})",
+                                self.name
+                            )
+                        });
+                }
+
+                let enriched = self.enrich_404_message(
+                    format!("{} API error ({status}): {sanitized}", self.name),
+                    status,
+                );
+                return Err(anyhow::anyhow!("{enriched}"));
             }
 
             let err = super::api_error(&self.name, response).await;
@@ -1879,6 +1960,12 @@ impl Provider for OpenAiCompatibleProvider {
                 });
             }
 
+            // A completion-only model 404s here and the /v1/responses fallback
+            // cannot rescue it — fail fast with actionable guidance (#3193).
+            if let Some(err) = self.completion_only_404_guard(status, &sanitized, model) {
+                return Err(err);
+            }
+
             if status == reqwest::StatusCode::NOT_FOUND && self.supports_responses_fallback {
                 return self
                     .chat_via_responses(credential, &effective_messages, model)
diff --git a/src/openhuman/inference/provider/compatible_tests.rs b/src/openhuman/inference/provider/compatible_tests.rs
index beb737c97..a02976a93 100644
--- a/src/openhuman/inference/provider/compatible_tests.rs
+++ b/src/openhuman/inference/provider/compatible_tests.rs
@@ -2181,3 +2181,123 @@ fn convert_tool_specs_dedups_many_duplicates() {
         .collect();
     assert_eq!(names, vec!["x", "y", "z"]);
 }
+
+// ── #3193: completion-only model 404 detection + actionable message ──────────
+
+#[test]
+fn completion_only_model_404_detected_from_openai_signature() {
+    // The exact body OpenAI returns when a completion-only/base model is sent
+    // to /v1/chat/completions.
+    let body = "This is not a chat model and thus not supported in the \
+                v1/chat/completions endpoint. Did you mean to use v1/completions?";
+    assert!(OpenAiCompatibleProvider::is_completion_only_model_404(
+        reqwest::StatusCode::NOT_FOUND,
+        body
+    ));
+}
+
+#[test]
+fn completion_only_model_404_ignores_ordinary_not_found() {
+    // A "model does not exist" 404 must NOT be misclassified — it should keep
+    // its existing fallback / enrich behaviour, not get the completion-only
+    // message.
+    let body = "The model `gpt-9o` does not exist or you do not have access to it.";
+    assert!(!OpenAiCompatibleProvider::is_completion_only_model_404(
+        reqwest::StatusCode::NOT_FOUND,
+        body
+    ));
+}
+
+#[test]
+fn completion_only_model_404_requires_404_status() {
+    // Same phrasing under a non-404 status is not the completion-only case.
+    let body = "not a chat model";
+    assert!(!OpenAiCompatibleProvider::is_completion_only_model_404(
+        reqwest::StatusCode::BAD_REQUEST,
+        body
+    ));
+}
+
+#[test]
+fn completion_only_message_names_model_and_remediation() {
+    let p = make_provider("openhuman", "https://api.example.com/v1", Some("k"));
+    let msg = p.completion_only_model_message(
+        "davinci-002",
+        "This is not a chat model ... Did you mean to use v1/completions?",
+    );
+    assert!(
+        msg.contains("davinci-002"),
+        "names the offending model: {msg}"
+    );
+    assert!(
+        msg.contains("completion-only") && msg.contains("chat-completions"),
+        "explains the capability mismatch: {msg}"
+    );
+    assert!(
+        msg.contains("chat-capable model"),
+        "states the remediation: {msg}"
+    );
+}
+
+#[test]
+fn completion_only_404_guard_fires_only_on_signature() {
+    let p = make_provider("openhuman", "https://api.example.com/v1", Some("k"));
+    // Matches → Some(actionable error).
+    let hit = p.completion_only_404_guard(
+        reqwest::StatusCode::NOT_FOUND,
+        "This is not a chat model. Did you mean to use v1/completions?",
+        "davinci-002",
+    );
+    let err = hit.expect("guard should fire on the completion-only signature");
+    assert!(err.to_string().contains("davinci-002"));
+    // Ordinary not-found → None (normal fallback/enrich path is preserved).
+    assert!(p
+        .completion_only_404_guard(
+            reqwest::StatusCode::NOT_FOUND,
+            "The model `gpt-9o` does not exist.",
+            "gpt-9o"
+        )
+        .is_none());
+}
+
+#[tokio::test]
+async fn completion_only_404_fails_fast_without_responses_fallback() {
+    // End-to-end over the wire: a completion-only 404 must short-circuit with
+    // the actionable message and NOT attempt /v1/responses (not mounted here —
+    // if the guard regressed, the error would instead read "responses fallback
+    // failed"). Provider has the fallback ENABLED (default `new`), proving the
+    // guard pre-empts it. #3193.
+    let server = MockServer::start().await;
+    Mock::given(method("POST"))
+        .and(path("/v1/chat/completions"))
+        .respond_with(ResponseTemplate::new(404).set_body_json(serde_json::json!({
+            "error": {
+                "message": "This is not a chat model and thus not supported in the \
+                            v1/chat/completions endpoint. Did you mean to use v1/completions?",
+                "type": "invalid_request_error"
+            }
+        })))
+        .mount(&server)
+        .await;
+
+    let provider = OpenAiCompatibleProvider::new(
+        "openhuman",
+        &format!("{}/v1", server.uri()),
+        Some("key"),
+        AuthStyle::Bearer,
+    );
+
+    let err = provider
+        .chat_with_history(&[ChatMessage::user("write a file")], "davinci-002", 0.0)
+        .await
+        .expect_err("completion-only model must error");
+    let msg = err.to_string();
+    assert!(
+        msg.contains("davinci-002") && msg.contains("chat-capable model"),
+        "expected actionable completion-only message, got: {msg}"
+    );
+    assert!(
+        !msg.contains("responses fallback failed"),
+        "guard must pre-empt the responses fallback, got: {msg}"
+    );
+}